TB-8 Monitor Reference Amplifier
Operations Guidelines And Considerations
The TB8-SEDS, "DaviSolidSingle" amplifier is described following the TB-8 description.
By Hayne Davis - Copyright 2002 - 2012 © by DaviSound
The original "WG" TB-8 ... circa 1998 version.
Our first of the newly "Re-Simplified TB-8" ... 2005 ... In a custom Walnut "WG" package.
Customized version of the new 2006 model to include custom applied, input monitor, VU Meters.
Rear View Of Same ...
The standard TB-8 rear layout.
Current top cover design uses a rear row of ...
small, optimumly spaced, air - vent portholes for heat dispursion.
Many users continue to request the VU meter customization option to accurately monitor the incoming signal level ...
In early 2008 we completed a beautiful, oversized, customized TB-8- 3RU, special for a client ...
which incorporated our first set of applied SiFam meters, the slightly larger size versions than our, now standard, 2 1/2" sized models.
This first, custom, "PedOak" unit to use a pair of these meters definitely conveys the charm of "vintage" styling!
Click the link below to view this model ...
TB-8 "WG" 3RU SiFam Retro VU Model
THIS EXOTIC EBONY CABINET VERSION DEMONSTRATES THE CURRENT, STANDARD, FRONT PANEL LAYOUT.
The TB-8, in keeping with DaviSound tradition, has always been a relatively electronically simple, yet an "as good as it gets", accurately transparent audio reproducer ... with power drive capabililty ... period.
Early in 2005, in keeping with our current "theme" ... "lower efficiency for higher fidelity" ...
which was brought about by extensive prototyping and comparative testing of all kinds of topologies in preparation of our newly designed "DaviSingle 3 T" (Single Ended Tripple Triode Triangle) powered monitor ...
(As an aside you can read the philosophy behind this new unit at Glass Ear ) ...
we decided to once again change the "innards" of our amazing TB-8 reference amplifier. This latest design we have settled with is what I call further "simplified simplification" as we have returned full cycle (pun optional!) from the bridge technique. As a result, although measurements are near identical in terms of electrical parameters in regard to those that affect "sonics" (power bandwidth, THD, intermod distortion & noise)... it seems to me this new version has a slightly "sweeter" sound.
Yes, I suppose I am waxing somewhat more "DaviSubjective" in my middle age!
I attribute this to our "simplification" away from bridge operation whereby we further reduce the crossover distortion potential factor (before applied corrective negative feedback). One push/pull stage must be be beter than two! The only "penalty" was 6 dB less theoretical output/headroom but I don't think that little extra margin was worth all the extra stages required for the signal to contort through.
I think it is now our best monitor amp to date!
Yes, "Bridge" (balanced) operation is more "efficient" in the traditional sense which is defined by how well the amplified signal is power transfered to the speaker load. But, if we redefine "efficiency " as how well an amplifier transfers ultimate signal fidelity to the load, then power may not only NOT be a contributing factor but, perhaps, it is even a detriment !
Long ago I learned that, to a great extent, both accuracy and "listenability" (a loosely defined term of mine to mean the ability of an amplifier to be listened to for hours on end in critical monitoring without creating listener fatigue) in monitor amplifiers was inversely related to high output power. In my estimation, given reasonably efficient speakers, more than 10 watts in a monitor amplifier is not only unnecesary, but undesirable as well.
OVERVIEW- "Ignorance Is No Excuse!"
Some of you may feel the following introduction might be more appropriately included in the DaviStatements section rather than in an "applications manual".
However, in order to fully educate would-be users who will be reading this data for more descriptive information before a purchase, as well as inform those who have already purchased a TB-8 on how it does what it does, I feel it may be necessary to also try and further "educate" many readers on power amplification in general. For that reason, please indulge the following preface.
My above assumption is based upon the large volume of email questions I get nowadays which tells me with marked assurance, that the masses, even very many within the ranks of the "Pro" audio engineering community, do NOT always know what they THINK they know when it comes to audio electronics these days! This seems to be especially true in regard to both audio preamplificatioon as well as POWER amplification and and what they both ACTUALLY involve!
This is particularly sad because so many will have read this or that "half-truth" somewhere, or bought into a semi-understood advertiser's exageration, and thereby founded and developed their cumulative knowledge on numerous, often quite shaky foundational fundamentals.
So, pardon me if, at times, this "applications manual" takes on a frustrated editorial viewpoint, but if you can really put yourself in our place for awhile, I think you will see the necessity of this approach!
This particular manual and "wordy" preface is, of course, about POWER amplfication.
Excuse the "weak analogy" but ...
Just suppose you KNEW for certain that you had developed the most pure, the most nutritional, and the world's most efficient food which, by the way, also turned out to be the tastiest as a side benefit. And, the few you could coax to try it always agreed with you to the point of astonishment!
However, you often had a hard time getting new tasters, let alone buyers, for your new super food because the masses were just not used to your methods of preparation and many were actually ignorant of the very virtues of the ingredients which made your product the marvelous thing it was! And, too, to make it even more difficult, you did not rely on "politics" and "payola" to promote your product!
Well, again, pardon the weak analogy attempt, but we at DaviSound often find ourselves in just such a dilemma when attempting to introduce our products to many of the "sophomoric" members of today's audio engineering school!
We often find it necessary to dispel popular myths and misconceptions about the "whats and whys" of audio equipment in general aside from actual comparisons of one type, or brand, from another!
So, we have made the attempt with our "on-line" manuals to try and impart as much information as possible about the APPLICATION itself, as well as the tool for the application, which is our particular piece of gear we are describing for the job.
The FACT actually is, really and truly pure and simple, the more you REALLY KNOW about audio electronics, the more you simply have to aspire to DaviSound's methods of concept, design and fabrication. Therefore, it is to MUTUAL advantage, for us to present as much honest, factual information as is possible within these "on-line manuals"!
We have found that, aside from microphone preamplifiers, POWER AMPLIFICATION is one of the most misunderstood areas of applied audio in general amongst many of today's practitioners!
So, we will spend a little time with our description of the TB-8 and, along with the reasons why we think it should be everyone's monitor reference amplifier, we will also include some basic information on power amplification in general! I trust that those of you who already KNOW and understand what I am about to cover will appreciate and tolerate my need to explain these parameters for those who don't already know.
Thank you all for reading and thank you for considering/owning a DaviSound TB-8!
"WATTS UP ...DOC?"
Aside from "What does it 'sound' like in comparison to "Brand X"?" ...the most common power amplifier question is, "How much output power does it have?" Or, in our case, "Why doesn't it offer MORE output power?". My answer to both of these latter queries in regard to power would be ..."It has enough"!
Back in the seventies, when the audio world was still struggling for STANDARDS in many areas of audio electronics measurement and specifications as stated by manufacturers, TRUE power, or RMS power became the power spec of norm. The adaptation of this "norm" was largely dictated by the Hi-Fi press to "equalize" maunfacturer's individual "specsmanship" (that being the creative ability of manufacturers to come up with new and different terminology to allow better numbers for their products on paper). And, rightly so because the adaptation of this standard helped put aside exhorbitant, manipulative claims (or was intended to do so) in terms of an amplifier's "power" output and made certain that the amplifer could deliver so much indefinite RMS output-period.
I became so entrenched in this idea myself that I, nearly forever, began to see all other rating considerations such as "Music Power", "Peak Power" or even "Momentary Peak Power" ("InstantaneousPeak Power") etc. as deliberately EVIL misrepresenations by their designers!
But, more recently, I can begin to understand the frustration and resulting reasoning of some of these old designer/manufacturers and, indeed, the many, particularly in budget car audio, who still use these techniques today. When such terms are so misunderstood and universally misapplied or misconsidered as sole motivation for purchase, then no wonder the manufacturers were/are tempted to "bend the facts" to give them what they want and, often erroneously, what they think is all important in a power amplifier ... the "King Watt"!
Now, no one would argue that, all things being equal, MORE power is better than less power. But, the point is, in the real world, all things are NEVER equal and it is this very imbalance, that often makes it desirable to choose somewhat LESS power in a monitor amplifier. The reasons for this will become readily apparent as you read further!
But, before we can ascertain and distinguish the various parameters of a power amplifier that contribute to "power", or the lack thereof, we must first provide some fundamental education in regard to audio power amplifier electronics.
To help with this, I will herewith "lift" some text from the "Power" section of our well known DaviSound TECH REPORTS and then we will elaborate even further.
Some Fundamentals On Power Amp Design EDITED from DaviSound TECH REPORTS ...
There are some very basic concepts that we will arm you with right now. First, hopefully based on some of our earlier discussions in regard to “LOADS” in AC lines, you can gain some insight as to just WHAT constitutes “power” in an electrical circuit. POWER, measured in watts, is the multiple of the current flowing in a circuit times the voltage potential of the circuit. Grasp that and you have it made!
Thus, if you have a 100 volts line potential at your outlet, and your resistive “load” across this line is such that it draws one amp of current, then you have a 100 watt circuit working. If you have a 100 volt potential and you short it to ground, you have a VERY powerful circuit, for a second or so, until all that current flow burns your wire into (the principle behind a fuse!).
From the brief example above, it should be seen that the lower the resistance in the load (a direct short is as low a resistance as there is!) the greater the current flow and thus the higher the power assuming the voltage potential remains constant with no voltage sag occurring at the power source as a result of the heavy load.
Now, in audio circuits, SPEAKERS provide this load. From the above example it can be seen that 4 OHM loads draw twice as much current as do 8 ohm loads and, assuming the voltage source potential, in this case the power amp, can maintain its output voltage without being “loaded down”, then the 4 ohms will give us twice the power!
Most amplifiers are designed to "swing", at their outputs, a voltage that is within just a few volts of their DC power supply. This means, if you are using a +/- 15 or 30 volt supply, you may likely get a +/- 12 PEAK or, ROUGHLY 6 volt RMS output swing. To illustrate another useful mathematical relationship, let’s say that due to circuit losses, our amp can swing 10 volts RMS into a 10 ohm load. Now, some very simple math can tell us what our output power is! That little “formula” is Vsq. / R. To state that another way, square the output voltage and divide by the load resistance. That is simple enough, no? In this case 10 volts squared is 100 ....divided by the load of 10 ohms...means that our amp is delivering 10 watts in this example!
An interesting way to increase the available output voltage swing of a power amp, WITHOUT INCREASING THE DC POWER SUPPLY VOLTAGE ... is to use a "BRIDGE" configuration. Perhaps it could be argued that ALL power amps should be bridge designs because of their voltage efficiency!
A "BRIDGE" amp is actually nothing more than an electronically "balanced" output amplifier consisting of two separate amplifiers, operating out of phase, each driving one side of the speaker (or other AC load) , each amplifying one HALF of the total waveform cycle.
Since a bridge amp can swing the same amount of voltage in both halves, it is an effective way to DOUBLE the output voltage across the load. But, while this fact is often confused by many, it provides you with FOUR TIMES the output power. This is because the current also doubles with the increased voltage swing. Using our earlier simple math for figuring power you can see this result for yourself.
Assume that one amp, one half of the bridge, can swing 20 volts by itself into 4 ohms. Using the simple formula, we square the output voltage to get 400. When we divide by our load of 4, we have a 100 watt output. Now, if we add a second amp to “push” 20 volts on one cycle while the other “pulls” 20 volts on the alternate cycle, we have a total of 40 volts across the load, thus doubling our voltage swing. But, do the math and see how doubling the voltage quadruples the power! 40 squared is 1600. 1600 divided by the 4 ohm load is...yep...400 WATTS!
Now, all of this math assumes that our real-world constants remain the same under all conditions. This means the amps must be able to, indeed, swing that much voltage and the power supply must be hefty enough to maintain said quadrupling of power. Very FEW can do this which is why you often see a spec like 100 watts into 8 ohms 150 watts into 4 (instead of the expected 200 watts into 4!) . Or, in the case of bridge operation you might see 100 watts per channel stereo, 200 watts bridged mono instead of the expected 400 watts bridge.
These diminished specs would likely be because of the real world power supply and/or output transistor current limitations. And, they are, very likely, what has confused many people in regard to output power ratings and capabilities!
Just remember, ALL THINGS BEING CONSTANT, halving your output load doubles power and using a bridge configuration QUADRUPLES output power! But, few real-world designs can achieve this when going from their highest quoted 8 Ohm Stereo output ratings to either 4 ohms and/or bridge operation.
End of text excerpt.
And ... WATTS MORE ...(Or Less) ...
One of the BIGGEST KEY FACTORS in terms of "how much power is required?" ... is the efficiency of the load that you are driving in it's ability to convert this power it receives into the work you want done!
With audio amplifiers, this translates specifically to SPEAKER EFFICIENCY!
Here is where we separate the "sophomores" from the "seniors" in our school of Audio Power!
It has ALWAYS amazed me why amateurs, and often even the audio press, concern themselves with power ratings in amplifiers and. for the most part, IGNORE speaker efficiency ratings!
Before we fully consider this MOST IMPORTANT aspect of what really constitutes "loudness", while also providing "headroom" in a system, we must, again, digress briefly to more fundamental discussion.
It is often surprising that, for whatever reason, some respected engineers do NOT always consider the efficiency factor of primary importance in their power transfer systems! I remember a conversation in this regard with my consulting Radio Engineer once years ago when I was building my FM Broadcast station. We were not discussing AUDIO power at that time but, rather, RF power.
I knew that I had picked the right man for MY job when he talked of antenna efficiency (often referred to as "antenna gain" in RF circles). He mentioned that some engineers suggest putting up relatively simple antennas for the FM broadcast band, with comparatively low efficiency, and driving them harder with higher power for the desired effective radiated output. He stated that HE preferred to use a 'HIGH GAIN" (efficient) antenna, and drive it with less power to the load for the same result. Though not an "RF expert" by any means at that time, this made very good sense to me because it was the same logic that I always instinctively employed when setting up AUDIO systems! I always made sure the speakers were of the highest efficiency I could obtain!
The area we are in now, this "speaker efficiency" area, while a consideration of utmost importance in a system, it seems to be the least understood and least applied consideration from what I see taking place in the audio world these days!
Though not often advertised this way specifically, I do see some designers implying this "perfect matching" between power amp output and speaker load design aspect when promoting their "powered speaker" monitors which are now gaining in popularity. I have no doubt that this is one of the prime considerations of such designs.
AND JUST WHY IS THIS "EFFICIENCY" PARAMETER SO IMPORTANT AND JUST WHAT DOES IT MEAN SPECIFICALLY?
Well, here's the "scoop". We have already determined that the two main concerns for power output in an amplifier are LOUDNESS, or "Sound Pressure Level", and system HEADROOM - the latter being important so that peak clipping will not occur at any operating level.
Well, consider this MOST IMPORTANT fact-
For every 3 dB increase in loudness in a given sound system, there must be a DOUBLING of power to acheive it!
That being the case, it should be seen just HOW important the relative Sound Pressure Level output capability of a given speaker really is!
For example, ALL speakers have a rating standard of 1W/1M. This means that Sound Pressure Level is measured at a distance of 1 meter from the speaker, while being driven with a power level of 1 watt measured across the speaker.
My 14 year old, who is as of this writing just discovering the wonderful world of audio, is constantly talking these days about this or that amplifier, or this or that speaker, in terms of WATTS! Unfortunately, like many of his more sophisticated complements in the pro audio media world, he believes simply that MORE WATTS means MORE sound, period. He has yet to grasp how all of what we are talking about here translates into LOUDNESS in a system!
In order to try and teach some of this to him, I looked carefully at the speaker packaging box in a local department store for one of the models he was considering. Sure enough, across the box, we saw ... "Brand" 200 WATT Woofer! I had to, somehow, get across to him that in speaker terms, all that meant was that the speaker was SUPPOSED to be capable of withstanding a driving power of 200 watts without self-destructing! He, like many other novices (or even those "sophomores" in our "schools of thought"), somehow naturally wants to equate this "WATT RATING" with sound output much the way you would with lightbulbs in terms of LUMENS or "light output".
And, you know, you can't blame him. Because, that is the natural inclination for most people who would likely assume such a falacy. And, to make it worse, this common misconception seems to be PROMOTED by the manufacturing and sales communities as a means of selling speakers!
And, based on a large volume of my email correspondence, this misunderstanding is quite "alive and well" to some degree in today's Pro Audio World as well! Thus the motive for all of this "preface" to our TB-8 description!
But back to the illustration. I looked carefully and, sure enough on one of the sides of the box in small print, I found what we were actually looking for. And, to their credit, this brand did, at least, list the figure in the specs. It read ..."SPL 84dB". It did not include the necessary 1w/1M but we give them the benefit of the doubt in that regard, and assume that measurement standard to be applied, because SURELY, nearly ANY speaker could acheive such a low efficiency rating as 84dB at 1W/1M!
Now, to try and give a reference to all of this, I pointed out that an SPL of 84dB at 1W/1M would actually be quite audible. When you consider that normal conversational speech level is around 70dB at 1 foot, and that translated to roughly 67 or so dB at 1M, then 84dB would be about like someone talking VERY loudly, if not shouting at you! And, it only takes 1 watt to give you that with that particular speaker! He seemed impressed!
I then pointed out that it is was QUITE common to find comparable woofers with an efficiency rating of 94dB...or even 96 to 98 dB ....at 1W/1M!
I think, at that point, I was beginning to strike a nerve as I could tell by the inquisitive look that he was pondering carefully what I was saying.
I went on to explain about how the logarithmic mathematical principles at work in the physics of sound caused it to be necessary, as our heading states, to have to DOUBLE our power just to get a 3dB increase in perceived volume output. THEREFORE, I explained, could he not see how he would be so much better off and ahead in the game if he just purchased a woofer with a higher SPL rating for his proposed system to begin with?
I had him calculate with me, just how much power the friend who had purchased this model woofer would have to use, to equal his "regular" 6 watt automotive amplifier driving a more efficient model!
I think he was beginning to get it! I hope our novice readers are as well!
Roughly speaking, one could get the same SPL from a system using the 84dB rated speaker and a large amp pushing 60 watts, by using a 6 watt amp and a speaker rated at 96dB!
To further understand what we are saying, let's look at it from another angle. Just add it up ...
Say we want to acheive a VERY loud sound output level of 102dB at a distance of 1 meter. Let's look at what we need. The LOW efficiency speaker rated at 84 dB at 1 Watt, would give us 87dB at 2 watts, 90 dB at 4 watts, 93dB at 8 watts, 96dB at 16 watts, 99dB at 32 watts, and our desired 102 dB at 64 watts. On the other hand, with our 96dB at 1 watt speaker, we only need 2 watts for an output sound level of 99dB, and then 4 watts for 102 dB!
It has always amazed me that MOST people, including many engineers and audiophiles, have no clue as to "how many watts" produce a given sound level in their system!
GIVE A TEST ...
It is a very simple thing to measure wattage, using even a simple Volt/Ohmeter. Of course, the better the meter, the more trustworthy the resulting measurement.
Here's how to do it. Set your VOM to it's very lowest AC scale. This will usually be 10 volts on an inexpensive meter. Then, play some music through your system with fairly steady rhythm patterns so that the peaks will be more easily observed on the meter. Crank it up to a decent volume, as loud as you can stand, and then measure the voltage across the speaker outputs.
The meter will be swinging in "VU" fashion, but you can easily obseve the highest peak readings with some practice. Note this measured AC voltage. it will represent the "RMS" voltage of your audio signal. Square this number (multiply it by itself) and divide the result by the speaker impedance.
Example: Your meter is peaking right at 6 volts so you square this reading for a total of 36. Your speaker impedance is known to be, say, 4 ohms so you divide 4 into 36 and you get 9. In this example, the system would be supplying 9 watts RMS.
Now, once you master this technique, you will surprise yourself by trying to guess how much power you are listening to! No doubt, if you are new at this, you will assume your system is putting out a hundred or more watts at that 9 watt level! That is, if indeed, you can get it to actually acheive 9 watts or stand to listen to it long enough to measure it while it is putting out this level of power (assuming an efficient speaker of course!).
Put your friends or a local "audio expert" to this test and give them an education as well!
As you will readily see, the old "Law Of Diminishing Returns" comes into play with audio power amplification in a very real manner!
Before we leave this subject of "watt measurement", let's elaborate a bit. Once you have played the loudness "guessing game", you can carry this process just a little further and conduct a TRUE RMS power measurement test on your amplifier. The proper way to do that (and the healthy, safe method for your ears!) ...is to LOOK at your amp's output as opposed to listening to it. You do this by substituting a "Dummy Load" for your amp instead of a speaker.
You will need a large (high power rating) resistor, preferably slider adjustable from below 2 up to 10 ohms. Measure the resistance first with your ohmeter as you set the slider for the desired reference load. You may want to test your amp at several different settings (say, 2, 4, 8 ohms).
Next, you will need a sine wave generator of some kind, a low cost audio oscillator will usually do the trick. While it can be a fixed frequency type, an adjustable unit will let you measure the amplfier's power output at varying frequencies across the band. Next, you will need an oscilloscope. Connect the probes of the scope across the resistor terminals which are , in turn, connected across your amps's output.
Finally, connect your AC voltmeter in parallel with the other connections. Then, with a signal applied and synced for viewing on your scope, turn it up until you see the sine wave start to flatten on the scope and then back off until it just resumes it's sinusoidal shape. At that point, look at your voltmeter and note the voltage. This will be your amp's true maximum RMS output voltage swing, which you square and divide by the resistor value for the total wattage as described above.
Now get ready for another surprise! Very likely you will find that your amp will NOT produce nearly the power you thought it would or at the rating it was advertised for! I have found very few in the real world that actually meet their published specs in terms of RMS power output!
SO WHAT'S THE "POWERPOINT" ? ...
So, aside from filling large arenas and concert halls with sound reinforcement, what is the need for all of this power? The ONLY justification for using somewhat high power amplifiers in critical listening applications would be - HEADROOM.
It is in this area of "HEADROOM", that is the ability of the amplifier to reproduce momentary peak transients without clipping or increased distortion, that the earlier discussion in regard to "Music Power", "Momentary Peak Power", and similar descriptions comes into play. In fact, this is the area where such application consideration takes on some very practical validity!
All other factors being equal, up to a point, an amplifier's PEAK POWER capability will normally be, roughly, about TWICE that of it's RMS POWER capability. So what is wrong with stating that in a specification as long as it is made clear? If it is not understood by the layman, then so be it but that is not the manufacturer's fault and does not imply that it is given to mislead the reader in any way! On the contrary, it is a very VALID consideration.
This is because, it is very typical for REAL WORLD MUSIC signals to utilize much different amplification requirements than a steady sine wave at a continuous setting! It is very typical for an amplifier design to faithfully deliver close to 100 watts for the milliseconds required of, say, a bass drum peak at a given gain setting ... yet maintain no more than a 50 watt continuous RMS rating! It is also reasonable to intentionally design such an amplifier with these parameters in mind!
One of the primary circuit factors which determines a power amplifier's output capability in terms of rated power output is its power supply. And, usually, that comes down to the current capabilities of the power supply's AC power transformer.
So, it is certainly no "sin", or in any way bad engineering practice, for a designer to choose to design a comparatively low RMS power amplifier as long as it's power supply will allow for any momentary peak power demands that will take care of the HEADROOM requirements for transient signals!
In fact, for professional recording monitor applications, it is arguable that LESS is better! Why? Simply because, aside from being lighter, less bulky, less expensive and just plain all around more efficient (that old "law of diminishing returns", remember?), there are always other circuit requirements and considerations which are necessary for very high power amplifiers that "get in the way" of transparent sonics! These are things like multiple output transistor stages and more complicated biasing and current limiting schemes ("clutter"!).
When we designed our first professional monitor power amplifier back in the seventies, we, like most everyone else, utilized every "in vogue" technique known at the time! This included elaborate output transistor biasing schemes and, since we wanted to boast a 100 watt RMS output, capability, we employed all the necessary protection schemes that go along with high power.
One of the most audible, at least to some of us, is always any form of output current limiting! In fact, it soon became obvious to us, that the ONLY way to get the kind of clean power our ears demanded was to simply eliminate all "protection" support circuitry and simply use the best over-rated parts we could find and just design a "barebones", faithful amplifier! This meant, quite simply, using a faithful voltage gain stage followed by a simple but accurate and dependable current boost stage. That combination is, of course, essentially what a power amp is- no more no less. And the fewer "frills" one uses to acheive it, the better!
The DaviSound circuit philosophy has always been "Less is usually MORE" in terms of audio quality. This applies to our complete signal chains from microphone amplifiers all the way to speaker amplifiers! We follow this philosophy because it is an EDUCATED, learned way of doing things and not because we may be "lazy", "cheap" or otherwise cutting corners. In fact, it could easily be argued just the opposite is true since we came by this philosophy "in reverse", so to speak! Our current circuits and fabrication methods have evolved over years of experimentation, lab work, field trials and many hours of "listening and tweaking" ... certainly no cheap, "corner cutting" in any of that!
Thus ... the TB-8 is, of course, one of those "less is more" designs.
We firmly believe that the microphone amplifier and the power amplifier should be the two most decidedly accurate stages in the professional studio's signal chain!
Any "artistic coloration" from assorted esoteric processing may take place in between, but not with, the mic amp at the front and the monitor amp at the end of the chain.
Even if one might argue that, in some cases, the mic amp might be deliberately chosen for "coloration" for desired, predetermined sonic "signature" effect, surely NO one worth his/her salt as an archive engineer would try an make a case for "coloration" or, so-called "character", in a reference monitor amplifier!
Always keep in mind the design principles described above and that the TB-8 was not designed as a "power" amplifier but as a faithful, minimal power Monitor amplifier. As such, it requires efficient speakers for highest volume levels. While some users find it fine for use with speakers of 88 to 90 dB 1w/1m efficiency, it is usually desirable to have better speaker efficiency in the 96 to 98 dB 1w/1m, or better, range.
Of course highly efficient speakers usually make better monitor speakers anyway due to their extra sensitivity to subtle signal variances.
When you listen to a TB-8 working into the proper, efficient load at the proper operating settings, you won't hear noise, you won't hear distortion, but you WILL hear your signal...ALL OF IT ... no more, no less ... with all of the faithfully reproduced volume you could want, because it is intentionally designed to be just this ... the most efficient, yet exact, power ampilfier ever offered for professional control room monitoring (or similar) applications!
TB-8 Specifics ...
The TB-8, as you might have summised from all of the "build-up" so far, is designed and fabricated based on the design philosophy reflected in the preceeding text.
This means that it, uses one simple pair of Class A/B operated push/pull current boost transistors to buffer the preceeding MasterPiece module voltage gain block stage, per channel.
Whatsmore, by using over-rated parts in terms of voltage / current / thermal handling capabilities, and utilizing these high power parts at relatively low power, we are able to avoid the support parts clutter of assorted protection schemes usually employed with designs that push their signal path parts to the limit.
TB-8 Electrical Specifications
Each channel of the two channel TB-8 is, specifically, formed with our highly regarded DaviSound MP-2 modules, front-end adapted for line level signals, each driving just a single complementary pair of "beefy" power transistors biased to Class A-A/B (meaning further into Class A than traditional A/B bias) incorporated within their feedback loop with a constant current load.
The MP-2, as especially configured for the TB-8, operates at a fixed internal voltage gain of 5 (14 dB) and the dual input level controls are configured as balanced, "H Pad" variable attenuators.
The input attenuator controls are, of course, relative and may be set as desired. However, for reference, "wide open" represents the full gain of 14dB from the TB-8 voltage ampfier gain stage. 12 O'Clock (straight up white indicator) settings are about the unity gain setting for an 8 ohm speaker load where 1 volt at the input equals 1 volt across the 8 ohm output.
Here are the measured results:
All known forms of distortion including Slew Induced Distortion, Total Harmonic Distortion, Intermodulation Distortion, Transient Induced Distortion and Phase Induced Distortion, plus Noise, are less than .008% combined at any level up to clipping!
Signal to noise is better than 96 dB.
Frequency response is flat from 2.12 Hz to 60 KHz at any level into 8 ohms.
RMS OUTPUT POWER is about 10 Watts per channel into 8 ohms, and 5 watts per channel into 4 ohms.
While the TB-8 is typically 4 ohms stable, keep in mind that some speakers claiming a 4 ohm impedance are, in reallity, of LOWER actual impedance. Obviously, the ideal mate for the TB-8 is a very efficient (high SPL rating), 8 OHM speaker system.
Peak power into 8 Ohms is right at 20 Watts per channel and instantaneous "music power" similarly increases further again.
This is because 20 separate 1,000 micro-farad (20,000 uF total) filter/storage capacitors in the power supply help provide instantaneous peak demand reserves well in excess of the modest, continuous RMS power limitations.
Speaker protection is inherent by design and, assuming connection is by reasonably low capacitance speaker cable, no peripheral componentry, such as a traditional Zobel R/C network as used on the earlier bridge design, is required nor necessary for stability under load.
Internal output impedance is well under 1 ohm for a very high damping factor into any rated load.
Input impedance is approximately 20 Kohms, balanced, across an H-Pad balanced attenuator.
Balanced XLR inputs with Pin 1 Ground Pin 2 High and Pin 3 Low.
The latest edition now features the inclusion of a front panel stereo headphone jack to more easily facilitate the amplifier's use as the ideal headphone amplifier. Headphones my be utilized simultaneously with speakers or the TB-8 may be used exclusively as a headphone amp without having to connect speakers!
ELECTRICAL SPECIFICATIONS FOR HOUSE IC USED IN MP-2 Modules
MORE ABOUT THE VOLTAGE AMPLIFIER STAGE ...
Frequency response, (as-is, un-compensated, unity gain, .777 V RMS input ) ... flat to 10 MHZ
Input Noise Voltage at 1KHZ - less than 4 nanovolts per stage
Typical Supply current - 11 mA (maximum- at full gain- full load)
THD at unity gain, per inverter stage less than .0002%
Slew Rate 12 V/uS
Maximum Supply Voltage + 48 volts
Lifespan- unknown - absolute minimum expected performance ...
under less than ideal thermal conditions - 20 years ...
Typical expected performance under average operating conditions - 100 plus years ...
Theoretical performance under ideal conditions - 1,000 years!
The inherent specifications of the IC itself are actually further improved in our audio application
by our circuit design techniques within the MP-2 module!
One such technique ... which we believe has so much to do with the exceptional, noticable "accuracy of detail" comment that we hear so often from users in regard to the performance of our amplifiers ... is a unique inter-stage direct coupling/biasing scheme which forces the operational amplifier into near full Class-A operation. Any extra heat generated in this process is harmlessly dissapated in the thermally conductive epoxy ecasement of the sealed MP-2 module!
The "WG" rack cabinet itself is made from hand sculptured SOLID, natural wood. As of spring 2001, we have opted for lustrous, hard penetrating oil finishes for the cabinetry as this provides added protection for the wood and, unlike other oil finishes, it is relatively maintenance free. Simply wipe it with a soft dry cloth to restore lustre.
The front panel as well as the rest of the cabinet are ALL solid oak (unless other exotic woods have been utilized custom by client arrangement/preferance). Like all natural things, every piece of wood has its own, distinct character and "personality". This makes EACH AND EVERY "WG" model unique unto itself which enhances the aesthetic value of the piece and the actual appreciative monetary value as well!
Keep in mind that as part of this wood "character" it is impossible to find/utilize a "PERFECT" piece of wood. This means there may be slight imperfections, or variations, in terms of grain pattern, color uniformity, and skew. However, any such natural variations in our selected pieces are very minor and are the very thing that gives wood its charm and "character" appeal!
There are NO user servicable parts inside. DaviSound Tool Boxes are fashioned from our "Master Pieces" modules which employ hand-laid circuitry, opaque epoxy embedded into plastic cased modules.
The TB-8 also utilizes four discreet (2 NPN and 2 PNP) power transistors fastened to two specially fabricated heatsink assemblies
inside the cabinet.
In the unlikely event that service should ever be required, simply contact us for return authorization and we will replace the defective components at no charge for the original owner.
THERE ARE EXPOSED, POTENTIALLY LETHAL AC VOLTAGES INSIDE UNIT NEAR THE POWER SUPPLY! DO NOT REMOVE TOP COVER UNDER ANY CIRCUMSTANCE WITH UNIT'S AC POWER CORD CONNECTED!
Though never having been enforced, it was original DaviSound policy to void warranties if the top cover is removed!
Internal tampering or unauthorized repair or modification attempts certainly void all warranties!
NEVER , UNDER ANY CONDITION, ATTEMPT TO REMOVE THE REAR COVER, THE BOTTOM OR THE FRONT PANEL FROM A TOOL BOX UNIT AS PERMANENT DAMAGE WILL RESULT!
DaviSound products are carefully assembled BY HAND, one piece at a time from start to finish, in prototype assembly fashion, with extreme care and attention. They are designed to perform from now on with no trouble.
However, there are always "real world" imperfections that can not be anticipated or accounted for no matter HOW careful one might be in fabrication. For example, we don't make the ICs and we don't wind the power transformers and these things can fail at times. Ours seldom do, due to extensive testing and pre shipment "burn-in" periods, but if you should ever have a problem, you are protected for as long as you own the equipment, as the original owner.
Simply carefully repack the unit in the original packing, ship it back to us (after notification) to
DaviSound, P.O. Box 521, Newberry, SC 29108 803-276-0639
and we will repair and reship at no charge to you whatever!
Thank you for your purchase and use of a DaviSound Tool Box!
And, we also thank you for allowing us to go these "extra lengths" within YOUR operating manual to elaborate in accordance with our "mission" in publishing these on-line manuals. It is all part of an ongoing effort to provide as much educational material as possible from this website in the sincere belief that ...
The more one truly KNOWS about audio electronics, the more one can truly appreciate the DaviSound methods!
We thank all of you who ALREADY know all of this covered herein for your indulgence as we present it for those who do not!
Please let us hear from you regarding your application of our unit and feel free to contact us with any questions or comments throughout the future!
We also hope that you will consider ALL of the DaviSound Tool Box models for your rack!
THANK YOU and GOOD LUCK!
There will, likely, be a generation or two of amplifier operators who will not be able to conceive of, let alone understand, a 1 watt monitor amplifier. These will also, no doubt, be amongst those of the latter generation who have, truthfully, never actually experienced the ultimate in audio reproduction excellence. It is unfortunate that many of today's "professional" operators have spent their entire professional life with ears wrapped around contemporary systems which inherently fall short, to varying degrees, of the ultimate, attainable in electronic reproduction. The old saying ..."they just do not know what they are missing" ... is most accurate in this case.
The "DaviSolidSingle" TB8-SEDS
Copyright © 2012 by DaviSound - All rights Reserved and enforced
SOLID State, SINGLE Ended, SINGLE Watt (rated), SINGLE Active Discreet Component
The TB-8, whose theory of operation is fully described above, was designed to offer them the opportunity of changing this unfortunate scenario when married with a good, low distortion highly efficient monitor speaker system.
Then, the TB8-SE was designed to take it even one step further with the inherent seamless waveform reproduction of single-ended operation! . So, if you want to hear the ultimate in amplfier reproduction "pure and simple", this just may, possibly, be your one, "single" (DaviSolidSingle) chance!
Inaugural version of the TB8-SE in sloping, Desk Mount Black-Walnut Cabinet
Photo Shows Original TB8-SE Prototype Rear Layout
Unique custom cabinet housing the first of the latest, final version- the TB8-SEDS
(SEDS for Single Ended, Discreet, Single component)
You can see the complete photo page for the first, custom cabinet, TB8-SEDS version here - UNIQUE CUSTOM DESK MOUNT TB8-SEDS PROTOTYPE
The TB8-SEDS is only available in 3RU cabinet size to accomodate the spacious layout of the internal components
and the heavy heat-sinking required for Class-A operation.
It has been said, with some accuracy, that an ordinary incandescent light bulb is an electric heater that emits some light as a bi-product. Well, it can also be said, with certainty, that a Class-A single-ended audio power amplifier is an electric heater that can be used to amplify audio as a bi-product. Furthermore, it is FAR more efficient as an electric heater than as an audio amplifer, which is why you seldom see them used anymore.
Why, then, deliberately design a new version the world's most inefficient audio power amplifier? Why, indeed! For those who enjoy listening to music at low to medium playback levels (and even LOUD levels on occasion with efficient speakers), all it takes is a few hours listening to the TB8-SE to know why.
Also, for those of us who are "minimalists" by nature, there is a true enjoyment and fullfilment in doing the most with the least. That is why it is so enjoyable for me, personally, to introuduce the latest, evolved, version of the TB8-SE in the spring of 2012 (although I must admit, as one who despises waste in any fashion, I enjoy using my Class A gear most in winter months when it can contribute useful heat to the room instead of requiring extra cooling!).
We have had the "DaviSolidSingle" in the lineup already for a couple of years now as of this writing. However, it has already reached its THIRD, evolved, state at this point. I can tell you that this is where it was, ultimately, headed but I just needed to find that "just right" transistor, and support topology, for my "single" active component per stage. After much prototyping and testing the past year, I have now settled on one and have built the first of the new, SINGLE component, SINGLE ended, amplifiers!
When we started this edition, the SINGLE in "DaviSolidSingle" referred strictly to single ended power stage rated at a single watt output. But the preamp stage driving the output power transistor(S) was our renowned MP-1 "MasterPiece" module. I then decided to create an ALL DISCREET version, and settled on a power version of our famous MP-4 discreet amplfier module topology which utilizes two transistors in Class A, a driver and a power output stage. Quality wise and performance wise, this could NOT be improved upon. It is, perhaps, the ultimate low power amplifier.
However, I still wanted to offer a version that truly lived up to the SINGLE aspect of the name all the way. Thus, I began experimenting with assorted, modern power transistors and support topologies that would offer some gain and drive in the power stage itself while developing enough power in Class A for a good solid watt of output with acceptably low, primarily second harmonic distortion signal component. The device I settled on, could easily be used in this manner and could, with a large enough supply and heat sinking for components throughout, be utilized for up to SIX watts or more of audio output with a 1 volt RMS input signal.
Our chosen device easily handles the smaller supply in our cabinet version for its rated 1.5 watt (at < .05% THD).
To use this amplifier, one MUST employ highly efficient monitor speakers. Often, in today's power hungry world, this requires building your own or having someone build them for you from hand picked speaker components. With an amp like this, you really want to be somewhere above 92 dB 1w/1meter efficiency and the higher the better. Those with 98 dB 1w/1m, or better, speaker systems will REALLY rave about this amp!
But, the beauty of designing for around one to two watts maximum power, is the fact that you can utilize ORIGINAL, simple, small signal amplification techniques and employ them for a, albeit very low, POWER amplifier! Furthermore, it can be done with a single, BI-POLAR transistor operarting with some voltage gain (fairly unique configuration of the textbook common emitter amplifier), with NO IRON (transformers/chokes/inductors) in the audio signal path. The ONLY transformers used within our design are the necessary AC power line transformers in the power supply. We utilize TWO of these, by the way, in TWO entirely separate power supplies so that each channel has its own, exclusive, isolated, power supply thereby meeting the so-called, "mono block" requirement.
Frequency response - FLAT from 10 Hz to 200 Khz - 8 ohm load
THD - .05% or less up to 1.5 watts RMS (mostly second harmonic)
Maximum output prior to clipping - 1.28 Watts RMS into 8 ohms - 1.32 watts into 4 ohms
Signal To Noise - Better than 100 dB
Input Sensitivity - Approximately 1 volt RMS input for 1 watt output
Total power Dissipation - Approximately 28 watts per channel - 56 watts total
Actual output impedance - Approximately 2.2 ohms
Input impedance - approximately 10 Kohms
Ideal speaker load - 8 ohms at minimum +89 dB 1w/1m - preferably higher (96 dB and above)
While the TB-8SEDS is typically 4 ohms stable, keep in mind that some speakers claiming a 4 ohm impedance are, in reallity, of LOWER actual impedance. Obviously, the ideal mate for the TB8-SEDS is a very efficient (high SPL rating), 8 OHM speaker system.
Speaker protection is inherent by design as is amplifier stability and, assuming connection is by reasonably low capacitance, standard speaker cable, no peripheral componentry, such as a traditional Zobel R/C network as often used on bridge, or push-pull designs, is required nor necessary for stability under load.
Input is unbalanced across a standard input attenuator volume control. The TB-8SEDS is now offered in only the unbalanced input configration with inputs via gold plated RCA jacks.
30,000 uF, total, individual, R/C filter/storage capacitors, per channel, in the power supply help stabilize and filter the high current demands of the Class-A output stages. The TB-8SEDS employs separate, isolated DC supplies for each channel, in essence forming a dual "monoblock" amplifier.
The TB-8 SEDS features the inclusion of a front panel stereo headphone jack to more easily facilitate the amplifier's use as the ideal headphone amplifier. Headphones my be utilized simultaneously with speakers or the TB-8SEDS may be used exclusively as a headphone amp without having to connect speakers!
It can be seen from the specs in the table above that even the most efficient Class A power designs, as this one is, waste a lot of electrical power in heat for the small amount of useful work they provide. In this case, each channel must draw around 28 watts to deliver the rated 1.28 watts of output and the idle current is just about as great as that under full load audio output. So, it is no joke when we mentioned above that these amplifiers are electric heaters ... in this case a 56 watt heater from which we extract 2 watts of audio!
The other noteable drawback of such simple designs (a single device with gain), according to some critics, is the relatively low damping factor compared to most modern day, multi-stage, solid state amplifiers. For this designer's ear and speaker systems, the "damping factor" is largely theoretical and not really a factor at all (so long as load impedance is, at least, twice the actual output impedance) . There are, in fact, some audiophiles who claim to pefer the "sound" of this slight inefficiency, if, indeed, there truly is one (this "sound character" is typically detectable to some degree with single stage OTL vacuum tube amplifiers of higher output impedance and, thus, much lower damping factors as opposed to our solid state version).
As for the ADVANTAGES of such an amplifier ... to some of us there are many ... which is the whole justification for utilizing them. One of the main, noticable, true advantages of SINGLE ENDED Class-A is the lack of listener fatigue over many hours of critical listening! This, we attribute, to the seamless full wave amplification of the single ended device which is unique from any other topology.
Another, real measurable, advantage would be the even order distortion harmonics inherent to this configuration as opposed to the odd order harmonics inherent with all push-pull designs (of course, with distortion below .05% in our design, it is of little consequence in any regard).
Suffice it say, for some of us, once we live with one of these amplifers over time, it is difficult to tolerate any other type for both critical listening and casual listening!
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