This preamplifier was designed for versatilty and a measure of control seldom seen with so-called, "high-end" designs.

Rather than use the traditional rotary switch method of selecting inputs, the "HAUPFEAR" utilizes a bank of double-pole, double-throw mini-toggle switches mounted at the top center of the control panel with which to select any of the FIVE, rear stereo inputs from their gold plated RCA jacks.

All RCA Input and Output jacks are, of course, GOLD Plated
for maximum conductivity and corrosion resistance.

There is also a single, front-panel convenience jack per channel, also with an
adjacent in/out switch.

You will note that this switching system allows for a measure of "MIXING", or more accurately, combining, by a properly designed matching network of low noise, metal film resistors.

Coming from first a broadcast and, then later, a recording studio background, I never understood the flawed logic of purposely avoiding a mix feature in a multi-source preamp because of some misplaced reasoning. In fact, I originally considered a full mixer design for this unit incorporating both individual level controls as well as "balance" controls for each input!

One of those "purist" "scoffers", you say? Well there is no reason for it to be, that's for sure!

I maintain that a properly designed, and matched, combining network will have no effect on signal purity other than, possibly, very subtle level changes due to the slight variations in loading when switching multiple sources in/out. This, of course, the whole point of having individual "gain" controls for compensation!

Furthermore, since the unit is NOT intended primarily for use with the combining feature, each combining resistor is totally OUT of the circuit when any input toggle switch is in the "down/out" position.

Our "old-school" hand wiring techiniques and layout, employing braided, twisted pairs, insure that there are NO effects, or artifacts, caused by the wiring from this input arrangement.

In reality, one may very likely never use the "mix" feature, especially for more than two sources at a time, but it is there just in case. Otherwise, the operator simply switches the input toggles on/off in succession, one at a time,
as may be desired.

While on this topic, it is worthwhile considering just why we need/use a preamp in any event now that high line level digital sources are the norm. I also find it amusing that I now hear discussions about "passive" "preamplifiers" ...
an oxymoron if I ever heard of one!

Assuming one could cope with about 6 dB overall through-loss, I could just have easily designed and offered a truly PASSIVE control center to be used ahead of one's power amplifier. I would have, of course, in that case called it a "passive controller" and NOT a "passive preamplifier".

But, even with the high levels of CD players and other similar sources available nowadays, it is still nice to actually have a true GAIN option when desired. In this instance, I chose to have our gain, about 15 dB per channel, provided by a uniquely configured triode vacuum tube per channel! This is plenty of drive and control to properly match the needs of ANY power amplifier!

And, while on this subject of the "INS AND OUTS" of this unit, though it should go without saying to any enthusiast who may be considering such a preamp, we will say it anyway. The inputs to "Haupfear" will accomodate virtually any line level audio source such as CD player, DVD player, MP3 player, computer audio line output, analog tape deck, TV audio, receiver, home theatre processor ... WHATEVER.

Likewise, the low impedance outputs will drive any variety of loads such as, of course, your power amplifier of choice, a recording unit digital and/or analog, computer audio line input and, virtually, anything else of the proper load impedance (10 Kohms or above) that you may want to bridge across the output divide network at any output jack.

And, of course, the newly modified version of "Haupfear" offers PASSIVE RIAA equalized phono preamplification now normalled through at the channel 5 inputs.

The extra gain buffering also allows for another very useful feature which is a totaly transparent, "either / or" function that we have included. It is a very gentle EQ "pre-set" which we call the "DaviSirkit". This feature provides for,
at least, some tonal enhancement at the flick of a switch when/if it may ever be desired. If not ...
simply never switch it in!

I assure all "purists" that the "DS" pre-set EQ ...
has absolutley no affect on the circuitry, or the audio signal, when not switched in!

The DaviSirkit provides a mild, (+7 dB Maximum) shelving, hi / lo boost curve
very similar to traditional "loudness" equalization.
It is accomplished, in this instance, with passive R/C nets ...
composed of polypropylene film/foil capacitors ...
and non-inductive, low-noise, close-tolerance resistors.

It is, in actuallity, a broad dip filter that, when used in conjunction with a few dB of tube gain
from the gain control, provides the HI /LO shelving "boost" effect.

OTHER FEATURES INCLUDE ...

You will note the unusual power switch bank to the right face of the control panel. There are three vertically in-line mini-toggles with three adjacent, associated, staggered LED indicators.

The lower most switch is the main AC power switch.

The second, middle switch is the DC powered, "Filament" switch for the tubes which energizes the tubes to "warm-up", or "stand-by", condition.

The final, top-most toggle controls the plate voltage and it is interlocked with the filament switch so that it can not be turned on unless the filament switch is on.

We utilize the familiar, instinctive "Traffic Light" indicator sequence with the bottom, "idle" indicator being RED ... the middle "warmup" (heater voltage) indicator YELLOW ...
and the top most "on" (Plate voltage) indicator GREEN.

We are amongst those who still believe it desirable, even with low power, small signal tubes, to switch power to the heaters for a minute or so prior to switching on the high voltage to the plates.

This system requires you to follow that 1-2-3 power-up procedure.

Continually switching high voltage to cold tube plates could, possibly, cause cathode stripping.

By the way, we also recommend that the unit be shut down by the same method - in reverse order.

Furthermore, it may be worth mentioning that despite the advice to the contrary by some of today's "expert" dealers, we are of the "old pro school" that believes tube gear should best be left on consistently,
twenty-four hours a day, for maximum lifespan and lowest noise, consistent electrical performance over time.

In the "old days", large studio complexes would even "cook" new tubes, in specially designed equipment with
"burn-in" sockets, for extended time periods before retubing gear and the studio equipment was, subsequently,
NEVER powered down!

So, you might want to consider, at the very least, minimizing the on/off cycles of your own gear!

Just as two, separate L/R input jacks are provided for auxillary front panel access, two output jacks are also provided on the front panel, one for each channel. In addition, there is a headphone switch which operates "either/or" routing the main outputs to their normal distribution networks and subsequent rear panel Gold plated RCA jacks, or to the front panel 1/4" T-R-S headphone jack.

Since there is no discreet headphone amplifier, there can be no simultaneous feed to both the normal outputs and headphones. However, this method does allow for monitoring the outputs, albeit at very reduced, low volume, with lightweight, medium impedance headphones, direct from the tubes, when desired for testing purposes. It will be explained how this low impedance drive is possible from an OTL circuit, when circuit topology is covered shortly!

ALSO SEE *TUBE NOTE* BELOW

The associated level controls are separate, one for each channel, and are highest studio quality, sealed, conductive-plastic potentiometers guaranteed for over a million, smooth, noise-free rotations.

Light Therapy?

Among the unique features of "HAÜPFEAR", are the rotatable,
white LED accent lamps as pictured above.

These feature tight-focused, white LED spot lighting mounted in penlight housings attached to ball-in-socket swivel mounts at the top ends of the control panel. This allows ambient lighting to highlight the control panel under dim lighting conditions and provides a very appealing aesthetic effect, along with the warm glow of the tubes and the LED indicator lighting.

These lights may be rotated to virtually ANY position thus accenting and illuminating any part of the panel, or adjacent surroundings, to the degree desired.

Just as the preamplifier offers the front panel tube ports for both viewing and ventilation purposes, the top cover provides similar, 1 1/2" diameter ports directly above each tube. This allows for air flow as well as easy viewing of the tops of the tubes.

Since the cover is too far above the sockets for easy finger access ...
whenever cold tube swaps might be desired for experimentation, or for replacement ...
the top cover must then be completely removed.

        Dimensions are: Faceplate 19" x 6 3/4"         Cabinet - 17" (width) x 10" (depth) x 6" (height)

We regret that the lighting and color balance in the photos abvove is not entirely accurate and does not convey the richer BROWNISH color of the Black Walnut.
The color rendition is somewhat more accurate in the photos of the panel lighting closeups shown previously.

The unit includes four one-inch diameter rubber cushioned feet mounted on the bottom.

THE NEW, COPPER-BACKED, 1/4" THICK WALNUT CIRCUIT BOARD FOR THE STEREO PHONO STAGE

The phono stage, added since original completion in April, is a classic passive RIAA EQ, "sandwich" ...
with gain distributed amongst two triode stages for both optimal headroom and S/N.
The first triode is the input buffer for each stereo channel...
and the second tube serves as the EQ buffer for each channel.




A NEW ANGLE "UNCOVERED"

Another photograph of the unit, with a different camera from the last photo shoot,
allowing a little closer to the true color representation of the black walnut cabinet.
It also better displays the gentle, tilted slope of the front control panel.

CIRCUITRY AND SPECIFICATIONS

And speaking of tubes ...
I know that all readers, by now, must be quite anxious to find out about the circuit topology...
and the tubes which are utilized.

With a name like "HAÜPFEAR" ...
you, no doubt, expect the use of Telefunken smooth plates, or similar.

As a matter of fact, we did consider using Telefunken 12AX7s for this design but a matched pair was not readily available at the time of completion and testing (But ... be sure and see the "Tube Note" included below!).

We did find a set of Brand New (NOS) 5751 tubes with near identically matched specs! For those who may not know, 5751s were originally introduced by GE in 1948 (coincidentally, the same year I was "introduced" so we are the same age!) for "aircraft, industrial and control" (ARINC) designated applications for the military.

While often seen in "hi-tech", state-of-the-art medical equipment of the day, such as one of the first, successful ECG machines, these tightly held, military specs tubes were very expensive to produce and were therefore considered much too "pricey" for general purpose audio use in those days.

However, they are, essentially, just very rugged 12AX7s with extra mica spacers, sturdier frames and extra parameter control maintained during manufacture, including must-pass tests for extremely
low internal vibration and microphony.

The heaters run slightly hotter, drawing a bit more current and they inherently offer reduced distortion over a 12AX7. Otherwise, they are essentially the same as a 7025 version of a 12AX7. With their linear characteristics, they happen to be ideal for the particular circuit topology that we chose for our preamp!

And, just as I can attest that this is an unusually VERY quiet tube preamp design overall ...
I can also assure you that the matched 5751s offer very low noise performance indeed! Your audio will "jump out" to you from a background of "velvety silence"! The recent noise tests with these tubes actually far surpass the published specs herein!

"Re-Inventing The Wheel"

I have an odd characteristic (actually MANY "odd" characteristics according to my wife!) of consistently attempting to "re-invent the wheel". I am noted for going off for days on "brain-storming/brain-bruising" sessions figuring out "new ideas", honestly and creatively ...
only to find out later that somebody else had already figured it out before I did!

Such was the case, to some degree, with the circuit I came up with that I felt may make a unique,
"ideal tube preamplifier" gain block!

I think that for me personally, as a designer/builder, the chief appeal of, and continuing fascination for, vacuum tube audio, (nostalgia aside) is, again that phrase, "elegant simplicity" as I have often seen it called. Those audio enthusiasts familiar with the DaviSound ... "DaviSimple" ... philosophy know that we aspire toward the most efficient, least cluttered circuit topologies we can devise for ALL of our designs!

I have had quite a bit of past experience with vacuum tube design and application but my most extensive past experience, and my expertise, is, admittedly, with making solid state circuitry "jump through hoops".

So, when I began my latest "brainstorming" efforts, scribbling various circuits on paper, trying to come up with what I thought would be a fairly original idea for a two stage, reasonably low impedance, high-drive tube line amplifier circuit, I was naturally drawing on my experience and knowledge base of solid state circuit design.

I had never actually employed direct coupled, two-stage triode circuits in operation before but wanted to try and implement the concept in this design if I could. I started with the same design I use in our DaviSound "MP-4" discreet transistor, Class-A, "MasterPiece" circuit block module. And, while, of course, they are not always directly interchangable, I began sketching out triode tube configurations with plates, grids and cathodes in place of collectors, bases and emitters or drains, gates and sources.

It wasn't long before I actually came up with my own "original" ... so I thought ... version of an unusual looking circuit that I subsequently learned had already been devised, even pattented ... just a "little while ago" ...
a little while ago back in 1943, no less!

Then, after being initially put-off a bit, I was both surprised and simultaneously elated to discover that a few of today's high-end designers are already using this circuit, or their tweaked version of it, in somewhat the same way that I intended!

The general circuit topology that I am referring to is known as SRPP. This translates to ...
"Shunt Regulated Push Pull" configuration. At least that is the most commonly accepted name of the circuit!

It seems that SRPP was the only designation listed for the circuit by the originator and everyone else has since just guessed exactly what SRPP refers to! Some think it is ...
"SERIES Regulated Push Pull", while some few say ...
"Series Reflex Push Pull".

Personally, based on the way it functions , as well as the terminology that was used in the forties ("reflex"), I tend to go along with the latter designation- "Series Reflex Push pull". But there were shunt voltage regulators based upon similar cascade structure so who is to say for sure where, exactly, the SRPP designation comes from or what it refers to. In any event, it is basically just a variation on the old "totem pole", bootstrapped, cathode follower layout with a few "twists" applied. In my case, I have even employed another subtle "twist", or two, (actually just a few well-chosen/placed resistors) to further balance the circuit and enhance performance!

Now, I have have had some correspondence relating to this topology that I need to address here and, thereby, clarify some misconceptions. Since our first listing, several "passionate", audiophiles have written to tell me that I should consider something besides an SRPP styled circuit for my new "tube gain block" for this or that reason. As is, more often than not, the case in such "biased" situations, each of these was basing a conclusion on wrong information in all of these communiques!

First of all, the circuit DOES , indeed, function in TRUE CLASS-A ! In fact, that is the only way this circuit CAN function ... it depends upon Class-A bias in order to work at all!

Secondly, yes I know it really is "push-pull" and, unlike some, I do NOT consider that a bad thing when properly applied. But, in fact, I do think that this unique circuit arrangement might offer the "best of both worlds" and that fact may lead to, yet, another name designation for it by some users ...
that being "SINGLE-ENDED REGULATED PUSH-PULL" (oxymoron time again?)!

Thirdly, yes, it truly does indeed act as its own "phase-splitter" for push-pull function and, although no curative, overall negative AC voltage feedback is utilized, it also seems to have a "magic" distortion-reducing arrangement by about a factor of 10 (based, I suspect, upon the 180' out-of phase, constant-current, cancelling effect)!
Some "simple" little circuit, indeed!

So, while there seems to be much misinformation out there about this, obviously, very misunderstood circuit topology, and I don't pretend to have all the answers to solving its mystery, I can say, based on my testing and experience to date, I believe it to be the most unique, all around tube circuit topology I have come across! Since there is no overall negative voltage feedback, there still can be plenty of "tube sound" although the measured distortion is lower than expected at nominal output levels and loading.

It DOES offer the ideal audio transfer characteristics of high input impedance, very low output impedance and excellent linearity with fairly high output drive capability. All of the parameters, of course, being governed precisely by the particular tube utilized. Some of the most common tubes seen in various spin-offs of this technique include the, low-gain, 12AU7 and its close cousin, the ubiquitous, high-gain, 12AX7!

Our particular "SuperSirkit" here operates in Class A with no externally applied global AC feedback!

Now, what we have learned is, the Sylvania, JAN (joint Army Navy) military grade 5751s we are utilizing herein, perform exceptionally well in this circuit and exhibit superb audio characteristics right out of the chute!

Our uniquely configured version of the SRPP circuit is already superb for audio, as mentioned, with the "upper half" of the tube acting as the direct-coupled cathode follower output stage while simultaneously serving as a constant current, plate load for the voltage gain, "lower half" configuration. That, aided by the extra linearity of the 5751 employed in this application, makes for an almost unbelievably low distortion tube amplifier!

So ... that's our topology ... almost public domain "textbook" but with a couple of our own little improvement "tweaks" and a great pair of tubes!

The new owner of the "HAÜPFEAR" can feel free to experiment and try other 9 pin, dual triode equivalents in the circuit, including those beloved Telefunkens, and see what his/her own ears reveal. That is, of course, half the fun of owning a tube amp!

* TUBE NOTE *

Substituting a pair of higher current, high-drive 12AT7s in this preamp design (in the main tube board) will actually yield listenable headphone levels from the headphone jack, direct from the tubes ...
if medium to high impedance (non-loading) phones are utilized!

So, that being the case, we will INCLUDE, AT NO EXTRA CHARGE ...
a selected pair of genuine Telefunken 12AT7 tubes for the new owner ...
inside the box, at shipment!

The "Uniqueness" Continues ...

Our 5751s are mounted on a unique, hardwood circuit board whose hand-wired, hand-soldered, surface mounted support components are securely surrounded by thermal epoxy. The 9 pin tube socket is, itself, epoxy mounted to the board and its silver-soldered pin connectors are also encased in a rigid, protective covering of thermally conductive epoxy!

NOTE:
All electrical connections within DaviSound "DaviSheen" preamps ...
including all internal to HAÜPFEAR ...
are first soundly, mechanically formed ...
and then SILVER soldered!
(This unit utilized almost an ounce of silver solder)

The discreet circuit boards are, in turn, mounted to the cabinet bottom with a sandwiched layer of double- sided copper ground plane between the boards and the Walnut floor.

Whether or not you have the West German Telefunkens installed in this particular model, the "HAÜPFEAR" can boast another "theme" component from the "Motherland" ... the inclusion of ROE, high quality, low ESR, "European style" high voltage electrolytic decoupling capacitors of West German manufacture used consistently throughout.

Three such 1,000 uF caps are seriesed, to triple their voltage rating, for a resulting capacitance of about 300 uF. These banks are, in every instance, by-passed over-all by high quality,
.68 uF 400V film/foil signal capacitors.

You will note one such decoupling R/C network at the B+ entrance to each tube. The caps are recess mounted in countersunk holes, again encased in thermal epoxy for stability. As a result of this method, each tube, in effect, has it's own isolated supply! Our decoupling arrangement has ultra low impedance at audio and allows ultimate dynamic range.

The power supply board is made of yet another section of discreet Black Walnut. It mounts two separate, high quality Toroid, transformers feeding a high voltage, full-wave bridge rectifier for the stable, high voltage B+ circuitry exclusively and a third transformer for the separate heaters and lamps supply.

The resulting DC is filtered on the main board by an RC net previously described and, in turn, by the individual decoupling nets on the tube board.

"HAÜPFEAR" utilizes RC filtered DC for the filament supplies.

The entire supply is fed from a fused, EMI filtered, IEC connector with detachable AC cord.

---

The Haupfear Consolette about midway during construction and prior to supply board installation.

Another interior view of The Haupfear Consolette upon completion.

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Electrical Measurements

Gain +15dB into 10 Kohm load Input Impedance ~ 10 Kohms Output Impedance ~ 150 ohms Bandwidth (-3dB) 2.12 Hz to 66 kHz Combined Distortions < 0.08% at 1kHz, 1.2 Vrms into 10 Kohms THD almost entirely second harmonic Odd order and all upper harmonics near nil SNR - Main Inputs to output > 87 dB unweighted SNR - Phono Inputs to output > 76 dB unweighted Output to input phase Inverted at center pins

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The following segment by DaviSound associate engineer, O.T. Hudson

DaviSound has been custom fabricating hand-made, top quality signal chain audio components for almost two and one half decades. Many of the things you read about that are heralded as "innovative" by various manufacturers in today's marketplace, DaviSound has been aware of .... and utilizing .... all this time.

Hayne Davis "wrote the book" (quite literally - Davisound TECH REPORTS ©1986/2000) on audio power supply design and capacitor relationships, along with special grounding techniques, and has always practiced what some are just now advocating .... such as multi-tier filter capacitor banks .... signal quality
by-pass capacitors throughout .... and decoupled, isolated supplies for each stage.

Likewise, truly unique grounding and construction techniques, many of which are exclusive to DaviSound in this day and age, also contribute to the unique audio clarity that distinguishes all DaviSound creations.

Another DaviSound tradition has been to use hand-picked, matched, selected
over-rated components and carefully, and skillfully, hand-solder them using tedious assembly techniques that are rapidly becoming a lost art amongst today's engineering society.

Interconnecting hard-wiring employs exclusive Twisted Pair Belden Ribbon Cable with alternating pairs configured as ground shields between the signal pairs
(based upon a technique taught by the late, great audio engineer ...
Norman Crowhurst!).

Other "DaviSecrets" amongst these utilized from day one by DaviSound, but only recently being boasted about elsewhere, are to keep circuit impedances comparatively low throughout the internal chain and to keep the signal path as simple and as uncluttered as possible with a minimum of active, and reactive, components all properly positioned and laid out with respect to each other.

I assure you this is a "living example" of the "perfect marriage" of "yesterday/today", handcrafted, audio engineering!

I am pleased to contribute my endorsement of Hayne's personal work on this one!