DaviSound Custom Octal Preamp
Operational Notes


This fully custom "audiophile" styled preamplifer was comissioned to employ something of the styling of the DaviSound TB-1, Pro Audio "Tool Box" but with custom features desired by the client. It was to employ all "Inner Tube" styled " vaccum tube color" circuitry of the type utilized in the TB-1 and the other DaviSound "Inner Tube" pro audio processors.

It was also one of the first cabinet styles to utilize the new, "Retro" standard, 2 1/2" Sifam VU meters. The VU meters are calibrated to read zero VU at 1 volt peak to peak with the rear panel mounted trim controls full CW. The trim controls may be reduced (CCW) as desired to accomdate zero VU for other, higher, operating levels.


The Octal Preamp was desired to impart vacuum tube "warmth" to the audio signal so classic 8 PIN early generation triode envelopes were chosen to be superior in this regard over latter generation 9 pin miniatures. The Generic 6 SL7 is High MU dual triode employed in the first stage of each channel and buffered by a Medium MU 6SN7, dual triode,in the cathode follower mode for optimum low impedance output characteristics. Russian millitary imports were used in prototyping and shipped in the unit. The tube chain gain and level structure is adjiusted by the front panel "active" gain controls which control the gain ratio structures of the vacuum tubes instead of acting as tradtional input attenuators. The preamp is, therefore, designed to accomodate a wide variety of input, line level signals from below .333 V to above 1.2 V by adjusting the front panel controls as needed.

DaviSound, unique "Inner Tube" circuit principles are utilized which force the tubes into early (low level) generation of even order signal harmonics, mostly second harmonic, while, at the same time, surpressing the non-musical and undesirable, harsh, odd-order signal harmonics! The result is the sound of full, rich overtones, imparting a "warm", lush low end and a "silky sweet" high end which, some feel, is particularly helpful in processing and listening to digital signal sources for long periods without listener fatigue.

This physical design incorporates the DaviSound "Inner Tube" principle with an MP-T circuit block floor mounted inside the cabinet. Care should be taken when pulling tubes to avoid weakening the floor mounts. It is suggested that a hold-down support, such as the rubber end of an ordinary wooden pencil, be held against the tube board with one hand as a safety measure of reinforcement support while pulling the tubes with the other hand. The octal sockets are very snug (which is why tubes are shipped in socket) and offer a significant amount of resistance when pulling the large octal pins free.

Operation is logical and straightforward so requires very little explanation. The left position of the input select switch selects the XLR input and mutes the RCA input jack, whereas the right most position of the toggle handle selects the RCA and mutes the XLR.

The gain controls will normally be operated between 9 O'Clock and 12 O'Clock knob handle positions. This will provide approximate unity gain operation to incoming signals . THE GAIN CONTROLS ARE NOT ATTENUATORS BUT TRUE GAIN CONTROLS AND WILL NOT, THEREFORE, BE TOTALLY SILENT AT FULL CCW SETTINGS! IT IS POSSIBLE TO CAREFULLY fine-tune THEM TO THE ZERO LEVEL "NULL" SETTING JUST PRIOR TO THE FULL CCW NEGATIVE GAIN SETTING.

Originally, the AC power switch was intended to be located in the center insert directly below the tube view/vent port. However, it was determined during prototyping that the switch would be best positioned nearer to the power supply at the right of the panel. Therefore, the AC power switch was relocated and a voltage status, yellow LED indicator was placed in the center panel insert. The main power switch, toggle up, activates the AC power supply and automatically applies the heater voltage and the safe plate voltage in the proper warm up sequence.

Input Impedance - 20 K Ohms       Output Impedance - XLR- 200 Ohms, Subs 500 ohms       Nominal operating levels - under .333 Volts to over 1.2 Volts.
Frequency Response - Input to Main Output - 1 HZ to 30 KHZ , +/- 3dB ...
Input to Sub output 1 Hz to 1 KHz with first order rolloff filtering beyond 1KHz at 6 dB per octave.
THD - (mostly enhanced second harmonic) - approximately 5% full CW Gain setting, 2% 12 O'Clock setting, less than 1% at 9 O'Clock, or below.
Signal To Noise - Greater than 76 dB unweighted      Heater voltage - DC Pi Filtered 6 Volts, 1 amp
Plate Voltage - decoupled and individually filtered by 3 separate Pi Nets and 2 input capacitors.

Dual rear inputs are XLR and RCA which are front panel switch selectable.
Outputs are XLR with dedicated RCA sub woofer outputs per client request.
This unit is configured for European standards at 220 volts, 50 HZ. A 1 Amp AGC primary fuse is utilized.

Copyright ©2008 by DaviSound