A simple preamp utilizing E88CC or 6922 double triodes in a classic long tailed pair configuration. It runs off relatively low voltage supplies, but at a pretty high current, at least for these small-signal triodes.
Gain is approx 20dB as shown, (single-ended in/out), with 500 ohm cathode resistors gain drops to about 6dB, I use the higher gain because my 8watt power amp is just a single-ended MOSFET follower without a voltage gain stage, (one per phase so it's fully balanced throughout).
It swings +/- 20volts, (double that for balanced outputs), and has a bandwith close to 500kHz. Most importantly for a whopping 1volt output, enuff to clip most poweramps below 100watts, the current varies less than 1mA and the voltage swing is minute compared to the 100volts at the anodes, so it's always near it's quescent state, hence very linear.
I paralleled two triodes so it could present lower internal resistance, better handle the low-valued anode resistor and high current, and as a bonus (theoretically) noise is reduced . The 47ohm anode-resistors were added to stop oscillations due to un-matched tubes. The cathode resistors also help the balancing, so matched tubes are not required, as a matter of fact unmatched tubes gives less cancelation of even order harmonics, perhaps 'enhancing' the sound, that is if it's used as a balanced preamp. We all got different tastes...
Squarewave response, 2V/div, balanced output, (83.8kHz):
On the right I zoomed in to measure risetime, approx 718ns, giving fc of about 490kHz.
Output impedance is 1100ohms (single-ended), not close to OP-amp circuitry, but those who believe that matters are missing the big picture anyways, just use it with an amp with input impedance higher than 10kohms, although distortion will not suffer into heftier loads, just gain.
The powersupply is in a seperate chassis except for a final pi filter utilizing 150µF polypropylene caps and 10H chokes per channel located in the forward half of the preamp's chassis. A delay of about one minute heats the valves a little before HT is applied.
The circuitry is hard wired located in the rear for short paths, it's got silver wiring, balanced stepped attenuator from Sonic Frontiers, passive selector switching between two SE and one BAL inputs. One set of SE outputs drive a Parasound 100watter for the subs, the other set of outputs are balanced for main amp.
Seen below is the ugly insides. I think most prototypes turn out a little messy like this, nothing really goes as planned. I always underestimate the need for space when I make the chassis, and since the chassis takes endless hours, I'm stuck with the problems.
Another shot of the preamp, here with the power supply. I hadn't planned for an external psu, but since the main chassis turned out too small, I had to.
