Okay, Saturday is schematic day.
I'm still figuring out some tweaks here and there but this one should already be functional. I tried to keep it as close to the original SSMH as possible. So let's dig in!
My build type number should say everything: 12AU7-1-48DC150. So we have a 12AU7 build using a single tube with an 48V PSU and a DC-coupled output stage biased with 150mA of current per channel. These are the original specs of the SSMH, except for the single tube. For the record, the heater is configured with the center tap grounded and a split heater at 6,3V and 150mA. This should give the same 300mA total quiescent current as in the original build.
I'll be working my way from input to output. Since I'm running on a single tube I added a dual gang (linear) balance pot to compensate for any channel imbalance. This then connects to a resistor and the typical dual gang (log) volume pot of your preference. Credit goes to Rod Elliott from ESP for this input network. The whole thing has 3dB attenuation with the balance pot centered and should give quasi constant loudness when turning it. The input impedance is somewhere around 10k instead of the 100k of the standard 12AU7-mod. This should be both less noisy and has enough juice to push the tube grid closer to 0V without distorting. The output of the volume pot then runs into a low pass filter composed of some grid stoppers and an extra capacitor to account for RF interference. The grid stoppers and the cap should be soldered directly onto the tube socket for maximum effectivity.
The tube gain stage and mosfet output stage are next. The cathode and plate resistors are approximately doubled in value because I'm only using one tube half per channel. Again, I added some DC bias pots for any channel imbalance due to the single tube. I decided to recycle the original SSMH mosfet source follower output stage and simply stack another voltage follower on top. The lower one is connected to a soft-start DC voltage reference, which sets the heater voltage. The top one follows the tube plate voltage. AC couple the audio out with some output caps and done.
The voltage follower mosfets are the logic level versions (IRL510, not IRF510!) and have a gate stopper and a 5V protection zener to prevent any oscillations and ESD casualties, while also preventing a large turn on thump from charging the output caps too fast. Again, solder these directly on the mosfet pins. In fact, this should be done before anything else to prevent dead mosfets. The input impedance of the mosfet is practically one zillion ohms so maybe solder the plate resistor directly to the gate stopper of the mosfet and run a wire to the tube socket.
I added some typical expected voltages for debugging. As I’m still guessing for the exact tube operating point, the plate voltage is just a guideline for optimal bias of the voltage follower. This stuff is still experimental so proceed only if you have some crappy test headphones and know how Ohm's law works.
Have fun prototyping!
Edit: Did a re-upload due to incorrect plate voltage. Should be right this time.
Edited by GlowGale - 9/13/14 at 9:47am