SOHA II Schematics

One Channel Amp Schematic

The common cathode input stage and the single ended buffer come together to make a single channel of the amp.

SOHA II One Channel

SOHA II - One Channel

There are two features of the final circuit that we didn't talk about in the Design section.

  • The grid of the second triode is a handy place to apply negative feedback. The values on the schematic inject about 6db of NFB into the amp, reducing the gain and distortion while increasing the bandwidth.
  • The triode pair is not restricted to a single tube type. It can be any pair that will come to a proper operating point at the voltages and currents of the stage. This means, specifically, that the cathodes must come to a positive voltage (making the grid negative) at the operating point. It is best if the cathodes are >300mV.
  • C3 and C4 are opamp rail decoupling caps to prevent the high gain opamps from oscillating.
  • C6 is a 1μ film cap in parallel with the electrolytic to handle fast transients where the electrolytic may exhibit too much inductance.


Typically, in an amp with NFB, the closed loop gain is set entirely by the applied NFB. In this case, however, the NFB is small and the gain is a combination of the μ of the triodes and the NFB. Here are some theoretical gains for different tubes:

Tube Gain
12AU7 8.7
6922 10
12AT7 13

One very nice feature of the minimal NFB approach is that the NFB reduces the widely varying μ of the triodes (from 17-60) to a much smaller range of gains. This keeps the volume pot rotation useful even though there are very different tubes in the amp.


The Power Supply

The SOHA II power supply topology is similar to the SOHA PS. It uses a single transformer to supply the split low voltage supply, the B+ supply, and the heater supply:

SOHA II Power Supply

SOHA II Power Supply

There are a few improvements to the original SOHA PS and as a result of extensive prototyping there are significant changes from the original SOHA II PS:

  • The voltage multiplier for the B+ is independent of the positive rail of the split supply.
  • The voltage multiplier is a 2.5X multiplier which yields about 105VDC at the output of the multiplier.
  • The B+ filtering section uses the TI TL783 HV adjustable regulator as the first filter section. The resistor values are chosen to permit adjustment from about 55VDC to about 100VDC.
  • The regulator is followed by two capacitance multiplier filters at the B+ output, one for each channel to further reduce PS ripple and to improve channel separation.
  • The heater circuit can be jumpered (or switched) to supply either two 12.6V/150mA tubes in parallel or two 6.3V/300mA tubes in series. For example, a pair of 12AU7s in parallel or a pair of 6922s in series.

WARNING - 100VDC is significant high voltage. Every precaution for handling HV should be exercised.

The maximum current draw from the heater supply is intended to be 300mA.

The heater supply includes capacity for three LEDs. Two of these can be used to light the tubes and the other for a power indicator.

The purpose of R3P is to reduce the voltage at the input to the regulator which will reduce the power dissipation. R3P dissipates about 1W leaving the regulator to dissipate about 1W also, preventing the regulator from getting hotter than necessary to do its job.


Epsilon 12

The SOHA II includes an simplified and modified version of the ε12 muting circuit and offset detector designed by Ti Kan (AMB). This version uses two 12V regulators from the SOHA II's 15V rails to get the split 12V needed for the ε12. It would have been simpler to run the TL081 from the 15V rails, but the small regulators isolate the 15V rails from any noise in the ε12.

Note that this version uses BC550C instead of the original 2N3904. This was done because the amp already uses the BC550C devices. Using them here keeps builders from having to source yet another transistor type.

Warning: The original 2N3904 cannot be substituted directly because their pinouts are different from the BC550. They can be substituted if you rotate the devices 180d.

Epsilon 12

SOHA II Simplified ε12

Notice that R1E and R2E are changed from 10kΩ to 33kΩ and that R4E is changed from 1kΩ to 2kΩ. The purpose of these changes is to make the SOHA II ε12 less sensitive and a bit slower. This is because the SOHA II SE Buffer can generate a bit more offset under certain signal conditions before settling down. However, the SOHA II ε12 is still sensitive enough to prevent damage to headphones.

If the modified ε12 in your SOHA II has problems, don't blame AMB since his original design has different component values.