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  • start with the smallest parts (resistors)
  • continue with larger parts (TO-92 transistors, trimpots, capacitors)
  • mount the heatsinks to the TO-220 transistors before you solder the transistors in
  • turn bias trimpots left turn to minimum ( = maximum resistance = minimum/no bias current)
  • adjust offset trimpots the the center of their adjustment range (aprox. 12 turns from minimum/maximum)
  • connect the buffers to the amp

click on pic for full scale image

installing JISBOS boards on a SOHA amp

Upgrading a SOHA with JISBOS boards means replacing the dual opamp on the SOHA with two JISBOS boards. The JISBOS boards have to be air wired to the SOHA. Basically you replace one opamp of the dual opamp chip with one JISBOS board by connecting the non-inverting input pad, e.g. (+In A) to the JISBOS input and the JISBOS output to the opamp output pad, e.g. (Out A).

changes on the SOHA

It is mandatory to replace the 78L12 / 79L12 TO-92 package voltage regulators with TO-220 package regulators (78xx / 79xx). It is further recommended to pick 15V regs (7815 / 7915) instead of 12V regs. This will reduce heat dissipation in the regulators a will give a little more voltage swing capability to the buffers. Additionally, depending on the bias setting of the buffers, it may become mandatory to install heatsinks on the regs. These heatsinks may interfere with the capacitors in front of and behind the regulators. You can gain some space there by replacing the post reg caps with smaller ones, e.g. 33µF electrolytic or 10µF tantalum caps. If you want large electrolytics after the regulator you can install them on the JISBOS boards.

Intended wiring scheme for the SOHA

  • Your headphone jack should be wired directly to the OG, OL and OR pads on the SOHA board.
  • Run a wire from each JISBOS output "G" terminal to the SOHA board ground plane, close to the 78xx / 79xx regulators and rail capacitors. There is an "unused" pad there at C7, C8 and C2 that could be used for this purpose.
  • Leave the JISBOS input G terminals unconnected.
  • Run wires from JISBOS V+ and V- pads to SOHA, you can use the opamp's pin 8 and 4 for this.
  • Run wires from JISBOS input to SOHA opamp pins 3 (R channel) and 5 (L channel).
  • Run wires from JISBOS output to SOHA opamp pins 1 (R channel) and 7 (L channel).
  • Keep all wires as short as possible, you can twist the V+/G/V- wires together for each channel, but don't include the input or output wires in that bundle. Route them separately.
  • You may optionally reduce the R6 and R16 resistor values to take advantage of the low output impedance and high output current from the JISBOS, but it will raise the effective voltage gain of the SOHA to be very high.
wiring scheme 1
click on pic for full scale image

alternative (perhaps superior) way of hooking up the JISBOS to the SOHA

  • The headphone jack ground is connected to SOHA's OG pad, but its left and right tabs are wired to the left and right JISBOS output pads.
  • Jumper the SOHA opamp socket so that it's pass-through: Install one jumper across pins 3 and 1 (R channel), another jumper across pins 5 and 7 (L channel).
  • Run wires from SOHA OL and OR pads to the JISBOS inputs.
  • The JISBOS V+, V- and output ground are wired as described in the original scheme above. Leave JISBOS input ground unconnected as before.
  • You can leave the SOHA R6 and R16 output resistors at 150Ω, or jumper them. It makes little difference because these resistors will now be in series with 100KΩ on the JISBOS board.
wiring scheme 2
click on pic for full scale image

opa2134 on SOHA

operating points

for trouble shooting or just checking after assembly please refer to the attached charts with operating points

operating points 12V voltages
operating points 15V voltages

add attenuation to the JISBOS boards

If you suffer from the "too much gain" problem with your SOHA (particularly with low impedance headphones) you can add attenuation as follows:
Change both JISBOS R1 to 100KΩ, and R2 to 33KΩ. This forms a voltage divider with approximately 4x attenuation. Since the SOHA tube stage has a gain of around 20, this drops the effective gain down to around 5, which is a good middle-of-the-road setting and compatible with a wide variety of headphones. This setup scheme will reap full benefit of the JISBOS buffer of low output impedance and high output current, and solves the "too much gain" problem. The only downside that I could think of, is that the 4x attenuation would reduce the overall voltage swing limit (now the tube will clip before the buffer would). This is not a problem with low-Z phones, but the attenuation factor should probably be reduced to around 2-3x for high-Z phones (change JISBOS R1 to 100K and R2 to between 50-100KΩ).

attenuation factor: A = (R1 / R2) + 1
converting it to decibels: dB = 20 * log(A)

R1 R2 attenuation effective SOHA gain
100kΩ 33kΩ 4 aprox 5
100kΩ 50kΩ 3 aprox 7
100kΩ 100kΩ 2 aprox 10
when adding attenuation you may consider to change C2 and C14 on the SOHA board from 100nF to at least 220nF or 330nF to prevent bass roll off