My build uses the latest board version that uses PZTAx6 SMD parts. Power supply is 2 GRLV set for +/- 30VDC rails with 2 100VA transformers.
Here are the detail steps I went through to adjust and bias my Dynahi. I gathered the information from reading this thread and other Dynahi build discussions on the web. Also consulted Kevin and reference the schematic.
I hand measured and matched all resistors, LEDs and transistors before soldering. I only matched the output devices (MJF15030/MJF15031) at very low current but this seems to work out fine at the actual working current (75mA). I replaced one that was about 5% off. The rest are all within 1.5 % of each other. I use matched 2SK170/2SJ74 pairs to replace the THAT340. They are BL grade. Feedback resistors (R52, R56) are 50K (200K on silkscreen) and I use 100R for R23, R24 (50R on silkscreen), 680R for R1, R4 (500R on silkscreen). This combination of R1/R4, R23/R24 works great for my target bias current of 75mA per output device. To bias the output devices for the desired current, it’s necessary to pick the right combination of R1/R4 and R23/R24. Higher R23/R24 resistance increases bias current while higher R1/R4 resistance shifts the bias adjustment range downward. As Kevin has warned more than once, do not test this amp without proper heatsink. The MJF15030/15031 is only rated at 2W maximum dissipation in 25C ambient temperature.
I took the following steps to bias and adjust the amp:
Step 1 - before powering up the amp:
Set RV3 and RV4 in their exact middle value. The Bourns trim pots I use have a 10% tolerance. I measured R9/R10, R11/R12 (100R resistors on top and below RV3 and RV4) to ensure RV3 and RV4 are both centered. RV3 and RV4 are located on the PCB right next to the THAT340.
Set RV1 and RV2 to their maximum resistance so the amp is biased at the lowest possible output current. I did this by measuring the resistance of R1 and R4 (the 500R on the PCB right next to RV1 and RV2). On my amp, they are about 640R.
On my build, I mounted RV2 backwards (in regard to the silkscreen) so turning both RV1 and RV2 adjustments clockwise decrease their resistance.
Step 2 – power up check:
Power on the amp and measure bias current (indicated by the voltage drop across the bank of 20R resistors) and output DC offset. With my amp at cold start and before any adjustments, the bias current is about 34mA and the output offset measured to ground is -1 VDC on one board and -0.5 VDC on the other. The DC offset between O+ and O- is 19mV and 25mV. This step is a sanity check to see if anything is out of whack.
Let the amp warm up to steady working temperature before proceeding to next step while taking periodic measurements and checking the heatsink temperature. It takes 30 minutes or more for the amp to reach steady temperature.
For the following steps, make the adjustments slowly and gradually and allow time for the amp to settle following each adjustment.
Step 3 – compensate for transistors mismatch in the input stage:
Adjust RV3 and RV4 so the voltage drop across R7 and R8 are the same and the voltage drop across R13 and R14 are the same. This adjustment is tricky and delicate. Adjusting either RV3 or RV4 affects the voltage drop across all 4 resistors (R7, R8, R13, R14). Also the adjustment is made to a 10K trim pot (pre-adjusted to 5K) parallel to a 100R resistor. The resistance change is relatively small at first in relation to the turns of the trim pot. I adjusted RV3 and RV4 in small and equal amount in turn to get to equal voltage drop between R7/R8, R13/R14. Be patient and be careful! This adjustment affects the DC offset between O+ and O-.
Step 4 – biasing output sections:
Decreasing the resistance of either RV1 or RV2 will increase the current of ALL the 16 output devices on the board. At the same time, decreasing RV1 resistance will push the output DC offset (measured to ground) more negative while decreasing RV2 resistance will push the output DC offset more positive. Adjust RV1 clockwise to increase the bias, say 20mA at a time. The output DC offset to ground will go more negative at the same time. Then adjust RV2 clockwise (CCW if RV2 is mounted in accordance to the silkscreen) to bring the output DC offset close to 0mV (the bias current will increase at the same time). Repeat these two adjustments back and forth until you reach your desired bias current while keeping the output offset as close to 0mV as possible. The offset between O+ and O- is not affected by RV1/RV2 adjustment and will remain very stable through the process while the offset to ground will drift quite a bit. It’s normal.
Step 5 – nulling the offset between O+ and O- if needed and/or desired:
Adjust RV3 and RV4 in a convergent fashion (one clockwise, the other counter-clock wise) to bring the offset between O+ and O- to 0 V.
I consider this step 5 optional unless the offset between O+ and O- is excessive and cannot be taken care by the servo.
I did not do step 5 on my amp since, following step 3 and 4, without the servo, the offset is steady at 23mV on one channel and 25mV on the other and not likely to hurt anything. I can always engage the servo if desired.
I have tried other methods of adjustment including one that skipped Step 3. The difference is that, with step 3, I end up with more uniform output current across all 16 output devices, generally within 1.5% of each other. When I skip step 3, while the current between the PNP and NPN banks on each phase is within 1.5% or so, the current variance between non-inverted and inverted phases is about 8%. I did not try very hard but frankly my wooden ears could not tell the difference between the outcome of the two methods. Maybe someone with a scope can see the difference. I also consulted with Kevin and he does not feel strongly if there is a right or wrong method either.
One more thing, this thing runs hot! By my calculation, running +/- 30VDC rails and 75mA bias, each output device is dissipating 2.19W. That’s about 35W of heat needing to be dissipated by the heatsink per board. It’s a monster of a headphone amp!