Home > Amplification > First One Build: Adjusting Operating Parameters

First One Build: Adjusting Operating Parameters

November 11, 2014 Leave a comment Go to comments

This should be the last info gathering post for building the First One Amp. Now I need to get a drill press in order to drill the holes on the heatsink…

The module is set to the correct operating parameters and tested at the factory. In case you need to check and readjust, most of the instructions can be found in this post [link] and following. The trimpots and test points are clearly maked on the board




The bias current when the amp is in idle (no input) is 220 mA for the output stage  [link] (200 mA absolute minimum if you have heat problems [link]), plus 60 mA for the rest of the circuit. This is a fixed value regardless of the supply voltage [link]. Trimpot TR3 is used to set the output bias current.Thus:

  • Output bias current = 280 mA. (minimum 260 mA)

How to measure output bias current

The simplest way to measure the output bias current is with a digital voltmeter in current measurement mode. Connect the meter in series with the positive power supply wire. Alternatively you can connect a low value (1 ohm) resistor in series with the positive supply wire and measure the voltage across the power resistor. [link]


The DC offset and VAS bias are set as follows:

  • DC offset = 0v (+/- 10 mv)
  • VAS bias = 12 mA when cold or 15 mA when idle for 20 minutes

How to adjust DC offset and VAS bias

TR1 and TR2 sets DC offset and VAS bias current at the same time. Both works in pairs reciprocally as best explained by this post from the VAAS thread [link]:


  • Adjust (both together) TR1 and TR2 clockwise: increase VAS bias current
  • Adjust (both together) TR1 and TR2 counter-clockwise: decrease VAS bias

How much to turn TR1 and TR2 depends on the DC offset, so you must adjust both to arrive at the correct VAS bias and zero DC offset. Using two DMM would make the adjustment easier.

How to measure DC offset

  • To measure the offset short the amp’s inputs and measure DC at the outputs.[link]

How to measure VAS bias

  • Amp cold: 12 mA bias: measure 120 mV between TP1(+) and TP2(-) or between TP3-TP4 (doesn’t matter which pair of test points).
  • Amp idle for 20 minutes: 15 mA bias: measure 150 mV between TP1(+) and TP2(-) or between TP3-TP4


Here is a diagram of the 3 parameters that can be adjusted:

  • Use TR3 to adjust DC Bias to 280 mA
  • Use TR1 and TR2 to adjust DC Offset to 0 V and VAS current by measuring voltage of 120 mV (cold) or 150 mV (warm)


  1. February 1, 2015 at 23:36

    Thanks for carefully documenting your thought processes and information surrounding FO – it is the best resource on the net, the threads on diya are not easy to use, as the info is spread over hundreds of pages.

    • BlgGear
      February 4, 2015 at 05:22

      You are welcome. I just received 1 mil kapton MT to make some isolators

  2. Thanh
    February 23, 2015 at 16:20

    Did you tried this with R2R DAC from Soekis? Are they fit together?


    • BlgGear
      February 25, 2015 at 07:26

      Have not build the amps yet. Was waiting for kapton insulators which I just received. But too many projects and too little time.

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