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bias adjustment

installing the buffers

  • put the buffer onto the sockets
  • be sure that the pins fit into the sockets and won't get bend
  • carefully push the buffer pins down into the sockets
  • before initial power up turn the trimpots full turn right (clockwise) for maximum resistance = minimum bias current

bias target zone

adjust bias current (reference to bias table for voltages) to about 20mA
  • don't set the bias to 30mA or more. the more bias current is flowing through the transistors the more heat they have to dissipate. without heatsinking (unfortunately there is no room for heatsinks) the output transistors can't stand more than 30mA. You'll fry the transistors and destroy your buffer by over-biasing!
  • even in single output configuration and with 32Ohms headphones at very high volume levels 20mA bias are enough to keep the buffer in classA
  • do the adjustments in small increments. The output transistors are BJTs which have strongly positive thermal coefficient, which means that as you increase the bias, the current through the transistors increase, and that will heat up the transistor and cause its DC current gain to go up, which would then in turn cause the current to further increase.
  • it would be best not to aim for 20mA right away, or else a few minutes later you'd find that the current had gone up to more than 40mA. Increase the bias slowly and in small steps, let the transistors warm up and settle at a stable current before making more adjustments. Do this with at least 5 minute wait time between each adjustment until you get to the target without drift.

adjusting the bias

  • turn trimpot one turn left
  • measure bias voltage in mV across a output resistor: put one probe to the testpoint near the pot, the other probe to a testpoint next to a output resistor
  • if voltage measures zero continue with left (counter-clockwise) turns, you will probably have to do some turns until the bias suddenly kicks in
  • when the bias kicks in give the buffers a minute to warm up and settle, don't turn the pots any further unless the bias has become stable
  • carefully increase (turn left = counter clockwise) or decrease (turn right = clockwise) the bias by half turns until the target bias is reached. Wait a few minutes after every adjustment to give the buffer time to settle. With the temperature changing the bias current changes, too. Be patient while adjusting the bias.
  • fine adjust the last 5mV by quarter turns

bias voltage/current table

output resistor
[Ω]
bias current
[mA]
voltage across
output resistor
[mV]
comment
2.2Ω output resistors (color code: red red)
2.2 15 33 lower end of recommended bias zone
2.2 20 44 recommended default bias
2.2 25 55 upper end of recommended bias zone
2.2 30 66 don't set the bias to this point or beyond
2.7Ω output resistors (color code: red violet)
2.7 15 41 lower end of recommended bias zone
2.7 20 54 recommended default bias
2.7 25 68 upper end of recommended bias zone
2.7 30 81 don't set the bias to this point or beyond
3.3Ω output resistors (color code: orange orange)
3.3 15 50 lower end of recommended bias zone
3.3 20 66 recommended default bias
3.3 25 83 upper end of recommended bias zone
3.3 30 99 don't set the bias to this point or beyond
3.9Ω output resistors (color code: orange white)
3.9 15 59 lower end of recommended bias zone
3.9 20 78 recommended default bias
3.9 25 98 upper end of recommended bias zone
3.9 30 117 don't set the bias to this point or beyond
4.7Ω output resistors (color code: yellow violet)
4.7 15 71 lower end of recommended bias zone
4.7 20 94 recommended default bias
4.7 25 118 upper end of recommended bias zone
4.7 30 141 don't set the bias to this point or beyond