Hi All. I just cannot get my head around this BIAS measuring of the L12 Amp to enable to set the bias trimmer! QUESTION: What VOLTAGE should I set across the combined emitter resistors please in the shown example below to achieve a bias current of 40mA? It's a BASIC OHMS Law calculation, but do I use the SUM of the resistors (0.44 Ohms) or just the single value of 0.22 Ohms?. ... In my amp the emitter resistors are in fact just 0.1 Ohms.... Perhaps I should measure each collector voltage separately to ground across it's single resistor instead? Many thanks for your help.
Hi,
With input shorted, volume minimum and no load connected ... Is that total; output stage bias current, or each device? For total:
Measure between your two test points and calculate as voltage over 0.44 ohms. If you could accurately measure the resistance, do that. You would need a Kelvin, or 4 wire resistance measurement to do this.
Your meter must be accurate at that voltage, so a 3 1/2 digit meter with 100 mV scale will not cut it. Your target is 17.6 mV.
For each device, measure across the 0.22 ohm resistors in each emitter circuit. Your target is 8.8 mV, demanding a good meter. The voltage drops should be close to each other, indicating a good match. I would check this even if the total current was measured across the points you indicated in your diagram, but your target voltage for total is only 4.4 mV in that case.
Once you have it set, confirm your AC mains supply voltage is what it should be. Let it run for 10 minutes or so and check it again. Then if you haven't needed to correct the adjustment, connect a load (assuming DC offset is okay) and play some loud music into a dummy load for 10 ~ 15 minutes. Disconnect the music and load and measure the bias current again. Allow it to settle until it cools off and measure again.
With input shorted, volume minimum and no load connected ... Is that total; output stage bias current, or each device? For total:
Measure between your two test points and calculate as voltage over 0.44 ohms. If you could accurately measure the resistance, do that. You would need a Kelvin, or 4 wire resistance measurement to do this.
Your meter must be accurate at that voltage, so a 3 1/2 digit meter with 100 mV scale will not cut it. Your target is 17.6 mV.
For each device, measure across the 0.22 ohm resistors in each emitter circuit. Your target is 8.8 mV, demanding a good meter. The voltage drops should be close to each other, indicating a good match. I would check this even if the total current was measured across the points you indicated in your diagram, but your target voltage for total is only 4.4 mV in that case.
Once you have it set, confirm your AC mains supply voltage is what it should be. Let it run for 10 minutes or so and check it again. Then if you haven't needed to correct the adjustment, connect a load (assuming DC offset is okay) and play some loud music into a dummy load for 10 ~ 15 minutes. Disconnect the music and load and measure the bias current again. Allow it to settle until it cools off and measure again.
The voltage across each resistor is just the current through that resistor times the resistance of that resistor. As the polarities are the same, the voltage between your arrows is the sum of the voltages across the two resistors.
If the currents through them are the same, which should be the case to a good approximation when the output of the amplifier is left open, the sum of the voltages across the resistors is just the current times the sum of their resistances:
I R13 + I R20 = I (R13 + R20)
If the currents through them are the same, which should be the case to a good approximation when the output of the amplifier is left open, the sum of the voltages across the resistors is just the current times the sum of their resistances:
I R13 + I R20 = I (R13 + R20)
Very many thanks anatech and MarcelvdG. Very much appreciate your comprehensive replies. The voltage of 17.6mV is what I calculated to achieve the 40mA bias current. What was throwing me was the YouTube video by Mike Beeny, that shows just 8.8mV being set across the collectors (as per my red dot positions) . I thought this must be wrong but then I thought that as the junction between them is zero volts (ground) I might be wrong. Just as a final check for my amp with 0.1 ohm emitter resistors please, I should aim to get 8.8mV across the same test points (half of that for the 0.22 Ohm resistors)? I attach the original circuit shown for the bias mod, (note the Bias figures and typo error)
When R13 = 0.1 ohm and R20 = 0.1 ohm and you still want 40 mA of current, the voltage across each should be 4 mV and the voltage between your red arrows should be 8 mV.
I'm assuming here and in post #3 that the 40 mA bias current you wrote about is the target for the entire CFP/Sziklai output stage, so on the positive side, the sum of the currents of Q5, Q2 and Q10. Is that correct?
I'm assuming here and in post #3 that the 40 mA bias current you wrote about is the target for the entire CFP/Sziklai output stage, so on the positive side, the sum of the currents of Q5, Q2 and Q10. Is that correct?
Thanks for the correction MarcelvdG..... Yes I have 0.1 Ohm resistors and the sum is just 0.2 Ohms for the pair, so just 8mV is the required voltage then....Thanks..... I am only following the mod as originally shown on DIY Audio some time back. It would seem the figure shown was the TOTAL of the entire output stage, so the answer is yes. I should really set the bias up using an oscilloscope, as others have done. It seems as standard, there is some cross-over distortion. For reference, the standard fixed 1K ohm resistor set the total bias current at around 25mA. There is a temptation to use even more bias but I understand this could actually result in more distortion (Douglas Self)