I know there are a few ways to adjust the bias pots on some older car amplifiers and that's good. I could not find much searching around on the Internet, especially with pictures. I just wanted to share one method that worked fairly well on a ZAPCO Z100C2, Z100S2, Z400C4-SL and a couple of old linear power amps. In no way am I saying that anyone should do this but if you have to because someone messed with them or you did extensive repair's and want to check, this may be helpful. You need a scope, sig gen and and a 4 ohm 15 watt or better resistor for the method I used.
I welcome all feedback, good or bad, as I love to learn...
10Khz on the sig gen with enough signal to get a clean trace on your scope.
I welcome all feedback, good or bad, as I love to learn...
10Khz on the sig gen with enough signal to get a clean trace on your scope.
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You should really bias it with a typical use speaker attached.
A resistor is purely resistive but a speaker is inductive too meaning it tries to pass current back to the amp when the transistors are turning off.
I design my own amps so have to come up with my own biasing methods.
Going by your pictures I do it a similar way by applying a sine wave and monitor output.
Then increase bias until output loses crossover distortion.
However I find doing that I get incredibly small bias currents like 10mA for a mosfet amp.
Many people prefer a much higher bias current as they say it sounds better.
A resistor is purely resistive but a speaker is inductive too meaning it tries to pass current back to the amp when the transistors are turning off.
I design my own amps so have to come up with my own biasing methods.
Going by your pictures I do it a similar way by applying a sine wave and monitor output.
Then increase bias until output loses crossover distortion.
However I find doing that I get incredibly small bias currents like 10mA for a mosfet amp.
Many people prefer a much higher bias current as they say it sounds better.
I did try this procedure and it does work...(using a set of speakers)...I got the same results! My dog didn't like it though! 😀
I have also heard others say that as little as you can get away with sounds best.
The impedance of a speaker does change significantly with frequency. To determine what the amp will do with the toughest rated load (will require the highest bias current to remain distortion free), you need to test with the toughest rated load. You cannot do that with most speakers because the load will decrease (maybe greater than a factor of 10) at points in the spectrum. One example:
http://www.parts-express.com/pedocs/specs/295-514-dayton-audio-um15-22-specifications.pdf
Resistors do have some inductance but generally not enough to make a difference. As an example, a Huntington Electric 2 ohm 100 watt resistor (AVT-100) has a measured inductance of 2uH. That's not going to cause a significant rise in impedance in the audio spectrum.
http://www.vishay.com/docs/31841/avt.pdf
For resistors with less inductance, you can use resistors with Ayrton-Perry windings.
Ayrton-Perry winding - Wikipedia, the free encyclopedia
http://www.vishay.com/docs/30208/hlnhl.pdf
http://www.parts-express.com/pedocs/specs/295-514-dayton-audio-um15-22-specifications.pdf
Resistors do have some inductance but generally not enough to make a difference. As an example, a Huntington Electric 2 ohm 100 watt resistor (AVT-100) has a measured inductance of 2uH. That's not going to cause a significant rise in impedance in the audio spectrum.
http://www.vishay.com/docs/31841/avt.pdf
For resistors with less inductance, you can use resistors with Ayrton-Perry windings.
Ayrton-Perry winding - Wikipedia, the free encyclopedia
http://www.vishay.com/docs/30208/hlnhl.pdf
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