How to test bias

[adlaic]

What is the bias and how do you test it on a transistor amp?

Thank You,

Adlai

[darkfenriz]

You can measure idle current of output devices by measuring voltage drop on emitter/source resistors (>95% of amps have them) when no signal is applied to input. Every digital voltmeter can do this.

[lgreen]

Originally posted by darkfenriz
You can measure idle current of output devices by measuring voltage drop on emitter/source resistors (>95% of amps have them) when no signal is applied to input. Every digital voltmeter can do this.

we are not as smart as you, does this relate to bias? how so?

If I measure 1V drop across 0.5 ohm resistor, do you simply say V=IR and I = V/R so bias = 1/0.5= 2 amps per device. Then multiply by the # of devices to get bias/channel? Then multiply by # channels to get bias/ amp?

[darkfenriz]

Originally posted by lgreen

If I measure 1V drop across 0.5 ohm resistor, do you simply say V=IR and I = V/R so bias = 1/0.5= 2 amps per device. Then multiply by the # of devices to get bias/channel? Then multiply by # channels to get bias/ amp?

Yes, that's how I understand it, except idle current is the same through a pair of output devices.

[cpemma]

Originally posted by adlaic
What is the bias and how do you test it on a transistor amp?

For what bias is/does, see this guide.

On the actual setting procedure, I found it slightly misleading in places, "Then, turn the amp off if you have only one set of speaker binding posts, or turn on Speakers B, and measure the DC offset." I'd just turn the volume right down, switching the amp power off seems a bit counter-productive.

If you can find a Service Manual for your amp it will advise on the manufacturer's recommended setting (and the test points to use).

[anatech]

If you can find a Service Manual for your amp it will advise on the manufacturer's recommended setting (and the test points to use).

Yes. There are some general rules that experience will teach you. Also, when you have multiple output pairs, make sure the current is shared relatively evenly. That's an indicator of transistor matching.

-Chris

[Zen Mod]

Originally posted by cpemma


For what bias is/does, see this guide.

On the actual setting procedure, I found it slightly misleading in places, "Then, turn the amp off if you have only one set of speaker binding posts, or turn on Speakers B, and measure the DC offset." I'd just turn the volume right down, switching the amp power off seems a bit counter-productive.

snake oil....

just another "expert" article,from series of "how to make that sheeety amp little less sheeety".......

I agree with you about manual- in any case manual is good for knowing exact procedure