# Any way to tell Class A bias in an Amp

Discussion in 'Archived Threads 2001-2004' started by Yogi, Oct 25, 2002.

1. ### Yogi Screenwriter

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Is there a way to say how much an amplifier is biased into Class A by looking at its ilde power dissipation. For example an amp like the Parasound 2205 dissipates 150W at idle. In other words it dissipates 30W/ch @ idle. Now if Class A power is only 25% efficient (suppose) then can I say that the 2205 is biased into Class A for the first 7.5 (25% of 30) watts? Would that be a reasonable assumption or, am I missing something else?

Regards

P.S. I am assuming that the input drivers are dissipating 10-15 W at the most so I am neglecting their consumption in my calculation.

2. ### Yogi Screenwriter

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Where are all the amp experts at? Saurav, Larry B, John Kotches... Can one of you shed some light on this?

3. ### Ludwig Crumb Agent

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You have to measure the voltage drop across the emitter resistor by hooking up a voltmeter onto both sides of the resitor and measuring the voltage reading on it (the power resistor connected to the emitter leg of the output transistor.) Divide that number by the resistor value and it would give you the bias current on that transistor. Multiply that by half the number of output transistor per channel (even class-A has a push-pull output stage, unless it is single ended like the Pass Labs,) and that will give you the total bias current per channel. Square that number and multiply it by your speaker's nominal impedence, it will give you the class-A power. Note, if speaker impedence is 4 ohm, the amp will deliver half as much of class-A power compared to an 8 ohm speaker.

4. ### Yogi Screenwriter

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Ludwig, is there any way to measure the voltage drop across the emitter resistor externally without opening the amp up?

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6. ### Ludwig Crumb Agent

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You could measure current draw from your amp with a progressively louder signal. It would be best to hook a power resistor of known value across the amp terminal, or run the signal of a freq in which your speaker is 8 ohms impedence. When the current draw starts to increase, that should mean it is going into class AB. Measure the voltage output from the amp's output terminal, square it, and divide it by 8. That should give you the power output.

7. ### Ludwig Crumb Agent

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Saurav

Load shouldn't make a difference on SS amp, especially when one is measuring the voltage drop with no signal going into the input of the amp.

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Yogi:

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Saurav:

11. ### Yogi Screenwriter

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Aww cmon Larry, I am not kidding. You guys have a lot of experience with SS and tube amps of all varieties plus you all have an excellent understanding of the technical details of amps. Now Ludwig is also on my short list of amp experts. I dont know where John is. He probably hasn't read this thread.

In any case, I am going to count on you guys when ever I have technical questions about amps.

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13. ### Yogi Screenwriter

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Aren't there some multimeters that tell you the current through a wire just by clamping around the outside of that wire? I remember seeing one some place. Or maybe it was something else

14. ### Saurav Cinematographer

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That sounds familiar, I believe there are devices like that. If you find such a product, that could be a much easier way out.

15. ### Scott Oliver Screenwriter

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Some may not want to disclose this information, but I would guess a reasonable amount of amp producers would.

Couldn't hurt.

16. ### WillieM Stunt Coordinator

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Ever think of just emailing and (or) calling Parasound and asking them? Their tech support is west coast. I have a phone number for a tech support guy, but I hesitate to publish it in front of God and everybody. However, i got the number from their Customer Service department so I suspect that tou can do the same.

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Yogi:

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19. ### Saurav Cinematographer

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I was thinking about this. Even if that clamp-on meter is inaccurate, he doesn't really need to know the precise value of the current draw, he just needs to see when it starts to rise. So, assuming it's reasonably sensitive (and I don't know if that's a fair assumption or not), it could still work.

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