Do I have the right tools to measure/adjust bias on solid state amps?

JoshHendi

^ElectroNerd^
So I want to know if I have the right tool to measure my amps bias. I have never done so before, and have a few amps I want to check/adjust.

My meter is a Klein CL3000

For the first amp, Sansui au717, I checked the manual and it says the range should be 20mv +/- 1mv. Now my Klein meter can read DC mv

Then I read the manual for my Sansui au9900, its measurements range is 50ma +/- 1ma

My meter has an option to read DC voltage, but am I right when I see that the only ampreage reading option is AC only? Therefore I cannot check the bias on my 9900, but I can with the 717?

BTW I have read that there are other ways to check the bias of an amp besides what the manuals instruct, but with me being a noobie, don't want to get too 'risky' or outside the guidelines, at least right away. Please feel free to let me know other ways of checking, if they aren't incredibly risky or challenging.
Is it possibly to check/adjust bias of any amp with my current meter?

Excuse my ignorance, I am trying to learn!

EDIT: I have now picked up two Fluke meters that can read MAs as well as MV, with two sets of clips/grabbers

Attached pic of my meter
 

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The meter is able to read mv... there is range button which allows a mv reading, so I can do the 717 by the manuals steps.

But I was afraid I was right in not being able to read the 9900
 
So I am currently reading the au717 and got everything connected correctly and everything looks good.

Could someone possibly explain in a little detail how to check the au9900 using a mv reading? I did some reading on the linked thread http://www.audiokarma.org/forums/showthread.php?t=520954&page=2
but am still a little confused on where to hook up my leads on the emitter resistor exactly...
 
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Allow me to throw a wrench into this whole operation. That +/- spec given by the manufacturer fails to point out that more important than being within that tolerance (btw bias usually has a much larger safe margin than the spec given as a percentage) is the critical need for both channels to be the same. 1/10th of 1mv difference between channels is audible. That's about the range you have outside of which your stereo image disappears. And within it is where the tweaking range is. This is why I'm willing to wager the vast majority of members here don't know or have forgotten the experience of an actual stereo image. Outside of it all you have is two sources with varying levels of attenuation. As bias falls off slowly over time, you can easily be fooled by it. Bias can still be within spec on each channel but stereo image is out the window. It needs to be set sufficiently high in order to prevent notch distortion but way more importantly both channels set identically.
 
So I should go ahead and invest in another meter?

I guess I should go ahead and buy two meters then, both the same brand/model and that can also read DC ma?
 
If this is your hobby, you can't go wrong buying a decent meter. I started with one less than 10 bucks and now use a pair of Greenlee. Not the best but decent. I don't deserve a Fluke yet :)
 
There is NO amp that I can think of that is capable of holding a bias adjustment between channels of 100µV. And the claim that such a difference is audible is specious at best.

Yes, I'd recommend that you find a more suitable meter.
 
There is NO amp that I can think of that is capable of holding a bias adjustment between channels of 100µV. And the claim that such a difference is audible is specious at best.

Yes, I'd recommend that you find a more suitable meter.

I said the stereo image lives within a 1mv between channels range and that 1/10th of that is audible. At idle they will absolutely stay within 1/10th of 1mv of each other. In operation they won't, however unless both channels are set accordingly, they won't have a close enough interplay to hold the stereo image. This applies to every amp I've ever owned. Have you ever attempted to discover just what is possible in this regard? I know you're a tech and with all due respect, I discovered this phenomenon if you will because the times I've brought gear in for service I've always received it back lacking in performance and so learned about these adjustments. I challenge you or anyone to prove me wrong in this. Or better yet discover it yourself
 
You are the one with the questionable claims, so any proof falls to you. Furthermore, there's nothing to discover here. Stereo imaging has nothing to do with the amount of idle current. Furthermore, there's NO amplifier on the planet that can hold a idle current adjustment to within 100µV channel-to-channel. Idle current is by design is a function of many variables, but once set, thermal variations take over and while most well-designed amps should maintain a 'balance' from one channel to the other of 2 to 3mV, it takes very little to cause one channel to drift away from your intended setpoint. To claim otherwise displays a lack of understanding for how the biasing system works as a whole.
 
Superampman,

Bias is adjusted to minimize distortion at the crossover point. Could you please explain, technically, how a small change in crossover distortion would affect stereo imaging? Imaging is dependent on both channels having the same frequency response, hopefully the same volume level, and, of course, no phase difference.
 
You are the one with the questionable claims, so any proof falls to you. Furthermore, there's nothing to discover here. Stereo imaging has nothing to do with the amount of idle current. Furthermore, there's NO amplifier on the planet that can hold a idle current adjustment to within 100µV channel-to-channel. Idle current is by design is a function of many variables, but once set, thermal variations take over and while most well-designed amps should maintain a 'balance' from one channel to the other of 2 to 3mV, it takes very little to cause one channel to drift away from your intended setpoint. To claim otherwise displays a lack of understanding for how the biasing system works as a whole.

I respect your knowledge and expertise as I'm sure it's far in excess of mine. I am talking strictly about the sonic nature and degradation associated with less than optimal adjustments here; perceived sq through/at the speakers. I did not claim 'amount' of idle current is influential. My position as I stated it is that the stereo image is dependent on quiescent current being equal. Much more so than the actual spec suggests since it refers to each channel separately. That 1mv is the range within what both channels must stay of each other is certain. Not only does stereo image depend on this but so does transient performance, timbre, extension, resolution, bass definition. All of these can be seriously degraded with bias still within spec. Thermal influences are not strong enough to make a difference once the amp is in operation for a short while, ime usually about the fifth cut on a disc, barring any major affect like a cold wind blowing in an open window or door or a heat vent close to one side of the amp. I don't see drifting fluctuations when setting bias current the way I do when setting offset. It stays stable where I set it. You may very well be correct regarding the precision channel balance can be held in by an amp. However, I nonetheless maintain it's capabilities far exceed what the spec suggests is adequate and indeed those capabilities are critical to it's proper function and potential. An amp can be less than optimally adjusted right from the manufacturer because it's within tolerance.
 
I respect your knowledge and expertise as I'm sure it's far in excess of mine. I am talking strictly about the sonic nature and degradation associated with less than optimal adjustments here; perceived sq through/at the speakers. I did not claim 'amount' of idle current is influential. My position as I stated it is that the stereo image is dependent on quiescent current being equal. Much more so than the actual spec suggests since it refers to each channel separately. That 1mv is the range within what both channels must stay of each other is certain. Not only does stereo image depend on this but so does transient performance, timbre, extension, resolution, bass definition. All of these can be seriously degraded with bias still within spec. Thermal influences are not strong enough to make a difference once the amp is in operation for a short while, ime usually about the fifth cut on a disc, barring any major affect like a cold wind blowing in an open window or door or a heat vent close to one side of the amp. I don't see drifting fluctuations when setting bias current the way I do when setting offset. It stays stable where I set it. You may very well be correct regarding the precision channel balance can be held in by an amp. However, I nonetheless maintain it's capabilities far exceed what the spec suggests is adequate and indeed those capabilities are critical to it's proper function and potential. An amp can be less than optimally adjusted right from the manufacturer because it's within tolerance.

:rolleyes:


:lurk:
 
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Thanks for the backup Dave...:bash:

RE: Bias

People believe a lot of offbeat things about audio and audio gear. And if that's what you want to believe, that's fine. But when you jump into the forums and spout these things as if they were common knowledge, you will be challenged. Guys are here to learn something, and those that are here with years of work and design experience are tasked by default to help keep things steered towards the plausible. Your assumption that idling current must be equal between channels is a faulty one, since it never will be, even if you set it as carefully as you can. The list of reasons for this is a long one, but thermals DO play a huge part (regardless of your assertions) as well as the fact that each channel is amplifying a different signal. By all means, we'd like to see them as close to each other as possible, but on a Class aB amp I doubt if you could hear the difference between an amp with bias adjusted as closely as possible to spec, and one where one channel was set to 1/2 the bias current of the other (as long as the lower channel was adjusted above the threshold of crossover issues).

Not saying any of this to convince you -since it's easy to see that you have already made your mind up about this- but simply to see that we don't have 50 threads pop up with guys freaking out that the bias in one channel can't quite be adjusted to match the other within some ridiculously tight tolerance. Wastes the time of the guy agonizing over it, and wastes my time (and others here to lend tech assistance) telling them not to sweat it.

Speaking of wasted time, I believe I'm done here. Happy trails.:hdphones:
 
Service manuals. Nothing in the hundreds I have read says anything about EXACT bias.
 
So I want to know if I have the right tool to measure my amps bias. I have never done so before, and have a few amps I want to check/adjust.

My meter is a Klein CL3000

For the first amp, Sansui au717, I checked the manual and it says the range should be 20mv +/- 1mv. Now my Klein meter can read DC mv

Then I read the manual for my Sansui au9900, its measurements range is 50ma +/- 1ma

My meter has an option to read DC voltage, but am I right when I see that the only ampreage reading option is AC only? Therefore I cannot check the bias on my 9900, but I can with the 717?

BTW I have read that there are other ways to check the bias of an amp besides what the manuals instruct, but with me being a noobie, don't want to get too 'risky' or outside the guidelines, at least right away. Please feel free to let me know other ways of checking, if they aren't incredibly risky or challenging.
Is it possibly to check/adjust bias of any amp with my current meter?

Excuse my ignorance, I am trying to learn!

Attached pic of my meter

I explain in this post how to do it on a AU-9900 by measuring the voltage across the emitter resistors.
http://www.audiokarma.org/forums/showpost.php?p=6784728&postcount=5
 
Thx doc...

I've read the post, just don't understand exactly where to put my +/- leads on the 9900 emitter resistor to do so. I could do the math and adjustment once I get the reading, just still not 100% where to put my clips.

Also, I got my bias set on my au717... left it on over night with no signal going to it, and this morning I felt the top of the case and it was fairly warm for idling with no signal over a 7 hour stretch... is this normal? am I just being paranoid?
 
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