Should I Adjust the Bias?

dave1701

Active Member
I have a Denon PMA-860, a great amp. One thing I've noticed is that the thing never gets hot. It doesn't even get warmer than ambient at low volumes. At medium volumes, it becomes just barely perceptibly warm. At high volumes, it gets modestly warm. This is driving 4 ohm Infinity RSb speakers, and as we know, Infinity(s) tend not to be very efficient speakers.

I was wondering if it would be a good idea to adjust the bias to make the amp operate more in class A, as it clearly has plenty of ability to dissipate heat.


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Check the factory spec, don't assume it is correct. I've seen amp that adjusted way low.

How do you measure the bias? You cannot blindly adjust, you have to have a reference.

Usually you check the voltage across the emitter resistor of the power transistor. Check the voltage and post back and see.
 
Cool, in the service manual, it says exactly how to set the idling current aka bias. Unfortunately, it requires a signal generator to spply a 1khz 10mVrms signal to both channels inputs, which I don't have.

However, I might be able to use an a computer or app on my phone to create the suitable waveform.

HFhDfuA.png

HFhDfuA.png
 
OK, without schematic, I can just guess, so do it at your own risk.

For me to adjust the bias, I don't need a signal generator. I don't understand why they even want to keep adjusting. This is what I would do:

1) turn on the amp and let it sit without music or anything for 20minutes.
2) measure the voltage WITHOUT ADJUSTING VR501 and VR502. Write down the voltage. My guess it's about 15mV to 20mV.
3) Adjust to increase 5mV on both side......NO MORE.
4) Listen to your stuff to see whether there is any improvement in the sound.
5) Monitor the heat sink to see whether it get hotter. If it is still cold, you can adjust a little higher, but not too much.
6) Play your music loud for a while to double check. It should not get any hotter than before the adjustment and just play it loud.

As long as you can grab the heatsink and hold you hand on it indefinitely, it's not too hot.

If you have the schematic, please post here so I can look at it. Without that, I am only guessing and play it safe. I can tell you a lot more if you provide the schematic.
 
OK, without schematic, I can just guess, so do it at your own risk.

For me to adjust the bias, I don't need a signal generator. I don't understand why they even want to keep adjusting. This is what I would do:

1) turn on the amp and let it sit without music or anything for 20minutes.
2) measure the voltage WITHOUT ADJUSTING VR501 and VR502. Write down the voltage. My guess it's about 15mV to 20mV.
3) Adjust to increase 5mV on both side......NO MORE.
4) Listen to your stuff to see whether there is any improvement in the sound.
5) Monitor the heat sink to see whether it get hotter. If it is still cold, you can adjust a little higher, but not too much.
6) Play your music loud for a while to double check. It should not get any hotter than before the adjustment and just play it loud.

As long as you can grab the heatsink and hold you hand on it indefinitely, it's not too hot.

If you have the schematic, please post here so I can look at it. Without that, I am only guessing and play it safe. I can tell you a lot more if you provide the schematic.

Thank you sir this is exactly what I was looking for.
 
I think I found the schematic, can you verify this is the right one? Also, can you confirm that you are measuring at the place I labeled TP?
If that's the point, I normally adjust to 25mV after warmup.

Do what I said in the last post, adjust a little at a time, wait for it to warm up, hand test, listen, then adjust a little bit more and test. Don't go over 25mV. Make sure you can have your hand on the heatsink and not feel painful hot.

After you finish adjusting, use it for 30 minutes and hand test again.

Denon PMA-860.JPG
 
I think I found the schematic, can you verify this is the right one? Also, can you confirm that you are measuring at the place I labeled TP?
If that's the point, I normally adjust to 25mV after warmup.

Do what I said in the last post, adjust a little at a time, wait for it to warm up, hand test, listen, then adjust a little bit more and test. Don't go over 25mV. Make sure you can have your hand on the heatsink and not feel painful hot.

After you finish adjusting, use it for 30 minutes and hand test again.

View attachment 918581

It's probably easier if I just link you the service manual. I'm not very good with schematics. For reference, the instructions for setting the Idling Current are on page 12. http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Denon-PMA-860-Service-Manual.pdf
 
Got it, yes, that should be the point. I don't know why they need a 1KHz, I never adjust like this nor I ever heard people doing that.
 
Please post back and tell me what is the reading after warmup before you adjust anything.

I did calculation, your rail is +/-56V, if you adjust to 25mV, total idle current is about 230mA. So the power dissipation is 112X0.23=25.76W/channel. That is high judging by looking at the size of the heatsink. I don't think you can get up to 25mV without getting quite hot. So stop about 17mV first, use the amp for a little bit, do the hand test. If it is not too hot, you can increase like 3mV at a time. Be careful.

Alan
 
Got it, yes, that should be the point. I don't know why they need a 1KHz, I never adjust like this nor I ever heard people doing that.

In some Sansui Service manuals, prior to bias & DC offset setting, they ask your to "pre-condition" the amplifier, in other words 'use it', 'heat it up' using a 1KHz signal and run the amplifier for a time so that the ambient / running temperature is correct.

Regarding cool running.
I have a Sansui AU-D11 II, which runs as cool as a cucumber. However if I really 'crank it' it begins to warm up just a little. I know how to set the bias, and I am certain it is correct, it's just the nature of the beast - which incidentally is my one of my all time favourite amplifiers.
 
Actually the idle bias heating effect is totally different from when you play with high signal. The idle bias is to optimize the cross over distortion and also give you a bigger class A region so when you play really soft, your amp is running in class A. So theoretically, the temp rise when you play loud is the same whether you have more bias or less bias as the amp goes into class B at large signal.

This is like class A amp, they get really warn just sitting there. But it does not get warmer even you play loud because it is always stay in class A and does not draw any more current between idle and loud.

To optimize, you usually want to keep the idle a little warm.

The hand test is direct quote from Nelson Pass.......After warm up, if you can grab the heatsink with your hand and stay there, it's not too hot!!!! That's what he advice people how to adjust the bias with his Nakamichi PA-7.
 
Actually the idle bias heating effect is totally different from when you play with high signal. The idle bias is to optimize the cross over distortion and also give you a bigger class A region so when you play really soft, your amp is running in class A. So theoretically, the temp rise when you play loud is the same whether you have more bias or less bias as the amp goes into class B at large signal.

This is like class A amp, they get really warn just sitting there. But it does not get warmer even you play loud because it is always stay in class A and does not draw any more current between idle and loud.

To optimize, you usually want to keep the idle a little warm.

The hand test is direct quote from Nelson Pass.......After warm up, if you can grab the heatsink with your hand and stay there, it's not too hot!!!! That's what he advice people how to adjust the bias with his Nakamichi PA-7.

What temp would being able to grab the heatsink be?

I should have put it in the OP, my heatsink is 5 degrees F above ambient at idle, and under the highest volumes that I use it at it goes up to 35 degrees F above ambient. I took these measurements with my themocouple on my multimeter.
 
Please post back and tell me what is the reading after warmup before you adjust anything.

I did calculation, your rail is +/-56V, if you adjust to 25mV, total idle current is about 230mA. So the power dissipation is 112X0.23=25.76W/channel. That is high judging by looking at the size of the heatsink. I don't think you can get up to 25mV without getting quite hot. So stop about 17mV first, use the amp for a little bit, do the hand test. If it is not too hot, you can increase like 3mV at a time. Be careful.

Alan

OK. My amp has been on all day and I just checked the test points I get 3.45mV on one and 3.75mV on the other. This is quite a bit lower than expected, isn't it? Time to bring em up?
 
OK. My amp has been on all day and I just checked the test points I get 3.45mV on one and 3.75mV on the other. This is quite a bit lower than expected, isn't it? Time to bring em up?
Definitely bring it up to say 10mV first just to be safe and listen for a while. You should hear the sound get smoother and a little warmer. 3mV is way way too low.

I would say 50deg C is safe. Even 60 is ok.

EDIT, but I am more comfortable using calculation. I would say 14mV is safe as 14mV gives idle current of 138mA through the two transistors. The total supply voltage is 2 X 56V=112V. So idle power dissipation is 112V X 0.138= 15.4W per channel. The heat sink should get quite warm but not hot.

Listen after you adjust and see whether there is any improvement. YOu can even raise to say 20mV for a short time and see whether it sounds better, but back it down unless it's not too hot.

If increasing voltage make it sound better to you, you can always consider putting a small CPU fan blowing onto the heatsink so you can raise it a little higher.
 
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Definitely bring it up to say 10mV first just to be safe and listen for a while. You should hear the sound get smoother and a little warmer. 3mV is way way too low.

I would say 50deg C is safe. Even 60 is ok.

EDIT, but I am more comfortable using calculation. I would say 14mV is safe as 14mV gives idle current of 138mA through the two transistors. The total supply voltage is 2 X 56V=112V. So idle power dissipation is 112V X 0.138= 15.4W per channel. The heat sink should get quite warm but not hot.

Listen after you adjust and see whether there is any improvement. YOu can even raise to say 20mV for a short time and see whether it sounds better, but back it down unless it's not too hot.

If increasing voltage make it sound better to you, you can always consider putting a small CPU fan blowing onto the heatsink so you can raise it a little higher.

I just brought it up to about 12mV each, so we'll see what that does. Thanks!

I made a big mistake and turned the wrong pot, I turned VR503, which the instructions say not to turn until you apply the 1khz at full volume. Because of that, I had to go through and follow all of the instructions from the manual. After I did that, I brought the pots up to 12mV.

I don't have a signal generator, so I had to improvise. I downloaded a free signal generator app, which allowed me to set the frequency output through the headphone jack to 1000hz. I then connected my phone to an RCA cable using the proper cable and measured the outputs with my multimeter in AC millivolt mode in order to determine what volume setting would give me the 10mV RMS level that was specified in the instructions. It seemed to work great.

Those pots are super tricky to adjust. You have to basically turn in the tiniest incriminates possible and hope that it ends up at a suitable number.

Denon recommends setting it at 5mV +-.5mV, so I guess it seems possible that the current was set to 3.5mV at factory...or more likely they somehow got pulled out of range due to age. Even 5mV seems low for a standard setting. Anyhow..time to do some listening!
 
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Do me a favor, find where is <5B-2> circled in red goes. For the life of me, I cannot find it in the schematic. This is controlling the use of VR503, I want to see why they have that.

It is nothing more than changing the bias a little, if you did not touch it, I would just leave it alone.

5B-2.JPG



Yes, 3mV is low, period. I would expect 10mV or more. The ideal setting if you have a bigger heatsink is 26mV. This is called Oliver's condition that minimize the crossover distortion. Even 10mV is very low, I can tell you in my spectrum analyzer test, the higher harmonics goes up when the bias is too low. My Acurus had the same problem, the heatsink barely warm, measured and show only about 5 or 6mV. I set it to 25mV and I feel the sound is rounder, smoother and better. But I have a big heat sink that can take it. YOu are going to have to add fan to raise higher. I am not sure you can do that with a fan.
 
Something doesn't add up here. My Denon has larger heatsinks than most 80W amps that I've seen. It has a THD of 0.007%. And they suggest a setting of 5mV on the resistor. I can't believe that it has a glaring, unique issue like this.
 
Actually the idle bias heating effect is totally different from when you play with high signal. The idle bias is to optimize the cross over distortion and also give you a bigger class A region so when you play really soft, your amp is running in class A. So theoretically, the temp rise when you play loud is the same whether you have more bias or less bias as the amp goes into class B at large signal.

This is like class A amp, they get really warn just sitting there. But it does not get warmer even you play loud because it is always stay in class A and does not draw any more current between idle and loud.

To optimize, you usually want to keep the idle a little warm.

The hand test is direct quote from Nelson Pass.......After warm up, if you can grab the heatsink with your hand and stay there, it's not too hot!!!! That's what he advice people how to adjust the bias with his Nakamichi PA-7.

I think you misunderstand me just a little bit, I know exactly what bias does, about class Ab amplifiers and how they are different from class A amplifiers.

What I was explaining was that Sansui ask you, as part of the bias setting procedure for some of their class Ab amplifiers, to run the amp with a 1KHz signal for a while to heat the amplifier up - I.O.W get it to operating temperature - THEN set the bias. You wouldn't adjust the bias with the unit stone cold would you? :D
 
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