Amplifier Distortion, DC-Offset, and You!

The 385i has no speaker protection relay, so the output of the amp is always connected to the speakers. If you hear nothing untoward on power-up, then you really have no concerns.
 
Many thanks toAK

I have an electrocompaniet AW 65
Could anyone help me as to where to take the bias reading??
If itake it across those big resistors (3watts) 4 of them. 2 on each board it reads plus or minus 20 mv. Is that making sense??? I have located the four trimers for bias and offset

Help a beginner!!!! :)
 
Thank you for your answer echowars.
Keeping the bias as is i try to ajust the ouput but once i have it close to zero if i leave the volt meter on it for a while( 30 min) i come back and the reading is totally off!!!! Any idea?
 
EchoWars said:
Time for one of those long, boring semi-technical posts that no one here reads...

As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire amp and replaced all the electrolytic capacitors with the exception of the big power supply caps (not that expensive...maybe $20 in caps).

The previous owner described the amp as 'perfect' sounding, and compared with the big Mac's and Krell's and such. Before buying it, he described his current system which was quite high-end. I assumed that since he owned expensive equipment that he knew what he was listening to.
:withstpd:

Got the amp back together today (parts finally in), and fired it up with my small Dynaudio's (can't use the Heil/Dynaudio's for this, as they are bi-amped). Ummm...it sounded like ass. OK, it didn't always sound like ass, but at low volume levels it was obvious that something was wrong. I had a pretty good idea what was going on, so I grabbed my meter. Sure enough, there was 100mV of DC offset in the left channel, and almost 250mV (!!!) in the right! 250mV is almost enough for the protection circuitry to kick in!! Not good. I pulled the driver boards out and replaced the amp input differential pairs for both channels with new Zetex HG PNP's ($5 in transistors, no biggie). DC offset is now about 12mV in both channels. An input pair being as unbalanced as the Kenwood was when it arrived probably generates 10x the distortion as a properly balanced pair, especially at low volume. If you would like to read more about the benefits of a balanced differential pair, read here.

OK, sit down for another listen. NOW we're cookin'!! Amazing night and day difference. I can honestly say that it is without a doubt the nicest amp I have ever listened to, and there have been more than a few that impressed me. The bottom end on this thing is as clear as spring water, and it has an openess that is difficult to describe. As for power, my dummy loads cannot take the power output of this thing, but I can crank it for 10-15 seconds without destroying them. Left channel - 218W before clip, right channel - 220W, this into 8-ohms. Totally cool!! I just have never heard bass like this...wow...

Bottom line...if you expect to hear great sound, you just might...regardless of the reality. The guy I bought this from was well-meaning, but did not know how to listen subjectively. His new spendy amps could be performing horribly, and his expectations of what he felt he was supposed to hear would rule out anything to the contrary.

Your own subjectivity could be suffering too, so give yourself a reality-check.

As a semi-poll, I'd like to see those on this board whip out their multimeters and take a look at the DC that is being presented to the speakers. This means..

1. Speakers disconnected (or connect the meter to the 'B' speakers and set the front panel speaker control accordingly)
2. Input set to an unusued position (not Phono)
3. Volume control at minimum.
4. Balance in center
5. Tone controls either defeated or set to mid position
6. Set your meter to read DC, and set to a low scale (300mV scale is common) Connect directly to the Pos and Neg of the speaker terminals
7. Give the amp 10 minutes to settle. Report back...I'd like to see how healthy all these old amps are.

If you read:

0 - 15mV: Damn good!! If you read '0V', you may have a capacitor output, or your meter is set wrong

16mV - 50mV: An acceptable value, especially at the lower end of this range. 2nd harmonic distortion is probably twice to four times what manufacturer's spec calls for at higher frequencies. Probably not audible, as the distortion is mostly in the upper octaves. At the upper end of this range I begin to raise an eyebrow. :saywhat:

50 - 85mV: Something is certainly amiss, and while this is not enough to put your speakers or equipment in jeopardy, the amp is running nowhere near where it should. I'd venture to guess that most of the DC-coupled amps that are in use by forum members here fall into this range.

100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH..)

Soooooo...go grab a meter and tell me what you find...
Click to expand...
EchoWars said:
Time for one of those long, boring semi-technical posts that no one here reads...

As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire amp and replaced all the electrolytic capacitors with the exception of the big power supply caps (not that expensive...maybe $20 in caps).

The previous owner described the amp as 'perfect' sounding, and compared with the big Mac's and Krell's and such. Before buying it, he described his current system which was quite high-end. I assumed that since he owned expensive equipment that he knew what he was listening to.
:withstpd:

Got the amp back together today (parts finally in), and fired it up with my small Dynaudio's (can't use the Heil/Dynaudio's for this, as they are bi-amped). Ummm...it sounded like ass. OK, it didn't always sound like ass, but at low volume levels it was obvious that something was wrong. I had a pretty good idea what was going on, so I grabbed my meter. Sure enough, there was 100mV of DC offset in the left channel, and almost 250mV (!!!) in the right! 250mV is almost enough for the protection circuitry to kick in!! Not good. I pulled the driver boards out and replaced the amp input differential pairs for both channels with new Zetex HG PNP's ($5 in transistors, no biggie). DC offset is now about 12mV in both channels. An input pair being as unbalanced as the Kenwood was when it arrived probably generates 10x the distortion as a properly balanced pair, especially at low volume. If you would like to read more about the benefits of a balanced differential pair, read here.

OK, sit down for another listen. NOW we're cookin'!! Amazing night and day difference. I can honestly say that it is without a doubt the nicest amp I have ever listened to, and there have been more than a few that impressed me. The bottom end on this thing is as clear as spring water, and it has an openess that is difficult to describe. As for power, my dummy loads cannot take the power output of this thing, but I can crank it for 10-15 seconds without destroying them. Left channel - 218W before clip, right channel - 220W, this into 8-ohms. Totally cool!! I just have never heard bass like this...wow...

Bottom line...if you expect to hear great sound, you just might...regardless of the reality. The guy I bought this from was well-meaning, but did not know how to listen subjectively. His new spendy amps could be performing horribly, and his expectations of what he felt he was supposed to hear would rule out anything to the contrary.

Your own subjectivity could be suffering too, so give yourself a reality-check.

As a semi-poll, I'd like to see those on this board whip out their multimeters and take a look at the DC that is being presented to the speakers. This means..

1. Speakers disconnected (or connect the meter to the 'B' speakers and set the front panel speaker control accordingly)
2. Input set to an unusued position (not Phono)
3. Volume control at minimum.
4. Balance in center
5. Tone controls either defeated or set to mid position
6. Set your meter to read DC, and set to a low scale (300mV scale is common) Connect directly to the Pos and Neg of the speaker terminals
7. Give the amp 10 minutes to settle. Report back...I'd like to see how healthy all these old amps are.

If you read:

0 - 15mV: Damn good!! If you read '0V', you may have a capacitor output, or your meter is set wrong

16mV - 50mV: An acceptable value, especially at the lower end of this range. 2nd harmonic distortion is probably twice to four times what manufacturer's spec calls for at higher frequencies. Probably not audible, as the distortion is mostly in the upper octaves. At the upper end of this range I begin to raise an eyebrow. :saywhat:

50 - 85mV: Something is certainly amiss, and while this is not enough to put your speakers or equipment in jeopardy, the amp is running nowhere near where it should. I'd venture to guess that most of the DC-coupled amps that are in use by forum members here fall into this range.

100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH..)

Soooooo...go grab a meter and tell me what you find...
Click to expand...

Just tested the dc offset of a garage sale HK 770 and am getting some outrageous numbers.
Both L and R metered a 1 when set to 200mv. At the 2v position the R ch,which was running a little hot, is measuring 1.08v, L ch .276v.

Given what ive read it seems as though the amp should have triggered the protection circuit or at least hear a severe distortion in the R ch.

And yes, i foolishly did give it a listen on both my Fortes and Spectre 3300s. While risky on my part, tbe 3300 did confirm that the protection circuit is functioing as it was momentarily triggered during heavy bass passages when volume was turned up. Actually sounded quite nice when paired with a Lazarus Cascade Basic Pre, offering a differant sound profile, though not bettering, the a Bedini 150 mk2, MC2205 and. Belles 200 that are in usual rotation.

Are these readings likely incorrect given what I'm hearing orby the fact that it is operational?
Is there any hope of adjusting it back into the acceptable >50mv range or is replacing the differential pairs the likely solution?

Rest assured the amp is presently sidelined after its inaugural and only demonstration.
Thanks
 
1V and 300mV ought to give a bit of a 'thump' as the relay engages. But even 1V probably isn't enough to trigger the protection, or result in any blatant distortion.

The 770 does have an offset adjust potentiometer. If you can figure it out, and if you trust what your meter is telling you, then you can try to adjust to zero. Otherwise, consider having a professional look at it for you.
 
EchoWars said:
1V and 300mV ought to give a bit of a 'thump' as the relay engages. But even 1V probably isn't enough to trigger the protection, or result in any blatant distortion.

The 770 does have an offset adjust potentiometer. If you can figure it out, and if you trust what your meter is telling you, then you can try to adjust to zero. Otherwise, consider having a professional look at it for you.
Click to expand...

Doing a little digging around in the amp produced a few leaky caps that are easily accessible and seemingly within the realm of what i can do diy. Given the high offset readings, would you suggest i replacing those before attempting offset adjustment? My thinking being that these are likely contributing to the amount of dc offset.

As for reading technique, ive settled upon clipping the probe into the terminal as if they were speaker wire.This seems to produce the most consistent readings(if those are correct). Is this how you go about the process?
 
EchoWars said:
OK, then define:
Click to expand...

I mean that if i ajust the dc offset close to zero and leave the meter on it and come back 30 min later the reading is nolonger close to zero (1.1 1.4 or so)
But is sometimes around 14.0. Or 21.5 ..... I m I doing something wrong? Should i just set it closest to zero , remove the meter and thats it?
How long should it be for an exact reading??
Thanx again for your help.
I'm learning lots here.
Truely yours

JF
 
This isn't the place to troubleshoot your problems, but offset will certainly drift with temperature. Adjust it, monitor it for an hour, re-adjust, and you've done all you can do. If it's drifting as much as 50mV, then perhaps something might need attention, but the front end of the 770 is a bit of a bear and chasing offset drift may end up a fool's errand. I think if it can remain under 20mV you're good enuf.
 
EchoWars said:
This isn't the place to troubleshoot your problems, but offset will certainly drift with temperature. Adjust it, monitor it for an hour, re-adjust, and you've done all you can do. If it's drifting as much as 50mV, then perhaps something might need attention, but the front end of the 770 is a bit of a bear and chasing offset drift may end up a fool's errand. I think if it can remain under 20mV you're good enuf.
Click to expand...
Ok i'll do like that. Thanks for your help. I will check all my amps for bias and offset. I'm sure i'm in for some
surprises!!!!!

Thanks again. Very interesting forum.
For anyone who wants to learn and of course listen to music with the best possible equipement....,

Cheers
 
This is a really cool thread, and I thank you for sharing your knowledge with others. I have a 1976 Sansui 771 that hasn't been touched inside as far as replacing anything and there doesn't appear to be any popped capacitors.
Left: 17 Right: 25
The polarities were reversed on the meter, but I believe you mentioned earlier that this is not a problem.
However, something that I wonder if it could be an issue is that the number would constantly fluctuate up and down as much as 2-3mV at a time. For comparison, I did the same test in a 2002 Pioneer Elite receiver that had values of L:5.8 and R:8.9 but the held solid. Should this be a concern with the Sansui?
According to the service manual the DC voltage output is adjustable via a pot.
 
Had a Little heart spasm when I ran this test on my newly recapped Marantz 2230. Speaker terminal Voltage at idle was 13 V (!!!). Each side. Thought I was about to melt my speakers, but it sounded great. After reading MANY pages of this thread, it looks like my 2230 is cap coupled, and that type doesn't follow the rules, I'm hoping that's the reason for my high readings!

BTW - My 2270 measured 3.3mV on the left and 3.2mV on the right.
 
Last edited:
EchoWars said:
This isn't the place to troubleshoot your problems, but offset will certainly drift with temperature. Adjust it, monitor it for an hour, re-adjust, and you've done all you can do. If it's drifting as much as 50mV, then perhaps something might need attention, but the front end of the 770 is a bit of a bear and chasing offset drift may end up a fool's errand. I think if it can remain under 20mV you're good enuf.
Click to expand...
Thanks for the guidance. Ive managed to dial in the offset to 11mv in each channel with a 5mv or so fluctuation.

Its been a week now and decided to see where it stands. I get approx -25mv in each channel with the top on. Removing the top sees the dc steadily drop, eventually settling in the aforementioned 11mv range.

Is this a common phenomena?
Should i adjust accordingly (bump it up) so the offset is at an acceptable level w/ top on?
Thanks again
 
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