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Amplifier Distortion, DC-Offset, and You!

This is a great thread, and I am late to the party but glad to see that negative numbers can be considered the same as positive because:

Sansui 2000X: -7.2R, -5.7L
Denon DRA-425: 4.1R, 5.0L
Yamaha RX-V685: -3.9R, -4.2L
Sansui Classique A-900: 0.8 R, -2.2L,
 
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von.ah said:
we put the trouble in troubleshooting

OK. Per another recent thread on pioneer x3x protection relays, I cracked open the relay's plastic case and deoxitized it. Wisdom would dictate that I wait for at least a few minutes to turn the unit back on, but as soon as I got the relay cover, its screws, and the bottom tray back on, I fired it up. :nono: The relay engaged like it should a few seconds after power switched on, and then tripped several times in the next several minutes. This time much more frequent than in the past. Shiznit. So, I disconnected everything and after 10 min, no relay trip. Then I reconnected the speakers to A, and have been running FM for about 15 minutes with no protection relay trip. Now my theory is that my speaker wire was the problem. This time when inserting the wire, I made sure it went in smoothly... DC at the terminals is still 26 mV L and 18 mV R. I think I need more time to ferret out what the problem is. Maybe it was just my neaderthal wire insertions... Thanks for the support, EW.
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quiggs said:
Still taking data points?

Pioneer SX-1250:

Left: 1.5mV
Right: 3.6mV

According to EchoWars, there's no point in trying to improve on these numbers. But since I've already downloaded the service manual . . .
[Next up: noise, smoke, recriminations, pleas for help . . .]
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Harman Kardon PM 665 Vxi

Włączony zimny wzmacniacz.
t R L
0 -45,5 -58
10s -275 -100
1 minuta -248 -115
5 minut -107 -56
10 minut -70 -40
15 minut -40 -22
35 minut 0 0 (+-10mV)

Czy takie wartości są prawidłowe w tym modelu?
 
L322 said:
Harman Kardon PM 665 Vxi

Włączony zimny wzmacniacz.
t R L
0 -45,5 -58
10s -275 -100
1 minuta -248 -115
5 minut -107 -56
10 minut -70 -40
15 minut -40 -22
35 minut 0 0 (+-10mV)

Czy takie wartości są prawidłowe w tym modelu?
Click to expand...

Polish:

Włączony zimny wzmacniacz. = Cold amplifier turned on.

Czy takie wartości są prawidłowe w tym modelu? = Are these values correct in this model?
 
I only recently discovered this interesting post through browsing; lots of useful and relevant technical information here.

While I may be late to joining this forum, to share my experience of measuring and successfully minimizing the DC offset voltage in my Bryston 3Be amplifier.
'Bryston 3Be Power Amplifier - Adjusting and Minimizing DC Offset'
https://www.audiocircle.com/index.php?topic=188637.0
Note this investigation and report was completed before I was aware of this forum, good news is it corroborates the technical information posted in this solid-state forum.

Also mentioned in this report is my experience confirming the low DC offset voltage in my vintage Bryston 2B amplifier.

Thank you,
John
 
I just picked up a Kenwood KA-880D integrated amp. Warmed it up & tested DC according to SM. Both channels in the upper 20mV (27-28). SM called for 9mV. Question. Many amps, receivers and such I've tested are higher than what SM calls for. Why are they this way.
 
I'm going to assume you bought these used. That said, it's very possible they've repaired with either non-exact transistors and/or said transistors were not properly matched with their circuit counterpart. The incredible OLG, open loop gain, in these can compensate a lot for mismatched SS, solid state, but to get it closer to perfect correct matched SS is recommended for best results, assuming other biasing components haven't drifted.
 
AmplifierRight (mV)Left (mV)
HK440 vxi (#1)
-14.3​
-4.3​
HK440 vxi (#2, Antoine)
19.8​
-3​
HK550 vxi (#1, survivor)
-2.6​
4.8​
HK550 vxi (#2, gramma)
-37​
-38.5​
 
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...
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It's worth taking a look at any differential pairs. You can see on the schematic. Some amps have them on their protection circuit (I'm thinking the Yamaha ca-410 here) The relay was flapping and it was something that got worse as the amp warmed up. Pulling and measuring the long tail pair confirmed how far away from each other they were. Two new, matched SCA992s later and it was rock steady.
 
timblade68 said:
Just measured my Yamaha M40 Power amp, recap completed about a 3 weeks ago with at best, 12 - 15 hours play/break-in time. Channels measured as follows:

Right 2.3mv
left 17.7mv

After the recap, I reset the bias trim to within spec. The left channel always seemed to run a bit warmer, especially in class A mode. So the elevated voltage in that channel may parallel the read. Quite a moot point, to my ears all's well in Natural Sound land.
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Just revisited the M40 amp internals, checking solder joints (all good), checked the idle again off by 10th so I reset to 10mv left & right, offset measured at left ch: 0mv / right ch -3. Maybe better reads due to new Audio Note Kaisei caps installed?
 
Anyone has the procedure to adjust the bias on YBA YA-201?

I have an amplifier on which the BB PGA2311P was faulty. After replacing everything is fine. I would like to verify the bias and ensure the amplifier is running within specification.
 
DC offset of my Harman Kardon HK1400 jumps all over the place (oscillation over oscillation).

It starts with 500mV, quickly drops to 120mV, drops further and starts to oscillate and drift.
It is impossible to set de DC offset trimmer to keep the offset in check, it keeps drifting and oscillating, even an hour after turning on the amp.

Is there a defective component?
 
BassBias said:
DC offset of my Harman Kardon HK1400 jumps all over the place (oscillation over oscillation).

It starts with 500mV, quickly drops to 120mV, drops further and starts to oscillate and drift.
It is impossible to set de DC offset trimmer to keep the offset in check, it keeps drifting and oscillating, even an hour after turning on the amp.

Is there a defective component?
Click to expand...
I would start a new thread. It sounds like a repair is in order.
 
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