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Amplifier Distortion, DC-Offset, and You!

New to me Harman Kardon 330C after main amp repairs, bias setting, and a half hour run time:

Left - 0.004 Volts

Right - 0.023 Volts
 
Kenwood KS-400R after a good 15 years of storage, but all functions work. After 90 mins of use:
L = 6.3mv
R = 23.5mv

Just for grins, I played with the volume (made an impact...duh), the base adjustment impacted the readings somewhat, the treble did not.
 
How often should offset be checked? Rebuilt my Pre & power amps Christmas 2024, are they do for a checkup?
 
Just checked my Echowars recapped 600 and 700m, ca. 2005-8
600
L = 13.1 R= 40


700m
L= 15.1 R= .1 fluctuating to 0

Thanks Glenn, wherever you are, both for the original post and for your skill in recapping these amps. Trouble is, I'm now afraid to let anyone else touch them.
 
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Mike_S said:
How often should offset be checked? Rebuilt my Pre & power amps Christmas 2024, are they do for a checkup?
Click to expand...
Dear Mike,　

You posed an interesting question, but I think it's an "it depends" answer.　

Suggest it's prudent to check the output offset voltages whenever the amplifier; has undergone repair or modification, has been taken out of long-term storage, was purchased used or its history is unknown, or there are concerns about the sound quality.
Other factors depend on the amplifier's circuit topology and the selection of components installed, but if the measured offset voltages are low millivolt values and stable, I don't think routine checks are needed.
That said it's not a difficult measurement to make.

This reply was based on my limited experience repairing my vintage Bryston 2B amplifier where the DC offset measured across the output terminals was +4mV left channel and -6mV right channel.
Documented in "Bryston 2B Power Amplifier repair - Hints and Lessons Learned" Bryston 2B Power Amplifier repair - Hints and Lessons Learned
These DC offset voltages have been stable since the amplifier was purchased new in 1979, and while all electrolytic capacitors have been replaced there have been no changes or adjustments to the original input transistor differential pairs.

Thank you,　
John
 
Mike_S said:
How often should offset be checked? Rebuilt my Pre & power amps Christmas 2024, are they do for a checkup?
Click to expand...

John Tulett said:
Dear Mike,　

You posed an interesting question, but I think it's an "it depends" answer.　

Suggest it's prudent to check the output offset voltages whenever the amplifier; has undergone repair or modification, has been taken out of long-term storage, was purchased used or its history is unknown, or there are concerns about the sound quality.
Other factors depend on the amplifier's circuit topology and the selection of components installed, but if the measured offset voltages are low millivolt values and stable, I don't think routine checks are needed.
That said it's not a difficult measurement to make.

This reply was based on my limited experience repairing my vintage Bryston 2B amplifier where the DC offset measured across the output terminals was +4mV left channel and -6mV right channel.
Documented in "Bryston 2B Power Amplifier repair - Hints and Lessons Learned" Bryston 2B Power Amplifier repair - Hints and Lessons Learned
These DC offset voltages have been stable since the amplifier was purchased new in 1979, and while all electrolytic capacitors have been replaced there have been no changes or adjustments to the original input transistor differential pairs.

Thank you,　
John
Click to expand...


I agree entirely but for the OP, since he had it rebuilt a year back, a "one year check" is likely a good idea. if still stable, I would not worry too much gong forward..
 
OK this is super cool. a way to test the health of our vintage amps! I'm 23 years late to the party here but this is what makes forums so awesome. like reddit, if you look around you will find someone else has already been down the road we find ourselves on at our own pace. keen to report back what values I get when I test this!
 
My digital meter has a section for DCV with five choices. The top one is 1000, then 200, 20, 2000m, and 200m. If I choose 200m and it reads 2.45, does that mean 2.45 millivolts?
 
Marantz PM-17SA: left = .003; right= 0.00. I have owned this antegrated for over 12 years, it’s my main amp. I bought it from ebay, and have no plans to replace it. Spec for offset is 3+/- mv.
 
NAD 705 receiver, after bulb test, vacuuming dust, general inspection, 30min idle:

L = 16.3mv
R = 17.1mv

Tuned to +/-2.0mV
 
If the woofer doesn't move, doesn't whistle, and makes no noise, that's good, right? That means it's correct. So I know what the offset test is, and my 1965 The Fisher 200t early germanium woofer is doing wonderfully and sounds great. Is it even useful to do the tests if you hear no noise?
 
I tested my RAM 512 amp that I just reviewed: L=-30mv. R=17.5mv.
Looks like I’ll have to review it again once I get it fixed.
 
Bias! When concerned about distortion with a specific speaker load impedance, it is much safer to err on the high side when setting.
 
Musical concerts Hafler DH 220 and DH 110 preamp. Nothing done to it since 1982, but was in storage for 20 years.
Left channel 7mV
Right channel 3 mV
 
Just bought a beat up Kenwood KM-106 power amp from a record store. Didn't do anything except dust off the exterior. Measurements were taken with one of my nice HP 3478A bench multimeters.

Right: 5.65 mV
Left: 12.54 mV


This amp has Channel A and Channel B outputs with an On/Off switch for each so for kicks i decided to also measure DC offset with the switches set to Off and got some weird readings:

Channel A w/ switch OFF:
Right: - 0.0011 mV
Left: - 0.0007 mV

Channel B w/ switch OFF:
Right: 1.79 V
Left: - 0.17 V average, drifts a bunch

Channel A looks like what I would expect (very close to 0 V) so I'm guessing something is going on with Channel B. I will be taking this amp apart anyway because both of the input RCA jacks are way loose and make intermittent contact and the power meter lights on the front look much dimmer than normal. May as well also do a recap while I'm in there...

Still sounded pretty good on both Channel A and B; with an old cell phone pushing YouTube music into the input and a loose pair of Bose car audio speakers hooked up treble was nice and bright and bass was well... present. Likely needs an equalizer tied in to sound the way I prefer.

EDIT: I dug up a copy of the Service Manual. According to the circuit diagram when the Channel B switch is in the Off position the Channel B Right + and Left + terminals are completely disconnected and left floating; measuring their voltage in this switch position is therefore meaningless. I'll plan to run some contact cleaner through the switches but for now I don't have any concerns regarding the Channel B outputs.
 
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in case it's useful to anyone

Sony STR-AV970 I'm using while my vintage gear gets troubleshot:

set amp bias to 2 mV for L/R channels per the SM.

I didn't see a method to adjust DC offset but it hovers around 16 mV Right and 6 mV Left, at the speaker jacks.
 
Is it possible that a switch can cause high dc offset?
On my JVC amplifier (AX-1100), when selecting CD1 there is high offset on right channel, around 60mv, which seems to get better after some time, but all other inputs are with normal values offset.
Can it be a problem of the switch, or something else, maybe cd1 input is wired differently from other inputs.
 
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