Amplifier Distortion, DC-Offset, and You!

Sorry for the late reply. Yes Counterclockwise.

I adjusted the DC Offset the best I could. Both channels now fluctuate between 26-39. The meter keeps going up and down between these two numbers on both channels at the best setting. There's no way the numbers will be close to 0! The amp does feel cooler to the touch now. The bass does seem a bit fuller than before. I'm a happy camper with my NAD 3140 more so now! :music:
 
I have a Hafler DH200 (original owner) and measured the DC offset at 46mv/26mv. Of course, there isn’t any offset adjustment on the amp, you have to match the driver transistor pairs to minimize the offset.

I read some posts by EchoWars and purchased a dozen TX694B AND TX795A’s which he recommended from Mouser. Using my handy chinese universal component tester, I matched beta and actually go 4 well-matched pairs of each type out of the batch.

Substituting the 2N5550’s (Q1/Q2) with TX694B and the 2N5401’s with TX795A (Q5/Q6) with the best matched pairs from the dozen that I purchased. Since I have more transistors, I decided to swap all the remaining 2N5550’s and 2N5401’s.

I powered up the DH200 and let it warm up for half an hour. Both channels were down to 1.1 mV!

Listening to the amp, I think it does sound better. All the small caps have been replaced, now I just need to upgrade the power supply and renew the output transistor thermal paste.
 
Hello guys.
I'm new to the forum.
I have made minor repairs on my equipments like replacing capacitors and improving some soldering and although I have an engineer training my career has been in configurations of telecommunication equipment ... so the few school knowledge of electronics are almost completely forgotten :(

However, as all of us here, music lovers, like to take and enjoy the best sound that our equipment can deliver.
As such, I have already made substitutions of poor quality potentiometers for the multi-turn type, but I have some doubts that I'm sure you can clarify as well as others.

My actual gear are a almost 20 years old Pimare Pre-30 and Primare A30.2, love them, by the-way :)

For power (A30.2) as the Service Manual is explicit the doubt is in the procedure ... should I short-circuit the inputs? I.e. get the 20mV with the xlr pins 2 and 3 shunted? The service manual called it idle current adjustment, what about dc-offset?

Now for Pre-30, that's a hard-one for me... I really don't know where or what should be measured by adjusting that multi-turn's.

Please note, have attached both service manual. Hop that also helps everyone o cares.

Thank's great forum

Hi all,

Does anyone cares about this?

Any help well appreciate.

Thanks
 
I was given a Pioneer SX-939 today, it's in good shape but is full of cigarette smoke. Uggghhh. I measured 18.5 and 22.something...not bad for an all original survivor that's never been taken out of use in 42 years. I was going to sell it, but after an intense (cop calling loud) listening session, I may keep this one. After I go through it and clean it and all pots and controls it'll be like new again...minus that one scratch ontop the faceplate that really isn't bothering me right now...
 
Very nice tread! Havn't read all the posts here, but some of them. I remember to have a SONY TA-3140F in storage with a "little" offset problem. It has about 196mV at the right channel. Left is at 43mV DC.

Offset is not adjustable, so i have also to change some transistors. Can anyone tell me which transistors are affacted? Is it Q502/3? Usually the original types are not longer available, so i woud need recommendations for substutes as well.

Thank you in advance!
 

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I think it's better to post a fullsize picture of the plan. BTW: D501 is a termal bias compensator located at the power transistors heat sink. The circuit braker CB-1 is a lamp and a bimetal switch together.

Schaltplan TA-3140.PNG
 
just picked up a Kenwood KA 4006

R: 9.5 mV ( started at 12 mV but stabilized @9.5 mV after being on 10 mins )
L: 1.2 mV ( started at .8 mV and even bounced around 0 mV for bit, before stabilizing at 1.2 mV )

I plugged in a tuner & checked the headphone output, speakers off, and everything sounded fine. hooking speakers up now...
 
From original post

"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...)"

Im listening to the JVC A-X5 with DC offsets of 77mV left and 457mV right. It sounds fine. At least my ears cant tell that anything is off (most mid fi sounds about the same to me, good, especially when it comes to amplification). I dont think I could tell a difference in sound bw this JVC and my current model C326BEE with dc offsets of 2.0 mV left and 0.2 mV right.
it was my understanding that too much dc is going to heat up and put excess wear (or damage) to the speaker's voice coil....not necessarily something you'd hear
 
The original post seems (to me at least) to suggest a correlation bw DC offset and sound quality.

So will this (high dc offsets) mostly be taken care of by replacing all the transisters?
i'm not really a tech, but from the service manuals I've read, there's usually independent bias adjustment pots to take care of the L&R offset....although that's not something I've ever done. I can say for sure, that just because you can't hear it, does not mean that it's not a problem.
 
i'm not really a tech, but from the service manuals I've read, there's usually independent bias adjustment pots to take care of the L&R offset....although that's not something I've ever done. I can say for sure, that just because you can't hear it, does not mean that it's not a problem.
Bias and offset are two things. Many amps do not have offset adjustments. The offset is 'adjusted' by matching the differential transistors.
 
Bias and offset are two things. Many amps do not have offset adjustments. The offset is 'adjusted' by matching the differential transistors.
well there you go - at least i was correct when i said i didn't know what i was talking about.

perhaps i should consider running for office?
 
Thanks for the challenge and test instructions. I just joined and I am most grateful to be among other audio enthusiasts.

When my neighbors moved, their son handed me this Marantz and said thanks. I didn't think it was anything but an forgotten ancient amp that would be of no use to me. But it was free. I couldn't complain. It was sad looking, with broken feet, and really noisy when looked at, with lights out. But with all that, on a set of Infinity Beta 20's, this unit is delicious sounding once you left the knobs alone. Some Deoxit and some speaker relay and interior and cleaning, and this unit is a really sweet sounding amp.

MARANTZ 2238b
RIGHT: .3mV
LEFT: .3mV

WooHoo!

I am awaiting the arrival of a new pair of Q-Acoustics 2050i's, and I was wondering if this Marantz would be compatible with them?
and…
What Class amp is the Marantz 2238b? I saw in a review of the 2050i, and they mentioned that "Class D" amps need not apply.

Thanks and Happy listening.

Adam
 
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Re. Replaced differential pair in ADCOM GFA 545 (post 4416):
I've not been able to find substitutes that would have lower Vbe, so I think I'm at a standstill on this one. Suggestions welcome: the original transistors in the pair were 2sc2362, I replaced them with KSC1845 (Hfe about 415, Vbe .76V).

Later edit: After installing the KSC1845's and resetting Bias, resultant DC offset was: Right channel 26.5 mV (was 57) & Left channel 28.5 mV (was 35). Googling "GFA545 DC offset" turned up numerous references that seem to indicate these GFA545 amps often exhibit widely varying DC offset (over 100 mV not uncommon), and replacing the differential pair in one other case similar to mine dropped the offset down to about 25mV. I also found a reference from Jim Williams (who apparently offers [offered?] a commercial refurbishment of these amps) that mentioned "A manual DC offset trim could also be installed if this (DC offset) is a major problem." Now that sounds interesting and is perhaps what this amp needs to bring DC offset down to nothing.
 
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Tough to make a blanket statement. Components can change value over time which is why things need to be adjusted. Are you talking power supply , decoupling caps?
It's too broad question I believe. Someone more knowledgeable please feel free to correct me or add to what i said
 
Can someone confirm this statement(generally speaking, but feel free to point out exceptions)? "Aging capacitors do not affect DC offset". This is what I have gathered.
If you are talking about the aging power supply filter caps ( the big ones after the rectifiers). No, they have NOTHING to do with the DC offset at the amplifier output. Absolutely nothing......unless it short out the power supply.......then all hell break loose!!!

In normal operating condition, output DC offset is governed by the offset of the input differential pair. Some amps use a trim pot to trim out the DC offset and it can drift over time.
 
Technics SA-GX170 Class H+
Been running for years in a dusty and sometimes hot shop environment....

Left... 4.8 mV
Right... 4.0 mV
20170711_131915.jpg
 
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