• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Amplifier Distortion, DC-Offset, and You!

Yet another thanks for the great procedural info on this topic, Echo!

I've done a little home electronics repair myself -- got a degree in E.T. and all that (actually, decades ago) -- but I would no doubt have missed a step or two on checking the offset.

I asked a new tech that I tried here in Denver to check these levels on my Sansui AU-919 while it was in for an unrelated problem, since it also seemed to be running a little extra hot. When I got it back, I asked about it, and he just shrugged and said that neither channel seemed to run hotter than the other, so he assumed it was OK.

He also recommended AGAINST replacing the original supply caps in my AU-9500 ('72 vintage), unless and until I start hearing 60 hz hum in the output.

Does this sound right to any repair gurus out there???

On the plus side, he did do a heckuva cleaning job -- used Deoxit and all that. And he seams to do good work with intermittant problems. But then, he also has a 4-month backlog.

It sounds to me like it may be time to re-assemble my old Fluke meter, replace the batteries, dust it off and actually do something with my own gear. The old addage holds true once again: "If you want something done right..."
 
Hey John...

Sorry for not replying sooner.
When I got it back, I asked about it, and he just shrugged and said that neither channel seemed to run hotter than the other, so he assumed it was OK.
Click to expand...
We all know the saying, assume and make an ass out of U and me. It takes 5 bloody minutes to check bias and offset, and failure to do so on any unit that leaves a shop is plain negligence.

He also recommended AGAINST replacing the original supply caps in my AU-9500 ('72 vintage), unless and until I start hearing 60 hz hum in the output.
Click to expand...
There is no reason not to replace them if you want. Perhaps he didn't want to do it because it's a PITA, but the only reason I'd recommend skipping it is because it can get expensive on some units.

EW
 
I suppose the 4 month backlog had something to do with not wanting to do the extra work.

The guy is probably somewhat of a hack -- probably didn't even realize the importance of checking those levels, but at least he seems diligent with the knowledge he does have. He seems willing to actually find the problem and fix it, no matter how well it tries to hide. I'll give him that much. That's more than I can say for the last guy that touched my gear. The guy swore up and down that there was no problem, and that his bohemian "rubber mallet" technique for revealing intermittants should be able to reveal any problem under the sun. And therefore, of course, I am profoundly dillusional when I claim there is a problem.

Well, I've been playing with the 919 for a week now. Problem has not re-occured. I'll try to find time to bias test it tomorrow or Monday.

Cheers,
Jonh
 
There's a lot of good techs out there that are so burnt out that they just don't care. I can sympathise, but not excuse. You specifically asked, and that's what makes it worse..
 
My first AK reply

Echo:

Hope you're not overloaded with responses to your original challenge. Just got a new multimeter, and this is what it tells me:

Sony STR-6800SD
L 34.2
R 28.4

Sony STR-V7
L 9.3
R 12.2

Luxman R-1050
L 2.1
R 11.7

Not too bad for 3 receivers I bought used over 20 years ago. Never spent a dime on any of them. They don't make 'em like they used to.

Thanks for starting this great thread!
 
Nothing terrible about any of those...

Welcome to AK. Kick yer shoes off and make yerself at home..:D
 
Old Kenny needs some attention. There is no offset adjust on the 9600:( , so the only option is to replace the amplifier differential pair input transistors. The 9600 uses an NPN diff pair input, and there are a lot of good high-gain NPN's that would be nicely suited to the job, such as a matched pair of 2SC1775's.
 
OK, I finally got a chance to check these:

AU-919 --

L: 5.8 mV
R: 63.5 mV :saywhat:

AU-9500 --

L: 19.6 - 22.0 mV (fluctuating)
R: 32.9 - 36.3 mV (fluctuating)

AU-717 --

L: 100.9 mV :(
R: 83.6 mV :(

So, maybe B/F was right about my previous A/B amp comparison. Maybe one major reason the 919 sounded THAT much better to me than the 717 was the fact that the 717 is so horribly biased!

Looks like I'd better pick up another service manual.

I won't write that repair guy off entirely. But I will keep looking.
 
Ok. I finally tested my H/K 730.

Results:

Speaker 1
left 16.4
right 6.3

Speaker 2
left 16.4
right 6.3

Pretty good? Does this indicate that the "Twin Powered" means one power supply for the left speakers and one for the right? Hence, the identical readings on speaker 1 and Speaker 2 left and right? Is the 10 mV difference something that needs attention or is that ok. Very interesting info!
 
OK you guys....what makes you think that the 'B' speaker terminals are going to be different from the 'A' terminals?

These are stereo amps/receivers. Two channels. Same amplifier for 'A' speaker terminal as for the 'B'.
Click to expand...
Just thought I'd toss in that reminder... :p:

Twin Powered likely means a separate supply for the left and right channels... While the reading you have is not *ideal*, it is perfectly acceptable.
 
Originally posted by EchoWars
Just thought I'd toss in that reminder... :p:

Twin Powered likely means a separate supply for the left and right channels... While the reading you have is not *ideal*, it is perfectly acceptable.
Click to expand...

Sorry. Didn't quite make it through all the pages before I replied! :p:
 
test results for DC offset

Great post and Thanks for the very useful information.
Here are the results I obtained:

Pioneer SX 1080 - R.Ch.1.9 MV, L.Ch. 1.7 MV
Sansui AU 5500 - R. Ch. 0.297V, L. Ch. 0.330 V !! (HELP!)
Sansui 7000 - R. Ch. 2.3 MV, L. Ch. 1.8 MV
Yamaha R-2000 - R. Ch. 65.5 MV, L. Ch. 28.7 MV (Any ideas for the right channel????)
Lloyds H430 - R. Ch. 2.6 MV, L. Ch. 4.6 MV
Pioneer A-30 - R. Ch. 23.3 MV, L. Ch. 23.8 MV
Sansui A5 - R. Ch. 32.4 MV, L. Ch. 20.7 MV
 
Originally posted by EchoWars
Time for one of those long, boring semi-technical posts that no one here reads...

[snip] read it and enjoyed it!

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...

Pioneer SX-650 receiver:
31mV left channel
28 mV right channel

Kenwood KM-8002 power amp:
15mV left channel
18mV right channel

I guess this is good, yes? I don't know if I can hear it or not, but I do think that the Kenwood sounds cleaner - and I did before I checked this with my multimeter.

I have a question though - I may be reading my multimeter wrong. It's a cheap Radio Shack digital that I bought to fix a doorbell recently. It has a 2v DC setting, that's as low as it goes. My readings were .15, .18 (or -.15, etc, if I had neg / pos reversed). Am I reading this right? I just move the decimal over two places for millivolts, right?

Thanks!

Best,

Wigwam
 
dc offset

echo . Any help with my postings re the Au 550o or the YAMAHA R-2000 HELP REQUIRED WOULD BE GREATLY APPRECIATED , THANKS , GARY .
 
Re: dc offset

Wigwam...normally, since millivolts is a thousandth of a volt, the decimal gets moved three places to the right, so I suspect that the readings might be high. But then, I dunno what your meter is telling you either. I would find it odd that both the Pioneer and the Kenwood are reading in the hundreds of millivolts, so your interpretation of the readings could be right. Check the manual for the meter for clarification if you can.

Originally posted by Deadear
echo . Any help with my postings re the Au 550o or the YAMAHA R-2000 HELP REQUIRED WOULD BE GREATLY APPRECIATED , THANKS , GARY .
Click to expand...
The Sansui and the Yammi need a tuneup. I am not familiar enough with either of these to say if offset is adjustable, but even if not, it can be drastically reduced (though if not adjustable it gets a tad more complicated). If these amps are important to you, then consider letting a pro jump in there and set offset and bias for you. It should not be a spendy proposition (i.e., if you were local, I'd do it for nothing...pretty minimal affair).
 
Re: Re: dc offset

Originally posted by The Sansui and the Yammi need a tuneup. I am not familiar enough with either of these to say if offset is adjustable, but even if not, it can be drastically reduced (though if not adjustable it gets a tad more complicated). If these amps are important to you, then consider letting a pro jump in there and set offset and bias for you. It should not be a spendy proposition (i.e., if you were local, I'd do it for nothing...pretty minimal affair). [/B]
Click to expand...

Thanks Echo . Up here where I live there is no one to do that kind of work available , that only leaves me .
I am somewhat capable of doing the adjustments , just need to know where the pots are located .
If there is anyone who has access to the AU-5500 or the Yamaha R-2000 schematic I would be forever grateful . Thanks .
 
I wish I had manuals for those...If I did I'd be happy to help. Perhaps you might start a thread in the Sansui forum and see if anyone bites...? Same with the Yamaha in this forum...
 
Sony 6060F - I have cleaned it up a bit and when I tried to measure the DC offset I get readings all over the place. Swings from +150 ish to -40 ish with no stabilization??? Any idea... Here is a pick of the inerds front and then back.

Thanks...
 
You must log in or register to reply here.
Back
Top Bottom