Amplifier Distortion, DC-Offset, and You!

redk9258 said:
You're overthinking it. The speaker connects to two connections. Measure the offset there- with the speaker switch on if it has one. If the amp is stereo, you do it two times.
If your service manual has a procedure to adjust offset then you should follow it. Don't mix offset for setting bias. That is completly different and not measured at the speaker terminals.
Click to expand...

Ok think I got it. Again these are monoblocks...

to measure the COMBINED offset (both channels) you put the probes across the (=) and (+) terminals.

to adjust each channel individually, measure from a ground to the (-)terminal, and use that channels trim pot to adjust.
then same for other side, measure from a ground to the (+), and use the other channels trim pot to adjust.

then each channels offset is additive and that is the reading you get when you measure between the (-) and (+) terminals on a mono.

isn't it better to have each channel of a monoblock offset set to as close to zero as possible? only way to do this is individually.
or maybe it doesn't matter if one channel is -200mV and other +200mV, sum is 0mv right, so we're good?
i feel more comfortable (maybe its my lack of knowledge) in setting each ch to as close to 0mV as possible, so that the combined (across the terminals is as close to zero as possible).

just my thoughts and results of my measurements. i just haven't had anyone explain this process well enough that it makes sense to a non electrical engr.

thks for input!

happy thanksgiving!
 
tripwr1964 said:
isn't it better to have each channel of a monoblock offset set to as close to zero as possible? only way to do this is individually.
or maybe it doesn't matter if one channel is -200mV and other +200mV, sum is 0mv right, so we're good?
Click to expand...

I would say each channel would be set separately- as close to 0mv as possible. I didn't realize you were using a bridgeable amp.

Hopefully Echowars will reply. Oh and BTW, what amp is it?

Happy Thanksgiving!
 
If you're talking about a BTL amp (bridge-tied, or simply bridged), then there is usually a more complex procedure for setting offset. But, measurement is accomplished the same way.

A monoblock simply means one amplifier channel to a chassis. Doesn't automatically mean that it's a BTL amp (and most monoblock amps are not).
 
redk, they are odyssey khartago "monos".

so yes to echowars point i believe they are "BTL" amps (ther are 2 channels bridged mono).
 
Hello! Thanks to EW for taking the time to help all of electronically challenged on how to check DC offsets and everything else he does.

I made it through 20 pages but didn't think I could make it through the other 198. I seen a similar question but not sure I fully understand and was hoping you had an idea.

After letting the amp warm up for a half an hour the results on my HK 430 are
Left channel 0.14 - 0.20 mV it never settles.
Right channel .25 - .45 mV with the occasional .10 and .55. It is constantly jumping around very quickly.

I'm apologize as I'm sure this has been asked by multiple people but this thread is incredibly long and my understanding is lacking.
 
I didn't see a question.

I'd also say that you are likely reading the offset incorrectly (well, actually you're probably misinterpreting the reading the meter is showing you). The 430 doesn't even have an offset adjust, so the real-world likelihood of measuring offset in the microvolt region is not good. With a well-matched high-gain differential pair, I'd expect to see something around 5 to 15mV (0.005V to 0.015V). You are saying you are seeing 0.00014V to 0.00045V. That's terribly unlikely, and even with the fairly expensive meters I have here, I'd have trouble measuring DC voltages in that region.

More likely you're reading 140 to 200mV (0.140V to 0.200V) on the one channel, and 250 to 450mV on the other (0.250V to 0.400V). If that's the case, sounds like the amp needs a bit of work.
 
EchoWars said:
I didn't see a question.

I'd also say that you are likely reading the offset incorrectly (well, actually you're probably misinterpreting the reading the meter is showing you). The 430 doesn't even have an offset adjust, so the real-world likelihood of measuring offset in the microvolt region is not good. With a well-matched high-gain differential pair, I'd expect to see something around 5 to 15mV (0.005V to 0.015V). You are saying you are seeing 0.00014V to 0.00045V. That's terribly unlikely, and even with the fairly expensive meters I have here, I'd have trouble measuring DC voltages in that region.

More likely you're reading 140 to 200mV (0.140V to 0.200V) on the one channel, and 250 to 450mV on the other (0.250V to 0.400V). If that's the case, sounds like the amp needs a bit of work.
Click to expand...
Or I'm a total idiot and in my haste before leaving out of town and while packing I wrote this out from memory. I went back and checked and it was indeed
14-20 mV on the left channel and 24-45 mV on the right channel.

My question which I also failed to ask was about how the right channel especially was jumping around so much and what could be a potential culprit for that?

I had seen in another thread that there was no pots to adjust on this amp for DC offset so I imagine that will have to stay as I'm not sure I'm comfortable pulling and replacing components at this stage. But if there's something I can check that's causing the wild swings I am all ears and welcome to help if you have the time.
 
With low-gain diff-pairs, I'd expect the offset to fluctuate a lot. Probably a good chance that that is what's happening.
 
Most modern amps have, as the first stage, a differential amp. Here is the part of the amp responsible for zeroing out DC offset. If this stage requires fine tuning, many amps have an adjustment pot to allow offset to be set to close to zero. If the designer chose not to include an adjustment, then the only recourse is to replace the differential amp transistors with a new matched pair.

http://www.audiokarma.org/forums/in...tion-dc-offset-and-you.5634/page-2#post-44429
 
Tested out an SX-950, R 2.5mV, L 2.6mV! Now hoping a quick lube job fixes the scratchy sound and left speaker cutting out on the main volume knob.

Thanks for this thread EchoWars, it inspired me to my first post.
 
Luxman's, Offset and Me.
Great thread to find while searching the immense AK archives.:music:.
I got the bug and "rediscovered" Luxman a few years ago and went a little audio crazy and ened up with a 404, 406, 113, 115 and 1120.
Everything works perfectly still, no scratchy knobs or switches, especially considering the 4ohm Infinities they usually drive.
The 113 and 115 have clicked into protection only a few times but admittedly deserved at the 3-4 o'clock position on rock and loud classical.
My 1120 is not used too much, semi-retired, but works great in stock form still and after a good warmup.
It has a unique sound that is like the stereo from one channel slightly phasing into the other. It is not unpleasant in any way and I assume this is the way it was meant to sound.
I wish to keep them all working well for years more with basic PM as they get swapped in and out.
AK is big on the maintenance thing and I am literally a maintenance guy at a local resort.

But now I feel like a housewife fretting over the tire pressure and the oil in the minivan.:oops:
Is there a Luxman value spec for these receivers and a way to nip potential problems in the bud without dismantling too much on the insides?
And, are tube measurements checkable and adjustable the same way?
I've an old Scott 200-B that probably needs tuning up too.

And can Anyone recommend a GOOD meter that measures most everything you'd need for re-capping a SX-950?
No luck with the junk ones fritzing out on me too many times. :(
Gracias....
 
I want to check/set the amplifier on a fully working SX-939, but the SM instructions are quite basic, and I've not found much more on the process for this unit on AK. Really, my question is not specific to this receiver:

SM specifies an 8v dummy resistor across speaker terminals. I don't have the proper bench dummy resistors, but since the manual doesn't specify, am I correct in assuming that the volume is turned all the way down for this process? What would happen if I'd insert, say, a pair of 1/4w 8 ohm resistors? Or, a pair of 8 ohm speakers? If the volume is turned down, what difference would this make? Thanks.
SX-939.jpg
 
If you already know that the offset isn't going to blow up a speaker, then you can connect them and measure offset. But most of the time, offset will read about the same whether a speaker (or dummy load) is connected or not.
 
So I received a 0 on my Sansui AU-7500, does that mean I have a "capacitor" output? And if so, what does that tell me?

Or did I just do it wrong? :)
 
Maybe this was mentioned on some of the 217 previous pages, but in case it didn't here goes my suggestion to those that some electronics knowledge.

If your amplifier or receiver has no speaker protection, get yourself a speaker protection board from eBay. They are very cheap and worth the expense.

Get it from a reliable eBay retailer, as the relay quality is important.

That doesn't mean that you don't have to check and adjust your offset, but at least you won't blow your speaker due to a malfunction offset. Millivolts are not so dangerous, but several volts will be. Certainly more costly than a protection board.
 
First things first, thank you EchoWars, for this informational and epic thread. I pickup up a Nakamichi SR-4A today, and all I could think about was this thread. I reviewed your instructions, and hooked up my DVM. Right channel 8.7 MV, and the left channel was 10.5 MV. YES!!! A little cleaning and I'm off to test it in my system. Thank you very much for educating me.
 
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