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Old 2nd July 2002, 03:31 AM   #1
fender4 is offline fender4  United States
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Default Diagnosis needed!

I have a pair of LM3875 based amps used as a stereo pair, with a single transformer/supply powering the pair. I am very happy with the sound...but there is a problem.

About 10 minutes into a listening session, I begin to hear faint distortion, mostly in (but definitely not limited to) the vocal range. The distortion gradually increases until it is audible even to my girlfirend (she's just not into good sound...weird).

Here is the weird part. I can increase the volume to near max for a few seconds, return it to normal levels, and the distortion "disappears" for several minutes. Also, it seems that the lower the listening volume, the more likely I am to experience the problem (it's not just the volume covering it up at loud volumes). BTW, the LM3875's are barely warm at all times. I am using an 18-0-18v 6A toroid, so it is not getting too much juice.

Have you ever heard of a problem like this? Could it be dirty AC in my apartment? I'm new in this apartment, so if this amp is telling me something, I would like to look further into the AC thing. Any ideas?

Thanks for your help and patience!

f4
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Old 2nd July 2002, 05:13 AM   #2
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Default Marginal stability

I would rather suspect that you have an instability problem that induces a slight amount of high frequency oscillation as the chips warm up. This instability is likely on the verge of not been there when cold, but crosses the point were it is triggered on when warm. Changing the volume may cause this oscillation to stop, but since it so close to being there all the time it will come back when the trigger point is reached again. It may be there at lower lower all the time but well above the range of hearing.

I would suggest looking at the output with a scope to see if you can spot it. This is the only likely cause in my opinion.

John Fassotte
Alaskan Audio
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Old 2nd July 2002, 05:19 AM   #3
Duo is offline Duo  Canada
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Sounds to me like you have capacitor problems somewhere.
It could be bootstrap or something like that but I'm not sure...
Are there many components in the amp other than the chip??
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Old 2nd July 2002, 06:45 AM   #4
fender4 is offline fender4  United States
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Thanks for the quick replies!

John,
You might be right...your description is accurate. It is very possible that it is there all along and a threshold is crossed. Unfortunately, I don't have access to a 'scope, but I will look into adding some stability.

Duo,
The design is basically an application-sheet design (see below), but it is in inverted configuration.

UPDATE! I just listened to the amp for 20 minutes until the "enemy" attacked Steely Dan - The Royal Scam. This time, I turned the power off for 15 seconds, and then turned it back on. The distortion was not audible for another 10 minutes. Also, if I turned the source off (silence) for 30 minutes and then played the source, it was VERY distorted. But when I gradually increased the volume to a low listening level, the distortion decreased (or it was masked??? I can't figure that out).

Everything is point-to-point with a very tight layout and a star ground. The grounds for the power caps, signal, output, and pot all connect at star ground........could that be part of the problem??

I am hearing the same problem from both channels simultaneously. I took great care in making the joints nice and clean. I measured 0mv of dc offset.

Any tips? It is frustrating, because it sounds GREAT until the problems start. 20 minutes of listening won't cut it.

Thanks again for your help, and sorry about the long posts.

-f4
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Old 2nd July 2002, 06:51 AM   #5
fender4 is offline fender4  United States
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Default Pic didn't work...

Here is the non-inverting version from the spec-sheet:
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File Type: jpg lm3875.jpg (23.9 KB, 267 views)
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Old 2nd July 2002, 07:04 AM   #6
Duo is offline Duo  Canada
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seems really weird if both chips are doing this...
Are you sure all you capacitors and resistors are the right values and int the right places? Are all the electrolytic caps in the right way around?

I don't think a star topology for the grounding could cause this...
Did you check with your meter to see that the power supply voltage stayed the same for the time it took to start distorting??
Or put a meter in series with the amp and PSU and leave the amp idle to see if the current draw changes after a while??


EDIT: i just checked the prev messages in the thread...
Does the spec sheet for that chip suggest any kind of inductor
or zobel network of some sort on the output? It could be very likely that, due to a reactive speaker cable, your amp is oscillating at high frequencies, generating heat in the OT devices in the chip since they cant turn off fast enough at that frequency. In other words, you might want to try putting zobel network on the output of the amp before the speaker, it consists of a resistor of about 10 ohms and a capacitor of about 100nF in series from the OT to common ground. If that fixes the problem, then your amp almost certainly was oscillating
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Old 2nd July 2002, 07:25 AM   #7
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Here's a pic of your amp with a zobel net attached, it's really easy to add on, heh, you could even add the network right at the speaker jack if you didn't want to open the box if it has one...


EDIT: never mind the caps labeled "1n" on the power supply rails, my diagram editor just picks that as a starter value and I forgot to change the them
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File Type: jpg zobel.jpg (16.0 KB, 254 views)
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Old 2nd July 2002, 08:13 AM   #8
vbd is offline vbd
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Hi, f4
How old is your amp ?
Is your volume potentiometer in good condition ?
I met similar effects coming from dirty pots, that were fixed by using the "magic" contact cleaning spray.... or a new pot !
Also check connections to the speakers, from the source, and the input switch... Urban atmospheres are quite corrosives and dirty contacts not good for sound
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Denis
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Old 2nd July 2002, 10:43 PM   #9
fender4 is offline fender4  United States
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Default Thanks, everyone!

This place is great. Thanks for all the good advice.

Duo,
I will give the zobel a try. I was being stubborn with my minimalist approach, so that simple solution might do the trick. I am only using about 3 feet of speaker wire right now, but it is junk wire.

vbd,
The amp and all its parts are about 6 months old, so it should be relatively clean. I have, however, dealt with the urban corrosion problem in the past with some older equipment...it makes you wonder if breathing this stuff is a good idea!

Thanks again, and I will post with any good news.

-f4
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Old 3rd July 2002, 12:04 AM   #10
Duo is offline Duo  Canada
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Your welcome

Also, it doesn't really matter what quality your speaker wire is...
I've heard of many oscillation problems even with the most expensive speaker cables. Either way, cable is cable, metal and plastic and rubber. It will always support some kind of oscillation under the right circumstances. And the length would determine the resonant frequency, along with the impedance of the cable itself. So anyway, good luck, I hope that's all the problem is
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