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Old 4th October 2004, 02:06 PM   #1
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Default Odd distortion problem

Hi Everyone.

Skip to the third last paragraph if you just want to read the question

Today I decided to start playing with my amp again, after a quite a break. I saw a post the other day by Nelson Pass about using a rectifier bridge to isolate signal ground from Mains ground, and decided to give it a go.

I did this, but didn’t really notice any difference in the noise levels. I decided I should crank it, and to my horror the left channel started distorting very badly. I put every thing back the way it was and the problem was still there  I hadn’t cranked the amp up for a while so I don’t know when the problem started.

After many tests (swapping speakers, swapping inputs, driving one channel only at a time, etc) I found that the problem is definitely in the amp, and not the speakers, or the source.

If I drive the amp with the right channel speaker disconnected, the left channel doesn’t distort.

I figured maybe I had a Power Supply problem, even though the PS doesn’t have separate caps for each channel (just 16,000uF per rail +- 63V). I resoldered all of the leads for the left channel, no difference. I unsoldered the left channel power leads from the caps, and soldered them to the caps on the other side of the bridge, see attached pic, and the distortion got worse…. So much so that it now distorted with only the left speaker being driven 

I find this really odd!!!!! I checked the quiescent current on both channels, (supposed to be 100mA) right was about 90, and left was about 80….. reset both to 100, still the problem is their….

I was planning on rebuilding the PS…. Now I’m wondering if I should spend the money or not!!! I recently (in the last year or so) replaced all of the electros except for the PS ones…. The amp is 17 years old.

Could faulty electros in the PS be causing weird distortion problems with only one channel (when both channels are running off the same caps)?? Is it more likely that there is a fault in the left channel, which only shows up if the rail voltage drops somewhat due to heavy loading on the PS?

Any suggestions where I should start looking? I’m thinking set up a dummy load crank it up and start poking around with my pc based scope…… see if I can find out at least where the distortion is occurring.

No nasty comments please, about the soldering of the PS wiring It’s rather difficult sodering 8GA wire you know

schematic of the amp (at least the power amp) here --> http://www.diyaudio.com/forums/showt...803#post228803


Tony.
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Old 4th October 2004, 04:45 PM   #2
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Tony,

Are you absolutely sure that you went back to the exact situation as before when you again removed that bridge? The most likely situation is that there was an unintended change. Especially when you say that the left chan doesn't distort with the right chan disconnected. That points to a chan interaction which point to a grounding thing.

Jan Didden
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Old 4th October 2004, 09:51 PM   #3
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Hi Jan,

definitely the same all I did was desolder one wire from the chassis lug, and solder it to the bridge rectifier, which was in turn connected to the chassis lug, when I put it back I simply desoldered off the bridge and back to the original lug.....

However I haven't really cranked the amp up since I completely rewired the PS the last time (with the heavy gauge wire) The ripple improved after I did this, but I never did a high power test..... So It is possible that you are on the money! I remember that I did try changing the way the earth cabling was configured and that rmaa tests showed worse THD when I did!

The center large black cable (with the center taps of the transformer soldered onto it) goes back to the star earth point on the PCB, both channels run back to that Star Earth point. The wire to the chassis lug runs from that point on the pcb too.

I was going to scan a pic of the original PS wiring scheme, but the scanner isn't playing ball. Maybe I need to revert to that original wiring) and see how it goes.

Tony.
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Old 4th October 2004, 10:26 PM   #4
ingrast is offline ingrast  Uruguay
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If you have the chance to scope it, don't waste a single more minute, do it and with the speakers connected. You may have parasitic oscilations, that usually appear at higher loads and may be affected by the other amplifier.

The design is a very high gain one, which explains the sensitivity to layout etc.

Parasitics should manifest as dense "sausages" strung in the slower audio waveform.

If you do not have a signal generator or other sine wave source, try a microphone if you can hook one and whistle (at a distance to avoid wind effects), you will be surprised what a good sine wave you can generate.

Check this and come back.

Rodolfo
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Old 5th October 2004, 03:05 AM   #5
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Thanks Rodolfo. I won't get a chance to do this till Friday ( have the day off)..... was thinking of using a dummy load, to save my ears, the neighbors ears, and my speakers but I guess it may only manifest under the load that the speakers are presenting..... I guess also I might see (on the scope) problems at lower levels, that I might not be able to hear.

The amp used to suffer from oscillations until I made a few mods to the preamp section.... (changed some cap values, and added a resistor or two) more recently (same time as replacing all the electros) I made some other mods in the preamp section, bypassing some stuff, and changing some caps from Bipolar electros to polyproylene ones... so it is possible that maybe that has re-introduced a problem.

arrrgghhhhh that'll teach me not to fully test it!!!! I only tested into a dummy load with a sine wave, not a loud listening test

It's a real pain that I don't know at what point the problem started looks like I have some fun ahead of me

Tony.
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Old 5th October 2004, 10:43 PM   #6
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Default Re: Odd distortion problem

Quote:
Originally posted by wintermute
Hi Everyone.

Skip to the third last paragraph if you just want to read the question

Today I decided to start playing with my amp again, after a quite a break. I saw a post the other day by Nelson Pass about using a rectifier bridge to isolate signal ground from Mains ground, and decided to give it a go.

I did this, but didn’t really notice any difference in the noise levels. I decided I should crank it, and to my horror the left channel started distorting very badly. I put every thing back the way it was and the problem was still there L I hadn’t cranked the amp up for a while so I don’t know when the problem started.

After many tests (swapping speakers, swapping inputs, driving one channel only at a time, etc) I found that the problem is definitely in the amp, and not the speakers, or the source.

If I drive the amp with the right channel speaker disconnected, the left channel doesn’t distort.

I figured maybe I had a Power Supply problem, even though the PS doesn’t have separate caps for each channel (just 16,000uF per rail +- 63V). I resoldered all of the leads for the left channel, no difference. I unsoldered the left channel power leads from the caps, and soldered them to the caps on the other side of the bridge, see attached pic, and the distortion got worse…. So much so that it now distorted with only the left speaker being driven L

I find this really odd!!!!! I checked the quiescent current on both channels, (supposed to be 100mA) right was about 90, and left was about 80….. reset both to 100, still the problem is their….

I was planning on rebuilding the PS…. Now I’m wondering if I should spend the money or not!!! I recently (in the last year or so) replaced all of the electros except for the PS ones…. The amp is 17 years old.

Could faulty electros in the PS be causing weird distortion problems with only one channel (when both channels are running off the same caps)?? Is it more likely that there is a fault in the left channel, which only shows up if the rail voltage drops somewhat due to heavy loading on the PS?

Any suggestions where I should start looking? I’m thinking set up a dummy load crank it up and start poking around with my pc based scope…… see if I can find out at least where the distortion is occurring.

No nasty comments please, about the soldering of the PS wiring It’s rather difficult sodering 8GA wire you know

schematic of the amp (at least the power amp) here --> http://www.diyaudio.com/forums/showt...803#post228803


Tony.

What is this a picture of??? Have you not heard of heat-shrink???? Hope it doesn't short...
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Old 6th October 2004, 11:47 AM   #7
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Default Re: Re: Odd distortion problem

Quote:
Originally posted by jerk



What is this a picture of??? Have you not heard of heat-shrink???? Hope it doesn't short...
hehehe you should see some of my posts in the ugliest prototype thread

edit ahhhh here we go http://www.diyaudio.com/forums/showt...789#post321789
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Old 8th October 2004, 02:30 AM   #8
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Default I hate intermittent faults :(

Well after stuffing around for about 2 hours only to come to the conclusion my cd burner is cactus, I decided to connect the sound card output direct to the amp (was going to burn a cd of test tones)....

Everything is working fine no sign of the distortion problem..... not even on the cd that it was painfully obvious on (Secret Garden track one)..... Hmmmm

One question though for Rodolfo.... the oscilations, should these show up within the audio freq range??? because my scope is pc based it has limited resolution, and starts to run out of steam by about 8Khz, and is completely useless above about 12Khz.... Not sure if the problem is with the graphics refresh rate or what, as the sound card can sample the data fine.

I tried varying frequencies from 20Hz to 12Khz, all produded nice clean sine waves at the speaker out, and even when pushed to clipping, only produced slight flattening of the wave form.

One thing I have discovered though, is that it appears the second set of outputs on my DV-18 are suspect.

Maybe if I leave the amp on for another hour or so the problem may manifest, could be heat related.

monitoring the track off the cd didn't show anything weird either.... grumble grumble grumble.....

Tony.
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Old 8th October 2004, 04:43 AM   #9
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Default 1Khz sine wave

Well here is the output from the amp, at just before (or just starting to) clipping.

both channels driven, into speakers with quilts over them to try and suppress the noise a little.... (1Khz sine at 100W/channel is rather ear peircing!!) traces picked up from second speaker connection on the back of the amp. full power before clipping so probably about 100W.

the pink wave is the channel that was misbehaving.... no sign of any problem, and the other channel, looks like it might be going into clipping a little before (based on the bottom of the blue wave form)...

One thing I have noticed today though is that the bad channel, is running much cooler than the good channel.... weird... (not that it is being bad today).

I've got no idea at this point, except that I seem to have a faulty rca lead, which is (I think) what I was using when I tested earlier in the week... I'm very dubious though that it could have been the lead... today it wasn't working at all on the channel that was playing up.

It was one thing that was different, and I didn't realise, and didn't swap with another.... maybe it was the culprit all along.....

Tony.
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Old 8th October 2004, 10:59 AM   #10
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I understand you were not experiencing the former problem at the time of these tests.

Waveforms look clean. You are right in parasitics should not be visible with the scope bandwith limited to 8 kHz - 12 kHz. They may be anywhere from tens of kHz to MHz. Yet their effect should be noticeable in distortions mainly in the sloping portions of the sine waves. As much as your ear - which also filters out high frequencies - notices their ill effects.

Still there is the possibility of higher frequency components varying in nature depending on wiring and setup. This could also explain the heat dissipation differences.

Do not hesitate to post further results of your tests, lets see where the gremlin is lurking.

Rodolfo.
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