strange GC Distortion....

[sa_schulz]

5{finished a Loesch Inverted GC exactly as described. First playing this amp all is great, less than a minute goes by and all is distorted top to bottom. If its oscillating I can't tell per se, but something is amiss. Any ideas?


Thanks,
Stefan

[Nuuk]

We may need some more information Stefan!

When you say that it is OK to start with do you mean that at the start of a track, where it usually starts quietly, the sound is OK, but when it gets louder, you get the distortion?

In other words is the distortion related to the level of content of the music?

[UrSv]

Originally posted by sa_schulz
5{finished a Loesch Inverted GC exactly as described. First playing this amp all is great, less than a minute goes by and all is distorted top to bottom. If its oscillating I can't tell per se, but something is amiss. Any ideas?


Thanks,
Stefan

Exactly as described can be many variations. Could you post schematic?

[sa_schulz]

My appologies. It was late; upon reflection my question was vague. The design is from the following link. http://f18.parsimony.net/forum31999/messages/38318.htm This is the inverted GC. The only difference is I realize now is that I don't have a 1uF cap between the pins 1 and 4. The supply is 25.5 V at 2.5A X 2 with 2200uf each at the chip. The pos and neg are floating. I did not however ground the cnter tap of each trans.
I power the amp and play music from my DVD Player or using my Sansui AU-D11 as a pre for phono, after less than a minute of breathtaking music from either source the amp distorts at all frequencies. All is well for 30 secs or so then fairly rapid distortion. The chips do not feel hot and i have done vibration tests and voltage/resistance tests. Nothing stands out. I am using homemade speakers using FE167 FR drivers. The only other thing I can think of is the 100K pot is a cheap RS dual audio taper. The pot however does not sound scratchy. I can for the brief time at the beginning run to very loud volumes without distortion. So as far as I can tell it is not a power dependent phenomena. Do I need a Zobel ntwk? If I am still vague on something let know.

Again Thanks,
Stefan

[JohnG]

I assume you have a good heat sink on the amp chip itself, and have paid attention to insure good thermal contact between the chip and sink. If not, this may be the source of your problem, which would be the chip protecting itself from overheating.

John

[Nuuk]

If not, this may be the source of your problem, which would be the chip protecting itself from overheating.

In my experience, when the thermal protection cuts in, it just decreases the volume but doesn't add distortion.

It could be a weak solder joint!

[carlosfm]

Originally posted by Nuuk

In my experience, when the thermal protection cuts in, it just decreases the volume but doesn't add distortion.
It could be a weak solder joint!

Yes, it lowers the volume and then it cools a little and then up again.
Nothing to do with distortion.
Stefan, check the PSU caps and see if there's any mistake there, like wrong polarity.
Sometimes they don't blow in your face when mounted with the wrong polarity, but they do those strange things.
I also hope that you have the PSU caps near the chip's pins.
If you have them far away you may run into those problems.

[jackinnj]

something like this sounds like a bad connection to a filter capacitor, bad capacitor, wrong connection (as suggested)

[fedde]

Originally posted by Nuuk

In my experience, when the thermal protection cuts in, it just decreases the volume but doesn't add distortion.

???

In my experience you get awful distortion. On the scope you can see that the output is switched on/off, like pulse width modulation...

Fedde

[carlosfm]

Originally posted by fedde

???
In my experience you get awful distortion. On the scope you can see that the output is switched on/off, like pulse width modulation...
Fedde

Fedde,
I've been there.
Pushing the volume to silly levels, connected to 4 ohm speakers, I could hear the sound up-down-up-down (in intervals of just a few seconds).
It never completely cuts the volume.
This is the thermal protection in action, and that was when I had the chips mounted on the bottom of the case.
Now I mounted two heatsinks and it shouldn't happen again.
What I'm describing is documented on the datasheet.
The chip has other forms of protection that may act as you say.