Need a plan to solve a distorted buzz coming from my Crown D75a- Help, por avor

[djk]

No, that is not correct.

The feedback injects the distorted signal back into the opamp and it now shows up in every stage of the amplifier.

[Don Richard]

The feedback circuit subtracts whatever is in the output signal that is not present in the input signal. That is how negative feedback lowers distortion. In this case either something is wrong with the FB circuitry or the problem is outside the FB loop. Does this amp use a feedforward loop?

[BentMike]

The feedback circuit subtracts whatever is in the output signal that is not present in the input signal. That is how negative feedback lowers distortion. In this case either something is wrong with the FB circuitry or the problem is outside the FB loop. Does this amp use a feedforward loop?

The service manual shows a feedback loop going to an error amp. Balanced Input to Error Amp to Signal Translator to Last Voltage Amp to Output Stage. Feedback is from the Output Stage to the Error Amp. Error implies correcting for the undesired signal. But how successful is it? Anything it doesn't scrub is degraded signal out. (I am really BSing now).

[djk]

Give Crown a call, have the serial number handy.

They are generally helpful and patient.

[russ69]

Russ,

All I have is a lubricated cleaner. Is that a bad idea for the time pots?

Also, How do I get the pots back right after exercising them?

That is why I hesitated to screw with them.

When you said trim pots, I thought you were talking about the front panel level controls. My bad.

[BentMike]

Figured it out. I am red-faced to be sure.

It isn't the amp, it is the speaker. You guys wisely challenged me early on. I should have paid closer attention.

When I first checked out whether the distortion moved around when I changed speaker wires L to R, I wasn't careful. The problem only happens after the speaker is on for a few minutes - it has to heat up. I switched the "good L channel" to the bad R speaker and I heard no distortion. Then I swapped them back, "bad R channel" to bad R speaker and after some time elapsed there it was "back" on the R side. I was assuming the R speaker was OK and the amp messed up, rather than thinking about how to validly check it.

What is wrong is an axial misalignment of the voice coil and the pole pieces or magnet. I can see a little kink in the surround, and I can feel it rub if I push just so on it. I should have been more wary of this, but I was too proud of a repair I did to the driver. When I bought the speaker the driver cam apart during shipping. I built a fixture to get it back together and it seemed good for a while. I was pretty puffed up about figuring how to get the driver back together and working after finding the magnet and pole piece loose in the cabinet. No way the distortion could have been from that speaker.

I can probably fix it right, but it aint't easy so I bought a couple on eBay for not much.

Thanks for working with me, I learned a lot.

BentMike

[djk]

A shifted magnet can be repaired, but it is usually cheaper to buy a replacement driver.

I get them repaired if they are a rare hard-to-find item, otherwise they get replaced..