Van Alstine amp problem...

[rtate]

I recently got a Van Alstine model 3 amp that I just finished
"freshening up" I replaced all the electrolytics with black gates and most of the film and all of the ceramics caps with polyproplyene. Also I upped the main ps caps from 20,000@75v
to 32,000@80v.
The amp plays music fine but it has a hiss that sounds like white noise.
It starts out almost inaudible but after about 2 hours I can hear it from 10 feet away!!
I have a schematic for the amplifier boards but nothing for the power supply, speaker protection circuit or the "bootstrap" boards.
I don't know what the problem could be or where to start looking
Can someone point me in the right direction please??
This seems like a good amp and I would really like to fix it!!
I do have pictures but thay are too large to attach...

[TerryO]

Originally posted by rtate
I recently got a Van Alstine model 3 amp that I just finished
"freshening up" I replaced all the electrolytics with black gates and most of the film and all of the ceramics caps with polyproplyene. Also I upped the main ps caps from 20,000@75v
to 32,000@80v.
The amp plays music fine but it has a hiss that sounds like white noise.
It starts out almost inaudible but after about 2 hours I can hear it from 10 feet away!!
I have a schematic for the amplifier boards but nothing for the power supply, speaker protection circuit or the "bootstrap" boards.
I don't know what the problem could be or where to start looking
Can someone point me in the right direction please??
This seems like a good amp and I would really like to fix it!!
I do have pictures but thay are too large to attach...

Why don't you ask on the Van Alstine forum over on Audio Circles for help, who knows maybe Frank himself "might" be willing to help, although many designers aren't necessarily delighted with people modifying their designs.

BTW: what year and what is the Model 3 amp?

Good Luck and Best Regards,
TerryO

[rtate]

The QC stamp on the transformer says 1978.
I believe that Frank had left the company by then and was not involved in the design of this amp.

[TerryO]

Originally posted by rtate
The QC stamp on the transformer says 1978.
I believe that Frank had left the company by than and was not involved in the design of this amp.

Yes, I'd forgotten that. Frank might have some ideas pertaining to that amp since it has his name on it. You could always ask and plead ignorance.

Best Regards,
TerryO

[danville]

My guess is that your problem is with one of the input op amps. I think these were LF357 JFET devices which sometimes got noisy, Is the noise white or 1/f? If it is 1/f then I am almost certain this will be the case.

Frank Van Alstine had nothing to do with the design of the Model 3 and I doubt he even has a schematic.

Al

[rtate]

I would describe the noise as "white noise"
It is not audible until the amp is on for a couple of hours...

If it is the opamps are there any better choices to replace them??

[llwhtt]

Did the amp make the noise BEFORE you "freshened it up"?

Craig

[rtate]

The noise was there before but is more audible now,
I had thought that it was the speaker protection relay so I took it apart and cleaned the contacts and it seemed better for a while but like I said , after a few hours it is quite audible,
It does not affect the music though...

[llwhtt]

I know freeze spray gets a bad rap but sometimes it just plain works, freeze that opamp and see what you get. I don't know how hard it is to change the IC, socket or no socket? Also check the DC voltages on the IC, pins 4&7. Got a used Sumo Nine years ago that some one had repaired. It would work for awhile then would crap out. Pulled it apart and found a rectifier diode had been installed in place of a zener. The voltages to the IC are zener regulated. The LF353 input IC did its best to hang in there with one DC voltage way too high but just couldn't handle. The Nine runs it's IC at 20 VDC anyway which already too high. It must "selected" to run in the Nine circuit.

Craig

[danville]

The op amps were never socketed. It probably has a round TO-whatever footprint. It is possible that it is a DIP footprint.

You can certainly update the opamp with something better today. I haven't tried anything recently but I am intrigued by the National LME49713 Current Feedback Amplifier.

Of course, the amplifier may need to be recompensated. There is a trim C that may help but be careful. It may sing.

If you are concerned with measured distortion, you should cut the cable harness and separate the power supply wires. These magnetically induce noise into the output lines. Since it is a very simple harmonic distortion, I doubt if it has much audible influence.

Al