Fluctuating high-pitch noise in op-amp circuit

[Rolox]

Originally Posted by indianajo

Yes if it is not self-oscillating. Plate has to be bonded so it is connected electrically to the safety grounded case. Transformer can stop RF but the wires to it from line cord have to be short so they don't act as transmitters. There are IEC sockets with RF trap in them if this has unprotected one of those to the line cord.
I tack the 33 pf caps right on top of the feedback resistor with short leads. I don't see them. 22 or 47 could help too.

I assume the feedback resistors are in the signal path, is it ok sound wise to use 33pF ceramic there? Would 33pF polystyrene work as well, or is ceramic the only type to use?

[Mark Tillotson]

Any 33pF will do, its not critical, it increases HF stability and has only the slightest effect on audio frequency performance.

[Rolox]

Originally Posted by Mark Tillotson

Any 33pF will do, its not critical, it increases HF stability and has only the slightest effect on audio frequency performance.

I went to the nearby electronic store but all they had was 39pF, so I took a handful. I suppose 39pF instead of 33 won’t hurt?..

[indianajo]

No. Hope they are ceramic, wound plastic has more inductance. Inductance resists high frequencies, you want inductance of RF short capacitor at a minimum. Impedance of inductance is directly proportional to frequency. Capacitance of X7R diaelectric varies with voltage, which is between +- 15 at this point, negligible IMHO. 1 uf ceramic caps in between RCA input & op amp input, you might be able to hear X7R versus tantalums, but the X7R will last longer. 1 uf caps are 30000 times bigger than a 33 pf cap.
I'm listening to 4.7 uf COG ceramic caps in the sT120 right now, sound fine to me. Speaker distortion is way above the 3rd digit of HD where John Curl made some money on all those books.

[Rick PA Stadel]

39pF will be just fine. Inductance and dielectric won't matter as much since this is only a 'does THIS make any difference' test.

How do you get along with the people upstairs? If it's an office, they have printers/fax machines. Ask them to let you know when they're about to run a multi-page print job. (So you would at least have a stable, consistent symptom.) Or at least trade phone numbers -- maybe they wouldn't mind an occasional 'Hey, what do you have running Right Now?' call.

My bet is still on line-conducted/transmitted, vintage CPU-sourced -- but I've been wrong before.

Rick

[Mark Tillotson]

39pF doesn't need a highly wound construction, it doesn't take much area with a thin film dielectric.
1 cm square of ordinary paper is about 20pF if between metal foils, and plastic films have higher dielectric that that and are thinner.

[Rolox]

Okay so I tried to locate the feedback resistors around the op-amps; the thing is, I don't have a schematics, and I can't recognize what is what :-( I'm not good enough

So I started with putting those caps on the RCA plugs; turns out those caps must be pretty bad because I immediately got some hum (or maybe I shorted one by spreading its legs too far apart, they seem to crumble easily - **** quality, I don't wanna use them), so I removed them.

So I'm back where I used to be. I think I will buy a Schaffner Filter and go from there...

[jan.didden]

Originally Posted by Rolox

I understand of course, the point is, I get the whistle only when this crossover is in the chain. if I substitute it with my other crossover, leaving all the rest exactly the same, there's no whistling :-) so it's fair to assume the problem is in the crossover... I'll add that lowering the output potentiometers of the crossover lowers the whistle in the same proportions

OK, clear. Now, what happens if you short the input to the crossover, disconnect the input cables?
If it disappears, what happens if you only connect ONE input cable, L or R? Does it then whistle?

Jan

[Rolox]

Originally Posted by jan.didden

OK, clear. Now, what happens if you short the input to the crossover, disconnect the input cables?
If it disappears, what happens if you only connect ONE input cable, L or R? Does it then whistle?

Jan

well that is interesting: disconnecting input cables yields a total absence of noise (bare the faint hiss that's always present, a normal level of hiss). Made me realize there's also some hum when the cables are connected.

connecting the preamp straight to the high pass amplifier using the same cable yields a very slightly higher level of hiss, barely noticeable difference; that's the noise floor of the preamp coming trough probably

then why does connecting the cables to the crossover input causes noise?

I've noticed: there are two ground planes as far as I can tell: one for the PSU section of the PCB, that one is grounded to the aluminum enclosure; and one for the signal part of the PCB wich, as far as I can tell, isn't connected to the enclosure... should the signal ground plane also be connected to the enclosure?...

[Rolox]

How about connecting the outer part of the RCA to the enclosure using a small ceramic cap?