Limiter hum

[emosms]

Hi, I've built this limiter from Rod Elliot's website:



But it adds hum to the signal. Is there smth wrong with the schematic?

I have other op-amp stage with gain 3-4 and there is no such problem.
For this circuit, as long as I turn the gain above 1 (TR1), there is audible hum coming. It increases with the gain.

Moreover, at min setting there is still a tiny bit of hum, compared to if I take it out of the chain.

So I wonder either the schematic is bad (seems ok) or I have something ground loop like in my layout (but I can't see).

[Mick Bailey]

How is it powered?

[emosms]

LM317 +-15v, the same psu as for all the other opamp stuff inside (a bass amp)
- the highlighted is a 2 channel limiter (full range/ horn speaker)
I am surprised that an opamp with gain over unity brings hum...

[Mick Bailey]

If the circuit otherwise works OK apart from the noise, you may want to check the signal ground point(s) for the circuit to see if the original grounding scheme of the amp isn't being compromised in some way.

[J M Fahey]

That.
The schematic is fine, and it does not generate Hum by itself, that certainly comes from grounding and layout.

Which are always tricky.

[emosms]

I found an error (the LDR part of the vactrol was across the feedback trim-pot only).
Anyway, I can't relate the hum to this very error.. it was rather that the limiter 'sensitivity' won't be adjusted by the trim-pot.

Now there is 'reasonable' hum (still there is a level change when I turn up the op-amp gain).

And yet another problem ... There is a feedback in the 'horn' channel. (It is after a crossover, 2k5 HPF)

The feedback is present if I turn down the preamp completely and disconnected the speaker.
The limiter circuit led glows after I turn up the volume at some level (the limiter is after the master volume).
Plugging the speaker confirms the led glows when there is a feedback squeal.

First suspect was 'optical feedback' - the circuit led is close to the vactrol and the vactrol LDR seems to 'leak'.
I put a barrier but the feedback is present.
There is no feedback on the full range channel.

[emosms]

Got it, getting lost in the project...

I was setting up some low pass filters at ~8k (to eventually minimize the horn speaker hiss).

So I used the input circuit R560 with some cap input to ground and this created some osilation, interacting with the volume pot...