SSL dbx emulation circuit question

[Nele]

Hi,

As I am still suffering from some hum in my ssl compressor I decided to go over the unit again and discovered I used a 100nF cap in stead of a 100pF cap between the opamp and the vca in the vca-emulation circuit. Could this cause the hum?

Thanks,
Corneel

 

[bluebird]

Hey Nele,

Yea I still have the hum in my SSL. Since we have the same exact problem I'll look and see if I have the 100nf in there as well.

I still haven't messed with it and have been using the unit regardless since when compressing you can really hear the hum.

I guess we're both waiting for each other to figure it out..... :green:

I'll take a look when I'm in the studio tonight... or tomorrow.

 

[Nele]

just checked jakob's site and he seems to use 100nF caps as well... so does kev at the group diy site.

I did manage to lower the hum by making an extra earth-connection diagonally across the board but it's still there...

adios,
corneel

 

[bluebird]

Man I thuink it has to do with how the bypass switch is hooked up. I swear I wired it right, but who knows.

 

[clintrubber]

I was wondering about this too, but got the impression that it needs to be 100nF (and not 100pF as indicated in the schematic) - this based on the various PCB-pics around (great additional source of info, thanks everybody !)
And it also fits the parts-list: 25*100nF.

In addition, the R//C for 1k & 100pF for BW-limiting would make little sense in this place in the circuit.

If I understand this correctly, with 100nF & 1k the additional 'delay' of this stage would still be less than the fastest attack-time so the 100nF will be OK.

Please correct me if I'm wrong. :roll:

Regards,

Peter

 

[thermionic]

If you can give the forum an idea (using an FFT / SpectraFoo / your ears etc etc) of the centre frequency of the hum it will be helpful to Jakob et al in suggesting potential cures.

Have you tried removing the bolt from the mains toroid and rotating it around / upside down etc whilst connected to a monitor? I recently had a killer hum on a preamp which was cured by rotating the mains tf 180deg and inverting (with a large rubber washer to cushion leads).

Justin

 

[Nele]

It's a 50Hz hum so it's definitely a ground issue. I made a connection diagonally across the board between two earth-points and the hum is a lot less. Still no idea where it came from..

 

[daArry]

i think 100pf is right no? it's marked 100pF on the pcb overlay and theres 9 100pF's listed in the parts list...however, on the pcb overlay (in the PDF) theres 10 100pF's total - I used 10 where it was marked and I dont seem to be gettin any hum...

Jakob, can u clear this 100pF count up?

Cheers

 

[clintrubber]

I don't think this all is related to hum; it was an initial assumption here above that it could be related to this cap-value but looks like it's been solved now by doing something else.

The differences in caps give either a BW (LPF) of 1.6kHz or 1.6MHz (or perhaps already a limiting of BW by the 5534 with its compensation cap, don't have the circuit in front of me).

The BW 1.6MHz seems unnecessay for a circuit passing the A/ & R/-shaped control voltage so I guess the 100nF will be fine and is being used by most people (based on their PCB pics).

But otherwise just wait for the final confirmation of Jakob,
since he's of course the one who really knows it because he has designed the emulation-circuitry.

Bye,

Peter