OT: Help with Albert Kruezers bass preamp

[rubberlips]

G'day guys,
I'm at the wits end and pretty much tried everything but still can't get rid of the hum etc from this particular circuit. Here's what I've done so far
* checked all component values and orientations
* buzzed all connections
* checked supply voltages
* the EQ is bypassed
* no switches in circuit

The only parts changes I've made to the circuit was R23 (2M2), C1 (330nF) and C4 (4n7). The FETs I ended up using 2N5459's. I also tried 2N5484's which worked but the other gave a little more gain.
Oh and C2 and C6 I only had 6.3V 220uF caps on hand, but when I measured the voltages they were only around 2V anyway.

Here's what I've found so far
* turn the gain or the volume pot up and it starts to hum, mainly the volume pot. However at min or max it's quiet.
* The first two FET gates are at 0VDC, T3 sits at about 6V which is where it would if you've got 2M2 and 1M voltage divider  with 15V

This is how I've wired it
* Both inputs use shielded wire.
* The output is shielded
* The circuit is in a wooden case, the back of the pots is grounded
* The controls and switched don't have shielding
* The power comes from the poweramp (+/- 15V) and I've tried two other  different preamps with the same supply and there's no hum like this

Okay, what am I overlooking???

Here's the circuit
http://www.albertkreuzer.com/pics/el/pre/schem/preamp_sch21.gif

Pete

[Albert Kreuzer]

Hi rubberlips,

could be a ground loop and/or bad shielding.

Try two things:
1. Shield the case (or use a metal case)
2. Ground shielded cables on one side only

This usually helps, supposed you've got everything else correct.
The voltages look ok. I used 220µ/6.3V caps also, no problem.

Cheers,
Albert

[kusi]

hi albert!

this circuit looks intressting!

but, little question: where is the "midrange-potis" and the "midrange-select" switch connected?

thanks!

greetings kusi

[Albert Kreuzer]

Hi kusi,

the switchable midrange option can be connected instead of the normal midrange pot/capacitors (P4 and C11/C12).
Just for people like me who are unable to decide what is best for them  :wink:

Cheers,
Albert

[rubberlips]

Hi rubberlips,

could be a ground loop and/or bad shielding.

Try two things:
1. Shield the case (or use a metal case)
2. Ground shielded cables on one side only

This usually helps, supposed you've got everything else correct.
The voltages look ok. I used 220µ/6.3V caps also, no problem.

Cheers,
Albert

Thanks for the reply Albert,
Yes sounds exactly like a ground loop as I just finished fixing the poweramp which had numerous loops.  I can't shield the cables only one side because they carry the 0V from input to output. Being a wooden case, there's no loop there.

I actually thought that there might have been a problem with the board with maybe a ground lifted off a resistor or cap etc. But they're all present when I buzzed it.

I guess with your builts of these preamps they're fairly quiet even at mid range??

I might try shielding the wires to the pots first, then wrap the wooden case in foil and see if that makes any difference. Maybe because of the gain of the FETs they're picking up nasty RF signals

Failing that, it might be time to whip out the vero board and start from scratch. Might be a short that I'm not picking up on the PCB

Pete

[Albert Kreuzer]

Yes sounds exactly like a ground loop as I just finished fixing the poweramp which had numerous loops.  I can't shield the cables only one side because they carry the 0V from input to output. Being a wooden case, there's no loop there.

I actually thought that there might have been a problem with the board with maybe a ground lifted off a resistor or cap etc. But they're all present when I buzzed it.

I guess with your builts of these preamps they're fairly quiet even at mid range??

I might try shielding the wires to the pots first, then wrap the wooden case in foil and see if that makes any difference. Maybe because of the gain of the FETs they're picking up nasty RF signals

Failing that, it might be time to whip out the vero board and start from scratch. Might be a short that I'm not picking up on the PCB

Pete

Yes, mine is absolutely quiet, so I know the circuit is ok.
Everything you listed is worth trying, my first bet is on case shielding.
Sometimes it helps to run separate ground wires and not use the cable shielding for the signal return path.

You mentioned earlier that the V+/- rails are derived from the power amp. Did you filter them well? Perhaps try a bench power supply for comparison.

Cheers,
Albert

[rubberlips]

You mentioned earlier that the V+/- rails are derived from the power amp. Did you filter them well?

yes, they go through two regulators (+/-) with big enough caps. Then when they reach the preamp board there's another two 100uF caps just incase. The PS doesn't seem to make any noise with other preamps, that's why I was curious.

I'll dig around a bit more and have a fresh look

Pete