hum/buzz with pot all the way up

[woodturner-fran]

OK, a better description:


Its a passive preamp using a broskie stepped attenuator and a Darwin source selector from www.twistedpearaudio.com. This is a relay based selector. So in the pre, I have a simple 240V-12V transformer, which then goes through a cap 3300 uF - LM7805 - 3300uF cap to give a simple 5VDC supply for the relays. The idea of the relays is that the signal doesn't have to go through a switch in the normal way (and gives you 6 inputs and 2 outputs).

So going though the thing tonight, I find I have DC (2mV) on the broskie attenuator, but its also there before the attenuator. As I think now, I didn't check the CDP I was using, but its there on any source I've tried. Somehow the CD is getting into the signal on the relay board - I must open it up again and see if I can see any short anywhere. Problem lies within the preamp (tried out the power amp on its own and no hum/buzz).

However, I think now I put in the cap wrong: The non-polar should have been in series on the hot of each signal, not across the ground and hot. Could someone confirm that this is the way it should be...

Sorry I have no drawing or photos, I'll do some tomorrow.


Fran

[Zen Mod]

Originally posted by woodturner-fran
................
However, I think now I put in the cap wrong: The non-polar should have been in series on the hot of each signal, not across the ground and hot. Could someone confirm that this is the way it should be...

Sorry I have no drawing or photos, I'll do some tomorrow.


Fran

you certainly did it wrong;
if you want to kill DC on hot ,you must put cap inline with hot;

besides-check grid resistors (from grid to gnd) on first tube after pot;
try to lower it's value

[Johan Potgieter]

No, Fran, definitely in series, if I understand correctly.

Also it would seem that you are more or less solving your own problems. If further serious problems a diagram might help. Strange about the dc on the inputs - as said, it will need investigating.

I just saw a screen-flash that someone else has posted, so you may now get 2 replies.

Regards.

[woodturner-fran]

OK,

I added the cap in series and it has cured the hum. Still a tiny bit there, so I will need to investigate this further. However, now when I turned the broskie attenuator, I get a good loud pop on each step (but only on the step of the centre control, not on the others). The lord giveth and the lord taketh away!!!!!!!!

So I now have to investigate a new issue!!!! Any ideas anyone???


This is now very off-topic and I'm sorry for that.....

Fran

[woodturner-fran]

[Johan Potgieter]

Ideas? Yes. You are switching over a voltage step, however small. But I am surprised you have it after the capacitor was added and not before. Where one switches with caps in the circuit one needs to bring both sides of the switch on the same (dc) potential, as it were, mostly by bypassing switch contacts with a resistor value large enough not to influence the operation itself.

But to solve this a circuit of what you have is necessary. I will look up the Broskie (must run now) and come back later. Perhaps others are more "hands-on" than myself.

Regards,

[Johan Potgieter]

Fran (follow),

You will notice that we posted simultaneously. I did not have the photos when I posted before. Not quite enough there to come to a conclusion, but perhaps later.

Regards.

[woodturner-fran]

I'm guessing that something like a ground connection somewhere in the broskie is needed. I don't have a schematic here for the broskie but more info can be found here:

PDF of broskie stepped attenuator

tubecad page for stepped attenuator


Fran

[DanDini]

Originally posted by Zen Mod
you certainly did it wrong;
if you want to kill DC on hot ,you must put cap inline with hot;

besides-check grid resistors (from grid to gnd) on first tube after pot;
try to lower it's value

But is is essential to use a non-polarised cap for output decoupling? I thought it wasn't...

Dan

[mrwhy]

Hello Fran
There are two kinds of hum: electrostatic and magnetic.
You get electrostatic hum (pickup) in any high resistance circuit (like your 100K pots). For a quick sheck put your finger onm a signal wire (not the hi volts!). This is because all things have capacitance beteween them so your input "sees" the mains wires in your house.
Magnetic pickup is different. If ANYWHERE you have a closed loop of wire (any thing grounded more than once) then the magnetic fields in the air of your house (and from the supply transformer) thred this and it acts as a shorted turn - many anps can flow!. This causes voltage drops (resistance of wires) and your amplifier picks them up eand amplifies them - even in "ground" and "chassis"..
To test for electrostatic, enclose the whole thing in a grounded metal box. If the hum goes it is electrostatic.
Magnetic is harder. All you can do is disconnect earth and chassis wires one by one and see if the hum goes.
Good luck and have fum
John