Help solving some minor hum...

I have an SP12/PH12 combo that I built a couple of years ago, and it performs really well - very pleased with it. However I do have a minor amount of hum audible when there is no music playing. It's not terrible by any means, but it's there and I'd like to reduce/eliminate it if at all possible.

With the volume all the way down and no inputs connected it can be heard - it's no worse on either the line or phono inputs.


I know hum is often caused by ground loops and I have tried to follow what I think is best practice, however I'm not sure if I have missed something.

Any help appreciated.

The chassis 'star' earth point is connected to the 'earth' prong in the power connector via a 10ohm resistor and 0.1uF capacitor shunt.

Each input connector is isolated from the chassis, all of the ground tags are connected together with a piece of solid copper wire - one end of that copper wire is then attached to my chassis star earth point. The only exception is the input for the PH12 - here the grounds are connected via shielded cable to the input of the PH12 board.

Each line input is a twisted pair - the ground is connected only at one end (at the input connector) and each goes directly to the selector switch.

The ground for the SP12 is at the CT location on the board - this goes directly to the chassis star earth. The only other ground connections on the board are on the input from the volume pot and to the output connectors (again these are connected only at the SP12 board, they are disconnected at the outputs (I disconnected them to test and it made no difference to performance)

The ground for the PH12 is connected back to the chassis star ground - this was connected to one of the ground points on the perimeter of the SP12 board, but I moved it this morning and it made no difference.

The output from the PH12 is a twisted pair to the selector switch, ground connected only at the board, not at the switch.

I had hum issues with the SP-10 and SP-12 preamps I bought that were built by Roy or one his techs also. I was told the hum was the fault of ground loops in the places I was living. My solution to the problem is to use John Broskie based ACF-2 Tube Buffer as a preamp.built it and never had a problem. Dead quiet.

Short all the unused inputs. Most especially the phono input(s). Shorting plugs are cheap enough and also very easy to make.

That will cure it.

Peter W. wrote:Short all the unused inputs. Most especially the phono input(s). Shorting plugs are cheap enough and also very easy to make.

That will cure it.

Thanks, I'll buy some cheap plugs and make some. Worth a shot.

I use the phono and 3 of the line inputs

Shorting plugs made no difference....

Any other suggestions?

Short all of the inputs and see if there's still noise. Another possibility could be noisy tubes.

corndog71 wrote:Short all of the inputs and see if there's still noise.  Another possibility could be noisy tubes.  

I've ordered some more tubes to try... in the mean time I'll make some more shorting plugs....

Neither of these have solved the problem. New tubes and shorting plugs. Still got hum...

Without the tubes fitted it's perfectly quiet, but then I would expect it to be as there's no signal path. So where else should I look? It's inaudible with music playing, but when it stops you can hear it - it's quiet but it's there.

Fixed it....

Disconnect the ground wire on the ground bus at the input/output connectors. I knew it was a ground loop issue, but finding it was a pain. In the end, so simple!

Now totally silent! Woop!

flak monkey wrote:Fixed it....

Disconnect the ground wire on the ground bus at the input/output connectors. I knew it was a ground loop issue, but finding it was a pain. In the end, so simple!

Now totally silent! Woop!

Can you elaborate? Did you disconnect all of the RCA grounds from the rest of the preamp?

OK... all of my RCA grounds are connected together via a copper wire soldered between the tags - the only exception being the phono input. The end of this wire was connected to the star ground, but it's not any more.

All of the twisted pairs from the inputs go to the selector switch, only the 'hot' wire is connected at the selector.

The output from the pre-amp is connected both hot and ground at the outputs - I tried disconnecting the ground previously and it didn't solve the problem, so I reconnected them.

The ground of the amp (CT point on the board) is connected back to the star ground on the chassis.

So everything is still grounded as it should be, but removing that cable from the RCA's has eliminated a ground loop while maintaining a fully grounded amp

Flak

Just curious:
Did you leave the RCA output ground tabs attached to your single copper ground buss wire and simply disconnect that wire from the earth/star point or did you remove or clip the buss wire altogether so that the RCA ground tabs are no longer connected to one another?