VAC Signature 2a Preamp Question

Before you say, "Dude, there's already a thread posted about this question!" Well, forgive me, I could not find it. This is an inquiry for Signature 2a owners who have the built-in MC/MM phono stage: When the selector is on #6, which is the turntable, I get a background hum and some of what is probably tube rush, and it's especially audible with no music playing with the volume up. When I switch to the DAC setting, with the same volume setting, everything is dead quiet. Do I have a problem or is this normal?

Your feedback is welcome and appreciated,
Ag insider logo xs@2xalonski
The phono stage uses additional tubes plus the line stage tubes. So you could have more tube noise. That said, did you buy new, or a used unit? How many hours on the 6 12AX7 phono tubes?

I would contact VAC, Kevin and Brent have always been very helpful and responsive to my queries.
Alón, a couple of experiments that may shed some light on what is causing the issue:

1)Disconnect the turntable from the preamp and insert shorting plugs into the preamp's phono inputs, and assess the hum and noise levels under that condition. (Have everything turned off, of course, while you make the change). Googling "RCA shorting plug" should turn up a number of such plugs you can buy, or else you could simply buy a couple of RCA plugs at Radio Shack or wherever and solder a wire between the center pin connection point and the ground sleeve connection point.

2)Temporarily, as an experiment, put a cheater plug (a 3-prong to 2-prong adapter) on the preamp's AC power plug, without connecting its safety ground (what would normally be connected to the wall plate screw) to anything. That will break any ground loops between the preamp and the other components it is connected to. Ground loops, of course, can sometimes contribute to hum and noise.

Those two experiments should help to determine if the problem is due to something internal to the preamp, or to some external influence.

Also, as I recall you purchased the Signature 2a a couple of years ago. Has this issue been present all along, or did it just arise recently?

Best regards,
-- Al
I've had Signature 2a before, and can tell you with confidence, that the problem you are experiencing, is the EMI induced. And its generated by the power supply. EMI affects SUT in the MC section of the phono stage. If you switch to MM, or any other input, the hum disappears.

The only cure is the physical proximity from the control unit.
Keep it as far away, as the umbilical will allow.
This is "normal" for that design.
I think you have told us. I hear what I think is tube rush! There may be a bad tube in the phono circuit, which BTW can also make humming or other baseline noise. Check your tone arm wires and check the tubes. You can get very quiet tubes from Jim McShane at his website McShane Design over at audio asylum. I have no connection to him other than being a customer.
Thanks everyone, your advice is appreciated. I would normally call Kevin or Brent, but it seems like in my audiophile life, the hum-chasing episodes only happen on the Weekends.

I know you're right, Maril555. With my components in a rack, it's difficult to find much physical separation of the Sig2a from its own power supply, which is one shelf below.

Today, in addition to the hum, I heard crackle and pops, so I went all Sherlock Holmes on my amps and found the bad tube in the right channel mono block that wouldn't take a bias adjustment. Replaced it, rebiased all the tubes, and now the pop and crackle is gone, but the hum is still there, so it's not the Phi 200 mono blocks. Everything is grounded well. It could be tube rush from the phono section tubes (the line stage is quiet). I'll probably call Brent for some advice and buy some really quiet tubes from VAC if necessary.

Almarg, I've been wanting to try shorting caps, so your advice seems well-timed to help find the source of the problem. I'll report back if I encounter some big audio life lesson that I can share!

Thanks again,