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,
Alón
alonski
Did you connect the ground wire from your interconnect running from the tonearm to the preamp chassis?
Thanks for responding. Yes, it's grounded and I checked the connection. It's good.

Any other potential culprits you can think of?
It could be a few different things, but I think the most likely cause would be that you have too much gain on your phono section. If you're using a high output MC cartridge, you need to use the MM setting. Its a common mistake. Other than that, you really need to list your entire system.
If you're not 100% sure, try the MM setting regardless. You can't damage your system in any way. You're essentially lowering the volume, so don't be afraid to try it.
Is there any other equipment above or below (in close proximity)to the phono board? If so move it/them away and hear what happens (after doing Zd542's excellent suggestion, of course).
Adding to Dweller's comment, is the control section of the preamp sitting on top of or close to the power supply?
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,
Alón
"Everything is grounded well. It could be tube rush from the phono section tubes (the line stage is quiet)."

In case you missed it, read my post from yesterday.
Alonski,
I can save your time and effort- I've had that EXACT same problem, called Kevin, and that's what he advised me to do- move PS away from the control unit. By doing so, you will get rid of 90% of the hum. And that is THE ONLY solution. I even played with some additional shielding with m- metal, no difference.
Zd542
You are right saying, that switching to MM will likely alleviate the hum. But for a different reason.
It has to do with the SUT in the MC section picking up EMI from the PS, and not with the higher gain of the MC input.
"Zd542
You are right saying, that switching to MM will likely alleviate the hum. But for a different reason.
It has to do with the SUT in the MC section picking up EMI from the PS, and not with the higher gain of the MC input."

I agree. But its just too simple not to try. Also, it can't be ruled out either because we don't know what the OP has, or how its set up. You're probably right, though, given this is a common problem with this model preamp.
07-27-15: Maril555
Alonski,
I can save your time and effort- I've had that EXACT same problem, called Kevin, and that's what he advised me to do- move PS away from the control unit. By doing so, you will get rid of 90% of the hum. And that is THE ONLY solution. I even played with some additional shielding with m- metal, no difference.
Maril555, Just curious did you try covering all unused input and output connectors with Cardas or AQ caps?
Knghifi,
No, I didn't, but I used shorting plugs, that I feel is a more effective options, than Cardas- made no difference anyhow.
I tried mu-metal sheets to shield PS and control unit- no difference.
Actually, the advice of moving PS and control units farther apart comes from Kevin Hayes himself.
I'm pretty sure he knows what he's talking about giving that advice.
I had this problem with my Phi Beta preamp, which is much like the 2a Sig. As others have said, you simply have to move the power supply as far away as possible, at least 3 feet.
Gentlemen,
Two issues with moving the power supply out of the rack: first, it looks so darn cool to have all four VAC logos lit up at night while tubes are glowing... (See rack photo on my system page). So, now that I've outed myself as one who cares about the esthetics of my system, not just the sound... Here's the real issue: I have nowhere to hide the PS, unless I stand it on its side between the rack and the couch, something I don't want to do to this beautiful component. No other place is available, and the amps are too heavy to go to higher shelves, so I think for now, if it's indeed a proximity issue, I'll just have hum along with the hum... Thanks everyone for trying to help!

Alón
You're right, it really does look cool, that's one of the things I like about my Phi 200 as well. I know it's not as aesthetically pleasing, but could you move one of the amps down off the rack and put it on a lower platform in front of the rack, then put the power supply on the bottom shelf? Otherwise, maybe get a high output cartridge? :^)
Just take the amps the hell out from the rack and pur PS on the lowest shelf.
You have a beautiful system, but I'm sorry to say, but it is a rack update time for you
I have nowhere to hide the PS, unless I stand it on its side between the rack and the couch, something I don't want to do to this beautiful component.
That's probably a non-starter for technical reasons as well, at least without an ok from Kevin. It would cause the heat generated within the PS to concentrate at unintended internal locations, very conceivably resulting in premature failure.

Best regards,
-- Al
Huh, I turn the VAC logo off on my Sigma 160i. I find it a bit ostentatious.
Different strokes for different folks I suppose. Hum away!!

Wait...can't the amps go on separate amp stands by each speaker?
it is a rack update time for you
I think he's right, Alon.
I think he's right, Alon
In the end, rack is an essential part of the system, so it's quality should be commensurate with the quality of your components.
Plus, I would try to avoid placing hot tube amps right under your other components.
They (amps) belong on a separate stands, next to the speakers
07-29-15: Maril555
Just take the amps the hell out from the rack and pur PS on the lowest shelf.
+1

Plus, I would try to avoid placing hot tube amps right under your other components.
They (amps) belong on a separate stands, next to the speakers
+1
Hey All – your suggestions are good and appreciated. I agree with the ideas and warnings about where things "should" go... I've been looking at this set-up for a long time trying to find a way to give my gear more breathing room... but it's not realistic in my living room. A new (wider) rack won't fit unless I sell the armoire, which would not earn me any WAF points. And putting the amps near the speakers, which I would love to do, would require spending about a thousand bucks on my electrician, buying two amp stands, buying a third AC Power Regenerator and acquiring two reference quality interconnects of 32ft in length!
That original hum in question is sounding like a very minor issue about now. Maybe one day I'll build a dedicated listening room with lots of room to play with and no WAF considerations. If you all have your own space, I'm envious.