VAC Preamp: Is my Phono stage failing?


My VAC Standard LE preamp is exhibiting a loud, strange hum, heard in both speakers equally when MC Phono is selected. However, the hum doesn't show up until about 10 minutes of normal sounding LP playback. It gets louder with volume.

I think it may have to do with the unit getting to a certain temperature, since it starts the process over again if I turn everything off for 30 minutes – quiet for 10 minutes or so, then Wham! a loud hum that has me diving for the mute button!

If I switch the selector to any other setting, such as CD... dead silence. Could it be a phono tube going bad, and if so, how is it in both channels?

Any ideas (aside from calling Kevin, which I will do)?
alonski

Showing 7 responses by alonski

The techs at PurePower were awesome, and they're in Canada, so they worked on thanksgiving! We narrowed it down to a safety ground loop. The unit is fine, as is my VAC. Seems this safety ground cannot legally be disabled, so I'm counting on my electrician to at least separate the safety grounds for each circuit so they don't bleed over. What remains a mystery is how this hum got into the signal path... Any ideas?
SOLVED! Well, almost.

Maineiac, Thanks, I really wonder how much I would need to spend on an outboard tube Phono Preamp to be significantly better than the VAC's.... sounds like it might be a lot. I would love to audition the Fosgate Signature, it's received good reviews and it looks so cool.

Pubul57, I spoke with Brent at VAC and he never heard of this problem and said to send it in for a check up. I guess the very notion of being without music for a few weeks was bad enough to kick my brain into gear and identify the problem's source.

My system is on two 20amp dedicated lines and all components are fed power from a AC regenerator that completely isolates the system from the wall power. I also have a hot tub in the back yard on its own separate 20a circuit.

Every time the hot tub motor turns on, the hum shows up in my phono stage. If I then turn off the tub, the hum disappears.

Considering the system and the tub are on completely separate dedicated circuits AND my gear is completely (supposedly) isolated from the wall power... how could this be?

I'm calling the electrician back in and I'm going to call PurePower and have a little talk with them. I don't think this has anything to do with my VAC.
Sorry Al, I should have clarified. My two channel analogue music system is my first priority. The PurePower regenerator is on this system plugged into the first dedicated line.

The other dedicated line is feeding the home theatre system which I keep completely separate. Although they are the same room, when I'm listening to vinyl, all power to the digital HT system is off. I use two Panamax power conditioners on this system.

When I'm in critical listening mode, i even turn off the refridgerator, which is quite loud in our open-plan home. Now, if I could only remember to turn the fridge back on, I would get fewer of those looks from my wife!
Just rolled a pair of NOS Telefunken 12AX7 to replace the stock VACs. And for about 40 minutes of horrible sounding crap coming out of my speakers (how long does it take to break in new tubes?) the hum started again – it took longer, but it did show up.

So it's probably not the tubes after all. If my phono stage is indeed needing repair, which means packing up and shipping my pre to Kevin way across the country, which would be another expense on top of the repair costs... I may consider an upgrade to an outboard phono stage instead of fixing the internal phono stage.

Does anyone know what outboard phono stage would be equivalent to the VAC's built in (which is awesome BTW)? And once I get a bench mark, what would you recommend as a significant upgrade from there?
Maineiac, Thanks, I tried having my Phi 200 amp direct to the wall and it sounded great. Then I tried plugging it into a Panamax power conditioner and it sounded like someone threw a wet blanket over the sound. Then i bought the Purepower AC regenerator and plugged the amp into it... WOW, it blew away anything I've heard from this amp. Surge protection of the highest order, clean power, lots of AC headroom for even more attack and a blacker background. A significant improvement from direct into the wall. But, to be fair, I'll A/B it again next time I move the 400 lb. armoire that's in front of the wall plug.

Al, I agree with your speculation on the pathway this rogue hum snuck through. I hope to have an answer on Monday when the electrician comes back.

Thank you both for your insights and help.
Alon
Same here, increased coverage to $50K replacement value, but no special rider. Who do you insure with?
Tpreaves, I would, but I neglected to have spares on hand.

Ghasley, Thanks Geoff – I'll PM you.

Maineiac, Yeah, I've been scratching my head about whether this could be a ground issue because it came out of nowhere. Just started happening a few days ago.

Wearing my Sherlock Holmes hat, I disconnected the tone arm while the hum was happening, but in stages: First I disconnected the right channel... hum was reduced. Yanked out the left RCA, hum reduced even more. Removed the ground wire... even more reduction. However, even with no tonearm connected the hum was still loud and unacceptable when selector was set to Phono. Then I tried every other source selection on the knob... dead quiet. Using one of the quiet settings, I switched the other selector from "Source" to "Cinema Bypass," which is connected via IC to my Marantz HT receiver, and the hum comes on strong as ever! So I disconnected my VAC from the Marantz and yes, the hum is still there!

Any tech wizards out there have a diagnosis?