Hum - preamp on, or off

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Dr. W
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Hum - preamp on, or off

Post by Dr. W » Fri Nov 16, 2012 11:48 am

Good morning gang,

I need some assistance from this amazing body of knowledge. I will try to make this not longer than it needs to be.

I have a hum that occurs when the preamp is in the circuit. When I use the DAC output (W-4 S DAC – 2) and connect directly to my amplifiers, Manley NeoClassic 250 mono blocks, the system is dead silent.

If I take two sets of interconnect and connect them with a coupler from DAC to amps, it is still dead quiet.

When I replace the couplers with the preamp there is an audible hum, whether the preamp is turned on, turned off, or if the power cord is entirely disconnected. The hum is at the same volume level in all three conditions.

Suspecting some kind of a common problem within the preamp I use one coupler and then only one channel of the preamp. I still get the same result on the channel with the preamp.

The hum is not as loud as an open input, nor as loud as a faulty(open) ground .

The preamp is a simple line tube preamp, with dual modo construction, using tube regulation and to rectification, and two tubes per channel for gain. It has four relay switched inputs, and one volume pot.

Because the hum level is the same whether one channel or both channels are used, whether the preamp is on or off, or whether a power cord is connected or not, I suspect that there is some kind of a grounding problem with in the preamp that allows an induced hum from some sort of lack of shielding.

HELP!

Thank you in advance for any responses. I am flummoxed.

Bruce
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Re: Hum - preamp on, or off

Post by radioeng2 » Fri Nov 16, 2012 12:58 pm

Next thing to try is to add a ground. The power cord may not give the pre a ground, depending on how it's made, or you could be giving it a different ground from power if coming from a different outlet.

If the power amps are using a grounded plug, then that'd be the first place to try getting a ground. Use any wiring you have handy and try touching anything that should be grounded on the amp chassis and go over to the preamp and do the same. Just look for something not painted over and experiment on the pre with both the chassis and then on one of the RCA's ground side. Just fish around a little and see if you can impact the hum level.

BTW....a powered down preamp might tend to cause it's own noise, though I'd expect you could hear it change the hum some between on and off. Also, you might try the pre to the amp's without the DAC to see if the pre has a sensativity that the amps alone don't.

Good luck with it and pass along what experimenting shows.

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Re: Hum - preamp on, or off

Post by Dr. W » Fri Nov 16, 2012 1:54 pm

Worth a try. I will get back to you.

Hold on to your hat. :-#

Every thing in the system is on a two prong plug/no ground. I would think that would decrease the potential for a ground loop.
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Re: Hum - preamp on, or off

Post by Dr. W » Sat Nov 17, 2012 1:58 pm

Update. Some improvement.

Mark, thanks for suggesting I investigate this

With sub-panel box off, zero impedance between amps, amp to pre (ea), pre to dac, dac to ea. amp.

With power on 0.07 volts AC between all pieces. Grounding one to another, no difference, butttttttt.....

I have "active" shielded cables. Before I was using my Van de Hul "The Ultimate First" mk II. Still somewhat infamous for hum with some tube gear. So, since the hum level seemed to be very similar I assumed that the new cables could not be involved. Wrong!!

While bridging a wire between pieces I found that with the chassis screw for the tubed power supply on one set of cables I could decrease the hum audibly. I next found that touch the screw with just one prob from my VOM (set to measure impedance) had the same affect. Actually it had the same affect not mater whether on or off.

I think my VOM is putting a resistance at the end of the probe/lead. The other lead was not touching anything.

I will continue to pursue this until I can get it to the lowest hum level.

Any ideas what I can attach to the screw in lieu of my VOM?

Bruce
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Re: Hum - preamp on, or off

Post by mbskeam » Sat Nov 17, 2012 3:40 pm

at the old house I was living at I had the same issue.
I took a wire and put a spade terminal on each end, and slipped it under a case screw...
you could not see it from the front, hum gone.... :D
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Re: Hum - preamp on, or off

Post by Dr. W » Sat Nov 17, 2012 4:02 pm

mbskeam wrote:at the old house I was living at I had the same issue.
I took a wire and put a spade terminal on each end, and slipped it under a case screw...
you could not see it from the front, hum gone.... :D
Was the other end just hanging out in space, or did you attach it to something? :-k
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Re: Hum - preamp on, or off

Post by mbskeam » Sat Nov 17, 2012 6:28 pm

it tied the pc together, when testing I used wires with alligator clips till I found the combo that knock the hum down, then I made it more permanent...
are you saying your cables have a outer metal shield ?
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Re: Hum - preamp on, or off

Post by Dr. W » Sat Nov 17, 2012 9:09 pm

This will explain my current interconnecting cables.

click on RCA cables and then biased cables, looked at the AC version.

http://www.auralthrillsaudio.com/
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Re: Hum - preamp on, or off

Post by mbskeam » Sat Nov 17, 2012 10:53 pm

ummm, maybe the cable box is having an issue.
have tried a different tube?
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WTL Amadeus GTA+DV DRT XV-1t
2 sets of ALS'3, set of SILVERS, set of PLATS
AUDIOCONTROL: phase coupled activator
KENWOOD: BASIC M2a (2)

“Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”
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Re: Hum - preamp on, or off

Post by TNRabbit » Sun Nov 18, 2012 12:07 am

Is there a ground lifted or short in one of your interconnects? Check 'em with a Multimeter.
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Re: Hum - preamp on, or off

Post by OconeeOrange » Sun Nov 18, 2012 4:49 pm

If possible, try to have everything on the same circuit.

A little hum ( bleed forward) is not cool, but has little effect on the music.

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Re: Hum - preamp on, or off

Post by Dr. W » Sun Nov 18, 2012 6:24 pm

TNRabbit wrote:Is there a ground lifted or short in one of your interconnects? Check 'em with a Multimeter.
Already done. But thanks for the suggestion.
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Re: Hum - preamp on, or off

Post by Dr. W » Sun Nov 18, 2012 6:29 pm

OconeeOrange wrote:If possible, try to have everything on the same circuit.

A little hum ( bleed forward) is not cool, but has little effect on the music.
All on one single dedicated 30 amp circuit isolated with 4 Tice Power Blocks wired in parallel feeding all sockets from a single "hot" wire and a single "neutral" wire from secondary side of the Tice Blocks.

You can see a picture at tne system link below.
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Re: Hum - preamp on, or off

Post by Chauss » Sun Nov 18, 2012 7:18 pm

Dr W- if you are using shielded cables- I think only one end of the cable jacket goes to ground. May want to check that-
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