VTL hum

[directdriver]

I am helping my friend fixing his VTL Stereo 90 amp using KT90 in push-pull and input and driver tubes are two 12AT7 tubes for each channel. The amp was being used in a video post-production studio so it was being used on a daily basis. Recently my friend said there's hum from the amp and sent the amp to me. I was inclined to think the filter caps needed to be replaced since the amp was in used constantly for years. It wouldn't hurt to replace all the electrolytic caps anyway. I replaced the caps with new ones of smaller value since original value ones are out of stock.

There's no hum when I don't have input cables plugged in. That tells me the caps are fine now. But when I plugged the my preamp into the amp there's hum. And most mysterious of all, there's hum when I plugged both channels but there's NO hum when either ONE channel is plugged! My preamp is a very quiet solid state preamp that never had any noise issue connecting to other power amps and I have sensitive horns speakers.

Again, when NOTHING connected, NO hum. Plug in either ONE channel, NO hum. But plug in TWO channels, there is HUM.

I don't think I have bad capacitor issue here anymore. Not tube issue either. This is clearly a grounding issue. The stock arrangement having the speaker negative connected the the RCA ground and then a cable to the circuit board ground. I tried to connect the speaker negative to chassis ground, no luck. I am baffled. Anybody had issue like this before or any suggestion? Thank you for any response in advance.

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[alexmoose]

I had an issue similar to this once. I kept getting a nasty ground loop from my USB tv tuner that put a loud buzz in my SE amp, I fixed it with two baluns back to back. You could use an input transformer, but that seems like a band-aid fix in this situation.

What makes it bizarre is that the buzz started randomly. so you really have to think about what changed in the amplifier to cause it. Are the RCA grounds connected together? thats what I would try first.

[directdriver]

alexmoose: "Are the RCA grounds connected together? thats what I would try first."

The RCA grounds are connected separately; they are located on two both ends of the chassis' back.

All ground points are routed to the circuit board and then a ground cable to the chassis. Seems like a typical ground scheme. The only odd thing is the each speaker negative is soldered to RCA ground. I tried both to RCA and directly to PCB ground, same result.

When nothing or only one channel is plugged in, extremely quiet, no hum, no buzz, no hiss. Only a faint hiss if I put my ear into the horn! So it's a quiet circuit and no faulty component. But when I plug in both channel, there's a hum. However, when I lift or reposition the cable the hum lessens. It could be shielding so I used a short shielded cable connecting directly from my source. Same result. Yes, it's a ground loop issue here. Something somewhere is not grounded internally or grounded properly. I just don't know where. I don't want to resort to input transformer.

I even solder both input to the same RCA jack as mono, same result.

The current hum level is not too bad for some people and with less efficient speakers it's tolerable but clearly it can be better. The problem is that I never got the amp when the amp was in stock form so I don't know what s/n level originally. Maybe it was like that originally but I doubt it.

The hum is driving me nuts!

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[Zen Mod]

knowing their schematics and having monoblocks made by these schematics , I'm sure that culprit is in physical layout only .
unfortunately - I can't remember anything from it , just because I saw original one many moons ago .

try - for test - to connect gnd to one RCA (internally in amp ) via 10 ohm resistor and inform us what is happening .

few pics can't harm , too

[directdriver]

Zen Mod: "I'm sure that culprit is in physical layout only."

Below is the picture and as you can see the layout and soldering are rather sloppy. It's a rat's nest! The power tubes use two ceramic sockets and the other two are plastic on the pcb!

Zen Mod: "try - for test - to connect gnd to one RCA (internally in amp ) via 10 ohm resistor and inform us what is happening."

That was going to be my next step. Thanks for the advice. I will do that and report back soon.

I did replace the stock input cables because they were so thick and stiff to work with. The replacement cables are well shielded cable. But I might solder the original back in there so everything is back to stock form just for troubleshooting. Other than that, almost everything you see in the picture is stock.

Again, the problem is:
NO channel connection = NO hum
ONE channel connection = NO hum
TWO channel connection = YES hum

It doesn't matter which channel I connected to, as long as only one channel is connected, no hum. It is only both channels connected, then there is hum.



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[kevinkr]

I notice that the grounds for the two inputs on the board are about as far away from each other as they could get, this is a great recipe for hum as you have discovered. Try connecting them both to the same point on the ground bus, and preferably close to the chassis mecca ground connection. You can just solder wire to the shields and then connect both to the same place.

You can instead try lifting the ground on one rca jack and running a wire from that jack ground tab to the other jack and see what happens. I had this very problem in a Citation II (internal ground loop) and doing this fixed it completely. Quick and dirty fix.

Another thing you can try is a 1 - 10 ohm resistor between one input jack and its ground connection, this too will break an internal loop.

These amplifiers IMLE hum with some pre-amps and not so much with others. The input ground configuration was not well thought out.

[Zen Mod]

in fact - it's almost neat for tube amp ;

put NTC of ~10R between audio gnd and chassis
unsolder shield of input coax cables at pcb side , route gnd from central gnd to RCA's with tiny wire ; that way shield will be just shields .

[directdriver]

I have tried adding a 10 ohm resistor in one channel between RCA shell to ground. No luck. Noisier on one channel.

I have tried adding 10 ohm resistors to both channels. No luck. Noisier on both channels mixed with some buzz.

I have tried jumping one RCA ground to the other RCA ground. No luck.

I have even tried even tying the two separate power supplies into one rail, by also snipping out one set of bridge rectifiers, in case "maybe a rectifier is leaking in the reverse direction causing small ground potential within the chassis," as a friend suggested to me. And both supplies come from the same AC winding. Stereo separation or crosstalk is not a concern here. Alas, no luck.

I have tried using two kinds of preamp, one with 3-prong AC plug, and one without. No luck.

Zen Mod: "put NTC of ~10R between audio gnd and chassis unsolder shield of input coax cables at pcb side , route gnd from central gnd to RCA's with tiny wire; that way shield will be just shields."

Can you elaborate? What's NTC? Thanks!

At this point, I'll try anything. If I have to jump up and down to do a rain dance to make the hum go away, I would do it.

kevinkr: "The input ground configuration was not well thought out."

No kidding!!

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[kevinkr]

Next thing to check is that neither of those rca jacks is connected directly to the chassis.

Have you tried moving where the input grounds are connected? And placing them together? Might want to try this with jacks and wires external to the amp and figure out what works first.

[Zen Mod]

NTC is negative temperature coefficient resistor

Pass usually use CL60 for that , but you can scavenge one from any broken PC PSU

you can mimicking one with 10 ohm resistor , between safety gnd ( chassis ) and audio gnd

but - check continuity between RCA's and chassis ( must be isolated) as Kevin sez , and try that "shielding just on sourcing side" trick