Bah hum bug

[fanboi]

Some months ago a friend purchased a pair of monoblocks and a preamp to upgrade his system.
At first all was well, then the system developed a buzz or hum (rather a combination of the two) - on one channel only.
Most of the usual suspects were tried and did not resolve the problem.
He sent the amps back to the distributor in Melbourne where they were checked and found to be OK.
Connected them up on their return from their holiday and still the same.
I went over there for a day with another preamp, pair of speakers, assortment of I/Cs and speaker wires etc.
There was no resolution to the problem, we tried turning off all other circuits in his unit as well as every combination of cables, speakers and preamps we could.
As a final attempt at cure, we ordered a line transformer, which when installed between the preamp and monos reduced but did not eliminate the problem.
Worth noting is that with the monos powered up and the speakers connected but no I/C into them, there is no problem.
The noise occurs only when there is another piece of equipment connected to them.
Somewhat miffed by all this my friend went OS on holiday, on his return we decided to bring his monos and preamp to my place and run them there.
They ran flawlessly, with no hint of a problem, regardless of how awfully we connected them (WRT powerboards etc) with either his or my preamp. (previous experiment at his place with other speakers had ruled out his speakers being the problem)
It would appear then that problem is somewhere in the wiring of his unit block.
As a further note, the noise is apparent with no signal passing and is entirely unaffected by the volume control on the preamp.

[Deleted]

fanboi
Does your friend live fairly close to the Sydney TV transmitters, or a nearby TV repeater (Translator) ? "Frame buzz" from a TV transmitter can cause similar problems. The level of the hum goes up and down in accordance with the channels picture brightness level. In that case the interference usually enters via the speaker leads. The cure was to wind a couple of turns of the speaker cable through a large toroid. I had this trouble years ago, when living at Lane Cove, which is only a couple of KM from the nearby Gore Hill TV transmitters.
Did you try swapping L and R input cables to see if the problem hum moved to the other speaker.

SandyK

[rowuk]

It sounds like a classical ground loop problem. The units are each "earthed" through the mains cables, for some reason there is a difference in that "ground" and the difference flows through the interconnects. I am pretty sure that if you plug pre and power amp in without interconnects, that you can measure voltage between the RCA grounds.
The solution is to get the grounding sorted out. Sometimes attaching a plain piece of wire between the metal chassis "solves" the issue. If you plug a computer into the preamp, the same thing can happen.

[Deleted]

" Sometimes attaching a plain piece of wire between the metal chassis "solves" the issue. If you plug a computer into the preamp, the same thing can happen. "

rowuk
On my home brew peamp and Class A power amp I have fitted banana sockets which are connected to chassis. I made up a single core lead with a banana plug each end, and this is used to further reduce any low level hum. With the headphone amplifier used with my PC, I did similar, but connected the other end to the top of the metalwork of the PC. This completely killed a hum problem at high volume settings.
However, fanboi's problem is a little weird, in that it only affects one channel !
SandyK

[fanboi]

Thanks guys for the suggestions.
Brief recap - with all 3 components connected to the same power board - at his home = problem - at my home = no problem.
Previous testing at his home changing speakers had no effect - so not the speaker.
I do not recall everything we tried, but we are both trained in a methodical approach to problem diagnosis and virtually nothing we tried made any difference, including changing preamp, I/Cs, speakers, speaker leads, power leads, power boards and turning off other circuits in the unit to eliminate noise from fridge etc.
An isolation transformer, Jensen CI-2RR, inserted between the preamp and monos reduced but did not eliminate the problem.
The problem was not evident at first installation of the amplifiers but occurred some weeks after.
A suggestion from elsewhere was that building maintenance to the common TV aerial system may have connected the two grounds (aerial and building) introducing noise into the electrical system.
Sandy, he does live in close proximity to the Gore Hill transmitters and that thought was one of the remaining possibilities. (interestingly the side with the problem is closer to the outside wall facing the transmitters)
Items to be checked on next attempt, building grounding, all shielded Blue Jeans I/Cs, toroids/ferrite cores on speaker leads, power conditioner (will try using one).

Miguel, both preamp and monos from same manufacturer so should be matched polarity and no problem at my home, may still be worth a try.

Rowuk, will try measuring for potential difference (and using ground jumper) but again there was no problem in my home.

Just to add to the confusion, the most significant difference we achieved in our first session at his home, was, that by inserting a fairly cheap, discrete component, passive linestage in place of his preamp; we managed to get noise on both channels, with the further side effect that when the volume pot was in the halfway position we had no noise but with the noise rising as the switch was turned to either maximum or minimum positions.
This, to me, suggested RF as a culprit with variation of the volume having some sort of tuning effect, perhaps by varying the capacitance of the attached circuit with the variation in resistance.
Will keep you posted after further messing around.

[fanboi]

Miguel, we both have pretty busy schedules and have not yet had another serious attempt.
Possibly this weekend.
If we find a solution I will definitely post it here for info.

[fanboi]

Well, went over there yesterday, armed with various bits and pieces including a power conditioner (transformer based, passive).
Connected just the preamp, monos and CDP to the power conditioner, unplugged all other TV, DVD etc on the same outlet.
Lights, camera, action - beautiful music - NO problem. Mark one up for the good guys.
Take 2, take power conditioner out of the equation and replace with his surge protected board. Still only the basic 4 components connected and again NO problem.
This is better!
Connect all other bits and pieces originally connected to outlet.
Take 3 - still NO problem.
Divine intervention assumed and advantage taken by attempting to wear out as many CDs as possible - trouble shooting session degenerated into speaker positioning exercise and playing around with plugging the rear firing ports to reduce bass boom etc.
Since he had not tried the amps after finding them to work OK at my place and it is now some months since we had the lengthy day of attempting to resolve the problem at his place with no success the best guess we can come up with is that in the interim some work on the building electrical system has resolved the assumed ground problem.
A happy result for him but we are none the wiser as to the actual cause.

[Deleted]

What an anti-climax !
Let's hope it doesn't come back again !