My tube amp causes isolation transformer to buzz

I guess what I’m saying is it’s easier said than done

last_lemming OP
Geoff,

I wish this was as easy as you think it may be.

>>>>>I don’t recall saying I thought it would be easy.

Geoff,

I wish this was as easy as you think it may be.

Here is what I did:

Step 1: unplugged everything in room but amp. Went to house circuit breaker, turned off entire house but room with amp. Remember, the only thing hooked up in house is the amp.

Step 2: Amp is plugged into power outlet - only the amp - no interconnects are connected to amp - no other components are even plugged in. Speaker cables are connected to loudspeakers. Amp on, no sound from speakers, almost completely quiet with barely a hit of the buzz.

step 3: plug in interconnects to amp, but NOT to preamp = buzz - relatively loud. Remember, nothing else in house has power but amp.

step 4: plug in interconnects to amp and to preamp = buzz - much quieter bit still audible, can be heard from sitting position. Also, Amp and Preamp plugged into same outlet and a ground wire connected to chassis of amp and preamp.

step 5: plug amp into Variac - amp still connected to preamp via interconnect = buzz almost completely gone. I have to put ear right next to speaker to hear it.

Ground is good at outlets, that is confirmed and cables are shielded.

I am confused when you say you can't find the cause of the ground loop.

Disconnect all inputs to the amp.  Turn on amp.  Noise through speakers?
yes? amp is the problem.  Try a different outlet or use a conditioner. No noise? Plug in input from pre-amp (with no other connections to pre-amp).  Noise?  yes, pre-amp or interconnection cables are the problem.  no noise?  not the pre-amp or cables from pre-amp to amp.

Plug a source into pre-amp.  Noise?  yes?  Source or interconnects from source to pre-amp the problem.  no noise?  plug different source and so on.

Check to see if outlet has ground connected.  

some cables aren't made very well, so better  cables with better shielding helps.

Also, don't run interconnect cables near power cables.

I agree with many hear that if it is a ground loop.  Find the cause and fix it.  masking the sound with cheaters (not ever a good idea because you actually lift the safety ground and now in the event of a fault, you, your kids, your pets, etc.  become a better path to ground).

take the time to find find the problem.

enjoy

Yuk, yuk! 😬

It’s probably obvious but transformers produce magnetic fields that produce....you guessed it, distortion!

I dunno, without magnetic fields there would be no loudspeakers, so I'm actually a big fan.

It’s probably obvious but transformers generate magnetic fields that produce....you guessed it, distortion!

“The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy”

Sure. Harmonics are the result of distortion. The big nasty is the 5th harmonic (300Hz) which is the one that causes the problems. Any transformer will make more distortion as you put more energy through it, since they are inductive devices. Fluke Instruments published a paper on this topic back in the 1990s; I'll have to poke around to see if I can find my copy, but I think its online too. Here's one I found that deals with this topic in greater depth:
http://support.fluke.com/find-sales/Download/Asset/1260362_6001_ENG_K_W.PDF

So can someone explain the “why” to this statement:

“The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy”

There is a gage on the front that as you increase the voltage goes up. It is labeled in “volts” and has a green “safe” zone (approx 105-120v).
Is this what you mean?

Yes.

I don’t know what the accuracy of that particular gauge/meter is, of course, and personally I would want to verify the voltage with a good digital multimeter. But I’d feel confident that it is accurate enough for safe operation of the amp, although perhaps not accurate enough to be fully optimal in terms of sonics.

Good luck. Regards,
-- Al

There is a gage on the front that as you increase the voltage goes up. It is labeled in “volts” and has a green “safe” zone (approx 105-120v).
Is this what you mean?

I originally bought the Variac when I replaced the main caps in amp so I could slowly bring up the voltage the first time it was turned on.


Btw. My room is very small and I don’t use that much power (watts) to power my speakers.

also the Variac is temporary, though I am thinking about a 1000w transformer. I cannot find the ground loop. 
I’ve gone as far as turning everything off at the elec panel and still get it. 

The full specs on the power consumption of the Classic 60 are:

105-125VAC 60Hz (210-250VAC 50Hz) 500W at rated output, 700W maximum, 370W at idle.

I suspect that the 700 watt number doesn't reflect normal usage, but rather is approximately based on the current rating of the 6.25 amp mains fuse that is used in the design. (6.25 amps x 120 volts = 750 watts). And you are most likely using the amp's rated output power capability only rarely, meaning that the amp would be drawing significantly less than 500 watts most and perhaps all of the time.  So I suspect that you are not exceeding the 50% guideline Ralph cited above (corresponding to 400 watts for the particular Variac) by much if at all.

A basic question, though: Does this Variac have a built-in voltmeter? Or if not, how have you adjusted its output voltage?

Regards,
-- Al

Variacs are not built with the intention to be particularly quiet. The only one that was to my understanding was made by MacIntosh back in the 1960s.


Also something to be aware of with any isolation transformer- 50% of rated capacity is a safe limit. The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy.


I think Erik might be on to something. If you have a ground loop, its not uncommon for an isolation transformer to fix it. But its usually better if you find what is causing the ground loop and fix that. Even if they don't hum or buzz, ground loops degrade the sound overall by increasing intermodulations.

It does sound like you might have a ground loop, but testing on a variac isn't how it's normally diagnosed.

Disconnect all inputs from the amp, see if the hum persists. Alternatively, lift the ground with a cheater plug (temporarily only).

Miller,

The unit is not modded. 350 watts is per spec.
it uses 700 watts at max output., again per ARC.

so the buzz being present in the Variac only when there is a load is normal? Because when I plugged a light into the Variac to see if I could replicate the buzz, did not buzz.  The light didn’t have a ground prong on the plug however..  though I don’t know if this means anything. 

Oh, and a variac isn't an "isolation transformer," it's an autoformer where the pickup wiper is on the same coil.

It's buzzing under the relatively heavy load as the coil is energized by the alternating current. You could try the ubiquitous  red $110 20A 2000watt model. 

Here's a question: do you know how transformers work?
A certain amount of buzz is inherent in the design.

So the real question then is, what exactly are you hearing? Since the first "buzz" was eliminated by plugging into a different path that means it wasn't buzzing at all it was ground loop hum.

What you have now is probably very normal amount of transformer hum. But could also be DC offset. Or ground.

But yes, the answer to your question is yes.

I find it interesting that your 60 watt amp draws 350 watts at idle. Was it modded when you got it, or what did you do?