Peavey VCL 2 - pwr xformer oscillation?! / stereo mod / etc.

hi all,

just got a VCL2 second hand, works fine overall, but some perculiarities:

1) when the unit is under power, there is an acoustically audible mid & hi freq noise from the inside of the chassis (electronic signal path seems to be clean, though). i attribute that to some mechanical vibration of the mains xformer?!?
i have the US 117V version and am using a step-down xformer since i'm in europe. the step-down is for 200W max but not really heavy -- are there switching step-downs? any possible interaction with the mains xformer maybe?
anyway, i don't consider mechanical vibration any good in the long run. any thoughts...??

2) does anyone have detailed info on the stereo-mod which has been mentioned in another thread?

3) the gains of the channels differ by a few db, even with compression out (threshold turned all the way up). would that be due to unit-to-unit variation of tubes, or something else? once i open the unit, one of the first things i'll be doing is swapping tubes btwn the channels of course, but maybe there are other known / probable issues. in any case, i don't think that the unit left the factory with a few dB's difference between the channels...

thanks for any input,
cheers,
volker

The transformer is vibrating because it's slightly undersized. Peavey had to cut some corners to make their price points on this unit (and the VMP-2), and the transformers do hum. Dipping the whole thing in paraffin might help, but really the best thing would be to replace the transformer with a bigger one (higher VA rating).

It's quite possible that your unit left the factory with one channel a few dB different from the other. The pots are probably not very well matched, and the front panels are not labeleld with any calibration marks (at least on the VMP they aren't). Tubes could be a factor too, if their gain isn't consistent.

Cheers,

Kris

[quote author="DrFrankencopter"]The transformer is vibrating because it's slightly undersized. Peavey had to cut some corners to make their price points on this unit (and the VMP-2), and the transformers do hum. Dipping the whole thing in paraffin might help, but really the best thing would be to replace the transformer with a bigger one (higher VA rating).

Cheers,

Kris[/quote]

sorry to extract on this from the original poster. But Kris, or anyone else wo wants to clime in on this... would you be kind enough to answer one round more about "undersized" power transformers.

Could one say that "hum" is one of the "main warning" signs that a transformer is a bit undersized...??? or could there be many other reasons for hum in a transformer ??? And what would be good design pratice regarding power transformer ratings...double as much dc current as needed ??? or more..???

say if I needed 30V/2amps..would it be best to get a 30V/4 amps transformer..???

Thanks and

kind regards

Peter

Hi Pete, here's a quote of Scott Dorsey from rec.audio.pro regarding transformer noise:

If you have a magnetic material, and you put it under a magnetic field, it will contract in one direction and expand in a perpendicular direction. This is called magnetostriction, and it's useful for all kinds of things.

Unfortunately, it's also a problem in transformer design. A power transformer core is made up of soft iron laminations which are under an alternating magnetic field. So, under power, they are constantly expanding and contracting, and this causes hum.

The solution to the problem is to shock mount the transformer, to oversize the transformer so it's further away from operating in saturation or to use a better core material. Peavey cut corners on the VMP-2, and they mostly cut them in places which would save a lot of money without significantly changing the sound (like in using a cheap power transformer). You don't get something for nothing, and it's a real wonder that you can get something that good for that price.

So, you have to put up with a few little things like this. I'd recommend
trying to mount the transformer on rubber grommets if you are personally annoyed by the hum.

Click to expand...

So, no humming is not an absolute sign that the transformer is undersized, but could be a combination of cheap core material, poor lams, and being run a little closer to saturation than it should be.

FWIW, I tend to double the VA requirement when I choose my transformers. You never know when you might want to change something inside the design (like swap out opamps for higher current draw versions). Plus, I hate the sound of 120Hz buzz and 60Hz hum...

Cheers,

Kris

I used to scoff at highend audio products with "absurdly" oversized trafos, but to some extent it is a successful, if brute force, solution to a number of problems.

The culprit in a lot of this stuff is the nature of the peak currents pulled by diode-capacitor circuits, which have high harmonic content as well as high magnitude. If you throw away a little efficiency by starting with a higher voltage and have some extra R in series with the secondary windings you do substantially better, and the magnetic fields don't radiate as efficiently either.

There is always magnetostriction though, even when you are not saturating. Some core material does it less than others, and damping the vibration by a lot of various substances helps. The latter is much easier to do from the outset, when the trafo is first assembled---look for the description of "vacuum-impregnated."

Toroids tend to be better than E-I cores.

Thank you ;-) Kris.. :sam:

Kind regards

Peter