[Martyn]

Hi, guys.

I'm building an AKSA 100 using Plitron transformers. These have dual primaries and will run off a 120V supply. So far I've installed the transformers in the chassis and have wired them to a double-pole, single throw power switch. I have used one pole to switch the 120V live (hot) input to both primaries of both transformers (I think), and the other pole to switch the neutral line. For the Plitron transformers, this means that I have switched the live line to the black/brown wires of both transformers, and the neutral line to the white/orange wires.

I've tested the transformers' outputs (open circuit) and seem to get the appropriate rail voltages, but I also get a discernable hum when switching on. The hum subsides over a few seconds but is still just audible. I doubt that this will be audible when the top cover is on, but I still wonder whether this is normal. Any comments, please?

I do have a photograph, but I haven't figured out how to insert it yet so I hope the description above is sufficient. Thanks,

Martyn

[andyr]

Hi, guys.

I'm building an AKSA 100 using Plitron transformers. These have dual primaries and will run off a 120V supply. So far I've installed the transformers in the chassis and have wired them to a double-pole, single throw power switch. I have used one pole to switch the 120V live (hot) input to both primaries of both transformers (I think), and the other pole to switch the neutral line. For the Plitron transformers, this means that I have switched the live line to the black/brown wires of both transformers, and the neutral line to the white/orange wires.

I've tested the transformers' outputs (open circuit) and seem to get the appropriate rail voltages, but I also get a discernable hum when switching on. The hum subsides over a few seconds but is still just audible. I doubt that this will be audible when the top cover is on, but I still wonder whether this is normal. Any comments, please?

I do have a photograph, but I haven't figured out how to insert it yet so I hope the description above is sufficient. Thanks,

Martyn

Hi Martyn,

If you only get hum during power up ... don't worry about it!   

If it was continuous, you would have a problem but that sort of hum can be due to "dirty" AC mains.

More important is whether you get any hum with your ears close to the speakers.

Regards,

Andy

[PSP]

Hi Martyn,
I agree with Andy.  I've used Plitron transformers for numerous AKSA amps and Avel-Lindberg for one; they all do this... I hear sort of a twangy, buzzy, groan-like hum for the first second after power is applied, then they're quiet.

Continued low-level hum may be due to dirty mains, DC on the mains (there is an old post here on how to block DC to reduce transformer hum), or maybe you need to tighten down on the transformer mounting bolts just a tad.  Don't tighten the bolts too much or you might deform the bottom of the chassis--now don't ask if that's ever happened to me!!!   

Good luck,
Peter

[AKSAphile]

The only reasons for hum at start up are:


a) high main voltage - that mean that your voltage is couple volts higher than that Piltron`s one


b) DC component on sinusoidal waveform - this DC component make your transformer go "easier" to saturation


c) bad transformer quality - The most important thing in transformer is ferromagnetic core. Ferrom. core transfer flux coused by primary winding and induce voltage at secondary winding, with loses of course.


So the most important thing here is quality of the core and max. flux flowing in ferr. core. If you minimize flux, you get a quieter transformer, but you need more copper winding for desired voltage  which increasing transformer inductance, price (copper is expensive) and weight.

Manufactures for their toroids use low cost ferr. cores at flux density around 1.7 Tesla. That mean their transformer will be cheaper (less copper winding needed) and lighter. Only problem is that flux density is near to ferr. core limit. If you go above (this you actually do with AKSA startup due to charged capacitors which acts like momentary short circuit) you get into saturation and electromagnetic force (Lorentz) shake your core or (and) windings.

How to avoid this:

- use transformer with quality ferromagnetic cores (amorphous core ..) with lower flux density

- the transformer winding and core should be sprayed with special "glue" and then baked to minimize mechanical noise coused by   
  electromagnetic force


Hope this helps

Andrej

[Martyn]

Humm...(sorry, couldn't resist), OK I'm not going to worry about it, at least not for the time being.

Peter, I understand exactly what you mean by a twangy, buzzy, groan-like hum, but thankfully it's nothing like that bad. I've no doubt that it will not be an issue once the top cover is on, as long as it's not a symptom or cause of something more serious. Not much chance of deforming the chassis - I already have a "penny" (fender) washer under the head of the bolt. I'm also sitting the transformers on thin pads of latex foam that I happened to have lying around. I did this to mechanically isolate them somewhat from the chassis, although you seem to be suggesting that a more rigid connection might be better. Maybe I'll tighten them down a bit and see what happens.

Andrej, I happened to check the mains voltage - it measured 119 volts on my low-cost multimeter. Also, I took my measurements with nothing but the switch and the transformers in the circuit, so there were no DC components or capacitors present.

Since none of you is telling me that this is terrible news, I'm not going to worry. I hope to be a lot further along by the end of the weekend. Thanks for your help,

Martyn

[AKSA]

Martyn,

Peter, Andy and Andrej are dead right - this is characteristic of most of the present toroids, and mostly reflects the very heavy inrush current drawn by all toroids as they switch on.  This is a result of the low DCR of the windings - there is not much copper in a toroid - and it's typically around 8 times the max load current for the first two cycles of operation.

If it persists, it is usually because there is a small DC voltage present on the mains - typically half a volt or so in most grids - and this creates a standing current in the windings which in turn produces a skewed magnetic curve in operation.  This means that one half cycle saturates slightly, and this creates in turn a buzzing sound in the core due to the magnetostiction effect - effectively a dimensional shrinking of the core material leading to a slight buzz.

It's all normal, don't give it a second thought.  These inrush currents are very big issues in toroids over about 1200 watts, and a lot of larger equipment has soft turn on circuits.  EI transformers do not suffer these effects to this degree.

Cheers,

Hugh

[PSP]

Just to be fair to Plitron and Avel-Lindeberg:  my "buzzy, twangy, groan" characterization on switch-on is accurate, but the overall sound level is quite low... you would never notice the sound if there was reasonable background noise or conversation levels in the room.  IME, a couple of seconds after power is applied the transformers I have used have been so quiet that I've never been motivated to listen very closely to see whether the sound level is zero or simply very, very low.

Peter

[Greg Erskine]

Hi chaps,

Are you guys saying buzzing transformers are normal? 

I must be deaf, I've only ever noticed one toriod and my autotransformer to make any kind of noise. Better go and do some A/B listening tests.

regards

[AKSA]

Hi Greg,

We are privileged Downunder.  Our mains is clean, but other countries are not so lucky.  DC on the mains is a common problem in the States and Europe, and reflects a dirtier supply (and many heavy users!).

Cheers,

Hugh

[G Georgopoulos]

is the hum coming from the transformer
during power up then the problem may be
the transformer itself this sort of symptom
is rare and could well be confused with normal
power up noise

hum is inherent to power supplies and its components
also hum can have widespread implications throughout
the amplifier through ground loops,psrr,etc



George