What makes a good audio power transformer?

[nania]

fdegrove

I think I can shed some light on this. I passed your email address to her when you couldn't get past her mail filter. Why she felt the need to point that out in here, I don't know.

[fdegrove]

Hi,

My goodness...indeed.
Spamfiltering and all that he?
Got it now.

Gina,

This is an important thread...wish more people would chip in.
Fred, Peter Daniel and so on...

Powerxformers are important.

[analog_sa]

Okay, how important is the quiescent current leakage of a transformer in determining how it will sound? What is an acceptable level of leakage for a 1kVA transformer? Does it even matter at all?

Hmm, what is 'quiescent current leakage'? Leakage inductance? It probably makes a difference but i sincerely doubt if anyone will come up with numbers.
I have to add my agreement to Frank's dislike for toroids. Based only on subjective experience no two power transformers sound the same. There must be a lot of factors and i wonder how much the waveform distortion has to do with it. I have measured some pretty high numbers and this distortion can only make the switching noise of the diodes worse. Apparently using more turns in the primary than the minimum required can reduce it. Whenever i have the time i'll try using a valve output transformer in a gainclone power supply. It will be interesting to observe if the low leakage inductance and low core distortion will benefit the sound.


cheers


peter

[nania]

analog_sa

Hmm, what is 'quiescent current leakage'?

This is defined as the current which can be measured across the coil when it is connected to the mains and is usually less than 100mA. I have also heard it called "excitable current" but I imagine that inductance leakage is the reason it happens. For the purposes of convenience, let's call this current "Aq". I agree that this probably is a factor in determining the sound we get out of a power supply but does a higher Aq necessarily make for worse sound? For those of you that have proposed bigger is always better, have you ever built an amp with a 3kVA variac as the transformer?

[traderbam]

nania wrote: May I politely ask that we limit this thread to discussions of Solid State amps

You are assuming, reasonably, that talk of power output transformers is unique to tubes. This pompts me to ask why this should be so. After all, much of the sonic quality of tube amps derives from the restriction to n-channel devices and devices that are well matched. Presumably, a correctly wound power transformer can be driven equally well, if not better, by a pair of npn transistors, or FETs if you must insist. Has anyone tried this? I'm not convinced that a complementary push-pull semiconductor direct coupled output will always sound better than a homogenous (opposite of complementary?) push-pull with a transformer. Perhaps Circlotron has a view on this.

[ThorstenL]

Koinichiwa,

I have considered this in quite some detail.

1) Low Core Flux - This means using an oversized core and using more primary turns than commonly done, the result is a drastic increase in cost. A larger primary inductance also stores more energy to be released on the rectifier current peaks.

2) Ability to handle DC offset - many mains lines cary a distorted waveform, whose 2nd harmonic content causes a DC offset, which in turn causes some DC offset in the mains transformer core, which is usually already marginal in size to allow for high flux.

3) absolutely balanced winding arrangements with added electrostatic screens - using such arrangements will maximally surpress any common mode noise from the powerline and minimise leakage currents anywhere in the powersupply.

The result is a transformer that in terms of winding complexity, size etc approaches a good quality SE Output Transformer and will thusly also attract similar costs.

There are of course approaches that can be taken to minimise the problems caused by using normal commodity torroidal mains transformers. Such measures are often much less expensive than actaully having small series custom mains transformers made.

Such measures would include primary side common and differentail mode filtering of ultrasonic noise and DC blocking circuits in the primary. Also the use of mutiple identical transformers arranged CAREFULLY with regards to leakage capacitances to the chassis so that balance is attained can be applied.

Using compared to conventional rules heavily overdimensioned transformers can also be used to keep core flux down. A really freaky option is to use multiple transformers with arrangements of primaris and secondaries in series for halving the voltage across each winding. In this case just make sure that the transformers are sufficiently identical and that you always make one final winding up from equal sections of windings of the two cores, to make sure each core/transformer sees equal loads.

I personally feel that discussing and testing such means of better using commodity transformers is more beneficial for most cases where SS gear is concerend. This holds especialy true for most DIY'ers wish to keep costs controlled.

However, given that few companies make commodity priced mains transformers for Valve gear it would likely be of interest to develop better mains transformers for these applications, incorporating the above points. In such cases the cost penalty should be much more modest.

Sayonara

[Peter Daniel]

Originally posted by Kuei Yang Wang


1) Low Core Flux - This means using an oversized core and using more primary turns than commonly done, the result is a drastic increase in cost. A larger primary inductance also stores more energy to be released on the rectifier current peaks.

Would it be advantageous then to use 115/230V transformer with primaries in series with 115V? Providing of course that secondaries meet the load requirements in such setup.

[nania]

Kuei Yang Wang

I personally feel that discussing and testing such means of better using commodity transformers is more beneficial for most cases where SS gear is concerend. This holds especialy true for most DIY'ers wish to keep costs controlled.

First of all, let me thank you personally for the inverted gainclone. The transformer in a SS amp will make up 30-60% of its cost and is probably the best place to start. We may be able to compensate for transformer deficiencies with other components but that will be a design decision. With all decisions, the better informed we are, the more likely we will be to make the right decision. Welcome to our thread.

[joensd]

I bought a transformer from a friend of mine that has an additional "shield-winding" between primary and secondary.
Does anybody know why you would need that?

TIA
Jens

[nania]

Kuei Yang Wang

I am using a 16V 120VA transformer from an old JVC receiver in my Gainclone and have found it to sound pretty good feeding 2x560uF Panasonic FC caps. Oddly, larger caps and transformers have made it sound worse and in some cases introduced oscillation. I concur with Peter's observation that the diodes play a big part in the sound of this amp, but that is currently being discussed in another thread so I won't bring it up here. Do you have any thoughts on what is causing these unexpected interactions?