Transformers (doughnuts in disguise?)

[mcai7et2]

I have almost completed my first power amp. It is a stereo device using two NCC200 boards from Avondale. I am now sourcing components for the PSU, I have two 63V 22000uf smoothing caps on the way, and am about to pick up a nice discrete Schottky rectifier. This leaves the transformer, I have a couple of questions.

First, are toroids always best. This assumption is based on seeing them is a lot of high end designs, but can there be a benefit to using other types?

Second, is bigger always better? Will using (say) the most powerful transformer I can afford be best, or is there the possibility of damage by overpowering other components.

Finally, is there anything to look out for when ordeing the transformer, will any low noise toroid do or are the special brands or types to look for. Also, are there any good reasonably priced sources of these in the UK?

Thanks,

Ed

[MRehorst]

Toroids are not always best. The old standard EI core transformers are fine in many applications.

Toroids are used in high end equipment because they have lower stray magnetic field than EI types. That means they are less likely to induce hum into the active circuits. Mounting them is usually a single bolt through the center. There is the also the ever-present marketing factor- toroids cost more, therefore, they must "sound better".

From everything I've read here, toroids are sensitive to power line noise/DC offsets and will buzz/humm/ etc., if there's crud on the power lines. Additional circuitry is required to prevent this sort of problem. EI core transformers seem to be less susceptable to such things.

Bigger is generally better except from the standpoints of weight, stray fields, and power consumption. Don't underestimate the problems that weight can cause. A BIG transformer needs a VERY sturdy chassis. Transformers are not lossless and the bigger they are, the greater the losses. A really big transformer can make life hell for your power switch, so some sort of inrush limiter circuit or triac switch often must be used. Also, a real big transformer rated for the same voltage as a smaller one will put out a little higher voltage than the smaller one for a specific load. If the small transformer is already pushing the voltage limits of your circuit, a big transformer may just push it over the edge.

MR

[jackinnj]

in the appendix to the application note there is a small monologue by a fellow from Coiltronics regarding stray noise from torroid transformers -- seems that pot-core is best, and you can direct the "noise" eminating from the transformer away from sensitive stuff.

[MRehorst]

Pot cores are used for high frequency transformers in switching power supplies.

MR

[fezz]

both www.farnell.com/uk and www.rswww.com sell ok priced transformers, but the best and cheapest transformers are avalable from rapid eletronics, you will need to contact them on 01206 751166 to recive a catalouge but they are definatly the best supplier in the uk

[dhaen]

Hi,

The UK price of torroidal transformers has dropped over the last couple of years.
This follows an influx of cheap transformers from Eastern Europe and China.
I don't know if the UK winders can maintain competition in the long term.
The quality of the imports seems to be OK at the moment. Can't comment on long term reliablility though.
But I've seen this situation before; where the home industry gets decimated, then the imports turn to rubbish.

Cheers,

[fezz]

Originally posted by dhaen
Hi,

The UK price of torroidal transformers has dropped over the last couple of years.
This follows an influx of cheap transformers from Eastern Europe and China.
I don't know if the UK winders can maintain competition in the long term.
The quality of the imports seems to be OK at the moment. Can't comment on long term reliablility though.
But I've seen this situation before; where the home industry gets decimated, then the imports turn to rubbish.

Cheers,

well, im not sure where these are made, all i know is i have bought expensive transformers from both farnell and rs which have not been as good as the cheaper ones sold at rapid.

most of rapids components are cheaper and the same or higher quality then others. so im happy with them

[dhaen]

Hi,

The Rapid new ones are now made in China. The old range is mostly out of stock now.

The RS are made in various places, but the cheaper ones are from Eastern Europe.

As I said, there appears to be no problem...at the moment.

Cheers,

[Ryder]

A few online articles on transformers if anybody is looking.
Menno Van der Veen knows a little about donuts.

Transformers and Coils (Chokes)
***Jan Rowland, Roll Your Own Power Transformers: http://members.tripod.com/~schematic...m/xformer1.htm
***Patrick Turner, Output Transformer Design Calculations: http://www.turneraudio.com.au/htmlwe...esigncalcs.htm
*******ransformer Basics, from Lessons in Electric Circuits: http://www.ibiblio.org/obp/electricC...s/AC/AC_9.html
***Determining OP Transformer Impedance: http://www.geocities.com/vintage_radio/outimp.htm
***Basics of Inductors: http://www.hills2.u-net.com/electron/induct.htm
* VoltSecond and Mike Lefevre, What does the Transformer Inductance do to the Loadline of the Tube? http://www.siteswithstyle.com/voltse...LOAD_LINE.html
✔Gabe Velez, Transformers: http://members.tripod.com/~gabevee/transform.html
* Michael LaFevre, Transformers and Phase Shift: http://www.magnequest.com/tech2.htm
* Color codes (MarVac Electronics): http://www.marvac.com/funpages/chokes.htm
* University of Surrey, Producing Wound Components http://www.ee.surrey.ac.uk/Workshop/advice/coils/#turn
* David Berning, An Audio Amplifier Design Philosophy: http://www.meta-gizmo.com/Tri/otlology/BERNINGS.htm
✔Randall Aiken, OPT’s Explained http://www.aikenamps.com/OutputTransformers.html
* Richard Sears, Trany Math: http://richard984.tripod.com/transformer_math.htm
* Lundahl, Technical Papers from Lundahl: very short pieces from Lundahl lab: http://www.lundahl.se/papers.html
* Pete Millett, Power Transformers for Audio Equipment: http://www.pmillett.addr.com/images/ax_power.pdf
* VoltSecond, Damping Ringing in LC Circuits: http://www.siteswithstyle.com/VoltSe..._in_xfmrs.html
* Plitron Site:http://www.plitron.com/Pages/sitemap.htm. Link takes you to site index. Under “Publications”. A collection of articles including:
-Secrets of Output Transformers, Menno Van de Veen
-Measuring OPT Performance, Menno Van der Veen
-Theory and Practice Of Widebandwidth Toroidal OPT, Menno Van der Veen
-Modeling Power Tubes and their Interaction with OPT, Menno Van der Veen
* John Atwood (One Electron), Transformer Measurements: http://www.one-electron.com/Trans_Tests.html
* Max Robinson, Understanding the Transformer: http://www.angelfire.com/electronic/...nsformers.html
*** Dean P Currier, A Biographical History of Induction Coils: http://radiantslab.com/quackmed/Deanbio.html
*** Edcor, Transformer Tek Notes: http://www.edcorusa.com/gadgeteer/tek-notes_xformer.htm
Toroid Corp. of Maryland Tech.Bull. #1 ( Application Notes on Rectifier Transformers) and #2 (International Line Voltages and Frequencies): http://www.toroid.com/technica.htm

Craig Ryder