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Audiofng 2nd December 2002 09:40 PM

Transformer Selection
Being new to all of this, maybe this question may seem simple to the more experienced amongst us, but here goes anyway, and please be kind.

I am looking at building Stan Curtis' 60W Class A amp but I am a little confused by the transformer requirements. Here is how it is written in the article: "Requires 500VA Toroidal." "The power supply has to deliver two split rails. The main supply to the output stage is nominally +/-40V at 4 A, derived from the amin transformer windings and rectified by bridge rectifier BR1. This rectifier can get very hot so it is bolted onto the chassis. The secondary supply is a low current +/-50V to power the voltage amplifier stages. The output from the extra windings is rectified by BR2 and fed to smoothing capacitors C12 and C13."

I e-mailed Plitron and they said they only custom wind "production" run transformers but recommended to use two transformers. That is not what is in the original design.

Any suggestions to clarify this point for me??

Thanks in advance.


haldor 2nd December 2002 10:10 PM

Hi Clint,

You need +/- 40 VDC rail voltages which sounds reasonable for a 60W amp. Here is how you can figure out what transformer voltage you need to get there.

- Add 2 volts to allow for the drop in the bridge rectifer = 42 V.

- Add 5% to offset line droop = 44.1 V.

- Divide by the square root of 2 to get RMS = 31.2 VAC.

The nearest standard value is 31 VAC so get a Plitron model number 757041201. This is the standard, dual 31 VAC, 400 VA transformer and should work fine in your amp.


pinkmouse 2nd December 2002 10:15 PM

Hi Clint

Yes, it's as easy as it sounds, just buy a stock 500VA 40-0-40 and connect this to BR1 input, as in the design, and then, (without knowing the design it's hard to be exact on power), use a 50VA 50-0-50 conected to BR2 input.

:att'n: Each transformer should be protected on it's input with an appropriate fuse for your local mains supply:att'n:

haldor 2nd December 2002 10:42 PM

Sorry Clint,

I blew off the part about needing a set of +/-50 Volt rails as well. Strange sounding design, what do they use +/- 50 V supplies for?

A second, small transformer is going to be the simplest and lowest cost solution. Just follow the same procedure as outlined for the +/- 40 V supplies. I come up with dual 38.6 VAC secondaries. The 30 VA version will output 0.38 A (PN 017035201). The 50 VA version will output 0.66A (PN 027035201)

Is this amp class A ? 500 VA seems excessive for a 60 W amp. If you really need 500 VA on the +/- 40 supplies then order (PN 087041201)

Pinkmouse. If you use a dual 40VAC transformer you end up with +/-54 V supplies and dual 50 VAC will result in +/- 68V supplies. Remember to convert from RMS to peak use square root of 2.


pinkmouse 2nd December 2002 10:48 PM

Hi Haldor, yes, you are correct, i just misread the Q, and thought the transformer was 40-0-40, not the rails. I was going to edit it when your post came through, but I was also outside the 30 min limit.

Audiofng 3rd December 2002 12:09 AM

Hey guys,

Thanks for the responses. The design I am looking at is here:

The design calls for a 500VA trans but if I go with dual trans will I need that much just for the output stage? Like I said I am fairly new to this and am just trying to learn.

You are being very helpfull.


NickC 3rd December 2002 01:11 PM

my two cents
with 2*40ac volts with should get 56 dc unloaded
to bring the down the voltage
1) install power resistors before the transformer bring down the primary ac
2) install diodes between power supply cap
-lets say u guys are using 10 caps normally we connect them in
parallel to each other to increase cap size. Put the diode
between the connections and u will get a 0.7 volt drop plus the caps are isolated from each other.

for stan curtis amp i feel u should use a bigger transformer something like 1000va i think plitron should stock this . why not go for the JLH. easier more support(Geoff,chrisma,me and much more people) bigger amps require more skill, exprience and time plus they are much cheaper to start and rebuilt if the projects fail

peranders 7th December 2002 04:24 PM

Re: Transformer Selection

Originally posted by Audiofng
...get very hot so it is bolted onto the chassis. The secondary supply is a low current +/-50V to power the voltage amplifier stages. The output from the extra windings is rectified by BR2 and fed to smoothing capacitors C12 and C13."

You don't have to have extra windings. Just use one or two small transformers and then simply add the voltage. I have used this technique in my QRO power amp. +-63V "power" plus +-20 V "drive power" = +-83 V and then stabilized down to +-70 V.

I think you must have 200 VA at least but more doesn't hurt. The VA depends on if the amp is "pure" class A or class AB.

Thumb rule: If you have BIG smoothing caps the VA rating should be twice the DC power. 100W DC power (feed to the amp) = 200VA at least.

No smoothing at all 100 W DC = 100 VA.

Why? Due to high current spikes (charging the smoothing caps) which creates lots of reactive power which warms up the transformer.

djk 7th December 2002 07:58 PM

A 30-0-30V 500VA seems about right for a stereo 60W class A amplifier. The efficency of a class A amplifier is very low, at best 50%, and you will want some margin for safety. If you run any transformer at full rated power for hours on end it would run at the maximum operating temperature it was designed for, anywhere from 85~105*C. This would be a change in temp of 60~80*C above ambient. If you derate to 50% the change would be 30~40*C, or 55~65*C assuming 25*C ambient.

To get the 50 volt tier for the front end you do not need a second transformer. Just wrap a dozen or so turns over the existing core and measure the voltage. Since you are only drawing a few mA 20~24 ga bell wire would be fine. To get the extra 10V you will need about 7V RMS. Unwind a turn at a time until you get the required voltage. For most toroids this will only be 5~10 turns. Add a second section of the required turns and wire to each end of the existing secondary.

You now have a 37-30-0-30-37V transformer that will give 50 and 40 after being rectified and filtered.

Audiofng 7th December 2002 08:12 PM

Transformer Requirement
Thank you all for the input. It has been most helpful. I emailed my requirements to John at Victoria Magnetics. He replied quite quickly that he can supply exactly what I require.
I am glad that it wasn't as commplicated as I was making it out to be. Hopefully with some time and alot of your help I will eventually be confident enough to go ahead and build something. I have a problem with needing to know how and why before I do anything. So hopefully everyone will be patient with me and my "newbie" questions.

Thanks again

Any other input is gladly encouraged.

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