What size transformer?

I am building one of Randy Slone's designs...Referred to as his favourite amplifier of any type in his Audiophile Source Book. It is rated as standard at 140W into 8Ohms and 200W into 4Ohms. It has been explained how it can be modified very simply to make 300 or even 400W!

I intend to run this at its intended power (140/200W). What size transformer would I need? I know that P=VI and that VA=VI but what about overheads etc. I studied power for a little while and understand there is a difference between VA and W. Is there a rule of thumb?

I forgot to ask. I have four transformers (2x500VA and 2x1000VA). Here are the specs of each:

2x115V Primary 2x50V Secondary
2x115V Pri 2x55V Sec.

After a bridge rectifier per side and two 6800uF electrolytic caps per side, what would the output voltage be? Any ideas?

I personally work on the principle that your transformer should be rated about 1.5 times more than the rms power you expect your amplifier to output.

So, in your case you are building a 200x2 into 4 ohm.

So that's 400W
so you want 400W x 1.5 = 600VA

Your 500VA should then be good enough for most applications.
1 If you have large smooting caps (or very large) you are able to take out half the VA rating. 100 W DC (not output power) requires a 200 VA transformer(*). Small or no caps The DC rating and VA will be the same. I don't have the formula right know.

Full power out with smooth power supply voltage creates an efficiency of 78%. In real life not more than 60-70%.

My amp with 600 VA, 2 x 50 V (=+-75 V DC) delivers approx. 2 x 500 W / 4 ohms. The transformers can take "normal" signals, music, not continious sinewave at full power.

For music and a monobloc I think a 500 VA will suits you fine.

(*) the caps (large ones) creates lots of reactive power (current spikes or peaks) which only warms up the transformer.
You have to think of the top value of the AC voltage 50 x 1.414 = 70.7 V. You will get a voltage around this due to losses in rectifier and over voltage in. This formula gives you a start value.

If you are nice to the transformer use two bridges and then connect the volatges in series. The other way is to directly connect the transformer windings and then connect the "upper" and "lower" wire to the bridge. The centertap is ground.

Normal colours of the wires: Green to bridge, blue to bridge, red and brown connect to ground.
I think I may have got my wires crossed (pun intended)...I have two bridges - one for the +ve side and one for the -ve side. In addition, there are two 6800uF audio grade caps for each side (4 caps in total per PSU). These will be mounted in Parallel after the bridge. I want about +-50V (I think...will check when I get home)

I have never been confused about PSU design before - but I am now! ;)

Sorry for being slow...

BTW...Are the colours different in the UK...I will check when I get home from work.
Rarkov, you can also look at the parallel thread. +-50 V use transformer with 35-40 Volts.

BTW: Toroids are expensive but the can also take short heavy loads. Use as big transformer as you think is needed. The only you have to worry about is the temperature. 1.5 times the max output power is a good rule but it depends on wether the amp is indended for PA or at home.

8 x 6800 µF seems alright as a start.
The design you intend to make has L-Mosfets on the output stage so you have to take their voltage drops into consideration. These devices are not as efficient as some BJT's.
So you want 1 transformer per channel (monoblocks). 200W RMS into 4 Ohms is 40V rms and 5A rms. This requires 56V(peak) so far (40V x 1.414). The power supply droop will be about 5V so add that on (61V). Since you are using mosfets Mr. Slone suggests a 7% increase in voltage because of there inefficiency, were at (61V x 1.07 = 65.3V). Now we add 1V for the drop across the rectifier. So you need about +-66VDC. Working backwards the voltage on the secondaries is 66V/1.414=46.7V and the current is 5A rms (2.5A a rail). We add 20% for losses and you get 3A rms a rail. So your ideal transformer would be (3A x 47 x 2 = 282VA), so your 500VA transformer could power almost both channels. You can regulate your power supply down to 66VDC as previously mentioned.
transformer for 100W amp


i just bought a 100Wrms (into 4 ohms) single channel amp kit on special from Dick Smiths. First power amp project. It requires a power supply, which i'm doing myself. I have the caps (4x3300uF), and the bridge rectifier. I just need to know what size transformer i need.

The amp requires +-40V, and suggests using a 28-28V. Will this be enough, with the drop across the bridge, etc? How many VA do i need?

I want to use a toroidal. Can anyone suggest where i can order one from? I'm in Tasmania, Australia. Thanks for the help.

Oh, and while i'm here, can anyone point me in the direction of a schematic for a dethump circuit?
I personally work on the principle that your transformer should be rated about 1.5 times more than the rms power you expect your amplifier to output.
For the transformer rating you need to take into account that when a transformer is loaded with a rectifier bridge and smoothing caps, the loading of the transformer is far from optimal. And so are the losses in the transformer.

A general rule of the thumb: VA rating should 2 to 3 times the average power needed. This also depends on the size of the smoothing caps. And more or less on the power duty needed. If you want to figure it out more accurately Pspice simulations can be of great help here.

For a 140W amp, a 500 VA transformer is a good choice, but a 300 VA will be reasonable also in most cases.