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 nigelwright7557 10th May 2008 12:26 AM

Transformer VA versus output power VA

Would I be right in thinking that a power amp can only put out half (ignoring losses in output transistors) of the transformer VA rating into the speaker ?

I am using a 1000VA transformer which is 0-40 0-40 each winding at 12.5 amps.

If the output voltage is 40 volts RMS then that only gives 500 watts RMS.

If so where is the other 500VA going?

 giacomo_pagani 10th May 2008 01:03 AM

Re: Transformer VA versus output power VA

Quote:
 Originally posted by nigelwright7557 Would I be right in thinking that a power amp can only put out half (ignoring losses in output transistors) of the transformer VA rating into the speaker ?

It's not the correct way to look at the problem. One circuit will deliver X power on a given load at a given input, and, depending on its circuit topology, class of operation, and so on, will have a certain efficiency (a number between 0 and 1, always lower than 1). If you calculate X/efficiency you have the power requirements from the supply. Then you size the transformer accordingly. To stay safe and be happy audiophiles within our particular applications, it's usually recommended that you choose a transformer with twice the power you need. So, if your circuit is supposed to draw 200W max, you pick a 400VA transformer.

In some other applications, technicians would laugh right in your face, if you proposed such an oversizing, but here we want to keep the supply particularly responsive and far from its limits.

Quote:
 I am using a 1000VA transformer which is 0-40 0-40 each winding at 12.5 amps. If the output voltage is 40 volts RMS then that only gives 500 watts RMS. If so where is the other 500VA going?
You have two secondaries there, each one is 40V @ 12.5A, so you're simply forgetting one of them. They are 500VA each .

If you prefer to think this other way, put them in series, you end up with 80V, and those, multiplied by your 12.5A rating, still give you 1000VA.

Giacomo

 Mooly 10th May 2008 07:29 AM

The VA rating is assuming a resistive load, which a bridge rectifier and caps is not by the way. The 1000 VA is a max for that transformer and, it's like saying a car engine is rated at 100 BHP but at 20 mile an hour your using 10 BHP. The 90 BHP has not gone anywhere, it's just not being used in the first place.
Also power in to the amp must equal power out, but that means all loses, heat mainly.
Your transformer on it's own will draw very little power, just enough to maintain the "core magnetic field".
Regards Karl

 Merlinb 11th May 2008 05:42 PM

As a rule of thumb, for a conventional transformer + rectifier + reservoir capacitor, the VA rating of the transformer should be roughly 1.5 times the continues power demanded by the load. (2 times if you want to over-engineer, which is good.)
In your case, using 1000VA transformer, you should demand no more than 1000/1.5 = 666W continuous.
If the amp is class A, and therefore 50% efficient (in an ideal world!!) then you could expect 666 x 50% = 333W into the speaker.
Obviously in real life you won't acheive this, so maybe 280W would be more realistic. However, for normal music you won't be using so much power continuously, so perhaps 300W isn't so unrealistic after all?

 nigelwright7557 11th May 2008 07:29 PM

Quote:
 Originally posted by Merlinb As a rule of thumb, for a conventional transformer + rectifier + reservoir capacitor, the VA rating of the transformer should be roughly 1.5 times the continues power demanded by the load. (2 times if you want to over-engineer, which is good.) In your case, using 1000VA transformer, you should demand no more than 1000/1.5 = 666W continuous. If the amp is class A, and therefore 50% efficient (in an ideal world!!) then you could expect 666 x 50% = 333W into the speaker. Obviously in real life you won't acheive this, so maybe 280W would be more realistic. However, for normal music you won't be using so much power continuously, so perhaps 300W isn't so unrealistic after all?

I am using a class AB amp with 6 MOSFETS on the output.
The MOSFETS dont quite allow rail to rail operation due to VGS being a couple of volts.

I run the amp into 4 ohms so this gives 12 amps at 40VAC which gives approx. 480 watts RMS.

 nigelwright7557 12th May 2008 12:13 AM

Quote:
 Originally posted by nigelwright7557 I am using a class AB amp with 6 MOSFETS on the output. The MOSFETS dont quite allow rail to rail operation due to VGS being a couple of volts. I run the amp into 4 ohms so this gives 12 amps at 40VAC which gives approx. 480 watts RMS.

I think the reason why Iam only getting half teh transformer VA is because the transformer is providing 2 lots of 40vac at 12 amps but the output is only one of lot of 40vac at 12amps.

 djk 12th May 2008 08:38 AM

A 40-0-40V 1KVA transformer will give about ±57V no load (with filter caps).

With a pair of 10,000µF filter caps the power factor will be about 0.7, so there will only be 700W for the amplifier circuit to consume.

Lateral FETs in class AB with moderate bias make for about 60% efficiency, or about 210W/4R per channel.

This is about what a commercial design (Hafler amplifer) did.

 AndrewT 12th May 2008 07:44 PM

Re: Transformer VA versus output power VA

Quote:
 Originally posted by nigelwright7557 Would I be right in thinking that a power amp can only put out half (ignoring losses in output transistors) of the transformer VA rating into the speaker ?
no,
the amplifier can put out a lot more than half the VA rating of the transformer in the short term due to the transformer's tolerance to short term overload.

 nigelwright7557 18th May 2008 02:57 PM

Re: Re: Transformer VA versus output power VA

Quote:
 Originally posted by AndrewT no, the amplifier can put out a lot more than half the VA rating of the transformer in the short term due to the transformer's tolerance to short term overload.

Its not just about what the transformer can give but also about what the speakers take.

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