transformer VA

[pjanda1]

I'm beginning an LM3875 project. I've read over many of the archieve threads on tranformer selection. It looks to me like 160VA should be more than adaquate for a stereo amp, yet many choose a larger unit. Is there any empirical reason to go larger? I'm specifically looking at the 22v 160va Avel from PE. I also want it small because I want to squeeze it into the smallest possible chassis.

thanks!

[Nuuk]

160va is enough (IMHO) for a good stereo Gainclone.

Larger transformers may have an advantage for reasons that I will not state here.

BUT, and I welcome comments from others here, I have found larger transformers to be a little noisier (ie louder hum).

[peranders]

You could manage with a smaller transformer. Think average power outtake. I think down to 80 VA isn't too wrong.

[pinkmouse]

I have a mono LM3875 running from a 30VA 18-0-18. It's used as a loudspeaker test amp, but goes more than loud enough for most uses.

[chris ma]

Originally posted by pjanda1
I'm beginning an LM3875 project. I've read over many of the archieve threads on tranformer selection. It looks to me like 160VA should be more than adaquate for a stereo amp, yet many choose a larger unit. Is there any empirical reason to go larger? I'm specifically looking at the 22v 160va Avel from PE. I also want it small because I want to squeeze it into the smallest possible chassis.

thanks!

For mee.-self anyway
I picked larger ones so that it can be recycled for other projects in the future. There is not much different in the $ amount for higher VA, but the trouble of looking/buying one and wait for it.. the delivery time....

The Butcher

[mod_evil]

As i transformer VA to Ampers?

I have 127v in the frist and 15-0-15 em second.... of transformer.

Thanks.

[sam9]

You would think VA=Watts but it doesn't. On the other hand that's a way of thinking about it that may be crudely useful. I think it is more accurately considered a thermal rating -- the higher the VA the cooler it remains. Not surprisingly there is a relationship between weight and VA.

While you could determine the VA rating needed to drive the amplifier with a continuous (i.e., day after day non-stop) sine wave at max power as a way to ensure the VA rating is totally adequate-- as a practical matter, normal (or even vigorous) home use requires rather less than that.

Dopes the above sound vague? Good choice of transformer VA rating is a bit vague by nature.

[merlinx76]

Originally posted by sam9

While you could determine the VA rating needed to drive the amplifier with a continuous (i.e., day after day non-stop) sine wave at max power as a way to ensure the VA rating is totally adequate-- as a practical matter, normal (or even vigorous) home use requires rather less than that.

A lower VA rated transformer will have a lower voltage at the demanding peaks of the audio signal which is when the voltage is needed most. While I agree that a transformer wouldn't need to push that current continuously but it still has to maintain desired voltage when the peak current is required.

When you think about it there is not much difference in price of transformers to go up a size or 2 just to be safe. Personally, I don't want to skimp here.

[kmj]

I picked larger ones so that it can be recycled for other projects in the future

A wise choise, for as someone in the tube-forum said "You don't just build ONE amplifier"

[djk]

I used an 80VA 18-0-18 to run a stereo amp with 8 ohm speakers. It sounded a bit 'thin' when driven hard.

Changing to a 160VA 20-0-20 gave a much better sound.

Even though the AC voltage was higher, the DC voltage was about the same with the 160VA unit because it had better regulation.