elliot sound project 27b question

[AndrewT]

Hi,
the 50Vac CT 250VA transformer should just about power two 100W into 4ohm channels.
Expect about +-35Vdc from the PSU when the amps are connected and biased to ClassAB.

[rtill]

Is it the correct amount of Amps? 5 amps just seems high to me.

[AndrewT]

your amplifier only draws as much as it needs.

[wg_ski]

Originally posted by rtill
Is it the correct amount of Amps? 5 amps just seems high to me.

5A is probably right for continuous sine wave. Modern practice (for both home and professional equipment) is to size the VA rating of the trafo for current draw with 1/8 power with pink noise signal. Buy this amp and the trafo will be an amp and a half if you're lucky.

[AndrewT]

Originally posted by wg_ski
Modern practice (for both home and professional equipment) is to size the VA rating of the trafo for current draw with 1/8 power with pink noise signal.

I hope this is not you suggesting we lower our standards to match PMPO?

[wg_ski]

Of course not. But a 3 amp transformer will work, will make the amp put out full power, and probably be better than what you get in a store-bought unit. If you played a continuous sine wave, it would likely overheat in maybe 15 minutes. Chances are the heatsink would boil spit long before then.

Modern amps, even ones that put out full advertised power, have the trafos undersized. Sizing it for full sine wave is overkill. The real answer lies somewhere in between. Personally, I size mine for 1/3 power at 4 ohms and 1/8 power at 2 - continuously. That's still about 50% more VA than typical, which is 1/8 power at 4 ohms.

[Juergen Knoop]

Originally posted by wg_ski
Sizing it for full sine wave is overkill. The real answer lies somewhere in between.

Yepp!
regards

[wakibaki]

Here's what Rod says 'The transformer rating should be 150VA minimum - there is no maximum, but the larger sizes start to get seriously expensive. Anything over 250VA is overkill, and will provide no benefit.'

Transformer choice is as much about experience of the design in question in it's intended venue as it is about calculation.

The amplifier is intended to work down to 4 ohm load.

Since the current draw woud be greater @ 30vrms the 3A 180VA transformer is getting a bit marginal at 4 ohms, plus it leads into other complications.

It's much easier to build an amplifier exactly as someone has built it before in these circumstances, because if you have problems you have to debug there are less potential sources of error and more chances someone can tell you spot voltages.

As ostripper points out, the distortion characteristics would be more pronounced with the 30v transformer, but this is not necessarily desirable in a solid-state instrument amplifier. Distortion is easy to add, and if you can't get a clean sound then it can be very frustrating. Even if you play dirty now, this may not always be the case.

So your 5A choice might be slightly more rugged than is absolutely necessary but it is a good conservative one which improves the likelihood of your building a versatile, robust and satisfactory working amplifier.

You can save a few bucks and go right down to 3A if you can find one conveniently.

w