Stereo Chip Amp - one transformer or 2?

[groyne]

I've been reading a lot of these forums trying to come up with a power supply for my 3886 based stereo amp.(BPA - 200).

almost everywhere, there seems to be the unanimous consensus that you'd need a transformer and bridge for each channel. so far so good.

i just happened to come across Prof. Leach's webpage for the leach amp and there seems to be only one transformer and one bridge for both channels.(i've attached it below).

bit confused here. wouldn't this cause loading? if each channel needs a transformer rated at 6A each, how can one transformer possibly supply the required current?

or is there something i'm missing here? cause if i can go with only one transformer, that will equate to huge saving and i will do that.

[officeboy]

If each channel needs a transformer rated at 6A then a single 12A should do you fine.

People use a single transformer per channel because they are audio snobs (jj) But it does give each channel an output uninfluenced by what is playing on the other channel.

[Variac]

People building Brian Gt's gainclones use one transformer generally, and share the rectifier bridge.

A compromise is to use a separate bridge for each channel.

I think general opinion is that using one transfomer and one bridge won't degrade the sound much at all perhaps no difference- especially if the transformer is big!!

[groyne]

yeah, that would make sense, but this transformer is still rated like it would be used for a single channel.

(sigh).this stuff is probably easier than it seems, but as of right now...isin't there a white paper on audio transformers? or even a forum post somewhere? can't seem to find anything that explains my predicament.

[GregGC]

The GC I build has one 220 VA transformer and 2 bridges per channel. The sound is fast and clear. Though it was an overkill (I don't put more than 15W into the 90dB speakers). I wanted to build the ultimate configuration and see for myself how, if any different it would sound to my ears. If I build a new one I'd use 1 transformer (120-200VA) and 1 bridge for both channels. You may want to increase the value of the PS caps if you are going to push the amp more than 40W/ch. PSRR of the chip is verry good. Build it as simple as possible.


/Greg

[groyne]

thanks greg,
i'm going with 10,000 uF/50V caps. power supply is +/-35 rails so should be okay i guess.

[Nuuk]

I have used separate transformers on all my GC's but when I tried just a single 120VA for two monoblocks the sound has something very good about it!

I since read a review of the Gaincard where somebody said that it sounded better with a single supply instead of two!

I am about to try a regulated supply and have two of them ready, both with a 300VA transformer so I will try it with one transformer and then with two. (regulation stages are separate for each amp)

As ever, 'suck it and see'

[PeteM]

Don't use that pilot light " neon" they are dreadful for noise

If you want an indicator wrap about 16 turns of thin wire around toroidal and use a diode + LED.

[Nuuk]

If you want an indicator wrap about 16 turns of thin wire around toroidal and use a diode + LED.

Pete, please ecuse my ignorance but isn't an LED a diode? Why do we need an extra one? Is it for protection?

[carlosfm]

Nuuk, you need at least one diode (and a small cap would be better too) to transform AC to DC.
It's a dedicated PSU for the Led.