Parts Express DOTD 20-0-20 200W transformer $5

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Well an LM3875 can't pull more then 5A. With a 4ohm load that's what you have at max power. It's about 3A with an 8ohm load.
(btw this is max power consumption at 29DCV)

So you would be better off with a 120va transformer per channel but I think you would have no problems getting this to work.
I have a GC that uses 1 75va per channel, but at normal listening levels only draws 0.3A from each secondary.
 
Peter Daniel said:
With specified dimensions, it can't be 200VA transformer.

I have exactly same size unit from Frost and it is 90VA, which seems more appropriate for that size (its 30-0-30 VAC BTW).

Peter, how sure are you about this? I was always under the impression that the guidelines for rating transformers on dimensions or weight were very broad, like +/- 50% or more (I'm not trying to be argumentative, I know a lot of surplus parts are spec'd by "eye").
 
I've seen many transformers, and 200VA can't be that small.
I just happen to have few 200VA rated EI transformers and they are twice as big.

Of course I may be mistaken, but I rather doubt it.
 

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Peter Daniel said:
I've seen many transformers, and 200VA can't be that small.
I just happen to have few 200VA rated EI transformers and they are twice as big.

Of course I may be mistaken, but I rather doubt it.

Yeah, I guess I buy that. I have a couple of those that I bought last year from PE. They are slightly larger and heftier than other 100 VA xfrmrs I have, but are dwarfed by the 300 VA units I own. But at $5/each, even using 2/channel looks mighty good, if you can live with the cosmetics.
 
Dumb Question on Parts express Transformer

If Peter's assesment is right can I then use one of these transformers for each channel (use only a single secondary winding and leave the other one as is)? Will that get my enough current from a single secondary winding for a sngle channel. I can still use two transformers for a stereo gainclone then.

thanks
vsr
 
I wouldn't see a reason why one of those transformers wouldn't work per channel (it will even work for stereo amp). The only question is how much performance it offers at higher power (and current demands), but for normal use a single TX like this should be fine per channel.

You still need both secondaries (when powering one channel only) as the supply requirement is +/- V.
 
Using both secondaries for a single channel

Probably a very basic question here -

How do I use two secondaries per channel with Brian's GC boards?

The power supply boards have a AC Hot and AC neutral per secondary.

How can I wire up two transformers with two secondaries each to this power supply board?

thanks
vsr
 
Hmm, I hope no one gets hurt putting these gainclones together..
Might be the one down side to these kits, it may just be too easy, for anyone to put one together.

As to wiring the transformer to your gainclone. power board. Lm3875’s need positive voltage and negative. As well as a reference ground. So this transformer is really perfect for that. You take one secondary and hook that to the AC1 and the other secondary and hook that to the AC2 the center tap will be your ground and that gets hooked to the N.

Do not attempt to wire your transformer for 220 on 110 or try doubling these up if you don’t know what you are doing.
:hot: :dead:
 
Sorry if last post not clear. I was referring to using a separate xfmr for v+ and v-. By wiring for 220 and running on 110, should give a 20vct sec on each xfmr. By not using ct and wiring 1 xfmr to ac1 and the other xfmr to ac2. should give equivalent to 40vct xfmr. $10 is a lot cheaper than $42+.
 
To take things a little further, if you wanted to use a separate transformer for each channel it looks like it would be pretty easy to change that dual bridge power PCB into a single bridge one and essentially have 2 rectifying circuits on each power board.

!* the following will only use the left side of the group buy power PCB*!
Hook up one secondary to AC1 and the other secondary to N, I would suggest leaving out the 4.7uf capacitor -V1 and -V2 will be your negative DC power rail, and Gnds will be your positive CD power rail. the center tap on the transformer will be your 0v or ground, just hook that wire to your umbilical or to the power ground on the lm3875 PCB
 
Lee35210 said:
Sorry if last post not clear. I was referring to using a separate xfmr for v+ and v-. By wiring for 220 and running on 110, should give a 20vct sec on each xfmr. By not using ct and wiring 1 xfmr to ac1 and the other xfmr to ac2. should give equivalent to 40vct xfmr. $10 is a lot cheaper than $42+.

You will also cut your VA rating in 1/2 since the amperage that the transformer can pass will be the same, Just that you cut the voltage in 1/2.
Also there is just no need to do this since this one transformer has it all already, the only problem is that it’s just a center tap and not dual secondaries, just jumper from one N connection to the other, on the power PCB to overcome this minor shortcoming.
 
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