Transformer and GC question

[41]

I thought I would ask this question in a new thread rather than clutter up the big LM3875 GC thread.

I'm going to buy the Chia Yu toroidal transformer from Apex Jr. but I'm a bit unclear about the wiring, even after reading pages 89-93 of the LM3875 thread here.

The product description is as follows:

"Chia Yu Co Ltd

Input: 2 X 120Vac 50-60Hz 3" Dia X 1 1/2" Tall


Output 42Vct 21V- 0 - 21V

42Vct For 120V input tie the 2 Blacks and Brown together along with
Tying the 2 Whites and Blue together
Output the left over 3 wires"

So if I want 120 volt input and 2 x 21 volt output, which wires do I connect to what holes on the power supply PCB, and does my bridge rectifier configuration change from the standard configuration? Sorry about the n00b questions, but transformers are still a bit foreign to me.

[twitchie]

I have a few of these, but don't have (I guess you are referring to) BrianGT's pcb's (not yet at least).

The black is the return/ground lead for the yellow and red, which provides you with 2 x 21V connections, BUT what I have found is that you cannot connect these to dual rectifier bridges as you can with dual secondaries without modifying the transformer.

It turns out (as others have found out before me) that the centre tap (black wire) is tied together under all that wrapping, and I have mine hooked up to my rectifiers now with the centre wires separated and a lead wire added for a 4th output (true dual secondaries now).

Any method of hooking up with the common centre tap for me before to get 21V, 0 and -21V would cause the tranformer to vibrate and I never saw the output I was looking for.

I may be completely wrong about this and just didn't know that another way existed, but it wouldn't work for me, and I did have both my AC leads fused with 2A slow blow fuses and nobody was hurt in this experiement either.

[Nuuk]

This is very difficult to answer without seeing the wiring instructions.

You seem to have a lot of wires on that transformer:

Black
Black
Brown
White
Blue
Three others (why didn't you say what colours they are?)

This makes a total of 8 wires and suggests that there are two separate secondary windings. So 4 wires would be input and four for output.

Have a look at this and see if it gives you any clues.

I would always suggest buying a transformer from somewhere that can provide exact wiring arrangements, especially if you are new to this game!

[Nuuk]

Twitchie posted while I was writing my reply so please disregard the above.

[fireman]

If I were you, I'd take a look at Brian's users guide where he explains the whole thing.

Personnally, I decided to unwrap the transformers and split the center tap. Unwrapping these babies is a pain (to be fair, it's not that bad) but now I'm sure I'll have everything under control. I'm a newbie and just didn't feel comfortable with the way the center tap is wired. I'm sure it works OK but for some reasons, I just didn't like the configuration.

Now if Canada Customs were to release the kits to Peter, I might be able to build my first GC.

Hope this helps !

[Apex Jr]

I got some feedback on the hookup of these transformers and
the information is on the website. These transformers are
a dual input 115V/230V with 6 wires on the input side and
3 on the secondary which I assumed were center tapped....
Steve @ Apex Jr.

[twitchie]

I'd like to add that after I untied the centre tap, these toroids work great. As Steve says, the AC connections are listed on his site (2 whites tie to blue and 2 blacks tie to brown to get your 2 AC primaries - I braided mine after seeing BrianGT's picture, but may cut them and tie them together and output 2 primary wires in the future)

The only thing I'm not sure of is what the VA capacity of these little guys is. I just finished a stereo LM1875 with one of these, and actually had it running a stereo LM3875 'mule' (my first attempt at a gainclone), but I would bet on something around the 80-100VA range. Is there a (safe) way to measure this?

[41]

Okay, I was hoping that the transformer would be able to handle some bigger speakers than that. I was looking at some Avel-Lindberg toroids:

AVEL Y236503 160VA 22V+22V
AVEL Y236652 250VA 25V+25V
AVEL Y236750 330VA 25V+25V

Now I'm not sure whether I'll use 4 or 8 ohm speakers, but which of these is best suited for a small or medium size room setting? Also, are any of these not going to fit in Peter Daniel's chassis? The 330VA transformer measures 4.8" in diameter and 2.1" high.