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Old 26th November 2003, 04:41 PM   #1
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Question Double up transformers for IGC

Iím looking to build my first amp, and after looking at the some of the passdiy stuff, Iíve decided to try something a bit simpler and do a gainclone.
Problem is Iíve already used up almost my entire budget on connectors, blackgate caps, and riken/caddock resistors.

And I still need a transformer, I have a few options that I can do for really cheap, I have a 8-0-8 tranny from a 400va APC ups, that I could use, but I was really hoping for more then the 22V that would give me. (although this will be run into 4ohm speakers)
I also can get my hands on some pretty cheap ($2) 24V 95va trannys, but they only have 2 secondary wires (not center tapped),
Could I link 2 of them up in series so that it was like a center tapped 48V transformer something like shown below?

Thanks

Click the image to open in full size.
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Old 26th November 2003, 06:12 PM   #2
Mad_K is offline Mad_K  Norway
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yes you could do that. but even better= use two bridges like this http://ampchipdiy.com/invertedLM3875_r2_1.gif

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Old 26th November 2003, 06:22 PM   #3
PRR is offline PRR  United States
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> Iíve already used up almost my entire budget on connectors, blackgate caps, and riken/caddock resistors.

Lesson: Don't blow the budget on trimmings and run out of cash for the meat. Meat and potatoes with old cheap parsley is better than good parsley with no meat.

> Could I link 2 of them up in series so that it was like a center tapped 48V transformer something like shown below?

As drawn, it might work, but balance could be unstable. And you end up with a 24VCT winding, good for about +/-17VDC. However I'm really not sure it will be stable.

The same two 24V transformers, with Primaries in Parallel and secondaries in series, would give a solid 48VCT, good for about +/-34VDC. However that is rather a lot for most chips and heatsinks.

$2 for 95VA is a very good deal. But you can find 24VCT transformers at surplus prices if you look around.
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Old 26th November 2003, 06:24 PM   #4
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Hah, thatís the schematic that Iím using to build my igc... oh the irony.

So a transformer with dual center tap is really just 2 transformers put together.

So if I have 2 95VA (24v 3A) transformers, do I get 190va after? (48V 3A)
And thatís plenty for a two-channel Gainclone right?
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Old 4th December 2003, 05:45 PM   #5
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Question Single no center tap transformer

ok, so the 24V transformers didn't work out, but i got some 18V ones from this surplus place, two of em gives me about 52V DC

I hooked er up first thing this morning, and it ran great, at low volume, I'll be running this into 4ohm speakers though, and well, 52V is way to much for LM3875. After playing with the Overture_Design_Guide13.xls I found that 25V is more what I should be shooting for.

So without further ado I give you another wonderful drawing, I think this address my problem of not having a center tap on these cheap transformers I got. But I donít know if you can parallel rectifiers.

Click the image to open in full size.

And Iíd love to just try this and not have to ask here, but Iím a bit worried about blowing stuff up, and after a couple of hours of searching I havenít found much info on a setup like this. And tinkering with PSU designer II hasnít gotten me anywhere either.

Thanks for al your help.
This is a great forum
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Old 4th December 2003, 08:56 PM   #6
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blarg, looks like the Overture_Design_Guide13.xls uses the voltage for each rail, not the total voltage for it's calculations. So basically I'm dead on for voltage I wanted, about 50V total.
(Have I mentioned that this is my first amp? )
And I think I've covered that I'm using this design http://ampchipdiy.com/invertedLM3875_r2_1.gif. The only difference is that I have a 50K pot instead of the called for 100K, I don't know why I did that, but I seem to remember a few posts that said 50K would sound better.
Anyway, with the pot turned all the way down I still get quite a bit of volume (hooked up to old car 6x9 w/ output from a dvd player) and turning it up results in much squawking, and popping, what I think is the amp clipping.


Is there anything Iíve missed? Or suggestions anyone might have?
Could I have the pot hooked up wrong? Input and ground to the sides, and the center continuing on to the amp.
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Old 4th December 2003, 09:09 PM   #7
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Default pots

I use 25k pots (because i had them in my bin) seems to be working just fine..


I am at the end of the line with the voltage, 63 volts to be precise and 8 amps (toroid)

I hope the LM's last !


J-P
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Old 4th December 2003, 09:14 PM   #8
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Default Re: Double up transformers for IGC

Quote:
Originally posted by officeboy
I also can get my hands on some pretty cheap ($2) 24V 95va trannys, but they only have 2 secondary wires (not center tapped),
Could I link 2 of them up in series so that it was like a center tapped 48V transformer something like shown below?

Thanks

Click the image to open in full size.

it will work, but you wouldn't get 48v. Instead, you will get 2x12v, as now the voltage on each primary will be halved. Also, the "ground" on the secondary wouldn't be stable, as someone else had pointed out.

I would parallel the primary instead. You wouldn't get 24x1.4v. Instead, more like 24vx1.1 - 1.2 (26-28v) under load.
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Old 4th December 2003, 09:41 PM   #9
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Default Re: pots

Quote:
Originally posted by uvodee
I use 25k pots (because i had them in my bin) seems to be working just fine..

I am at the end of the line with the voltage, 63 volts to be precise and 8 amps (toroid)

I hope the LM's last !

J-P
And are you able to get a full range of volume control from them?
Thatís the problem I seem to be having


And Millwood, thanks for your post, it didn't take long for me to see the error in that first drawing. I've ended up with something more like this right now.
Click the image to open in full size.
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