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john65b 29th September 2006 11:05 PM

Transformer question for Chipamp...
Alright, so I picked up this old receiver on ebay cheap for the torroid and chassis..nice buy btw...

Anyway, got a torroid question. The label says 40V 0V (0V) 40V and should be around 400VA. I want to try to use it for a LM3886 and get 20VAC out of on...

Now here the tricky part...all four wires (blue black black blue) all have continuity and thus are the same winding. I get 40V between either of of the blue and black wires.

If I connect the 40V and 0V to the AC side of a single rectifier it should be the same as if I had 20V and 20V on the AC side of the rectifier, correct?

I tried it on a rectifier and it shows (after the smoothing caps) about 28VDC, the same as if I had 20V and I missing something?

What would be my virtual Ground if wired this way for a dual 20V power supply? Or is this the reason why this torroid cannot be used?

john65b 30th September 2006 12:07 AM

When I measured DC voltage across smoothing caps, I think I connected the Chassis ground as GRND between the + and - rails ....

I did this a few months back and I cannot remember exactly how I did it...

jerishi 30th September 2006 12:38 AM

I'm wondering if you're playing with a half-wave versus a full wave rectifier. Are you trying to reduce transformers output to please the 3886 requirements? F.W.I.W.... just got home(am a carpenter) and probably have little to offer here, none-the-less.., I'm also curious how you might use a salvaged transformer, with different specs than ones' specified (for instance) with the 3886 chip.

john65b 30th September 2006 12:44 AM

I am using a full wave rectifier. Yes, I would like to use this transformer if I can. At 40V secondaries, it is too much for the LM3886 straight up, but when connected as I have outlined...

I would like to dig up the rectifier and caps to try it again, but my basement is getting refinished and I can't get to it at the moment. I do have the torrid within arms reach...It came from a KLH KL2400. These receivers are quite ubiquitous on ebay - very cheap, and they are supposedly Chinese "fakes", as I read, KLH never made a model KL2400...

I took apart the receiver board and it was very cheap, but the torroid and soft start board looked reuseable...

Go figure...

john65b 30th September 2006 09:03 PM

Or how 'bout this...

Is there a way I can drop the 40V 0V 40V to a 24V 0V 24V using a couple of LM338 Voltage regulators BEFORE the smoothing caps? The torroid is 5A, so the LM338 is good for 5A.

Between the Rectifier and smoothing caps, the voltage should be lower than 40VAC rectifier input, correct? Putting it after the smoothing caps is no good, cause the 56VDC smoothing caps output is too high for the LM338, 40V is max... My guess is about 37VDC at this point, which would be perfect. Just wanna to drop it 10V - 12V.

I also could put the LM338 before the rectifier (on transformer output), as the LM338 regulates DC and AC.

But I remember some saying that you cannot use the dual LM338 regualated system on a center tap transformer, only a split winding. Is this true?

Don't see why I can't just use the centertap as grounds (0V) for both LM338 rails...

I have used the LM317's many times to regulate 20V laptop power supplies down to 13V for my Tripath amps, and think this should work here too...


Nordic 30th September 2006 09:41 PM

I think those are only good for 1.5A, you will need a couple of times that....

I think wanting to build a gainclone is one thing, wasteing a perfectly good transformer that you could use for something more substantial is another... I mean it in the nicest way though... its just trannies are expensive....

john65b 30th September 2006 10:32 PM

The LM317 is good for 1.5A
The LM350 is good for 3A
The LM338 is good for 5A.

Correct me if I am wrong.

I have two of these 40V 0V 40V, and would like to use one of them on the "special" LM3886 I am building.

I planned on using one for the UCD180 I had, but was pushing it with 40V too, so I bought another toroid at 30V 0V 30V.

Just want to use what I have as opposed to buying another trannie...that's all...

Nordic 1st October 2006 08:40 AM

Roger that....

Are those transformers 40v AC? If it is, you will have 56.6V - diode drop.

driving 8 ohm speakers at peak with 40V would require 5A before you start adding extra to deal with real world efficiency..

4 Ohm loads are almost not an option for you...

I'm not too familiar with the other chipamp brands, but still wonder if there isn't somthing that could use those rails with less compromises.

AndrewT 1st October 2006 10:01 AM

my short answer is buy the correct transformer.

The building of a high power regulator (that sounds good) will be more complicated than building a good sounding chipamp.
This extra complication and cost (and risk to the sound quality) completely negates the reason for going chipamp in the first place.

Either, build a discrete amp that suits 2*40Vac or buy 2*25Vac (or 25+25Vac).
BTW 40Vac can support 120W to 150W into 8r. It will need to be about 160VA to 250VA to do one mono channel. If you regulate it down to a suitable voltage for a chipamp, then the losses in the regulator and the amplifier result in less output power but still require the same VA rating for the transformer i.e 160VA to 250VA for 50W to 60W output.
Double these VA ratings for stereo and double again for 4ohm duty.

JesseG 1st October 2006 07:55 PM

From John65b


Just want to use what I have as opposed to buying another trannie...that's all...
A couple of points

You can (use the trafo you have) - If you are deterimed to run at +/- 20V, you just need to get hold of a couple of 8 - 10 mH chokes - one for each side of the PS - and use LC input filter instead of RC. THis will have 3 effects... 1) it will bring down the voltage of the trafo secondaries by about 25%, 2) it will do a way better job of filtering the ripple and 3) it will provide much better current stabilization.

However, you don't really need to. I am running a LM4766 chipamp in native stereo mode (not bridged or paralleled) off a 38.5-0-38.5 MONSTER conventional trafo. Because of the really high PSRR of National's audio chips (God bless 'em) mine is a simple bridge>really big caps(12,000uF)>bypass caps(0.01uF) and that's all. DC Voltage to the chip is +/- 34V. And, it sounds wonderful! A couple of very knowledegable hifi buddies (both tube nuts) have said: 'that's the best sounding chipamp I've ever heard'. This compared to Peter D's 4780s and a host of 3886s.

Seems that most people don't read the National spec sheet very well - they specify power input range of 20 to 80 volts. That is
+/- 10 V to +/-40V. Since the basic technology for all of the National Overture chips is the same, the same electrical rules and parameters apply, except for the current needed to drive them to performance.

I should add that my LM4766 rig barely warms up the heatsink running at half-throttle for hours at a time.

So, I am a big fan of running these devices at far higher voltages than seems popular. Considering that the chip costs about $5, build one a try it. It won't blow up and it will sound much better.

Lucky you is what I say. If you have any spare 40-0-40 torroids, I'll take a few ;)

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