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Old 17th August 2005, 07:30 PM   #11
PRR is offline PRR  United States
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> just being sloppy- 18v X 2.
> Sorry again. I have an 18v *2, 250VA tranny
> 18v *2 tranny will not quite give me 25 volts after the bridge and inductor.
> I have not been able to find a 21v *2 , 300VA tranny.

Is this for the plan you attached? That clearly shows 2*28VAC transformer. The DC output will be around +/-35VDC under load.

> was thinking of getting a 25v *2 300VA

Plan says 2*28, but I'd expect many audio amps to also work fine with 10% less.

I've been hoping Mr Pass dropped in. L-C filtering is the art of compromise, and he's faced this problem more critically than I have, than most folks have (because he HAS to give good performance-for-cost to feed his family). He'd also know if an "Aleph 30" makes sense at +/-24VDC, +/-34VDC, +/-36VDC, or whatever you end up with.

> with a continuous draw on the supply as with a class A amp, you can put the inductor as the first thing after the diode bridge (an "L" filter) and instead of multiplying the transformer output voltage by 1.4 or 1.3, you multiply it by 0.9 -it lowers!!!

Yes, IF the inductor is big enough. As you say, if it is not big enough, the output voltage floats up to 1.4*VAC. I don't have the formula handy, but I think it needs a much bigger inductor than you can easily buy in these values. And as you say, it works for Class A, but for Class AB the inductor has to have enough Henries to hold-down the voltage at minimum load (maybe 0.1A) yet not saturate at full load (maybe 2 Amps), a tough design and generally too expensive. Also L-C input filters love to oscillate; Duncan's PSU keeps bombing on wild PIV swings. These may not happen in a real PS due to other losses, still a concern. The nice thing about C-input filters is: if the diode doesn't pop, nothing exciting happens, they just work.
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Old 17th August 2005, 10:20 PM   #12
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I found some useful theory on the following pages.

Some useful equations too.

I guess my biggest problem at this point is finding a coil that can handle the power. I think that I will call some manufacturers (Erse) and see if they have any recommendations.

I had not made the distinction between ac and dc power, in part because I associated the ripple with AC. However, a cap placed before the inductor shunts AC to ground (better shunting with lower resistance cap), conducting DC.

I also had a problem running it on Duncan's program. I will need to experiment more.
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Old 18th August 2005, 12:16 AM   #13
Variac is offline Variac  Costa Rica
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I found some useful toroid core chokes here:

The can contains a ferrite toroid ring with about 12 ga wire wrapped around it, 3/4 encapsulated in epoxy. About 5" in Dia. Those specs on the label are hard to believe, but if it works its gonna be great!!

I ordered 4 - one for each leg of 2 monoblocks.

I recall he has quite a few, but those are really a deal at $20 ea.
They should do the job for most Pass amps. I haven't installed them yet...

Also L-C input filters love to oscillate
No one had mentioned this to me - Thanks, I'll be careful...
"The geek shall inherit the earth"
Burning Amp thread here on diyAudio
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Old 18th August 2005, 02:10 AM   #14
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Thanks. I will check out Apex. . . now to keep it from oscillation.
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Old 18th August 2005, 01:32 PM   #15
DaveM is offline DaveM  United States
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Take a peek at They have hammond chokes under their transformers page. They have open chokes rated at 2.5 mh and 10 amps (500 VDC max) for $20. These appear to be perfect.

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Old 18th August 2005, 04:17 PM   #16
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Thanks. I have a couple of emails out to manufactures to see what they say. I will reprint them if I get a response.
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Old 19th August 2005, 12:16 AM   #17
Tweeker is offline Tweeker  United States
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Anyone have any comments on using something like this as a huge air core on the cheap? 500' of 200C 12awg magnet wire on the spool. DCR should be about .85ohm, the DCR isnt all bad, itl keep it from ringing. Id think itd be good for 10 amps?
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Old 19th August 2005, 01:30 AM   #18
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I don't see why not. There are many online calculators for determining core diameter, windings, etc.

What is the physics behind "ringing" and DCR?
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Old 19th August 2005, 05:37 AM   #19
Tweeker is offline Tweeker  United States
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An inductor with more DCR has a lower q, the resistance damps the ringing. If your inductors and capacitors start really resonating youve got trouble, as mentioned by PRR.

Heres a fairly relavent link on the Damping of Power-Converter Front-End Averaging Filters

And one on Input Filter Design for Switching Power Supplies
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Old 19th August 2005, 06:42 AM   #20
flg is offline flg  United States
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If you investigated tweeker's links, the Coilcraft unit I suggested is for preciisly that same pupose. Although, most of that bis was detailing the fine points of a "Single Ended" input filter. There are also the common mode/differential inductors like I suggest, might be inexpensive and or useful(wiring the 4 pins for maximum inductance and using it as a choke). Not that anything there was wrong or inappropriate but, I work with SMPS every day (not National) and I would not be here if I wanted to read that stuff some more...
I will say, if anyone remembers the electric drill test??? That actually seems like a very good test to me. That guy probably wants some good EMI protection if he's going to be drilling to the tunes!
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
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