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Re: THANKS JOHN GREAT POST; drlowmu, why.........

Posted by drlowmu (M) on July 17, 2006 at 12:45:43

In Reply to: THANKS JOHN GREAT POST; drlowmu, why......... posted by radiance on July 17, 2006 at 08:21:20:

Radiance,

I'm not very technical but I will try to answer you. Whenever you employ a cap input filter, you put NASTY SPOKES on the B+, probably because the diodes are uncontrolled. So, you always want to control the diodes, avoiding problems at the outset ( rather than cleaning it up later on ), by using a proper choke input filter.

Now the real question becomes....WHAT sort of choke?? Dennis' NEW answer is to use as low a value choke as to control the RF coming off the diodes, but not so large as to "choke " the amps music delivery with too much wire and too much core mass.

You are winding a 5 Hy choke - that WILL meet "L critical", or, act like a choke input filter at 120 HZ, the measurement of L crit. that EEs have used for 100 years or so. So you are getting the difference between 1.41 times VAC ( secondary of the PT under load ) MINUS ( .9 times VAC ).............. or a difference of about .51 times VAC between the two!! It is expected to do that !!

A choke input filter ( meeting L crit. ) classically gives you a VDC output of .9 times the VAC of the power transformer's secondary VAC, ( under a load ) minus the rectifer drop. IF you take that SAME power trannie and make it a cap input filter after the rectifier, you get a VDC output of 1.41 times the transformer's secondary winding ( under load ).

5 HY is a high enough value to reach L crit in a lot of circuits IMHO. No wonder you are dropping VDC as B+ final !!

What Dennis is suggesting, ( and John has copied and reported on ), is employment of Ls UNDER 1/2 a HY, and in John's case, a .3 HY Hammond. Radiance, there is NO need to wind your own L's!! The Triad C-40X is available from Allied Electronics, all day long, is ten ohms in DCR, .32 HY, and does 600 mA., and they retail at $10.79 each in small quantities.

Now, when you use such a low HY choke as under 1/2 a HY, it DOES meet L critical, but NOT at 120 HZ, but rather, at a much higher frequency . ( Hence, Dennis' " R.F. Stripper " description.) These low HY chokes will NOT be anywheres near " .9 times VAC = VDC " and they wont be " 1.41 times VAC = VDC " as in a cap input filter. But instead, depending upon load, and as a guess, they will be " 1.35 times VAC equals VDC ". So, you must take this into account in the re-design.

Now Radiance, we are only talking chokes.

There are three components to a low DCR - per Dr. Halijack's " Figure of Merit " of a Power Supply configuration : 1) the power trannie 2) the rectification Z, and 3) the first L, L1, in the chain. If you use a 70 ohm DCR power trannie, you will NOT have the same overall listening result as John Swenson using a Ultra Hi Fi ( albeit kluged ) 3.5 ohm DCR seriesed-pair of Signal DU 1/2 Trannies !!!! Jim Dowdy has custom trannies wound for his customer's amplifier designs, and I've seen figures of 4 to 6 ohms for their DCR.

I hope this helps you.

Jeff Medwin

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