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In Reply to: RE: Schottky posted by Triode_Kingdom on April 01, 2017 at 10:49:35
I think it's fair to say that the "hash" filter idea is in the public domain. L_RD knows that I've suggested it on numerous occasions.
The filter is simply a LC section made from what would ordinarily be regarded as RF parts: a 1000 pF. cap. either in mica or NP0/C0G ceramic combined with a high current RF choke. Use the choke with the largest inductance that does not saturate under the PSU's load demand.
Mouser carries suitable Bourns/J.W. Miller and Vishay parts. Look into the 5900 series parts on the linked catalog page.
No diode voltage equalizing resistors are in a "McShaned Deuce".
Eli D.
Follow Ups:
I modeled this last night, and the PRV in each leg never exceeds the rating of a single diode. It's approximately the same as the output voltage (2.8 X RMS). I guess that's why equalizing resistors aren't strictly necessary when two are used in the Cit II. The A500 used single 500V diodes to produce +370V, and the A700 used single 600V diodes for an output of +410V. Today's higher line voltages squeeze those windows somewhat, so I will probably use two diodes in each leg of the A700 rebuild. The A500 can use 600V singles.
The general values you're describing for the hash filter - e.g. 0.001uF and 1mH - produce a huge output Z spike between 100kHz and 200kHz. The supply effectively turns into a current source at that frequency. I don't know whether this could have an impact on the power amp or other stages, but a 10uF electrolytic across the 0.001uF will flatten it.
Good discussion, my rebuild path is becoming clearer!
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Buy Chinese. Bury freedom.
Just wanted to add this image so you can see that the ultrasonic spike isn't only a small anomaly. If the goal of the hash filter is RF suppression, this doesn't do the job! Granted, it's well out of the audible spectrum, but any energy on the B+ line at that frequency could cause the PS voltage to swing like crazy. 10uF across the .001uF resolves this completely.
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Buy Chinese. Bury freedom.
Edits: 04/02/17
Go for it! My objective was to block noisy crud, regardless of source, from sneaking into the reservoir capacitor. There is always room for variations on a theme.
FWIW, I'd still like to see a simulation with a LC reservoir section located after the "hash" filter. Superior designs, like the "Deuce", some McIntosh models, and some Marantz models, include the LC reservoir section following the doubler stack. The choke is not particularly large valued. When big caps., ala McShane, are used at the I/P of a PSU filter, the ripple waveform is highly "triangular". Applying Fourier's Theorem to that "sharp" waveform led me to call for "hash" filtration immediately following the I/P capacitance. Energy well up into RF "territory" is present that can get past the principal choke, via its real world winding capacitance. Kill the garbage before it gets to the "back door".
Eli D.
Here's the model of the factory CIT II filter. I had to guesstimate the DCR of the choke and ESR of the caps. I'll be glad to change any of the values or insert a hash filter using your components. Output Z of this design isn't particularly good, probably swings quite a bit when a 30Hz note is present. Then again, it was only $160 in kit form. :(
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Buy Chinese. Bury freedom.
I've snipped from Jim's site the minimum level of Cit. 2 PSU overhaul he offers. Jim offers stuff beyond this.
For the Citation II Power Supply
1. (2) 820 uf 250 volt 105 degree Celsius very low ESR clamp mount caps that increase the capacitance from
100 uf total in the doubler to 410 uf. (2) clamps are included.
2. (4) General Instruments fast/soft recovery diodes that replace the existing top hat diodes. They fit nicely in the
fuse holder right where the old top hats are now. The GI diodes are very good, and quite reasonably priced too.
If you increase the capacitance, you really need to replace those diodes. And the DO-41 packaging is ideal for replacement.
3. (1) JJ 100/100 dual section 500 volt clamp mount cap, and...
3a. (1) Nichicon 220 uf/500 volt clamp mount cap. The Nichicon replaces the 40 uf 525 volt original twistlock,
and the JJ replaces the dual 50/50 V1/V4 decoupling cap. (2) clamps are included.
Note: This amp responds very well to improved decoupling of the input 12BY7A tubes!
4. (1) .01uf high voltage film cap to replace the existing ceramic unit.
5. (1) Gen'l Instruments fast/soft recovery diode to replace the bias selenium rectifier
6. (1 ea.) 220uf and 100 uf radial electrolytics, a new 1.8K ohm resistor, and a terminal strip for the bias circuitry.
Two new 4.7K ohm resistors to increase the range of bias adjustment.
7. A copy of the schematic, and kit installation notes and diagrams.
You end up with 630 uf (vs. 140 uf stock) of high quality capacitance for the B+, and twice the decoupling
capacitance.
Eli D.
Yes, Jim's basic mods to the doubler create a much better supply. I modeled it yesterday, based on the info on his Website. It's not likely I'll ever own a Cit II, but the concepts Jim is using would be applicable to any supply of this type. Below is the model of the modified doubler. It's scaled the same as the factory PS I posted earlier. Quite a difference!
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Buy Chinese. Bury freedom.
Hi TK,
Thanks for running the model! I know you didn't have all the best info to run it with either - so your effort was greatly appreciated.
Just FYI - the DCR of the Cit II choke is 11 Ohms at room temperature - it may be a bit higher when under the hood of a Deuce which does get warm underneath.
Also the ESR of the doubler caps is .187 Ohms each - so even stacked in the doubler the total ESR is under .4 Ohms.
Finally (again just FYI) there is 220 uf of high quality capacitance after the hash filter so the spike you saw isn't there.
One last time - THANKS!!
NO need to thank me, it was a necessary exercise for my own work. I was glad to see such a remarkable improvement from your upgrade design (not that I expected anything else). I have a number of HK amps and receivers to rebuild using the transformer sets on new chassis designs. Most will probably be sold off, but One of these, the A500, will be for my own use. I just picked up a spare power transformer for this amp, so now I'm thinking about re-configuring as two isolated channels or maybe monoblocks. Anyway, I'll probably be looking to you for the filter caps in some or all of these. The extremely low ESR of the components you're selling has really caught my attention. Send me a PM if you ever have any pending designs you'd like to see modeled in SPICE off the forum.
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Buy Chinese. Bury freedom.
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