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Old 31st May 2012, 04:01 AM   #1
diyAudio Member
Join Date: Mar 2012
Default Capacitor advice needed

I am building a simple grounded cathode SET headphone amp and decided on an LC-LC PSU. The chokes are 10H. At the moment C1 is 22uF Solen MKP and C2 is 2 x 40 uF ASC X386S, it already sounded good to me, despite according to PSUDII I need to bulk up the C2 to at least 300uF to prevent ringing.

I have ordered a 1000uF Mundorf 4-pole electrolytic (it’s the smallest available to me) to parallel with the 2 x 40 uF ASC X386S. On paper the 4 pole idea sounds good, but truth is I really can't tell how much truth is in it. And I thought the large reservoir would help with LF response, my power transformer's secondary has highish resistance (220R), tube rectifier and two chokes doesn't help either).

At C1 I would also be paralleling another 22uF Solen MKP, not really needed but I already have it.

But I am having a second thought now. Should I be using such a big electrolytic (albeit a somewhat fancy one) in such (some say critical) position? Would I be better off with paralleling 2 x 100uF worth of motor run capacitor instead?

Thanks for reading.
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Old 31st May 2012, 03:46 PM   #2
diyAudio Member
Join Date: Jul 2009
Location: Northants UK
Any ringing you may be getting will be damped by the choke resistance and while you will lose volts if you do so, adding a series resistor to the choke resistance will guarantee damped resonance. For optimum damping you need the total resistance to equal 2 X the sr root of L/C. That's about 1343 ohms for the first (Solen) cap at the moment.

In my view from having recently built a number of these filters, I would ensure that there is either enough resistance or capacitance to ensure at least critical damping in each LC stage. To me the difference was audible on a low gain preamp. I also found the greatest improvement when I opted for a larger capacitor and the minimum resistance for a damped circuit. You are limited by the series resistance of the choke of course. I was working at lowish voltages and ended up using 6800uF caps and having three stages of LC filtering. Another audible improvement was adding snubbers across the first choke as recommended by Morgan Jones "Valve amplifiers" latest edition. Message me with your email and I'll send you the details of this if you like. Another DIY audio member did the same for me and it was a great help.

Morgan Jones also recommends cascading the LC stages with different cut off frequencies. I ended up using lower value chokes of higher frequency cut off in each successive stage and having three stages. Choke specs don't tend to include the capacitance or self resonant frequency so getting this right took some hunting around and measuring of chokes.

Hope this helps

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