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Old 24th May 2005, 05:52 AM   #1
mmerig is offline mmerig  United States
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Default Phono recap questions

I am planning a refresh of the phono section of my preamp (Hafler DH110). It has the usual RIAA correction and gain stages. Attached is the schematic. Only the electrolytics are going to be replaced, and they are for one channel:

C6 22 mfd 50V nonpolar
C7 220 mfd 10V nonpolar
C8 220 mfd 10V nonpolar
C13 1000 mfd 6V nonpolar
C16 22 mfd 50V nonpolar
C17 470 mfd 25V low ESR
C18 470 mfd 25V low ESR

The rated output is 3 volts rms, and maximum output is 12 vrms at recording outputs (and to line amp). Rail voltage is +-23

I would like to put in some Blackgate N type caps, but there is a big jump in voltage, size, and of course price beyond 6.3 volts. Could I measure realistic voltages at the caps with a record playing? I think I can get away with the 6.3V versions up to C8, but I may be wrong on this. There is a 1000 mfd 25V that would work for C13, and 50V versions for C17 and 18.

The tough one is C16, which is after 3 stages and the voltage may be up to 12vrms. How can I calculate what the likely voltages would be? Any educated guesses based on experience with other phono stages? How critical is this cap to sound?

I'm not committed to Blackgates, so are there any suggestions on nonpolar caps in phono stages? A Solen film cap could substitute for C16, but it would have to be off-board. Nichicon ES would be cheaper alternatives to all of the NP Blackgates; are Nichicons good enough for phono stages? I have lots of LP's and listen to them often, and the phono sound is a bit gritty these days, especially in the mid and high range.

Any suggestions or advice would be apreciated, and I'll come back with my impressions after I finish the project.
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Old 24th May 2005, 06:14 AM   #2
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The tough one is C16, which is after 3 stages and the voltage may be up to 12vrms. How can I calculate what the likely voltages would be?

12v rms is of course pure scifi. The max possible output from such a circuit with a real cart is probably 1-2v rms so a 6.3v rated cap is fine. What may be more disturbing is the way the dc offset of this circuit behaves. Quite possibly on power on/off it jumps briefly close to PS rails.
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Old 24th May 2005, 06:15 AM   #3
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Ugh, here goes. Firstly I'd try to eliminate c5,6 by matching the input complementary pair to minimise base bias current - or substitute JFETs . Your c16 is obviously a utility part and I would suggest lightening it's load (whatever it is) to be over 10K when connected with the next stage - then substitute a 1uF polypropylene. If you lose bass, parallel 2.

Apart from that I've run out of suggestions.
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Old 24th May 2005, 06:28 AM   #4
AndrewT is offline AndrewT  Scotland
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the two 22uF can be substituted by film caps, probably cheaper than blackgates.
How accurate are all thes caps? For a riaa correction Hafler may have measured each one & corrected the resistor values to compensate for cap spread. Normally you would aim for 1 or 2% tolerance but not with electros.
regards Andrew T.
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