Questions on parafeed values

johnsonad · 1858

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Offline johnsonad

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on: July 31, 2013, 02:46:46 PM
I understand the equation used to calculate the ideal range of parafeed value and what will happen if the value is two low but I'm fuzzy as to what happens when you go say three to four times higher than the recommended value. I know PJ will say try it and see which I intend to do but I'm wondering if it has a negative effect at some point or if it starts to change the impedance or other values?

I'm going to try a 6uF cap which I like the sound (and it's the lowest value I have in it) in my S.E.X. which calls for a 1.5uF cap.

Thanks!

Aaron

Aaron Johnson


Offline tsingle999

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Reply #1 on: July 31, 2013, 03:34:53 PM
Try it and tell us how it sounds!

SGS iTransporter with Qobuz & Roon to Optical Rendu to BH DAC (Battery) / Wavelegth Cosecant to BeePre to 300b(ehemoths) to Jagers.
Bottlehead Stat headphone amp with Wavelength Brick DAC


Offline Paul Joppa

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Reply #2 on: July 31, 2013, 04:25:01 PM
(setting the stage first) In a series feed SET amp, the inductance of the output transformer causes the impedance presented to the tube to drop below some low frequency, usually in the 20-30Hz range. This shifts the load line so that the tube becomes current-limited, in creasing distortion as well as limiting the available output power. The bass falls off at 6dB/octave below that corner frequency.

(now the parafeed difference) In a parafeed amp, the plate choke has the same effect, but the parafeed capacitance raises the cap/output transformer/speaker impedance at the same time, and if the capacitance is chosen well this will maintain the impedance seen by the tube down to a somewhat lower frequency - maintaining both power and low distortion. It's not a lot, at best something like a half octave, but it's still a good thing. Below that corner, the bass falls off faster, 12dB/octave. No free lunch, you know.

By increasing the capacitance, you move the situation closer to that of the series feed. Fortunately, the parafeed plate choke often has more inductance than the average series feed output transformer, so when the capacitance is very large it just acts like a high-inductance series feed design. FWIW, this seems to have been the approach used by Japanese audiophiles when they started reviving the parafeed approach, 20 or 30 years ago.

All of the above analysis is based on a pure resistance load, i.e. nothing like a real-world loudspeaker. That's why I so often recommend experimenting.

Paul Joppa


Offline johnsonad

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Reply #3 on: July 31, 2013, 05:26:55 PM
Thank you Paul. I'm going to give a few different values a try and hopefully find a combo that works well with my speakers.

Aaron Johnson


Offline Grainger49

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Reply #4 on: August 01, 2013, 10:49:28 AM
So what PJ is saying is that at the low frequencies the XL of the OPT is matched by the XC of the Parafeed cap.

And I will be corrected if wrong.