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RE: TU-882R Headphone Amp - Possible Mods

Posted by Mike C (A) on April 12, 2016 at 17:18:23

In Reply to: RE: TU-882R Headphone Amp - Possible Mods posted by Tweaker456 on April 11, 2016 at 23:49:03:

Hi,
Simply remove R1 and R2.
The two input caps have to be replaced by a link.

I left all PSU caps as standard including C12 and C13 but I may replace these with good quality Panasonic.

You must have made an error in the calculation - we've all done it! :-)

Z = 1/ (2 x pi x F x C) where F is the frequency and C is capacitance.

Rearrange to:
F = 1 / (2 x pi x C x Z) where Z is the impedance.

At low frequencies we can totally ignore the small filter cap C7 and the miller capacitance of the output valve, so the load is just R13.
When the impedance of the coupling cap equals the load R13 (1 megohm), half the signal will be lost across the cap and half will appear across R13 (look on it as a potential divider). This gives - 3 dB.

So use C = 0.047 / 1 000 000
Z = 1 000 000
and you'll get F = 3 Hz near as dammit and that's ideal.
RC coupling give phase shifts, roughly speaking these affect only up to about 10 x the -3db frequency, so there will be no appreciable effect above 30Hz (and nothing much to worry about above 15Hz).

Hope this helps; do ask about anything you are not sure about.

"The only silly question is the one that should have been asked but wasn't". :-)

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