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Input cap on Adcom 555II
Input cap on Adcom 555II
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Old 8th April 2009, 02:54 AM   #1
AcidJazz is offline AcidJazz  United States
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Join Date: Apr 2004
Location: NYC
Default Input cap on Adcom 555II

I have a couple 2uF Sonicaps I that I would like to substitute for the stock 1uF that are installed. What cons would there be with doubling the value of the input coupling caps? What would be the effects on the response of the amp?
I had installed some bypass .01uF caps on the stock caps and found the sound to be much improved, which leads me to the conclusion that the stock caps may be poor quality. Also I noticed that the stock caps were rated at 50v. whereas the schematic called for 100v.
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Old 10th April 2009, 03:25 AM   #2
gni is offline gni  United States
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Join Date: Sep 2006
Cap in series is a high pass filter at 6dB per octave); frequency will depend on
input resistance.

Increase value will lower the cut-off. (2x value = down an octave)

Decrease value will increase the cut-off. (1/2x value = up an octave)

10k input with 1.06 microFarads is 15Hz cutoff (down 3dB).
10k input with 2.12 microFarads is 7.5Hz cutoff (down 3dB).

10k input with 0.01 microFarads (10 nF) is 1,592 Hz cutoff (down 3dB).
Good amp for running a tweeter since the amp will mainly amplify frequencies
above 1500 Hz; it will attenuate frequencies at 6dB per octave below 1500Hz
and be down 36 dB at 23 Hz and -66 dB at 0.73 Hz.

C1 (fd) = 1,000,000 / (6.283 x Impedance x Crossover Frequency)

The cap will effectively block DC (you DO NOT want to amplify DC. . .BAD thing happen). Too high a value and low frequencies (below 20Hz down to DC might
be amplified more and consume more power at frequencies you can't hear and
your amp will run higher.

Probably won't matter but try to replace the cap with same or higher voltage.
Some engineer calculated it to be the right value. Some preamps (tube) can
produce greater than 20V! I have a mixer (SS) that can put out 12.3V.

I think I got the math right. Please, anyone, corrections.

A very good discussion with types also:
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