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james mitchell
 
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Default Passive Volume Control (Passive Preamp) Info

This post documents my findings on amplifier input shunt capacitance and
contradicts all information I have seen on internet websites. I have read
several websites indicating amplifier input capacitance is very small for
most amplifiers. I have found that amplifier input capacitance CANNOT be
assumed neglegable when considering an passive volume control (passive
preamp).

I recently acquired a Goldpoint SA-3 50KOhm stepped attenuator volume
control box (very nice box) to use between by CD player and amplifier which
bypasses my AV receiver when listening to CDs. The box has multiple inputs
so I can pass the signal through the A/V receiver or not.

A 50KOhm potentiometer (volume control) has a worst case output impedance of
12.5KOhm at -6dB. This is very high an care must be taken to reduce shunt
capacitance between the volume control and the amplifier. Sources of this
capacitance are the interconnect and the amplifier input. With a high
output impedance, even a small amount of shunt capacitance will cause some
high frequency rolloff. To minimize interconnect capacitance, I am using a
1/2 meter RG59 interconnect (17pF/ft) between the volume control box and the
amplifier.

As stated, the output impedance of the volume control box is 12.5kHz
at -6dB. Additionally, at 0dB (no attenuation) the output impedance is
whatever the source output impedance is. In my case, 1.2kOhm for my AV
receiver preout. With the volume control box hooked up, I tested whether I
could hear a difference between the vastly different output impedances
feeding the amplifier input at these two levels. I changed the volume back
and forth repeatedly while listening carefully. I thought I could tell a
difference but wasn't sure. I knew the capacitance of the interconnect I
chose was too low to cause any high frequency rolloff. The only other
possibility was the amplifier input. I created a test to determine the
shunt capacitance of the amplifier input.

I placed a 511K series resistor at each input RCA jack on the amplifier.
Then I reconnected the volume control box. Since the amplifier input
impedance (resistive part) is only 50KOhm, the series resistor dominates the
total impedance seen by the source. This setup produces a voltage at the
amplifier input almost directly proportional to the amplifier input
impedance. So if the input impedance is changing with frequency (ie. shunt
capacitance), the input voltage will change proportionally. Thus, the
output frequency response can be used to determine the input capacitance.
With this setup, the frequency response was down almost 10 dB at 20 kHz.
This corresponds to 500 pF of input capacitance. A MATLAB simultation was
used to determine this. I used Stereophile Test CD 2 as a tone generator
and a multimeter to read the voltage at the amplifier outputs.

I though there may be something wrong with my amp (Adcom GFA-5500). I
contacted Adcom and they confirmed the shunt capacitance to be 390 pF. I
wondered if this high input capacitance was unique to Adcom amps so I
contacted Bryston. I confirmed that the input capacitance on the 3B SST and
4B SST amps is 220 pF. This is less than the Adcom but still significant.
By the way, amplifier manufacturers add this capacitance to the input to
reject high frequency (RF) interference.

The moral of the story is find out the input capacitance of your amplifier
before choosing to use a passive volume control (passive preamp). My
experience has shown that the input capacitance of the amplifer cannot be
assumed small relative to interconnect capacitance. I have modified my
volume control box to 25KOhm to reduce it's output impedance. With this
setup, I have 0.6 dB attenuation at 20kHz at -6dB volume setting which is
tolerable. At most volumes, the frequency response is ruler flat. The
transparency of the passive control is hard to beat.

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