Ramsey Electronics Phone Patch Mixer Kit PPM3 - Manual (Page 5)

PPM3 Page 5
PPM3 CIRCUIT DESCRIPTION
The PPM3 is essentially a stereo audio mixer, with one stereo line input and
two specialized mono inputs, each feeding into the stereo line output. The
phone and mic inputs each have a pre-amp stage that acts as a complete sig-
nal conditioner. The mixing core is done with two op amps, U2:C for the left
channel and U2:D for the right channel.
The line inputs are AC coupled and volume adjusted by R2 and then fed into
the mixer. The maximum voltage gain from the line inputs to the line outputs
for each channel is 3.2, or 10 dB. This is set by the ratio of R22/R23 for the
left channel and R21/R24 for the right channel. The maximum voltage gain
can be calculated for each channel by:
G(left) = R22 / R23 = 15K / 4.7K = 3.2
G(right) = R21 / R24 = 15K / 4.7K = 3.2
If you are interested in the gain in dB format, it can be converted by the follow-
ing:
G(dB) = 20 * Log (G) = 20 * Log (3.2) = 10 dB
These are the maximum gains that are achieved when the line volume pot is
set to its full (clockwise) position.
The microphone input has a DC phantom power of +5V which is required for
some microphones. If not needed, this can be eliminated by not installing
R13. If you’re not sure what your mic needs, install R13. The signal is AC
coupled by C33 before it goes into the pre-amp. A value of 5.6nF was chosen
because with an input impedance for the pre-amp of 180 K, a high-pass filter
is utilized that cuts out frequencies below roughly 150 Hz. In effect, this is
desirable for a vocal microphone because it will eliminate some annoying
“pops” and other low-frequency noise like 60 Hz AC line hum, for example.
U3 is the pre-amp that performs a few useful functions all in one part. First, it
acts as a noise gate which simply means that it effectively “turns off” the mic
when only background noise is present, and then automatically turns back on
when there is a minimum signal. It also acts as a variable compression ampli-
fier, which means that it will amplify a quiet whisper more than it would a loud
shout. Its compression ratio is determined by R9, which with the 39K resistor,
works out to be about a 3.5:1 compression ratio. C6 controls the release time
of the compression and the real-time rms averaging for the internal level de-
tector. This capacitor can range from 2.2uF to 22uF. The 10uF value seemed
to give a good compromise. Too small of a value and the gain will change two
fast, two large a value can result in slow response times to signal dynamics. If
other compression ratios are desired, this chart can be used: