How Does It Work - Ramsey Electronics STC1 Instruction Manual

HOW DOES IT WORK?

To help you understand where we are in the circuit you will want to look at the
circuit diagram in the center of the manual. We will work from the input of the
left channel to the output of the left channel. We don't need to look at the right
channel since it is identical to the left half.
The audio signal coming into J1 is a line level signal at about 1 volt peak-to-
peak, which is a 0dB line level signal (standard). The audio level is cut in half
by resistors R63 and R28. This is necessary due to the lower voltages supplied
in the circuit. This keeps the signal level well inside the operating range of the
opamps later in the circuit to prevent distortion.
The signal then enters into U1:A and surrounding capacitors and resistors. This
is an active low pass filter to prevent any signals from outside the audio range
from entering into our circuit. This keeps high frequency signals from interfering
with our special filters later in the circuit.
The low pass filter output is then fed into what appears to be three feedback
loops in an opamp circuit. Each branch is a different type of filter. One for low
pass, one for midrange, and one for high pass. Yes, you have it, it's the tone
controls for the left channel of the circuit. This arrangement allows us to boost
or cut frequencies from +/-12dB in three bands.
The output of our tone controls are then fed to the limiter circuit. This is where a
little magic comes into play when you see the two diodes in the feedback path
of the opamp. As you may know, diodes only conduct in one direction, but what
you may not know is that it takes about .5 to .7 volts to begin to conduct in the
forward direction. This is called the forward voltage drop of the diode. This .7
volt drop is used as our limiting voltage reference.
During signal levels under .5 volts peak-to-peak on the output pin 1 of U2:A ,
U2:A acts like a simple inverting amplifier with its gain controlled by R23, R18,
and the feedback resistor of R15. For example if we had R23 set at 7.8K ohms,
the gain of the circuit would be -1 in these signal conditions. The change comes
with signal levels over .5 volts. The diodes D1 and D2 begin to conduct on ei-
ther positive or negative going signals since they are connected in opposite
directions. This allows R51 to come into the gain equation. When the diodes
are on the gain lowers dramatically. In this case the gain goes to less than -.1.
Now the gain is 1/10 of what it used to be, and it prevents the output of this
stage from going much over .7 volts peak-to-peak.
The output of this limiter stage is then fed to two different stages, one of which
is the clip detector. This clip detector is set to detect any signal over .6 volts
peak-to-peak at the output of the limiter, which is where significant sound dis-
tortion begins. D6, C4, R55, and R58 make up a simple peak hold circuit by
rectifying the AC audio output of the limiter into a DC level related to volume.
STC1• 5