function is to switch the
120 VAC on and off with a
device that is triggered by a
three volt audio signal from
the preamp. A shielded
cable will deliver the signal
to the automatic switch.
Since a portion of the
circuitry is always exposed
(AC) and it will be remotely
located with the amplifier, I
wish to use good quality
components. Could you
suggest some alternatives.
My assumptions include a
high impedance input to
the triggering device
feeding an electronic and/or
electromechanical switching
device. Surface-mount
devices are not a necessity.
Thanks in advance for your
assistance.
— Paul Lunn
ASince the audio is
being used to turn
the power amp
on, and since the
signal will vary in amplitude,
I have designed a time delay
turn off when the signal goes away of
about one second (see Figure 2). If it
is possible that the audio will have
quiet times exceeding one second,
you will want to increase C2. The
design includes the power supply
which provides ±8V volts to the
switch circuit. I included L1 and L2 to
reduce current spiking and make the
supply quieter.
U1 is a dual op-amp; the first
stage is a half wave rectifier with a
gain of two. With a three volt peak
input, the capacitor, C2, will be
charged to six volts. The second
stage is a unity gain buffer to drive
the opto triac switch. The triac is a
zero crossing switch, so it should not
produce much noise. The opto triac
specs are: must turn on at four volts
input and must turn off at one volt
input. I included specs for a heatsink,
although I doubt it will be needed.
From an initial charge of six volts, it
takes about one second to get down
to one volt. The resistor, R1, is for
static protection; it has no effect on
■ FIGURE 2
gain. If the preamp output is ground
referenced, C1 and R2 could be
eliminated.
AUDIO LEVEL ADJUST
CIRCUIT
QI have a whole-house audio
system that I’d like to
connect to two different
TVs as audio sources. The
problem is, the TV line level audio
out is considerably lower than my
other components (CD player, FM
radio, etc.) and is not adjustable. So
when I switch to the TV source, I
have to turn up the room volume.
Then when I switch back to the radio
or CD player, the audio blares out
the speakers if I forget to turn down
the room volume. I’ve looked at some
of the electronic kit suppliers, but I’ve
not seen anything like this.
Can you recommend a circuit for
a two channel audio level shifter with
(I’m guessing) a unity-to-5x gain
adjustment? Something with a
frequency response as good as or
better than HD TV audio? A single
chip amp would be fine since it does
not need to have a high power
output and my preference would be
for thru-hole technology as my eyes
are not what they used to be.
— Jon Wilbert
ATwo channel mixer with
adjustable gain ... piece of
cake! The signal is undoubtedly stereo, so two pairs of
op-amps are needed. Separate
volume control for each channel is
provided (see Figure 3). Most audio
devices nowadays use RCA jacks, so I
included those in the parts list. The
power supply can be 12 volts (±6V)
or use the power supply of Figure 2
(±8V). The parts list is for a 12 volt
DC wall wart, but if you have one in
your junk box that is between 10 and
16 volts, use that. If you use the power
supply in Figure 2, R1 and R2 are not
needed. The gain in each channel is
August 2008 27
