Audio Switch Notes
Updated 4/11/12 rel.
1.3

(Note: some schematics were hand drawn for expediting this
site. They will be redrawn as time permits.)

There has been a request to propagate some of the techniques I have used
throughout the years to implement simple audio switches. There are a number of
complex chips out there that can accomplish this task, but many of them are
expensive, hard to diagnose and troubleshoot, and single sourced. As a
broadcast field engineer, troubleshooting and finding a faulty specialized audio
chip and replacing it is almost impossible when in the field. Discrete
components are usually stocked in a field serviceman's parts cabinet or obtained
from the local parts store or even cannibalized from another piece of
equipment.

The enclosed circuits have been used in many high quality, low noise
applications in recording and broadcasting equipment.

Some of the requirements are: low noise, no DC pop when switching, reasonable
amplitude performance, low distortion and good cutoff (muting).

The circuits shown have distortion of less than 0.1% and attenuation reaching
-66 to -70 db. Some specs. may vary slightly but should come close to this.

When I refer to BP, this is the bias point which is always 1/2 the supply
voltage, filtered. Keep in mind that the maximum voltage for typical CMOS is
+15 volts. Staying close to 15 volts will obviously give you the most headroom
although a single 5 volt supply will adequately switch line levels. Using a
single supply obviously requires DC isolation with capacitors between stages
and increases the parts count. Using a bipolar supply +/-5 to +/- 7.5 volts can
eliminate many parts.

There are two types of audio switches that will be shown. 1) J-FET 2) CMOS
switch.

A simple J-FET switch is easy to accomplish. The use of a P-channel J-FET is
required. The more common N-channel will not give the same results, especially
fading. A common device is the 2N5462 or the J176 or J177 available from Jameco (part number 140142CF $.59ea or
211414CF $.29ea at 2005 prices).

The circuit below is an example of a simple audio switch:

Values are not critical so experiment for best results. The
2N2222 or any signal NPN transistor is used to invert the control logic if
required. Note there is no power required except to the 33k pullup resistor.
This voltage can be from +5 to +15 volts. I suggest bread boarding the circuit
to insure you understand the working concept. The circuit should mute
sufficiently any audio from 15 hz to 20 khz. Mute level should *easily* reach
-50db. If not, you don't have the circuit wired correctly. This circuit will
also fade by providing a variable voltage at the junction of the 33k and 1M0
resistors (R3 and R4). The fade will be low in distortion until -20 db has been
reached. At this point some distortion will occur but by this time the signal is
almost gone.

The following is another type of audio switch. Clean on/off
keying

:

CMOS switches are easy to implement too. There are a few
standard devices: the CD4016, CD4066, CD4051, CD4052, CD4053. The first two
devices are quad bilateral switches that can pass audio or digital either
direction. The 4016 can fade audio because there are no gates or buffers on the
control pin. The 4066 has a buffered gate and will hard switch. The 4051, 4052,
4053 are mux bidirectional switches that can switch analog or digital, even
video signals. The 4051 is a 1-of-8 switch, the 4052 is a 2-of-4 switch (great
for stereo), and the 4053 is a triple 1-of-2 switch. Pinouts can be found by
Google or any Motorola/Fairchild data book. A good reference to these devices is
the CMOS Cookbook by Don Lancaster (H.W. Sams).

The following was taken out of an automation switcher I
designed back in 1979. The CMOS chip is the CD4016:

The following is a single cell switch using the CD4016:

The following can be used to fade audio. A simple R/C network
can be put in place of the 20k pot to fade the audio

Here's a
PICLIST requested circuit. A stereo A/B switch using one chip and a few
parts. Note that all values are not critical but typical. They can vary greatly
with little effect on performance. Just impedance and audio levels. With this
circuit you could eliminate the 10k input resistors. They were originally put
there to create an input match. If you have an extra output pin from the PIC and
want the option of turning both channels off or on at the same time, you can
eliminate the inverter transistor, two associated 10k resistors, and drive the
control pins (5,13 and 6,12) separately. A "1" on the port out pin will turn
the switches on. A "0" will turn everything off.

The following is part of a stereo multiplex generator circuit
I helped design many years ago. It shows series and shunt switches to insure a
hard, clean, audio cutoff.

As soon as I have more time, I will clean up the drawings and
provide some tighter specs.

Some of the component identification I use can be referenced to
this page.

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