Audio Switching Network
by John Bryant
What this simple circuit does
is take two audio inputs and put either signal in
both sides of a pair of stereo headphones, or it
puts one of them in each phone simultaneously. The
device will also, simultaneously but separately,
put either signal out to an outboard speaker. That
is it! Sounds simple, but it is a wonderfully useful
In a recent e-mail to my Northwest
DXing buddies, I mentioned that I was building a
new rack box to go under my new receiver and that
I was putting in not one but TWO complete sets of
"my much-beloved" audio switching network.
Guy Atkins immediately shot
back a query asking just which circuit I had in
mind. I realized that, tho' I believe that I wrote
a small article about this for DX Ontario, I've
really not shared my marvelous "find"
as I should have done. It is classified as a "find"
because Don Phillips published the original circuit
in DSWCI Shortwave News in 1998. My circuit is a
slight modification of Don's design.
I first built my original version
of this circuit to give me the best chance of comparing
the DXing or listening performance of two receivers.
Wearing stereo headphones hooked to this network,
you can switch instantly back and forth between
two different radios, with the flip of the Priority
Switch. Or, if you throw the Mono/Binaural Switch
to Binaural, you have one receiver in each ear.
In the Binaural Mode, throwing
the Priority switch swaps the inputs to opposite
sides of the head. I'm not sure what this latter
capability could be used for, but it is kind of
neat. Generally, I drive the network from the External
Speaker port of each receiver. I have never found
any way to do a listening comparison between two
receivers that is so effective as this. If running
receiver comparisons were the only use for the circuit,
it would still be worth having.
It didn't take me
long, of course, to find a second major use for
the circuit: "checking parallels". In
both SWBC DXing and International MW DXing, it is
often possible to at least tentatively identify
a station by simultaneously receiving a stronger
signal from the same network being broadcast on
another known station or frequency. (The two stations
are "broadcasting in parallel".)
The usual technique for "checking
parallels" is either by using two of the memory
channels in the receiver and switching back and
forth quickly or by using two radios and manipulating
the volume controls appropriately. Either technique
works well when both signals are relatively strong,
but when the target signal is really "down
in the mud" there is NOTHING like using this
network to put one signal in each ear, simultaneously.
There were several times when,
with the old techniques, I would have sworn that
two Indonesian stations were broadcasting national
news from Jakarta in parallel only to find out,
in "stereo" in the Binaural mode, that
I was hearing the same female announcer but actually
hearing her work in two different programs!!!
Also, only when I began to use
this network did I discover that All India Radio
feeds the national network news programs to some
of its regional outlets via satellite and some by
land lines. Compared to the direct broadcast from
New Delhi, about two-thirds of the regional broadcasts
are delayed by a second or so. The delay itself
is not very important, but not discovering it over
several years of using the "old techniques"
of parallel checking indicates how poor they really
I discovered a third use for
"my beloved" the first time that I took
a miniaturized version of it on one of the Grayland
DXpeditions. Since I normally only bring one receiver
on DXpeditions, I soon discovered that I could use
this network and a long audio cord to "listen
over the shoulder" of one of my compatriots.
I just plug the network into
my receiver (Receiver A) and my compatriots' radio
becomes Receiver B. I can hook to some radios directly
to my compatriots' Speaker Out port. Others require
us to use a "Y" connection at the headphone
jack. Hooked up that way, when Dave Clark shouts
"Holy S***, CHECK 972!!!" at 3 AM, I can
be listening to his radio with one or both ears
instantly while everyone else has to decide whether
to abandon what they have been listening to themselves.
On DXpeditions, this network is also useful to compare
each other's receivers while in pedal-to-the-metal
A fourth use for this circuit
is the one that has me buying more switches this
week. Many of us have begun to use MiniDisc recorders
rather than cassette tapes. Invariably, these wonderful
recorders are stereo machines. DXing using two receivers,
each feeding one channel of the stereo deck, sounds
just wonderful, frankly.
This is an especially attractive
idea since the Fast Forward mode of MiniDisc technology
uses a sampling rate that keeps the audio relatively
understandable while speeding through the track,
so you can whiz through the track later and not
miss the ID. Its probably taken you much less time
than it took me to realize that this networking
circuit is perfect as an interface between the deck
and a set of stereo phones, allowing you to listen
to either channel A or Channel B or both, at the
flick of a switch.
Use With Long-Playing MiniDisc Technology
Sony has recently introduced
decks that will record at standard 74/147 min. stereo/mono
length, and at two new stereo-only LP lengths of
160 and 320 minutes. The two new lengths are known
as LP2 and LP4. I used one of these decks, the MDS-12,
to test the whole operation.
The largest concern, of course,
was obtaining pristine channel separation. Notes
on the MiniDisc Community web page indicate that
a different algorithm is used to record at each
of the three speeds. The LP4 encoding strategy takes
advantage of the fact that, in music, about 90 percent
of the same information exists on each channel.
That is the only way, apparently, to achieve the
radical compression necessary to jam 5 hours of
quality stereo onto a MiniDisc.
I was concerned that this approach
would preclude the recording of totally separate
tracks in LP4 mode. I was right. At fairly low levels
of different audio on each channel, there did seem
to be full separation. However, when I did the ultimate
test - Buddy Holly's "Rave On" at full
blast on Channel A and WWV on Channel B - there
was some crossover bleeding both ways. When listening
to just one of the channels, it sounded like low-level
sideband splatter on AM was also coming in from
the other track. The problem was not very pronounced
and, if put in a desperate situation, I might try
to use the 5 hours of LP4 with two receivers, but
feeding the channels with only mid-level audio.
Running the same test Buddy Holly
Test at the LP2 recording speed produced excellent
results. Even Buddy's hottest licks stayed right
where they belonged, on Channel A! The same was
true at the conventional stereo speed of 74 minutes
per MiniDisc. So, with these newer decks, you can
record over 5 hours with one receiver in LP4 mode
or record two receivers for almost 3 hours in LP2
and this switching circuit works like a charm to
listen to the result.
I ran the Buddy
Holly tests using an early generation Sony MiniDisc
Recorder and found the same pristine separation
of the two stereo channels: seventy-four minutes
of excellent recording of two radios simultaneously.
Using this circuit to monitor the recording as it
happens and to review it later also works like a
Don Phillips' original DSWCI
article stressed using high-quality, new switches
since most users of this circuit quickly become
addicted to switch flipping. I agree whole-heartedly.
The best quality full-sized switches in a sturdy
mount should still hold the parts costs at around
$20.00 and will last you for many years.
Sony MDS-12 is a professional deck
MiniDisc Community FAC on MDLP
I'd like to thank Don Nelson
and Guy Atkins for their usual excellent behind
the scenes advice that refined this article considerably.
Written in February
published on DXing.info on June 24th 2002