Simple two line output combiner

Simple two line output combiner
Simple two line output combiner
Copyright Tomi Engdahl 1997,1999
Introduction
Sometimes there is need to combine the output of two line level audio sources to be fed to
one input. A standard way in professional audio is to use a mixing desk, but on home audio
system you might want to look for a simpler and less expensive alternative.
Some people have suggested of combining two audio outputs with an "Y-adapter" which is
ment to split one output. Those adapters are just one male connector and two feamle
connector directly wired to each other in the followin way:
OUT 1 -----------------+
---+
+------------ INPUT
|
|
+---OUT 2 -----------------+
|
---+
|
|
(shields) |
+---------------------+
This kind of circuit is just a correct construction for situation where you have one line level
output (connected to circuit input) which you want to split to two inputs (connected to circuit
outputs). Hardwiring two inputs in parallel and ot one signal source is perfectly OK in line leve
audio.
But if you want to do the opposite this arrangement is not adequate. If you use this kind of
"Y-adapter" to combine two line level audio signal you are effectively shorting two equipment
output to each other. Audio equipments are not designed for this.
When two outputs are directly wired in parallel, you are effectively fighting with each other.
When they are sending out different audio signal, then most of the time one of them is trying
to push the output to different voltage than other output. This effectivley causes stress and
overload to outputs. In best case you get uneven mix of the signals, lowered output voltage
and increased distortion in the sound. In some cases you even risk on damaging the
equipments because you are constatly overloading the outputs. The reason for this to happen
is that line-level outputs are low-impedance voltage sources and they just aren't going to be
very happy if you connect them to each other. Anytime the output differs between the two (as
it will almost all the time in stereo) each will be trying to ram current through the other -more current than they're designed to source, and way more current than they're designed to
accept (they're designed to accept practicly nothing).
So "Y-adapter" is NOT the right way mix two audio sources.
Correct approach
A right approach is to make a mixer circuit which combines the audio signals without
overloading the outputs. This kind of circuit will safely load the outputs as they are designed
to be and provides very nice even mix of the two signals to the output.
A mixer circuit of this kind does not have to be anything complicated. Two correctly selected
resistors will do the job very well. The circuit below shows a circuit which works very well.
INPUT 1 -------\/\/\/\---+
---+
R1 10K
+------------ OUTPUT
|
|
+---INPUT 2 -------\/\/\/\---+
|
---+
R2 10K
|
|
(shields) |
+---------------------+
This circuit construction provides 10 kohm input impedance and around 5 kohm output
impedance. Both of the values are well within what you would expect from an audio
equipment.
Building the circuit
To build this circuit you need three RCA connectors and two 10 kohm resistors (R1 and R2).
The resistors connect the audio signals (center connector) from inputs together to from one
output signal. You can see the wiring details in the figure below:
INPUT 1 -------\/\/\/\---+
---+
R1 10K
+------------ OUTPUT
|
|
+---INPUT 2 -------\/\/\/\---+
|
---+
R2 10K
|
|
(shields) |
+---------------------+
When you have connected the audio lines together using the resistors, then connect the
connect the connector grounds directly together using short piece of wire. Now you have
made a nice one channel mixer circuit which can mix together two line level audio signal
sources.
Component list
R1
R2
INPUT1
INPUT2
OUTPUT
10 kohm resistor (0.25W)
10 kohm resistor (0.25W)
Female RCA connector
Female RCA connector
Male RCA connector
Technical specs
•
•
•
•
•
•
Attenuation: 6 dB
Input impedance: 10 kohm
Output impedance: 5 kohm
Distortion level: So low that it could not be measured.
Noise level: Very low (temperature noise of 10 kohm resistor)
Power source: Passive device, does not need external power source.
Frequently asked questions
What does that attenuation mean ?
The attenuation causes that you need to turn up your stereo equipment volume a little bit up
from what you are used to do to get the same volume output.
What does it mean that the output impedance of the circuit is 5 kohms ?
The output impedance of the the audio equipments have effect on two things: loss of output
level because of load and loss of high frequencies because if long cables. At 5 kohm that
output amplitude loss is neglectable when connected to a typical audio amplifier input (10
kohm impedance or more). The only bad effect of this large outpu impedance is that you
might notice some loss of highest fequencies if you use long audio cable more quicly that if
the cable were connected directly to your audio source. If you need this circuit directly
connected to your audio amplifier audio input, there is no problems. Even something like 1-2
meters of typical audio cable does not cause anythign severe. If you put 10 meters of cable
after this circuit, you might notice some loss of highest frequencies though.
How do I use the circuit for stereo signals ?
For mixing you need to build two of those circuits. One for left channels signals and other for
right channel.
Can I adjust how loudly each signal source play at output ?
This signal itself does not provide you any possibility to adjust the levels of the signals. If you
want individual volume control you have to either have equipments which have output volume
controls on their line outputs or you have to build my more complicated Simple line mixer
circuit instead.
Can I mix more than two outputs with this kind of circuit ?
You you can expand this circuit to more channels if you add one resistor and one input
connector for each new output. Connect them to the circuit in the same way as the existing
inputs. Adding more channels adds the attenuation caused by the circuit. You can go up to
around 5 channels if you are not afraid of some extra attenuation and noise (background
hiss).
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