A 20 Input Audio Mixer for Multiple Receiver Setups

Posted on Feb 6, 2014

Because of this audio mixer, I can compare the various receivers just by twiddling the gain controls. No switching required. Makes A/B comparisons a snap. (They are all driven by the same antenna). All of this used to be easy I just used a different speaker on each radio. However, once the receiver count soared up through 6 or 7, this solution started to lose its luster. Not only the

A 20 Input Audio Mixer for Multiple Receiver Setups
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cost, but also the space, consumed by all these speakers seemed to be getting out of whack. So another solution began to seem attractive. I put up with various frustrating switching arrangements for years before I got around to doing anything about it. This desire to listen to multiple rigs at once led me to develop a 20 input audio mixer. I run the audio from each receiver to the mixer, which then drives one speaker typically a Collins 312B-3. This allows me to listen to whatever radios I want without worrying about any switching. I still have several speakers to choose from, but whichever one I use is driven by the mixer and, therefore, all the receivers. Some of the mixer`s inputs are 8 ohms and directly accept amplified outputs that would normally be connected to speakers. Other inputs are 600 ohms and accept line level signals. Some receivers, like the R390A, have only a 600 ohm line level output. Others such as the Collins S line - have both types of outputs. I use whichever outputs exhibit less distortion. The design has been very successful. The mixer provides adequate output power (up to a couple of watts) with very low noise, and is nearly bullet proof with respect to RFI. The noise delivered to the speaker when all the radios are connected but turned off is only about 6 microwatts, which is just barely audible when I put my ear directly on the speaker grill and the room is quiet. I tried to keep leads very short...

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