W3YY Audio Control System

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The number and type of audio signals that exist in a modern amateur radio system can be significant.  Yes, you can still build a simple station with just the microphone plugged into the transmitter and the receiver feeding a speaker, but if you have multiple rigs, computerized logging, microphone preamps, outboard audio processing, audio recording, etc., a centralized audio control system is needed.  I designed the system described here to control the routing and levels of all transmit and receive audio in my shack.  The system gives me complete control over my transmit audio and allows me to monitor, in my headphones or through external stereo speakers, all audio sources with each source equalized and panned in the stereo field as desired.  This includes (1) microphone audio (pre-processor), (2) audio processor output, (3) FT-2000 main-receiver audio output, (4) FT-2000 sub-receiver audio output, (5) Collins 75S-3C audio output, (6) Collins 75A-4 audio output, (7) PC audio output (Recordings of previous QSO's, N1MM audio, and MMTTY and Digipan transmit audio).

The audio control system at W3YY is built around a very high-quality, but moderate priced audio mixer, the Soundcraft EPM8.  I had previously used a Behringer Xenyx 802 mixer in this application, but after adding a new condenser microphone, external audio processor, and the Collins S-Line and 75A-4 to the shack, I needed additional input channels on the mixer.  Also, one thing I really wanted that wasn't available on the XENYX 802 was one or more pre-fader AUX SENDS so I could independently control the monitor mix without disturbing the audio levels sent to the audio processor and transmitter.  Most compact mixers sold these days usually only have one AUX send and it is post-fader.  Those that do have two AUX SENDS usually allow only one to be switched to pre-fader.  The Soundcraft EPM8, however, has two AUX SENDS, both switchable pre/post.  Very nice!  So, with this new mixer I have sufficient inputs for all my audio sources and my monitor mix, transmit audio level, and audio processor level settings are all independent of each other without any interaction!

One of the challenges that one faces when interfacing amateur radio equipment is the fact that it typically uses unbalanced audio connections.  Unbalanced audio connections are prone to hum and RF pickup.  Professional audio systems, by contrast, employ balanced audio connections and higher audio signal levels (+4dBu).  In the system described here, I use balanced pro-level audio whenever possible.  If I need to interface with an unbalanced audio connection, e.g., the audio input and output on the radio or PC, I use a high quality isolation transformer to provide the necessary balanced to unbalanced conversion as well as to break any potential ground loops.

The isolation transformers I use are EBTech Hum Eliminators, which contain two high-quality 1:1 isolation transformers, and EBTechLine Level Shifters, which also provide two high-quality isolation transformers, but with approximately 5:1 windings to convert audio levels from -10dBV to +4dBu or vice versa.  The Line Level Shifters are particularly useful when interfacing to PC audio cards that typically have -10dB unbalanced audio line inputs and outputs.

The audio system provides the following functions:

Main Receiver Audio

Sub-Receiver Audio

When I operate split I place the main receiver audio (the DX station I'm listening to) in the center of the sound stage and the sub-receiver audio (the split) over on the right.  Levels are adjusted so I can hear enough of the pileup to know what's happening, but not enough to interfere with listening to the DX.

Second Receiver Audio

Microphone Audio

PC-Generated Audio

Click on the link below to see a block diagram of the system. 

audio_system_w3yy.pdf - W3YY Audio Control System Diagram (Still shows old system with EPM6 mixer, but new EPM8 system with 75S-3C and 75A-4 is similar)

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