Audio Circuit

Hi Folks,
I am hunting for a reference design for or to purchase outright a
circuit board which will allow the combination of several audio signals
into an aux audio connection. (I am sure this has been hashed out here
in the past, but my searches on this group didn't end my cluelessness)

The particulars:

1. Single seat aircraft
No intercom or audio panel

2. Becker 4201 Comm
Aux audio input:
---Snipped from manual 23-10-01 of the Becker 4201- SN 1K and up--

"Auxiliary" audio input
The AF-AUX (P1/4) auxiliary audio input enables the switching of audio
signals from other equipment in the aircraft. The switched-in audio
signals can, however, only be monitored in the reception mode. The
facility to switch two units together will be used particularly in those
aircraft which are fitted with an Transceiver and an NAV receiver. An
audio input voltage of 1 to 8 V, 600 Ohm is necessary for modulation of
the audio audio amplifier (can be adjusted in the "Service" mode).

Audio auxiliary input
A second and third radio unit (navigation receiver) can be monitored
simultaneously via the audio auxiliary input. During transmission the
auxiliary input is switched off from the audio end amplifier. If
necessary, the input sensitivity can be matched to the noise level of
the aircraft (for setting refer to service mode).

-------------------------------------------

3. Several audio inputs:
AOA, EFIS(various warnings), EIS, gear warning, IPOD, (maybe another)

My goal is to integrate all of the audio inputs independently into a
well designed circuit board. Then have one input into the aux audio
port of the Becker 4201.


Jim Weir in Dec 06 issue of Kitplanes introduced and outlined the use of
OPAMP Circuits to integrate these systems. It is this technology I
believe is appropriate for my integration needs.


Before I begin to hack together my own circuit, is there a solid
reference design out there from which I can build? Any guidance is
greatly appreciated.

Ken