Four (or more) inputs can be mixed and produce stereo output. Gain of each stage can be boosted by adding RX, but it should be kept below 50 (RX above 2 K) to avoid poor frequency, response. If more than four stages are used, decrease RXto 6 K for six inputs, or 4 K for eight inputs
The op amps are 741 or other lower noise types. The power supply circuit is also given.