The current circuit produces good results. However, it does not follow standard design practises.
Although the gain of the current design is stated to be 1 -> 10x, it is actually 0 -> 5x if one input is connected due to the configuration of the amplifier. If both inputs are connected to the same source gain raises to 6.6x
Input impedance of the current design is 2K, which is fine, but a little low. Typical mixers are in the 10K -> 20K range.
The low frequency cut-off of the current design is 8Hz, again a little low.
Speaking individually, I would also like to see a 1/4" input for connection to synthesisers in addition to the RCA
See below for a more conventional design with 10K input impedance, three channels, a gain of 0 -> 10x, and a low frequency cut-off of 15Hz :
Construction notes
For v1.0 PCB boards - this omits my proposed 1/4" jack.
On v1.0 PCBs replace R4 and C1 with jumpers. Replace R1 and R2 with 10K. Replace RV1 with a 100K log pot.
Mount 1uF capacitors serially on the pin of the input jacks.
Be careful: the silk screen on my PCB did not the match the schematic for R1, R2 and R4.
BOM
2 or 3 of 10K resistor (1 per input channel)
2 or 3 of 1uF capacitor (1 per input channel)
1 of 100K log pot
1 of 1/4" chassis mount mono jack socket (optional)
The current circuit produces good results. However, it does not follow standard design practises.
Although the gain of the current design is stated to be 1 -> 10x, it is actually 0 -> 5x if one input is connected due to the configuration of the amplifier. If both inputs are connected to the same source gain raises to 6.6x
Input impedance of the current design is 2K, which is fine, but a little low. Typical mixers are in the 10K -> 20K range.
The low frequency cut-off of the current design is 8Hz, again a little low.
Speaking individually, I would also like to see a 1/4" input for connection to synthesisers in addition to the RCA
See below for a more conventional design with 10K input impedance, three channels, a gain of 0 -> 10x, and a low frequency cut-off of 15Hz :
https://www.dropbox.com/s/260vo72rjcyfoi1/OAS%20improved%20input%20stage.jpg?dl=0
Construction notes For v1.0 PCB boards - this omits my proposed 1/4" jack.
On v1.0 PCBs replace R4 and C1 with jumpers. Replace R1 and R2 with 10K. Replace RV1 with a 100K log pot. Mount 1uF capacitors serially on the pin of the input jacks.
Be careful: the silk screen on my PCB did not the match the schematic for R1, R2 and R4.
BOM 2 or 3 of 10K resistor (1 per input channel) 2 or 3 of 1uF capacitor (1 per input channel) 1 of 100K log pot 1 of 1/4" chassis mount mono jack socket (optional)