Help, audio mixer!!

[Seige332]

Ibwork for a local law enforcement agency; we were looking for a device to be able to record our phone calls during a serious incident like a hostage negotiation. I found the attatched schematic and attempted to build. I removed the variable resistors (potentiometers) thinking maybe i wouldnt need them. Here is the issue that i am running into... The mic level is extremely low, i installed an inline preamp for the mic and plugged only the mic in and it is still extremely low so its not being cancelled out due to the line in. My question is what would be my issue here? Is it that the potentiometers are a must or is there something else i could be missing. Please help thanks

 

[Alec_t]

Welcome to EP!
The potentiometers may or may not be necessary, depending on the two signal sources. In the posted circuit, R1 and R2 allow the input levels from the two audio sources to be individually adjusted (preset). This might be necessary for high level inputs, to avoid over-loading the amplifier stage. R3 is a balance control, and R4 is an overall volume control.
Can you post details of the mic amp (ideally a schematic) and what signal level is driving the other input?

 

[Alec_t]

..... in the meantime, try reducing the 100k bias resistor to 33k, or (to draw less battery current) increasing the 15k bias resistor to 33k. Simulation shows the transistor is incorrectly biased in the circuit as shown.

 

[Audioguru]

I agree that the transistor is biased wrong, it is almost cutoff.
Each 47k resistor feeding the input resistance of the transistor and its bias resistors reduces the level to 1/11th. Then the voltage gain of the transistor will be about 180/11= 16.4 times. A mic usually needs a gain of 100 to 200.