I have a situation perhaps someone here can help with. I'm currently testing a circuit (see attached screenshot) I designed from some hints and some knowledge. A simple two transistor mic preamp, and the trick is that its powered from the microphone input line from the system. The mic input line has 4.5VDC on it, and I've successfully made a preamp that takes my 5-15mV dynamic mic signal and boosts it as high as 160mV, which is plenty for the application, but could be higher on another system I use if I can figure the best way to increase the gain with this low voltage I have to work with. I need to make it an adjustable gain to be able to apply this same circuit to other systems as well. VOX thresholds are as high as 100mV on some systems, for example, and as low as 14mV on others.
So, looking at the resistors in the circuit, I'm thinking R8 and R3 set the gain of Q1, and perhaps R1 sets the gain of Q2? I admit to some guessing here. The R7 is there to limit current, and the amp is very sensitive to changes in it or in R9, which I use to represent the internal input impedance of the system the mic plugs into.
To control overall gain, I'm thinking make R7 a variable resistor. To increase the max gain of the TPv2 node, I'm thinking I need to tweak both transistors to perhaps operate at sqrt(40) gain each so I get a max gain of 40 (voltage gain, that is). Hence my question about which resistors really control the gain of the two stages. Admittedly, this has been confusing since it's not a typical topology for me. For example, is the 220K R2 feedback resistor really needed??
I'm open to suggestions on a better design, and would even pay for a solid one if anyone can offer it.
Many thanks,
Ed
So, looking at the resistors in the circuit, I'm thinking R8 and R3 set the gain of Q1, and perhaps R1 sets the gain of Q2? I admit to some guessing here. The R7 is there to limit current, and the amp is very sensitive to changes in it or in R9, which I use to represent the internal input impedance of the system the mic plugs into.
To control overall gain, I'm thinking make R7 a variable resistor. To increase the max gain of the TPv2 node, I'm thinking I need to tweak both transistors to perhaps operate at sqrt(40) gain each so I get a max gain of 40 (voltage gain, that is). Hence my question about which resistors really control the gain of the two stages. Admittedly, this has been confusing since it's not a typical topology for me. For example, is the 220K R2 feedback resistor really needed??
I'm open to suggestions on a better design, and would even pay for a solid one if anyone can offer it.
Many thanks,
Ed
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