Mic Preamp Gain

[Mfinzi]

Good morning, l apologize for diverting your attention, please help me. I would like to know how to calculate the gain accurately of this type of preamp circuit. which resistors give the gain. I want to do some frequency response measurements. thanks regards

 

[AllenB]

Question given its own thread.

 

[PRR]

> which resistors do you use to determine the gain.

R9, R14,VR1, R13, R12.

Why do you think the frequency response needs measuring? (Worst-case on lows is about 8Hz; highs is very supersonic, and dead flat between.)

 

[Mfinzi]

Mic Preamp

Thanks trying to use ARTA, will need a mic preamp with precise gain steps. User manual recommends a Monacor mpa-102, with a stepped gain, trying to build mine at a small budget. Phil Allison states that this circuit goes to 60db, how do you arrive at this with the resistors that you illustarated

 

[MarcelvdG]

In addition to what PRR listed, the current dividers R4 - R10 and R8 - R11 attenuate the current coming out of the first stage by a factor of 11 and therefore also influence the gain.

 

[Mfinzi]

Mic Preamp

if the pot was at minimum what would be the overall gain, conversely, if it was at maximum, what would be the overall gain

 

[MarcelvdG]

Assuming infinite loop gains in both the discrete part and the op-amp, Zmic << R1 + R5, Zload >> R16 and R4/(R4 + R10) = R8/(R8 + R11), the voltage gain is:

(R12 + R13) * R4/(R4 + R10) * (1/(R2 + R6) + 1/(R9 + ((R14 * VR1)/(R14 + VR1))))

 

[MarcelvdG]

Assuming +/- 15 V supplies, you have to add about 1.2 ohm to 1/(1/(R2 + R6) + 1/(R9 + ((R14 * VR1)/(R14 + VR1)))) to correct for the effect of finite loop gain in the discrete part, so the gain then becomes

(R12 + R13) * R4/(R4 + R10) * 1/(Rcorr + 1/(1/(R2 + R6) + 1/(R9 + ((R14 * VR1)/(R14 + VR1)))))

with Rcorr ~= 1.2 ohm.

 

[Mfinzi]

Mic Preamp

thanks for the formula, though, it wont give me precisely 60db on a spread sheet , when the VR1 is 0, its also complex to comprehend. but i will survive. great regards