Looking at many pro recording quality microphone preamps, I noticed that every design I looked at that uses an opamp (discrete or IC) limits the gain provided by the opamp to about max 60dB. While most preamps use another stage (transformer(s) or another opamp) to get to 70db or even 80dB, I wonder why they don't just use the first opamp to get there. From what I understand, there would be some advantages:
- better signal-to-noise ratio as voltage gain rises,
- simpler audio path,
- less parts & cost.
Does it have something to do with opamp stability over 60dB?
Here is a typical schematic. R12 limits gain to 40.1dB. I'm using these formulas:
$$A = 1 + (R_{fb}/R_{in})$$
$$gain_{dB} = 20 * log(A)$$
I also noticed that complete mic preamp ICs made by THAT-Corp also have a maximum gain of 60dB.
Best Answer
Gain/Bandwidth product, you want maybe 50KHz bandwidth at 60dB (1,000 times), so you need somewhere around 50MHz, gain/bandwidth product (And more would lower HF distortion)... Make it 80dB and now you need 500MHz GBP, which is getting difficult if you want low noise down near DC (And is getting really bad news to stabilise at low gain).
Also consider that the noise is completely dominated by the noise for the stage having the first 20 or 30dB of gain (Do the maths), there is a lot to be said for splitting things so that the first maybe 30dB of gain happens in a low noise stage designed for low Z sources and low 1/F noise, which now only needs a few MHz of GBP and will be easy to stabilise even with weird source impedance. Then do the rest in a second stage (where noise matters less and you have a known source impedance).
The other difficult thing is that control laws that make sense become increasingly tricky if going for a one knob gain control, a classic instrumentation stage with have a gain setting resistor varying from a few ohms to maybe a few k ohms, which if you think about it is only maybe 3 orders of magnitude, very difficult to make a reverse log pot have more range then that.