Op amp based amplifier noise source clarification...

[Newmarket]

However, as the gain is reduced from 60dB to 40dB, the noise at the output of the mic pre drops by nearly the same amount so the important thing, the signal to noise ratio, does in fact increase. Further illustration of the futility of using a single figure to describe performance.

Yes. It's a lot easier to explain with a graph / sketch.
I usually put it in terms of the gain goes down 20dB and the noise goes down too but not by quite as much. But if you need only 40dB gain to get the same output as you did with +60db - with  x10 different signal sources obviously - then you're winning.

 

[Tubetec]

Lots of good stuff here,
I could only skim over as Im out for the afternoon.

I did find a nice op amp noise calculator ,link escapes me just now ,but I'll post it later .

I generally use single stage valve mic pre's ,in conjunction with an input transformer of the right ratio ,it works ok on everything from dynamic mics with high level sources ,to condenser's with quiet or loud sources. I've seen a few say in the modern age mic pre gains of 60-70db are almost never needed and that fits in well with my experience in real world situations.

 

[Matt Syson]

Someone commented to me (many years ago) that designing for around 30 - 40dB of gain is more difficult than 60dB plus. I forget WHY (if he ever told me) but accept it. It is posibly why the early Neve designs had 3 stages available and at midrange settings the first (or second) stage was switched out.
At the time it was BC109 (selected) for your low noise transistor and (mostly carbon resistors (except the expensive 2% metal film that became available which offered 'low (er) noise and greater gain accuracy. Then again there are many other opportunities to mess things up, especially in the modern 'switchmode' age.
I suppose you could go the 'cryo' (seroious cooling) route BUT musicians would probably object to the cold (as would instruments).

 

[JohnRoberts]

"Someone" is wrong...

JR

 

[abbey road d enfer]

Someone commented to me (many years ago) that designing for around 30 - 40dB of gain is more difficult than 60dB plus. I forget WHY (if he ever told me) but accept it.

When the gain control is achieved by changing emitter degeneration, the input transistor(s) see a larger total source impedance at low gain, which increases the equivalent input noise. This is particularly noticeable on xfmrless mic preamps, as seen on the THAT datasheet.
With transformer input, the difference is not so great, partly because the gain range is smaller.
Now making a circuit with 60dB gain without a xfmr requires specific attention to other issues, such as HF THD and slew-rate.

A few years ago I designed a preamp optimized for gains between 25 and 45dB, because I had noticed that I very seldom needed gain outside this range. I had the luxury of having Sowter bulding me custom large-core xfmrs. Noise was never an issue, although the performance is quite respectable at less than -127dBu at any gain. I spent more time optimizing the transient response and interfacing with teh outside world.

 

[JohnRoberts]

I gave some though to this years ago since we don't always need 60+ dB of gain to drive a 5V a/d to full scale.

If you scale down both the feedback and gain resistors you don't suffer the Johnson noise penalty. Conveniently there are some modern uber opamps that can drive very low load impedances that would support lower impedance feedback networks.

As Abbey shared designing a intermediate gain preamp is mainly a slightly different design problem. The off the shelf mic pre ICs are optimal for circa 60dB gain.

JR