BA283/AV gain staging on 1272 type mic pre

[mhelin]

Typically BA283 is connected like on page 3 on this NeveInfo2 PDF:

http://www.celestial.com.au/~rosswood/data/images/NeveInfo2.PDF

So there is the input attenuator starting at -45 dB (well, it's just a 18k load then but anyway), the resistor to ground is connected to T terminal  for the first gain stage of BA283 is used to increase gain, the 10k pot between P and L terminals and for the second gain stage terminal K the 1.5k resistor in series of 100 uf cap is used to increase the gain of the second stage about 4 dB.

Now, Geoff Tanner lists the T and K terminal gain setting resistors and the gains get:

http://auroraaudio.net/ask-geoff/faq

Why isn't then  anyone building a 1272 type mic pre sharing the gain more evenly between the two stages?  Is there a catch?  Would the FR suffer from it or would it risk the stability of the amplifier if the second stage gain would also be increased that maximum of 20 dB? If so what would be the safe level of gain one can expect from the second stage?

 

[sodderboy]

Neve, the company, did a bunch of stuff that does not stand scrutiny today.  Change the circuitry to make more "sense" and listen to what you get.
My pet peeve, that I never understood, and the designers can not really explain, is why the Monitor Outputs of the 80 series desks used an unbalanced attenuator post output transformer to control gain.  Especially when the output module has an inter stage where a simple pot can be inserted.  The circuit as designed in real use has changing impedance and has been a bear to control with long machine room runs.  All sorts of hum or buzz issues across the sweep of the pot.  If it's such a great design, why not use it on the master fader?!?  I think it was stamped "boilerplate" at a point and copied henceforth.

No one has wanted to pay for modding an 80 series monitor output for a "logical" design so I still wait for the experiment.  For everything else Neve, well, I do not presume to know better than the designers.  The skills tip to their side.
I forget if I raised the attenuator issue with Ruff or not. . .  Geoff does not know why they did it that way.
Mike

 

[ruffrecords]

@Mikko

The gain in the second stage determines the output noise when then interstage pot is fully down or the gain is at minimum. With the recommended second stage gain settings the output noise will be around -90dBu.  As you increase the first stage gain its noise begins to dominate. For example, if the first stage achieves an EIN of -125dBu then with 60dB of gain, the noise at the output will be -65dBu.The noise will reduce as you reduce the gain until at low gain the output stage noise dominates.

So, if you raise the gain of the output stage by 10dB then the output noise will be 10dB higher all the time.

@Mike

An unbalanced pot was used only on monitor outputs as far as I remember. Monitor outputs were intended to feed monitor amps that in turn fed monitors in the control room. They were never intended to be sent over long runs to machine runs. Of course, they were designed in the days when just about everybody used transformers for balanced lines and these tend to have much better CMRR figures that modern electronically balanced inputs.

Cheers

Ian