Neumann U474A module mod question with schematic

[anticpunk]

Hello all,

I'm racking a pair of Neumann U474A modules as utility pres. Originally they were talkback modules, but they offer 64db of mic gain and balanced output, so they a worthy candidate for a general purpose mic amp.

My question is, they have a built-in FET limiter, feedback design, and I'd like to be able to control the attack and release time, even in an archaic way. Not looking for precision, and since it's feedback limiting I understand that precise times and cobtrol aren't feasible. I just need to know which component values could be made switchable to modify the times.

I've read that stock attack is 1ms and release is around 1s.

Disabling the limiter I believe can be done by removing the signal from the gate on the FET from the comparator circuit, but that's all I have pinned down so far....

Suggestions appreciated.

 

[abbey road d enfer]

Atk time is governed by R25 (1k) & C10 (0.8uF), release by R20 (10 Meg) & C10 also. In fact the atk time is shortened by the accelerating capacitor C12 (10nF).
You could replace R25 & R20 with potentiometers and see if you like it. I would replace R25 with a 22k pot and R20 with a 2.2Meg for a start.
I would also disconnect C12 or make it switchable.
You don't need the extra-fast Atk and the extra-long Release for recording.

 

[anticpunk]

Precisely the info I was hoping for! Thanks so much for the quick reply!

Now that that's out of the way, does anybody have any experience with these or similar Neumann mic amps? Just curious about the sonic quality of them......

Thanks again!

Jay

 

[anticpunk]

Abbey Road D Enfer, another quick question if I may, since C10 is a common element to both attack and release times, is it then feasible to add let's say a rotary switch, to the circuit to shift the entire range of the limiter?

Increasing capacitance seems as though it must slow down the limiter  and increase both times  if I understand the circuit flow correctly.

Just a thought. I realize the limiter is basic and probably not a huge asset for tracking anyway, but understanding how the envelope signal is controlled will help me better understand similar circuits later.

Thanks,

Jay

 

[abbey road d enfer]

Abbey Road D Enfer, another quick question if I may, since C10 is a common element to both attack and release times, is it then feasible to add let's say a rotary switch, to the circuit to shift the entire range of the limiter?

Yes, you could do that, but I suspect you'll really want to increase the Atk time and decrease the release time, which requires acting on the resistors.
Now, instead of blindly following the advice of a dubious advisor on the internet  ;D, you should go to your soldering iron and forge your own opinion. 

 

[timtom]

How did you make the limiter switchable? Did it work with cutting the signal at the gate of the FET?

 

[timtom]

Is there a alternative to the VCR2N FET? I got one preamp that is missing the limiter FET. It works but doesn't limit compress.

 

[abbey road d enfer]

Is there a alternative to the VCR2N FET? I got one preamp that is missing the limiter FET. It works but doesn't limit compress.

You may try to find a N JFET of similar characteristics:

VGS(off) Gate to Source Cutoff Voltage -1 V
rds(on) Drain to Source ON Resistance 20 Ω

I would look at J109, J112, J309, BF861C
Yiou would probably need to make ajustments and have inadequate tracking in stereo mode.

 

[timtom]

You may try to find a N JFET of similar characteristics:

VGS(off) Gate to Source Cutoff Voltage -1 V
rds(on) Drain to Source ON Resistance 20 Ω

I would look at J109, J112, J309, BF861C
Yiou would probably need to make ajustments and have inadequate tracking in stereo mode.

Thanks for the reply.

It doesn't need to be perfect. Even different characteristics are wanted, for example less limiting.

 

[abbey road d enfer]

Thanks for the reply.

It doesn't need to be perfect. Even different characteristics are wanted, for example less limiting.

Then you just need to experiment; there is no risk in it.