Modifying the Collins 356A Mic Pre for variable Negative Feedback Control

[jasavoy]

I've done it differently. 100R 2W wirewound pot in series with 7R6 across the feedback winding, 7R6 to pin 4, wiper connected cathode. There are enough connector pins to add this once you free up the input transformer center tap connection. This will give you 40-60 dB range, approximately.

The 100R value provides a bridging load to the feedback winding, so as to not disturb loading, while also minimizing overall cathode resistance change.

Fully removing NFB to get 68 dB gain destroys response, both top and bottom. You can lessen by 20dB satisfactorily, with loss of 1.5dB or so at 30Hz. The 7R6 stop resistor limits you to about 20dB.

The method above may limit useful gain to a range of 34-54 dB from the loss of the cathode bypass cap; I haven't tried it. That may be a more desired range.

Raising the cathode bypass caps to 100 or more will help linearity on the bottom end as well.

Noise goes up with gain, to be expected.


Who can tell us exactly what R205/C204 achieve? I have never seen anything like this in any other amp. A bit of modeling once suggested to me that it helps the extreme bottom end, but that modeling was likely flawed, and I'm not lending it credence.

Doug, this is cool- I saw in another thread you got this figured out. Could you give me the run down? Did you do exactly as above? What values did you end up using for the cathode bypass caps? I've got a prototype "Collins channel" built here with a 356a preamp and a 356e limiter and messing around with different options and curious what the NFB will do. Also wondering about adding a hi-pass somewhere, perhaps just in the sidechain, but would be useful in the preamp too. Can't figure out what's happening with R205 and C204 but disconnecting C204 at the grid of V202 didn't have much effect to my ear. Thanks for any help!

 

[emrr]

exactly as above, I probably put 100mfd in the cathodes, not certain.

 

[jasavoy]

exactly as above, I probably put 100mfd in the cathodes, not certain.

Thanks! Any ideas about adding a hi-pass switch to this pre?

 

[donivan]

Doug
would you time to post a drawing of your gain mod
thanks