Langevin 5116b/UA 1016 tube preamp - adding a 3rd gain stage

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Matt C

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I'm writing to get some help/ideas for a mic preamp build I'm planning.

I was going to build a preamp based on the Langevin 5116b (schematic below, very similar to UA 1016 circuit), but I'm interested in adding a 3rd stage to squeeze a bit more gain from it.  Stock, the gain maxes out at 46dB, I'd like to get something more like 60-65dB.  Seems like the thing to do is insert another gain stage in between the existing input and output stages.

I do have some understanding of tube circuits but designing something like this is a stretch for me.  If anyone has some help and insight into the issues to address, and how to make it work, that would be great. My biggest question is how stage 2 needs to be adjusted (compared to stage 1's design) to accommodate the amplified signal coming out of stage 1.  Is it just a matter of adjusting the bias point so Vg-k is larger? Or am I overthinking this?

Somethings I know I'll need to consider:
- Make sure stage 2 can handle amplified signal level
- The negative feedback will have to be moved to the 2nd gain stage, so that it doesn't become positive feedback.
- 2nd stage shouldn't need any serious output drive capabilities since it's just driving the 470k grid resistors of the output stage, so another 12AX7 or similar should work fine.

Any help or ideas are welcome, thanks!


 

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abbey road d enfer

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you may simply disconnect global NFB by disconnecting R5 and R6.  I guess that would increase gain by about 20 dB. But I wonder why you would need the additional gain...
 

Matt C

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abbey road d enfer said:
you may simply disconnect global NFB by disconnecting R5 and R6.  I guess that would increase gain by about 20 dB. But I wonder why you would need the additional gain...

I guess that would be the simplest place to start.  I assumed that completely eliminating NFB would increase distortion and/or mess with the frequency response to an unusable level, but maybe I'm wrong.

extra gain is for when someone decides to sing extra quiet into a SM7b (or whatever). 
 

gridcurrent

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Matt C said:
extra gain is for when someone decides to sing extra quiet into a SM7b (or whatever).
you can do like what was done a half century ago:  add an amplifier.
 

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abbey road d enfer

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Matt C said:
extra gain is for when someone decides to sing extra quiet into a SM7b (or whatever).
You may replace R5 & R6 with a dual 1Meg pot or a rotary switch. (If usinf a pot, make sure both sections track well).
 
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