Active Ribbon Preamp Circuit to MicPre for extra gain

I have some  racked Shure M67 micpres.
I really like the tone but would like to see more gain in circuit.

Could PRR's active ribbon preamp circuit be added somewhere for that extra gain?
Maybe single ended just before Shure circuit out (so to do 2 channels of gain for each diagram build of circuit)?
Prolly need switching in and out of circuit via relays.

I have a 48v rail on the PS so that's a start...

I'd rather add something after it (line amp) than modifying the input topology... Nothing fancy needed there, just a NE5534 with some trim for the gain and impedance balanced output should do the work quite well, I wouldn't break my head around it.

Nice day to start a new project! Happy New Year!

JS

stitch-o said:

I have some  racked Shure M67 micpres.
I really like the tone but would like to see more gain in circuit.
I have a 48v rail on the PS so that's a start...

Click to expand...

1.  it appears you have changed the placement of the 50k channel gain control.  By loading the 1:14 input transformer into the pot,  there is loss of over 2 dB, and more importantly, the sonic benefits of an unterminated secondary.
2.  the Shure M67 is specified at 90 dB of overall gain.  Considering how plentiful and inexpensive those mixers are on the used market, why not simply get additional units rather than doing the direct out mods ?
3.  if proceeding with added gain stage plan, a TL074 would take little space and provide enough drive for modern bridging loads.  A negative rail can easily be added to the existing power supply.
4.  the 0.47 uF coupling capacitor is a bit small even for bridging loads.

Thanks for the ideas gents!
I am looking around at IC line driver circuits now.
I am using Jensen JE-123 wired 1:2 for balancing output.

@Grid: the LF rolloff of the .47uf coupling cap would be better at 1uf or 2.2uf? Would 4.7uf be too large value?
Also, the circuit I followed is a mod. I did notice there is DEFINITELY not 90dB of gain.
These are cheap mixers but I don't need more channels, I need more gain in the channels I have.

Forgive the question: whats the benefit of unloaded input transformer secondary?
Maybe I should return circuit to original with larger value couple cap and that might give me some gain back?
How would returning pot to output of circuit affect the primary of output transformer, if at all?

EDIT: I found a line driver design at PAD forums that might work:

stitch-o said:

@Grid: the LF rolloff of the .47uf coupling cap would be better at 1uf or 2.2uf? Would 4.7uf be too large value?

Click to expand...

you do the math.

stitch-o said:

Also, the circuit I followed is a mod. I did notice there is DEFINITELY not 90dB of gain.

Click to expand...

the spec sheet lists 90 dB total available gain including the built in line amplifier.

stitch-o said:

Forgive the question: whats the benefit of unloaded input transformer secondary?

Click to expand...

the topic is covered in Tremaine's Audio Cyclopedia 2nd edition as well as in the microphone section of RCA Broadcast catalogs, available at AmericanRadioHistory.com.  To summarize:  unloaded secondary will manifest more transformer ringing, lower noise and generally more music.  loading the secondary will result in prettier square waves, more noise, and contribute to global warming.

The amp section post summing in the original shure
mixer accounts for some of the gain missing Im sure.


Hacked up schemo 1:

An additional gain of 78dB might be excessive?