Been doing some research into the Transamp mod of the instrumentation front end.
Jim has covered the conversion in the Soundcraft consoles 600 series etc. Likely can go with similar changes in this mic pre front end. For now I will keep the SB737 transistors and go with a more modern chip as suggested by Jim. Will copy previous discussions on this topic here.
Originally Posted by Jim Williams
It's a rather simple mod, you remove the 22k resistor and 100 pf cap in the opamp's feedback loop. Connect the 22k resistor with a 10 pf cap across it to the transistor's emitter, the transistor that feeds the - or inverting input. Don't connect to the non-inverting (+) input or you create an oscillator! Change the 100 pf cap from the non-inverting input to 10 pf, that open's up the bandwidth to 200k hz. You may need a small phase lag cap across the opamp's inputs, a 220 to 470 pf cap will usually work. That up's the phase margin so the opamp remains stable.
Or, get a ver. 7 or higher 600, the changes are already there. Changing the transistors helps at higher gains, try 2SA1084's or 1316's. The opamp is also very important, many can be used, try the LME49720, AD8599, LM6172, LT1358, etc. Local .1 uf psu bypasses are required for wideband opamps.
MoreNow, bypass the electrolytic caps with Wima MKP-2 .01 uf 250 v. Change C8 to 470 uf with a .1 uf Wima MKP-2 100v bypass cap. Change R10 to 15 ohms to make up the 6 db you lose at the top of the gain scale. Remove RIC 1 and add 4 4.75k 1% metal film resistors. Add a pair of .1 uf mono ceramic caps from pins 4 and 8 to a close by ground. Now you can play with opamps. Try LT1358CN8, AS8599, LME49720NA, LM6172. Check with a scope for stability as the second half runs the filter which is a follower, 6172's don't usually like followers.
Modifing the front end from an instrumentation design to a "trans-amp" does wonders for THD reduction. Since the mic preamp is also your line input for mixing, cleaning that up does wonders.
Pull off the feedback resistor from the inverting opamp input pin 2 and attach it to the emitter of TR2. Leave the feedback cap in place. Then remove R12/C10 from pin 3, the non-inverting input. Attach those parts from the emitter of TR1 to ground. This applies to versions 1 and 2.
Use 10 ohm 1/2 watt fuse resistors. Use 470 uf/25V bypass caps. Use Wima MKP-4 .1uf 250V for the ground decoupling and chassis ground de-coupling. Add two more for the .1 uf mylar psu decoupling, 4 are used total.
Use 4.75k and 2.21k for the fader amp resistors. Use a 22 pf feedback cap. I use a 100 ohm resistor in place of the 2.2 uf cap, short out the output cap and use a LME49990 opamp, lowest noise available. Then it's direct coupled, super low noise and fast. Then be prepared to hear more "uglys" in your music
Pic of mod in the schematic is attached