Opamp Labs 425 - Mic Preamp

[beezer4]

I recently purchased an Opamp Labs A-45 Distribution Amplifier with 4 of the 425 plug-in opamps inside. I'm hoping to turn this into a 2 channel mic preamp transformer balanced in and out. I'm removing all the unnecessary resistor distribution network.

Info on the 425 can be found here: https://www.opamplabs.com/products/logic-module-plug-in/d-c-amps-signal-level/425.html

I've been reading on GroupDIY / TapeOp and other forums that a single 425 wouldn't be able to drive a 600ohm output and to leverage a second opamp. I'm looking for some insight on how to implement this.

Rough idea:
1:10 Input Transformer -> Input gain control -> 425 -> 425-> Jensen JE-123-S 1:2 or 1:1 -> Bourns 600 ohm T-Pad Attenuator

My sticking point is which 425 the gain should come from and what if any resistor(s) are required between the two 425s.

The unit already includes the necessary  +-24VDC power supply.

If I can clarify anything, please let me know.
Thanks!

 

[shabtek]

starting points for 2 opamp transformer coupled pre
https://www.jlmaudio.com/JLM99XMB.pdf
http://www.jlmaudio.com/Baby%20Animal%20Dual%20Mic%20Pre%20Schematic.pdf

 

[PRR]

> I've been reading ....single 425 wouldn't be able to drive a 600ohm output

It says it will, on the can.

Do you really have 600 Ohm loads?

 

[rackmonkey]

What PRR said. They'll drive 600 ohms.

Most of what you want to know is here: https://groupdiy.com/index.php?topic=69637.0

My sticking point is which 425 the gain should come from and what if any resistor(s) are required between the two 425s.

gain comes from the opamp on the input. If you need to drive a couple of miles of cable then put the second 425 configured for unity gain (they are unity-gain stable) afterwards. 10k resistor between first and second 425. 

Better approaches for controlling gain can be found in some of the schematics attached in the thread above.

Good luck. The basic idea you're going for makes a great sounding pre.