Junk box mic preamp via JLM Audio GAIN

[skipwave]

Many thanks to @JLM Audio

I built this because I had the parts lying around. JLM’s GAIN PCB made it quicker and easier.




Input transformer is a random vintage with military code, I paralleled the secondary to use as a 1:7 turns ratio.

Output transformers
- Jensen jt-11-hfmpc (clean)
- Opamp Labs T-18 (dirty)

discrete opamp is the “Big FET Bloke”, a design born right here on this forum

The circuit I built matches the JLM Baby Animal, I just did it using the smaller JLM GAIN circuit board. The difference is that the Baby Animal PCB has the switches mounted to the front edge of the board and needs a deeper chassis.
Full circuit schematic: http://www.jlmaudio.com/BA/Baby Animal Ver2 Black PCB.pdf

Little power supply brick, if you want power inside the box:
JLM Audio Shop
Note: I have built Baby Animals with external power supplies, which you can get from JLM or find cheaper on eBay or Amazon

This is the little square “GAIN” PCB that I used:
JLM Audio Shop
Alternatively, this is the full featured Baby Animal preamp PCB that has the Phase / Pad / Phantom switches on the board:
JLM Audio Shop
If you can’t find a vintage input transformer, this one that JLM makes has the mojo:
JLM Audio Shop

 

[3nity]

Nice use of spare parts!

 

[mylesgm]

I originally asked Joe to make that GAIN pcb up to use spare parts I had lying around but I haven’t as yet done the projects, I have a dozen or so to do. Any pitfalls to watch out for? And what’s the big cap to pot for?

Nice work!

 

[skipwave]

@mylesgm the quote feature wasn’t working for me, so….


> I originally asked Joe to make that GAIN pcb up to use spare parts

Amazing! Happy to play a part in helping it fulfill its destiny.


> what’s the big cap to pot for?

The discrete opamp uses current feedback rather than the vastly more common voltage feedback. It requires a very large value capacitor which goes after the gain pot unlike the smaller cap before the gain pot in voltage feedback circuits. Search this forum for “gain bloke”, if interested.

 

[mylesgm]

@mylesgm the quote feature wasn’t working for me, so….


> I originally asked Joe to make that GAIN pcb up to use spare parts

Amazing! Happy to play a part in helping it fulfill its destiny.


> what’s the big cap to pot for?

The discrete opamp uses current feedback rather than the vastly more common voltage feedback. It requires a very large value capacitor which goes after the gain pot unlike the smaller cap before the gain pot in voltage feedback circuits. Search this forum for “gain bloke”, if interested.

So the cap is particular to this DOA and not generally needed?

 

[skipwave]

So the cap is particular to this DOA and not generally needed?

Yes, that is correct. With a voltage feedback opamp you would stuff the 470uF on the GAIN PCB, which I jumpered to accommodate the Big FET Bloke.

 

[skipwave]

Oh, and search this forum for “gain bloak” as it was misspelled initially and maybe always.