Some years ago, i got my hands on an old and obscure mono mixing console. It was labeled just as PM5200. It wasnt of much use to me so i tore it down and salvaged parts for my own projects.
The other day i was looking through my things and found the mic preamp modules from that console and i thought this would be a great weekend project. I remember i used one of the modules for a guitar recording rig. Battery powered and with a transformer input it worked very well with Shure SM57.
The circuit uses only solid state discrete components and uses capacitor coupling. It allows for 3 gain settings and a nice line in/mic feature.
I do like the shield. 1mm thick steel.
You can see the original module with the busted gain setting. First i had to do is to reverse engineer and draw a schematic. It wasn’t very difficult, old PCB single sided, resistors all 500mW clear marked. You can see the schematic below:
With +/-15V the circuit draws about 10mA so its running pretty hot.
T101 along with D101, D102, R109, R110 for a constant current source. T102 and T103 form a long tail pair. You can attack it with balanced/unbalanced signal. You can use an input for more feedback or if you use the inputs together you can get a nice line in input with about 10x gain. T104 and T105 forms the main voltage amplification stage.
Next i took down all the components. Nothing was worth keeping.
I changed the NPN transistors to 2SC2240 from Toshiba. I really like these transistors in audio applications. PNP transistors i used BC559C, low noise and i have alot of these.
Also i replaced R119 with a multiturn pot of 5k in value. I hope i can get rid of the last coupling capacitor. We will see. One thing that surprised me was the lack of any power rail decoupling on board. I added 2 100n MKT caps.
After that i recorded frequency response which you can see below. It is pretty linear with just 1dB dropping at high frequency.
In next part i will explore some improvements and just put it in a box, add controls and make it ready to be used.
Thank you for visiting.