Microphone Preamplifier Part 1: Restoration

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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.

Original Mic Preamp Module

Original Mic Pre PCB

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:

PM5200 MIC PRE Original Schematic

 

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.

MIC PRE PCB

MIC PRE PCB2

 

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.

Modified Mic PReamp

Modified Mic PReamp PCB

 

After that i recorded frequency response which you can see below. It is pretty linear with just 1dB dropping at high frequency.

PM5200 Mic pre frequency response

 

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.

 

2 comments

  1. Bebetto says:

    That is a device made in Romania, you can tell by the capacitors (IPRS) and resistors. Are you Romanian? Is there a year marked on the capacitors? Knowing the year they were made you can find out more about the device. It was probably made by Electronica Bucharest. Cheers!

    • Stefan says:

      Yes it was a Romanian product. I’m afraid i don’t have it anymore to check the old capacitors…
      Thank you for your interest.

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