Tag Archive for microphone preanplifier

Microphone Preamplifier Part 2

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Hello Project Ryu fans,

About a month ago i reversed engineered a mic preamp module from an old console. If you don’t remember please refer to the article here. After i rebuilt the modules with newer components i decided to build a nice 2 channel preamp with them.

Below you can see the circuit for the unit. M1 and M2 are the modules presented in part 1.

Dual Channel Mic Preamp

As you can see it splits a 12V rail into +/-5V duble rails. I did this because i wanted to make it more mobile so i can power it from a big battery.

Below you can see my example of chassis.

 

Dual Channel Mic Preamp

Dual Channel Mic Preamp

Dual Channel Mic Preamp

Dual Channel Mic Preamp

This preamp doesn’t have a phantom power although you could easily add a +12V phantom power. In this regard it is best suited for dynamic microphones.

Thank you for visiting.

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.