Telefunken AR51 DIY/Schematic/Layout

Hi guys. I spent last few days collecting values from pics on the web, and other resources to make simple layout for this project.

Now that i actually have made the mic in Sterling Audio st51 body i can confirm that it works great. I used regular chinese power supply modded to 120v. I did make a cardioid only version though

I chose this project because of simple pcb that can be easily built on perfboard, not a lot of components, and widely available capsule.

I used perfboard but high impedance part is floating in mine.

I used russian 6n16b-v tube in paralel.

I even used stock st51 transformer which is 5:1 but that caused some high end roll off. With this capsule it works great, so i kept it. After FoxAudio measurements the original has it as well

I believe original is 11:1

Some values vary in diferent pcb versions. Mostly high value resistors.
I couldn't find electrolytic cap value, but if it's like in Copperhead it should be 47uF.

I used info on bias, original tube, thd, pictures, etc from:

http://www.foxaudioresearch.ca/ar-51.htm

I hope people will get my hieroglyphics...

Original pics from foxaudioresearch

Note that not all values match this (older) version with k67 capsule.

Traces.

These two images are easy to overlay in photoshop for example. In that case front side with components stould be mirror imaged.

Stereo version AR70 with better visible components. This is newer version pcb.

My mic is still a mess, i used tube in parallel, so there are wires all over the place, i had 220k 1w resistors so i used two in parallel. Tranny is held by tape, excuse me for that. I will fix it eventually. This was really quick build. I made it in about 3 hours.

No B+ filtering caps, i compared it to ksm44 which is quiet, this one is in that range, slightly noisier, must be great tube. Really surprised...

Why would Telefunken bother thickening traces? I have to wonder how any PCB trace could have higher resistance than some  P2P wiring (30AWG in M49, I believe).

Are you after the low frequency rolloff of the Ar51 or trying to avoid it?

This is budget T mic. I guess p2p is out of question. Although i wouldn't pay over 300$ for a new one.

Quote from FAR:
Telefunken Electroacoustik mentions using heavier traces on their circuit boards and we can see that this is true. The technique of thickening the traces with solder was used on almost all printed circuit board equipment in days of old and it still has the same effect today; much lower resistance traces.

I don't see the point of doing this.
The difference between 0.1 ohm and 1 ohm can be neglected in a microphone ciruit.
(Although it wouldn't hurt also...)

Well this whole RFT series is about marketing mumbo jumbo, so...

I just made it as its simple and straightforward to copy and requires not so many parts. Not that it sounds bad, but i just laugh out loud every time i see the price and marketing plot.

Delta Sigma said:

Why would Telefunken bother thickening traces? I have to wonder how any PCB trace could have higher resistance than some  P2P wiring (30AWG in M49, I believe).

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In the absence of any coating applied in production it prevents oxidisation of the copper.

MagnetoSound said:

In the absence of any coating applied in production it prevents oxidisation of the copper.

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Usual that's why solder mask is so popular but i can bet that in AR51 it was only to make the look more "vintage".
Yeah real deal "mumbo jumbo"

ln76d said:

Usual that's why solder mask is so popular but i can bet that in AR51 it was only to make the look more "vintage".
Yeah real deal "mumbo jumbo"

Click to expand...

Yeah, I wouldn't argue with that.

Delta Sigma said:

Are you after the low frequency rolloff of the Ar51 or trying to avoid it?

Click to expand...

I really cant make up my mind about that. If this was a fet mic i would go flat, as i dont mind EQing clean low end afterwards.

However signal coming from capsule is saturating the tube and transformer in different maner if lows are rolled off. On larger scale it's kind of like OD guitar pedal vs fuzz. Fuzz goes as full range signal into clipping section while OD has rolled off low end. I am also not a huge fan of saturated low end. If i saturate kick drum in a mix for example i saturate above 500hz, and leave clean low end. On bass guitar i use clean DI for low end, and amp sound for mid range...

I might be wrong, but this is just the way my logic works. In most famous tube mics some kind of roll off is also present.  I am also huge fan of sm57 on kick drum, but not fan of it on snare 

I buy what you're selling! Certain saturations or colourations - to my ears - can wash things out.

Same logic as when you go too big on an output cap the low end of the mic kinda washes out - or becomes weak to some ears.