mercredi 4 octobre 2017

The making of a DIY Neumann U87 microphone

I needed to extend my mic closet and decided that a good old Neumann U87 copie should do the trick. I did start some tube mic, but the extra cables, deadly PSU kept me from finishing those yet.
So I wanted a "normal" condenser mic, one I could use on nearly everything, and the U87 was the safest bet. It's well known, respected and most of all easy to make with PCBs, capsule and parts easily accessible. And the doc to build one is extensive.

The components

I chose a kind of standard component set:
PCBs from Dan (amazing vintage mics PCB website), SYT5 body from Studio 939 (it's a bit bigger than the original), cinemag 13113 transformer, and Maiku k87 capsule. The capsule is made in Switzerland and I was lucky enough to get one of the first, with a massive discount. It feels much better engineered than the usual Chinese one.
I also chose to use Rev. 21 of the schematics, and use the styroflex caps set from Dan, as it seems to be the standard configuration.
The price
Capsule 115€, components 72€, body 150€ , transformer cm13113 42€, PCB and styroflex caps roughly 40€
Total: 420€ not including shipping costs for PCB, transformer and body.
I could have done it a bit cheaper if I had fine tuned the BOM as some components are doubled, and I also ate the extra 20€ components shipping costs. It's on the cheap side though, and the capsule is a really good deal.


The making

Populating the PCB was fairly straightforward, I just needed to remember that as the resistors are "up" they are higher than most of the caps. So build order would be:
Diode first then electrolytic caps, FET, resistors, and then styroflex caps.
The high impedance part of the circuit (everything before the FET) should be kept clean. Handle with care, clean the PCB after soldering. The middle leg of the FET is soldered to a special Teflon plot to keep it unspoilt, as are 1 resistor leg and caps leg too.

Switch PCB & transformer

Nothing complicated there, you just need to be careful with wiring. I soldered the switches on the pcb, remembered to use the 3 position switch in the pattern part, the other 2  in pad and lowcut. Nothing fancy. Luckily, someone had a build thread with the same transformer, so I just needed to "paint by number".

Don't attach your switch PCB to your mic body yet, getting capsule wires through it is a pain...

Bias

As I don't own an oscilloscope, I did the "multimeter bias", plugged the mic in a working preamp, fed him 48V, measured the drain voltage and adjusted it with a trimmer. And, oh god, I couldn't get more than 7,5V. I should have had 11,5V. After several hours reading, I just had a new look at the mic and found out I had 2 transformer cables shorted ! After correcting this, I could bias the FET to 11,5V. Hurrah!!!

Capsule

Installing the capsule requires a steady hand, I did some continuity check to be sure everything is screwed the right way, then soldered the capsule wire to the right spot. And then noticed that the wires got through screw holes. I had to unsolder them, put them through the right holes, solder them again. It's a shame to have to rework things in the high impedance section...

Result

I tested the mic with a Neumann pmv70X preamp and it sounded right. I needed quite a lot of gain, but was warned, and this preamp is crystal clear with a massive 70dB gain, so it worked out quite well. I feared some extra noise due to an unclean high-Z part, but was happily surprised to find none of it. I even normalized the audio, but it sounds clear to me. I then checked all polar patterns, everything running fine!
I'll need some real world test and comparison to confirm it's really working as it should and sounding as good as I want it to sound.

Thoughts

The build is an easy one, if you're careful. It might be quite different with an other body though. It would be interesting to compare the mic with others mics in this price range. Is it worth the time spent?

Ressources

GroupDIY resource thread
PCBs
Maiku capsule

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