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

Gyraf g7, a DIY tube microphone | part.1 ordering stuff

As I was waiting for my cases to rack my latest projects I thought about starting yet a new project. I had some PCB laying around that I bought a while ago in frenzy of "I want moooore PCB" and dug in my treasure chest to find a g7 tube mic.

A word about the g7: it's a tube mic designed by Jakob Erland, the guy at Gyraf Audio in Denmark. His contribution to the audio DIY community is invaluable, and beside offering some high end audio tools, Jakob has a nice DIY page. The mic is an original design based on the famous Neumann U47, AKG C12, with the Neumann U67 tube (EF86). That's quite a pedigree...
As with all the tube mics, you need an external power supply because tubes need high voltage to work. No phantom power for those. And as a reminder, those high voltages are lethal, so don't die building a mic...

The starting point is that I bought 2 of them, one had already quite a few components soldered and the second is just the bare PCB. So I started to complete the one already soldered. And the starting point is to order everything that is needed. And I suddenly remembered that a PCB and components are mostly cheap, but it gets expensive real fast with metal work, transformers, tube, connectors and so on... And you need to find a capsule...

The capsule

I bought a C12 capsule from microphone parts. Fast shipping, unbeatable price. It's not the best on the market, chinese product but with high standards. A lot of people report that their capsules just work really well.

The PSU,

I bought the parts from Conrad as they have cheap shipping costs. Still they managed to break this little order in 4 different parcels, one I'm still waiting for...

The transformer

I bought the expensive Lundahl 1538 as proposed by Gyraf. OEP is also fine, but I wanted a high quality build this time. The don audio shipped the transformer really fast, thank you.

The tube

I bought 4 used ef86 on eBay. Let's see if they are as good as the seller claims. At least they look clean and fairly unused

Let's build the thing!

DIY Monitoring controler from IJ Research | update

After a really long time, I finally got into debugging my monitoring controler from Igor (IJ research).
It didn't work for quite long and I eventually found 2 mistakes:
- wrong power connection... (OK, shame on me, this shouldn't happen)
- some pins of my test VGA cable were shorted to the shield so the power supply was not working anymore. I changed the cable and it's working.
Here a quick'n dirty video
It's moving! and it blinks!

Well I should be cautious with saying this, because it just responds as expected, display, LEDs and buttons. I need to make a custom db25 cable to test the input-output with audio. And then I can say if it's really working and passing audio.

And then I just need my brother in law to CNC a nice case for this one and integrate it in my setup.

As chunger says: human wins

Creating a JFET preamp: the FETREDD

So I decided to create a mic preamp.

In fact, it's not entirely true, I'd like to design a PCB for it. I bet it's not that easy and I have to say that I have no clue about how well it will go, but still, I find it worth a try.

The beginning of a mic pre

Everything started when ForthMonkey on GroupDIY posted some schematics and sound files of a mic preamp. It's a JFET design with only 3 active components in the entire circuit: the 3 JFET.
The part count seemed low so I thought about doing a little PCB because I'm curious. I never ever designed any PCB, I have no clue about doing such a layout, but GroupDIY is a community insisting on experimenting things so let's go.

The schematics of the FETREDD

They seem pretty easy: a first JFET, a bunch of resistors and capacitors, two other JFET and an output. No funny part, everything seem quite clear.

If I were into marketing, I'd call it a "pure class A preamp delivering an outstanding sound with personnality" and I would sell it a fair amount of money!

As a bonus, ForthMonkey included an extra little circuit called TNT. This switchable circuit kind of boosts the lows and highs. A kind of loudness thing as far as I understand. If I find it boring, it's still a place where I can add any kind of processing.

Here the schematics from the main circuit (thanks to ForthMonkey)

 And the TNT circuit:

The power supply & format

You need a single rail +24V power supply and +48V for phantom power. Well I'm lazy, I do own a 51X rack which provide 24V and 48V so I'll do a 51X layout. This way, I don't need to create/borrow an extra power supply, a complete 19" rack and so on. It will also help me with layout decisions, because the format is already there and it's a good start.

What I need to add

It would be too easy to just copy the schematics and call it a day, the FETREDD mic preamp wouldn't be complete without a few things:

an input and output stage

that's where it gets a bit complicated. At the moments, the pre is (as far as I understand) high impedance unbalanced In and Out. So I can plug a high impedance instrument directly in like a guitar or a bass and it would work fine and I can get a nice sound on the output as long as I don't need to drive long cables. Right now it's a perfect preamp stage for Instruments, but I want it good for mics and I want it to be able to drive long cables. SO I need an input and output stage.
The easiest for me is to put 2 transformers in the circuit. They aren't cheap but they sound good, they make my life easy and might even add some character to the sound.
I could add a kind of little circuit to adapt impedance and balance the input and output. It would probably be a bit cheaper, but even though they are easy circuits, I still need to interface and adapt them to this circuit. And I can't do this at the moment, because I don't understand electronic deep enough to know exactly what to do.
So transformers.
I decided to use the Haufe ST 8456 as an input transformer because it's small and sounds at least as good as the lundahl equivalent and it's cheaper. Oh and I can source it here directly in Germany which is quite convenient in regard to porto and taxes.
I haven't decided yet what I will use as an output transformer. I will need to test the circuit with several. ForthMonkey said I would need a 10k:600 transformer, but if the circuit could drive 600Ω I could use a 600:600 one. Which would be quite convenient because there are more 600:600 transformers than 10k:600.
So I need to build this circuit, try it with different output transformers and select the best. Well, let's see how far I go.

Adding transformers to the circuit seems to be easy, just put one before and one after the circuit, connect the right pins to circuit and ground and there you are!
Oh and it's easy to test transformers as they basically don't really make the preamp different.

48V phantom supply

this power supply is necessary for condencer mics. It's not a difficult thing to do because it's basically a switch, 3 resistors and a capacitor, but it needs to be done.
While informing myself on preamp design/layout I stumbled across the FETboy. It's also a class A preamp (from Scott Hamptone) and the schematics were kindly released in an audio magasine. And it shows a nice schematics on how to transform a basic gain stage (like the FETREDD) into a full blown mic preamp. Thank you Scott.
As I'll use an input transformer, I don't need to protect my circuit against phantom power with extra capacitors.
You can find more infos on the FETboy on DIY recording equipment. A great DIY audio wiki by the way.

Input PAD

Well I shamelessly copied Scott Hamptone input pad. It's a -14dB pad placed on the secondary of the transformer. That means that the pad is between the transformer and the preamp. Which means that the transformer still get the full signal before it's attenuated and it will add distorsion character to the pre. Scott writes that most modern good quality transformers can cope with this.

Instrument input

Do I need this one? It seems pretty easy to implement because it's just a jack between the transformer and the gain stage input, but I still need to figure out how to do this. I need the instrument input to bypass the mic input when a jack is in, that seems pretty easy. But I also need that the jack plug let mic signal through when no instrument is inserted. Do all the jack plug do this? If so, why do a lot of preamp have an extra switch for this? Because the instrument input goes through the transformer? It's such an easy question that I barely dare asking it...

What else?

At the moment there isn't any kind of LED to show anything to the user. Do I need shiny LEDs? I find it good for 48V as you don't want to fry your expensive ribbon mic. I also like the idea of a signal LED. Something that shows if a signal is coming in (or coming out?), I find it easier to troubleshoot if you at least know if there is something coming in, even though I don't believe in metering for analog hardware in our digital world.
There isn't any kind of phase inverting switch. They are quite handy for stereo/multitrack recording. How and where can I implement this?

Check list

So in the end I would "just" need this:
the PCB
the electronic components
the input and output transformers
the frontpanel (made by my brother in law)
a metal L bracket to hold the PCB and frontpanel together (also made by my brother in law)
Time and so on...

Jensamp 51X, a DIY germanium preamp

Origins

RCA BC-7A

This germanium preamp is based on a classic mic preamp: the RCA BA71. These modules came from the film and broadcast consoles of the 60s-70s and are considered as one of the finest of this era.
GroupDIY forum member Jensenman redesigned this old beast adapting it to modern recording environments constraint (48V, DI input, variable gain etc.) and optimizing it a bit for a better bandwidth.
Kubarth and [silent:arts] managed to put everything in a 51X form factor, a kind of big brother of the infamous API500 format. Note that you can plug an API500 module in a 51X rack (and it will even work!).

Build

An easy build

Jensamp kits are available through [silent:arts] white market thread. The kit comes with everything except 2 bits of wire for the LED. The yellow PCBs are really well made, and metal work is as you expect from a german production.
It is really easy to build, I didn't struggle on anything. Start with small things like resistors and solder your way up until the big Haufe transformers.
Plug it...
Enjoy the sound!

I had first a little oscillation problem that I solved by changing some vintage components. I also took care that no metal casing was touching the PCB traces.
Take care to discharge yourself from static as germanium diodes can be sensible to it.

Costs

The kit comes with everything and costs 220€ (excl. VAT and shipping costs). It is a really cheap preamp when you hear such a sound quality. Germanium preamps usually cost 4-5x more...

Sound

Here comes the real deal. The sound of the Jensamp germanium preamp is thick and large. It is not a clean preamp. And you'll love it for its sound. No wonder people are comparing it with Neve mic preamps...

As a test example I plugged a cheap chinese mic in it and it sounded good, full bodied! I then plugged the same mic in a Lindell audio preamp (6X-500) and it sounded harsh and aggressive.

 

Recall sheet

I wrote a little recall sheet for those who are trying like me to document their sessions.
Download it and tell me on what you used it.

Feel free to post any comments or questions about this unit. If you want to try it yourself, contact me at my recording studio in South Germany.

G9 51x DIY tube mic preamp

What is the G9 51x?

G9 51x tube mic preamp

I bought a PCB for a tube mic preamp some time ago: a G9 51X. The G9 is a famous tube preamp design from Gyraf Audio in Denmark. It has been tweaked so that it fits in a single 51x module. What a feat!

The 51x format is kind of an extension from the API500 format. 2 more pins which deliver +/- 24V. You can learn more about it here: GroupDIY 51X Alliance.

The 51x format is well regarded and used on the GroupDIY forum. You can find a list of 51x projects here: list of DIY 500 and 51x projects.

Build

I closely followed the advices from the designer of the board on GroupDIY. Those were important and helped having a smooth building process.

Basically, how to solder the little lugs so that your tube will fit when you insert it (you can ignore this and take the risk to break your tube of course), fitting from the elements on the faceplate, some mods are required so that the pre works better, how to solder the high voltage converter, and so on.

The build itself is easy if you proceed carefully. Solder from the smallest to the tallest elements. 

There is a vertical diode which is not silkscreened really clearly between the 2 caps C502 and C504.

For the HV converter, I would advice to solder C503, then D501, then the inductor L501 because D501 will get on the way of C503. Well at least keep an eye on C503 because you need to solder it flush to the PCB or you'll have clearance issues.

I used the OEP transformers because I had them laying around (well and because I couldn't afford to buy expensive transformers when I have cheap ones available).

Costs

I bought the PCB+frontplate long ago. I think I paid 25-40€. The faceplate is a new one from http://frontpanels.de which explains why it's different from the original faceplate.

Tubes Westinghouse NOS: 40€

Components: 61,30€ 

I used the BOM from the first post of the thread on GroupDIY about G9 51x. It was fine even though I had to find some replacement parts because some weren't in stock.

Transformers: roughly 30€

Total cost 150-200€

Well and I bought a 51x rack from Vandaal, so it's 550€ more expensive.

Pictures

You can use those pictures as a reference because the unit is in working condition.

Yeah, power supply is working!

Hi there,
I'm pleased to announce that I managed to have the power supply for the innersonix lucidity preamp work.
As I had a bit of respect for 220V, a friend helped me wire the transformer and explained roughly how such a supply is working. After that I got power but the 3,3V was missing and the LED wasn't working. After a message to Alex from Innersonix he told me I should cut pin 7 from one of the regulator. It was written in the groupdiy forums but I somehow missed it ( or forgot about it). so I cut the pin and it worked...

I just need to wire everything now and let you know