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!

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.

Innersonix Lucidity, first steps

The lucidity PCBs

I start populating

Soo, I got everything I need so far, at least enough to start. I checked every component for its value. Took care to match (as much as I could) the resistors R1 & R2, R4 & R5, R7 & R8, and R9 & R10, to improve common mode rejection ratio (CMRR). So far so good, it seems quite easy.
I started with the resistors, then added those small yellow capacitors. That way, I don't have to think about polarity yet and they are all small components, the board is still easy to manipulate.

I added the diodes, I needed to take care of polarity, don't want to burn everything... After the diodes, I looked at what components had a low profile and decided to solder the adapters for the chips.
I soldered then the red capacitors. BTW, in the original BOM,  C20 and C21 are rated
180pF and in the mouser BOM, they are 150pF. The innersonix team said it's not a problem, and they used the 150pF.

After the red WIMA capacitors, I soldered the relays and the resistors which are to be soldered straight (hard to get them straight). Now it starts to look like a real board, with lots of stuff on it.

Here the relays

The wima capacitors

To be continued...

Apex 205 ribbon mic mod

Hi there,

a few words about my apex 205 mod. Sorry, no pictures as I modded the mic late at night without thinking about taking pictures/blogging.

The Apex 205 can be a great ribbon mic, but can be also terrible when not set up appropriately. It was my case as I had a really low output. So I started modding it.

Mechanical mods:

I unscrewed everything and took the mic apart. I always moved the mic slowly in a dust free room. Never blow on those mics!
I removed inner mesh layers of the mic headbasket. Just pulled hard with plyers.
I removed the ribbon protections. I just needed to unscrew a lot of little screws for that.
I retensionned the ribbon as it was sagging. The trick is to use alcohol to hold the ribbon while you give the right tension. It was quite tricky to find the right position but finally got it to work. Now I have a normal output.

TO DO: change the transformer