Open source project: "Generic" Transformer-couple FET+BJT microphone PCBs

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midwayfair

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This is a PCB set meant to replace the circuitboards in inexpensive transformer-coupled microphones that use a FET + emitter follower circuit and a low ratio (almost always 2:1) transformer. Usually the transformer is actually fine though I will say I do generally prefer the sound of the Cinemag.

This project is a lot less generic than the Schoeps-style transformerless mic board was, so I'll explain all the changes real quick:

First, like the Schoeps board, I've made the assumption that you are going to be using a flatter-response capsule than the typical 67-style capsule in an off-the-shelf mic or in the Aurycle kits, mainly because that tends to be the first thing people modify. For this reason, I chose to omit the u87-style negative feedback circuit. It doesn't have that big of an effect and I've never had any particular luck with it anyway -- removing it from my aurycle build killed a good 3-6dB of white noise (not just better S:N). It was also quite a few extra parts to fit, and without standing resistors (ew) it would have meant sacrificing some of the more important changes, like an improved power filtering chain.

I did, however, manage to shoe-horn in a simpler low-pass shelf filter on these PCBs for use in a pinch if you aren't replacing the capsule. It's three fewer parts but it's similar to what's used in some C12/ELA251 projects that put a pF capacitor from the plate to ground. See the notes on the schematic for suggested starting points -- 470pF and 10K would put it -4.5dB centered around 10KHz (the cutoff would go up if the FET's output impedance is lower than 33K, so using a different FET changes things).

I made a few tweaks to the circuit, lowering the current draw of the audio stages and including extra power filtering stages for both transistors.

There are components for a switched high pass filter and a negative feedback pad like in the KM84, both with optional parts to be used with on-off-on switches (which requires some creativity in getting them into the microphone body ...).

Finally, I included one of my favorite modifications, which is optional parallel FETs for the input device.

Project background:
As MXL and other manufacturers selling basically the same product move to SMD, it's harder and harder to find a mic that can just be straight-up modded, and for the most part I don't think the PCB sellers on here are going to be very interested in making a project for sale that's a copy of a dirt-cheap mic.

These PCBs are fitted to the body that's used in the V63/V67 (and the numerous similar mics by Thomann etc), Aurycle (IMO the best completely blank bodies you can get for this size - unpainted but heavy brass), BM800 (I believe that's the same body shape and size), etc.

Standard warning:

******These documents may change until they are verified by both me and at least one other user!******

This is a work in progress -- I am going to make the PCB order this weekend, though, so I'll be able to test all the open source project PCBs at once.

Schematics and layouts:

fet-with-emitter-follower-schem.png


fet-with-emitter-follower-pcb.png


This one requires a polarization PCB. You can go hardcore and make it on perfboard like Audioimprov, or I've shared some on Osh Park, which can find in this thread: https://groupdiy.com/index.php?topic=65174.0

Osh Park share: https://oshpark.com/shared_projects/pOLxStQo
Mouser cart: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=c47896c57b

An explanation of some of the notes on the BOM:
1. There is a 2n3819 in the cart in case you simply can't get ahold of a 2SK30, 2SK170, or any similar FETs (e.g. 2N5457, 2N5485, 2N5952, BF245, etc. -- all of which have different pinouts but will work fine).
2. There are some alternate parts if you're building with a 4:1 transformer instead of a 2:1. (Cinemag sells both.) For instance a smaller output capacitor (2u2 instead of 4u7) and a 22K for the Q2 emitter resistor instead of 10K.
3. There is a multiturn or single-turn trim, your choice, for the bias voltage. Both will fit on the PCB.

fet-with-emitter-follower-no-oscillator-schem.png

fet-with-emitter-follower-no-oscillator-pcb.png


This one has on-board polarization. It's basically me getting all the parts I actually use from the Aurycle kit (which is the same as the MXL V67G) onto a single PCB. The polarization is lifted from the KM84. You only get ~40V on the capsule (the audio path sags the phantom power some even with the current draw lowered) but that's plenty good enough for single-pattern.

Osh Park share: https://oshpark.com/shared_projects/dV2qXGta

Mouser cart: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=fa56fe3804

Same notes as above about the Mouser cart apply here.

If you need a transformer, like if you're buying the Aurycle body by itself instead of modding an existing mic, Cinemag's CM-11021 and CM-24110 are both solid choices.

Technically you could build either of these with a higher-ratio transformer. But Dan has a better project PCB for that (the D847), and that's getting farther and farther away from the point of the project.

After I've verified the layout, if anyone wishes I can share the Eagle files.
 
I've kinda-sorta did a similar thing a while ago :p I've had a set of boards done a while ago (10 of each), just haven't quite gotten around to completing (ie. assembling) a set of each just yet, though. Life keeps getting in the way, and all that :p

https://groupdiy.com/index.php?topic=61845.msg783516#msg783516
 
I've verified that the boards work, but it'll be a little bit before I fully verify the project as I'm waiting on the screws for the body. I'll post pictures when it's all done.

EDIT: Forgot -- there was a small mistake on the board. The labels for the primary and secondary were backwards. Electronically everything is fine, it's just that I had the P and S markings backwards. I'll upload the replacements as soon as I can.
 
Eagle Files:

https://drive.google.com/open?id=0B5Sa1JDj8POIZEpyRlMzSXZCb1k

https://drive.google.com/open?id=0B5Sa1JDj8POILXRTNGU0OC1VWGs

In situ :)

condenser-mic-board-e1489370211903.jpg


Ran out of "good" switches so I was only able to use one pad and one low-cut setting, but when I can gt some more I'll mangle the body to fit them.
 
Glad you are finding them useful!

This reminds me, I did a mic build diary using the boards last year and should probably drop it in this thread:
 
An explanation of some of the notes on the BOM:
1. There is a 2n3819 in the cart in case you simply can't get ahold of a 2SK30, 2SK170, or any similar FETs (e.g. 2N5457, 2N5485, 2N5952, BF245, etc. -- all of which have different pinouts but will work fine).

Re: the 2SK170, Les Watts notes in another thread that the "2SK170 is not the best device choice for this [different but similar] circuit" because "it has HUGE input capacitance."

I don't know if it's a factor here because it's a different circuit (an MXL fet circuit Les is commenting on) but I suspect it's similar enough that I should mention it. It's too bad because I have a handful of 2SK170's on hand. :|

----edit----
More on input capacitance: Danish member GentleVoice1 seeks a very linear, very low input capacitance fet, something better than 2SK170.

Your thoughts welcomed! I'm assuming you've found 2SK170 perfectly acceptable and we're splitting hairs.
 
Last edited:
The notes on the schematic say that the inductors L1 and L2, and the resistors R6 and R7 should be of 0.1% tolerance. The Mouser BOM says not. I'm assuming the BOM is correct.
 
The notes on the schematic say that the inductors L1 and L2, and the resistors R6 and R7 should be of 0.1% tolerance. The Mouser BOM says not. I'm assuming the BOM is correct.
They might not have been available as 0.1% when I made the BOM or last updated it and I don't make it a habit to update the BOMs for these projects as they're more of a convenience than anything.
Get them as close as you can, it might make a difference for the inductors. If you buy extras you have a good chance of matching them manually if you can measure it.
 
Okay. Thanks for that. So it's not the absolute values that matter (within reason), but the values relative to the members of each pair? They should be within 0.1% of their partners, rather than within 0.1% of the stated value?
 

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