Newbie with toy project - Ribbon Microphone

Hi!

I'm planning to build a ribbon microphone and I have some questions. It seems as if this is the place to be for microphone questions.

Prior to this I've built and tinkered a lot with guitar amplifiers. Well, building guitar amps is not a HiFi thing, quite the opposite actually, MOJO. Now I want to build a microphone, that sound fairly good.
The plan, at the moment, is to build a ribbon motor and hook it up to a common source stage, or on stage for each line. The stages will be powered by 48V phantom. First issue I have is the impedance of the ribbon. It seems impossible to do this without a transformer between the motor and the gain stages. If I go with a transformer on the input, how do I choose transformer? I have seen some Ribbon Microphone Transformers vend on the net, but they all seem to be matched for passive ribbon microphones. Threading on my school knowledge I have a hunch that these 'passive transformers' will cut the cheese, but is there better transformers for active ribbon microphones, or lets put it, is there a more intelligent way of choosing transformer for an active ribbon mic? Second question I have is about the common source stage.  Are there some dos and don'ts I should be aware of. The plan is to buff the signal 25 to 30 dB, seems plausible given the phantom power of standard recording equipment. The first iteration of the gain stages will be more or less without filtering, possible a Miller capacitor to cut high frequencies.

Summary
Tranformer: How do I choose a transformer, where do I find them?
Gain stages: Dos and don'ts.

Please comment, I'll post gain stage schematics and a microphone layout in a day or two.

Cheers!

I've actually started considering grabbing a pair of GAP R1 Active Mk3's, possibly later this year, just 'cause the active-ness would offer room for tinkering / modding / (potential) upgrading.

During my research about'em, i came across some info from GAP themselves, something about removing a diode from the circuit board, to reduce noise. I believe this page is on their "personal" web-shop:

http://goldenagemusic.mamutweb.com/Shop/Product/Golden-Age-Project-R-1-Active-Mk3-Modification/1081074-M

Interesting project...

I've been looking for specs on ribbon microphones that are on the market. Most of them have a roll off somewhere between 10k and 15k Hz. I haven't gotten my head around it, but I guess it's due to features of the ribbon it self. I'll see if I can optimize the ribbon set up.

I've found some stuff that I think I can use in the microphone.

• Transformer - Lundahl LL2912 to adjust the impedance from the ribbon to the gain stage. Also gear up with a 1:37 ratio.
• jFET - LSK489 in stereo with common source gain stage settings powered by standard rec. equipment 48V phantom. I don't think I'll bypass the cathode, but gate to drain will have a 'Miller' capacitor.

I've done some spice simulations that seems promising, but I still would like to get input from experienced guys like you all. The common source gain stage I've set up in simulation has a 330k gate resistor and a 270 Ohm source resistor. Please comment on my idea. or idea for a set up of the microphone.

Cheers!

I only just saw your posts.

On the ribbon high frequency response... Being  a pressure difference microphone there is a response null when the front to back pathlength equals one wavelength, and a reduction of output when that point is approached.

With modern materials and careful control of diffraction and other parameters a ribbon mic can be constructed with a bandwidth of 20kHz with reasonable output.

Passive ribbon transformers are around 35:1, giving about 120 ohms output impedance with a 0.1 ohm ribbon. At low frequencies the impedance is very much higher.

A load like that dictates lowest noise will be achieved with bipolar transistors, although some of the low noise jfets work pretty well. With source resistor bypassing or simply low degeneration in common source configuration much second harmonic distortion will be generated...liked by many.

We actually use the single version of LSK489 in our condenser microphones. It has about the best input capacitance/noise ratio in the business. Our ribbon microphones are passive...we feel an outside pre without the current limitations of phantom powering can do a better job. Ribbon microphone signals can have huge dynamic range...130dB+. So attenuator switches are in order for active ribbons.

Les
L M Watts Technology

Thanks for the reply. The project is coming along nicely, takes time though. I'll post some stuff here if you or anyone else is interested.

Cheers!

The microphone is more or less finished by now. Here are some pictures.

I couldn't attach more than one picture in each post... Here's the interior:

I just saw this. Nice job! How does it sound?

Les

Yes, looks really good. Any further information about this microphone? Cheers

Nice work! Very tidy. How does it sound?