Multipattern electret?

[szegedin]

I have a very stupid question, if anyone wants to humor me:
You know how multipattern mics are based on two capsules back to back? I understand this is based on applying different voltages to the two capsules, to end up with different patterns.

Is it possible to wire two back to back (omni) electret capsules, onto the same preamp circuit, in order to get a sort of bidirectional mic?

I have reasons for wanting to try this -- just wondering what would happen if a second electret capsule is added to the same circuit. I'm not trying to make a multipattern mic -- just get an omni or bidirectional out of the two capsules in order to do M/S.

 

[analogguru]

It is possible - probably you will need two individual FET buffers.

The Samson C03 does exactly this:
http://recordinghacks.com/microphones/Samson/C03

(I bought one used for € 20,--)

or the Behringer C-3 (available new for € 35,--) :
http://recordinghacks.com/microphones/Behringer/C-3

The Blue Yeti uses 3 capsules.

But it is nothing really new under the sun, in the 80's Maruni produced a so called "Panorama Microphone" (sold e.g. as Vivanco EM 160) with multipattern (cardoid, stereo and bidirectional) using 3 electret capsules.

 

[rogs]

I'm not trying to make a multipattern mic -- just get an omni or bidirectional out of the two capsules in order to do M/S.

I think you may need to try  cardioid capsules - rather than omnis - to make a 'multi capsule' M/S mic?...

I've been playing around with Primo EM204 capsules,  fitted into a BM800 donor body and using a simple 'Schoeps' type circuit  to achieve just that ... Seems to work pretty well.

The attached image show the  capsule mounting I used for my  prototype  Mid/Side mic..
The 'mid' cardioid capsule faces towards the front of the head basket, and the  'side' capsules are mounted pointing left and right.  (Not quite coincident , but close enough to be effective)
The  2 side capsules are connected in anti-phase to create a 'fake' figure of 8....

Copy of the schematic here if it's of any interest:  http://www.jp137.com/lts/MS.EM204.Schematic.BC560.pdf

As I say, it seems to work quite well.....

 

[szegedin]

It is possible - probably you will need two individual FET buffers.

Cool -- thank you. I will have to try to figure out what kind of FET buffers and where in the cicruit (?)
Just a DIY project -- looking at electrets out there I thought the Shure KSM line is pretty decent/affordable, as a donor preamp.
Was inspired to do this by something somebody said -- a guy who worked at DPA said the preamps they use on those mics (the MMP) is "not actually anything special" -- just a generic electric really.

I think you may need to try  cardioid capsules - rather than omnis - to make a 'multi capsule' M/S mic?...

Great, thanks very much. Would love to hear your mic in action.
I think you're right about two cardioids for a proper bidirectional. I think omnis would sort of work for a wide stereo effect-- I had something in mind like the Crown dummy head.

 

[analogguru]

Just a DIY project ..... I thought the Shure KSM line is pretty decent/affordable, as a donor preamp.....

If you want to DIY and have Shure written on your mic then you can do a search for an el cheapo AMS26.... everything is inside there, 2 back to back electrets, 2 FET-buffers....

Attention:
even when these mics have an XLR-connector, they can NOT be connected to a standard XLR input.  On pin 2 of the XLR connector is the buffered signal of the front-capsule and on pin 3 the buffered signal of the back capsule.

 

[rogs]

Would love to hear your mic in action.
I think you're right about two cardioids for a proper bidirectional. I think omnis would sort of work for a wide stereo effect-- I had something in mind like the Crown dummy head.

Here's a quick spoken word stereo file I made testing the 3x EM204 capsule mic:  http://www.jp137.com/las/204.MS.mic.mp3

......Recorded  on a Tascam DR100Mk3 , set to record as 2 channel  - left (mid) and right (side)....

That master file was then decoded to stereo  using the free VST MSED:  https://www.voxengo.com/product/msed/  ...

The original left (mid) channel file can then be used as a mono version, with out any 'phase funnies' that can occur when summing a conventional stereo mic recording...

 

[abbey road d enfer]

True multi-pattern requires cardioid capsules.
For M-S, you need a figure-8 pattern, which is not achievable with omni capsules.
Two omni capsules connected out-of-phase result in almost no signal (only very phasey residual).

 

[szegedin]

True multi-pattern requires cardioid capsules.
For M-S, you need a figure-8 pattern, which is not achievable with omni capsules.
Two omni capsules connected out-of-phase result in almost no signal (only very phasey residual).

You're right.  I'm sort of mixing up dummy-head (two omnis) and m/s (figure-8) because I haven't decided which I would do.

 

[szegedin]

Here's a quick spoken word stereo file I made testing the 3x EM204 capsule mic:  http://www.jp137.com/las/204.MS.mic.mp3

......Recorded  on a Tascam DR100Mk3 , set to record as 2 channel  - left (mid) and right (side)....

That master file was then decoded to stereo  using the free VST MSED:  https://www.voxengo.com/product/msed/  ...

The original left (mid) channel file can then be used as a mono version, with out any 'phase funnies' that can occur when summing a conventional stereo mic recording...

Thanks -- that stereo imaging is quite good. Interesting that the capsules are side by side rather than back to back.

 

[rogs]

Interesting that the capsules are side by side rather than back to back.

It was easier to construct - and exposed the rear of the side capsules to more 'clear space' as it were.

So -although they're not precisely on a coincident axis, the small offset  doesn't seem to affect the left/right image too much - at least not as far as I can tell!

I must confess to being inspired  to this style of construction by reading about a similar project - see here:

https://www.instructables.com/id/Build-the-MS-Alice-Stereo-Microphone/

The electronics for that project are a bit more complex, and the TSB165A capsules used there are too expensive here in the UK, once you've added shipping costs.

You could always buy 3 x BM800s as capsule donors of course, but when buying those it can be pot luck what capsules you actually end up with...

 

[szegedin]

Yeah, makes sense, I am familiar with the Alice mic.
I had in mind using more high end capsules, so I'm looking for a middling sort of decent electret preamp.
Could you point me to what you 'added' to the circuit or 'doubled' from the donor mic for the two side capsules? It's a little over my head but I can sort of copy paste.

 

[rogs]

Could you point me to what you 'added' to the circuit or 'doubled' from the donor mic for the two side capsules? It's a little over my head but I can sort of copy paste.

Cardiod electret capsules like the Primo EM204  have internal FETs which act as impedance converters, so are much lower impedance (and thus easier to work with)  than the TSB165A capsules.

I constructed the circuit I posted above ( see here: http://www.jp137.com/lts/MS.EM204.Schematic.BC560.pdf ) onto stripboard (it was only a fun experiment, so it didn't justify designing a PCB)

It is just about the simplest way of constructing a Mid/Side mic to interface with a stereo balanced 48V phantom powered mic input.

The 'Mid' mic uses a BC549 low noise  BJT to act as a phase splitter, to drive the balanced output emitter followers for the left (mid) channel.

The 2 side mics don't need a phase splitter - each capsule drives one half of the 'Side' emitter followers directly, to provide the anti-phase 'figure of 8'  signal required for the 'Side' channel.

As I say, it's quite a bit simpler than the MS Alice circuit...

I found the EM204s to be as good  - if not better - than the TSB165A  capsule, but not quite as good as  a 34mm LDC capsule .

(Mind you, trying to get 3 x 34mm LDC capsules and associated electronics into a single BM800 body might be a bit tricky!