JLI TSC-2 dual sided capsule - biasing for 3 polar patterns

[Glenn Wardle]

Hello Folks. I've just ordered two TSC-2 capsules and I'm very much looking forward to designing all the support circuitry (pretty well from scratch) - which I'm more than happy to share once all issues are sorted.
But can some helpful folks advise on the biasing, and connections/phasing of the outputs in order to get the three polar patterns, Cardioid, Omni, and fig 8.
Cheers
Glenn

 

[Khron]

As always with answering (generic) technical questions - "it depends"...

 

[abbey road d enfer]

For omni, you need both capsules in phase, for fig-8, you one capsule rear) out-of-phase with the other (front). For cardio, you need only one capsule active. Whether you choose to let the passive one connected (which introduces a 6dB loss) or to disconnect it is up to you.
Putting the rear capsule out-of_phase can be done two ways: either biasing it with a voltage of opposite polarity to the front one, or biasing the backplates at a positive voltage, biasing the front capsule at twice this positive voltage, and varying the bias voltage from same as the front for omni, same voltage as backplate for cardioid, and zero volt for fig 8.
Each method has its pros and cons.
The first method requires disposing of positive and negative bias voltages, the second implies producing a higher bias voltage.

 

[rogs]

There are a couple of options I've tried which may offer some ideas......
Create a pre-amp with a separate impedance buffer for each side of the capsule, and then some simple inverting/ non-inverting switching using unity gain op-amp buffers.
I've found this works quite well : https://www.opic.jp137.com/index-multi.html

Also includes a 6dB adjustment to deal with any difference in output levels..... I've no idea whether the capsules you're describing have well matched outputs or not?

An alternative approach is to create a single pre-amp and simply change the polarity of the capsule polarisation voltages to suit.
I've tried that idea in this thread, and it too works quite well: https://groupdiy.com/threads/dual-polarity-voltage-multiplier.86416

In theory, the second option should be marginally quieter, but in practice I found little difference between the two.
The OPA164* op-amps offer a pretty good spec -- rail to rail ouput swing, very low distortion, and an acceptably low noise floor.

 

[Glenn Wardle]

Thanks for the almost immediate constructive comments.
My current thinking is indeed to do as Rogs suggests, and use opamp summing and phasing to sort out the polar pattern. One little bit of a new idea I'm considering is to use a buck converter off P48 phantom power to derive the opamp supply as this potentially gives me a bit more current to play with. I might even be able to derive the polarisation voltages off the same inductor as the buck. I suspect AKG might do something similar on their latest C414 as it includes LEDs and presumably a small embedded micro too which would be a tricky job at just 3mA with purely linear regulation.
With regard to the capsule, it's puzzling that JLI don't state the 'ideal' polarisation voltages used to derive the three polar patterns. To go to all the trouble to manufacture a complex capsule but without telling folks how to get the best out of it is really odd.
I'm not really interested in producing a clone of any particular mic. I want to create switchable pattern mics as a stereo pair that just 'sounds' lovely.
The instrument I play is a hammered dulcimer, and it's a tricky instrument to mic up.

 

[abbey road d enfer]

With regard to the capsule, it's puzzling that JLI don't state the 'ideal' polarisation voltages used to derive the three polar patterns. To go to all the trouble to manufacture a complex capsule but without telling folks how to get the best out of it is really odd.

AFAIK, most capsule manufacturers don't provide specs for an "ideal" bias voltage. I would think most of them assume a voltage between 40 and 70V, which is what most classic circuits employ, with the exception of measurement mics using elevated voltage, such as B&K 200V types.