Instinct tells me that trying to add switches around that kind of high impedance circuitry is asking for trouble! .. I may be proved wrong of course! :)
I'm not sure why you're not keen on the single sided output?? ... Many mics use that configuration.? My Rode NT1 does for example -- helps make it one of the lowest noise mics on the market!
By connecting the 'cold' side to ground, via the same size output resistor and capacitor, will keep the signal path 'balanced'.
Using the second op amp as a differential output certainly gives you another 6dB of signal, but it has little to do with the actual 'balance' of the system -- a similar (preferably identical) passive output is just as effective in that? regard.
The lower output signal from a single sided version does require more preamp gain, but most decent modern preamps should be able to provide that without any increase in measurable noise...
As I mentioned to Jerry in my reply to him, I did wonder if it might be possible to reconfigure component routing on the PCB, as? the amps are already configured as 2 x non-inverting buffers, and 2 x unity gain inverting amps?...
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On Sat, Apr 9, 2022 at 06:20 PM, <michaeljtbrooks@...> wrote:
Hi Arjay,
Looks interesting! Thanks for the idea. I'm still quite keen on a balanced signal output though.
How feasible is it to put switches on the high impedance side?
If so, how about this...
Still with four OpAmps, but we use a DPTT switch to change the rear capsule's OpAmp between non-inverting, null, inverting:
- Omni: Standard non-inverting
- Cardioid: + and - are connected together, resulting in null output
- Figure-8: Inverting. Positive input gets connected to virtual ground
We delay producing the cold signal until after mixing.
My two concerns about this approach are:?
1) Can you actually get away with putting switches on the high impedance input side? Or is that asking for trouble with spurious signal paths between the switch connections?
2) In cardioid mode, the Rear capsule OpAmp is still active, thus will be contributing some noise to the final signal.
I'm in the UK too. I bought Jules' PCBs via PCBway. I think it cost me about ?20 for 5 Dual OPA Alice plus 5 DC-DC Hex boards including shipping, which really wasn't too bad! I bought the components from a mixture of AliExpress and RS Components.
