Rogs,
First, thank you for all your work and for sharing it with the community. Really, greatly appreciated!
Several days ago I was discussing with
@mrgrooves666 regarding some proposed adjustments to the multi-pattern circuit, so output could be a little bit higher. Here are the ideas:
- Replace 1K for a 47K for R6 (1K is acting as a 1/2 voltage divider for AC signals)
- Add a 1G resistor (one side to the capsule connection and the other to ground) and a 1000pf coupling cap (maybe polystyrene o polypropylene, one side to the 1G and capsule connection and the other to the signal input) this will be at each of the capsule inputs (This will prevent V/2 reaching the capsule)
- Replace OPA1641 at the output for a OPA1692 (it's an audio grade, dual unit opa) and use the additional opa unit to drive a fully balanced output (as the OPA Alice), this will increase output level by -6dB, and also this dual opa uses only 0.65 mA for each unit. 1.3mA total, less that the 1.8mA required by the OPA1641, leaving more room for the capsule polarization voltage.
I know this will increase the component count by 6 (2x1G resistor, 2x1000pf coupling caps, and 2x2k2 resistors to drive fully balanced output), but maybe this will be well justified for some of the folks that would love to have a circuit with a higher output level.
Thank you and hope you could find this ideas useful.
Regards!
Homero - thanks for your kind comments.
I'm just a hobbyist, and I enjoy tryng out different ideas and sharing my results with groups like this one.
I do often post project notes - including the stripboard layouts - mostly to remind myself what I did at the time!
- but the notes may also serve as a starting point for other experimenters, who may want to try out derivative ideas? --- Bit like you are suggesting here.
The reasons I chose to do the multi-pattern as I did was for two main reasons - to keep it as simple as possible, while at the same time try and keep noise levels to a minimum.
The OPA1641 is used here to serve as a differential amp (Figure of 8 - link A) and a summing amp (Omni - Link C) amp.
It also serves as an inverting amp for the cardioid pattern.
To keep the noise levels low I've chosen 1k resistors for the resistors in the signal path. ( The lower the resistor values, the lower the noise. )
That's also the reason I've kept the audio single sided.... Adding an additional inverting output - similar to Jules' Alice OPA design - adds not only the noise from an additional op-amp, but also the noise from 2 extra 2k2 resistors in the signal path.
It's not a lot, but as well as increasing the audio output by 6dB it will also increase the overall noise figure by around 3dB.
I've found that adding the extra 6dB of gain required from either a good quality mic pre-amp, or digitally in a DAW, will add less noise.
The OPA1692 does - as you point out - have a lower noise figure than the OPA1641, but not by much, and the advantage is probably negated by adding the noise from the second op-amp and it's associated resistors?
As regarding using a second 1G resistor and AC coupling it, I've only done that in my
LDC version which uses Phantom Power to supply the polarisation voltage. That maximises the available voltage across the capsule, by grounding one side of the instead of referring it to the half rail.
In this multipattern version I simple measure the half rail voltage the capsule is biased at, and add that value to the output voltage selected for the capsule polarisation, by the voltage multiplier circuitry.
More recently I've been looking at creating a simpler multi-pattern version,
by fitting a dual polarity voltage multiplier to the simpler
LDC OPIC version which only uses a single op-amp. That should - in theory - be a lower noise version - although the differences measured on the first prototpye seem fairly minimal?
That configuration also doesn't allow for easy balancing of capsule output levels.
Experimenting with different polarisation voltages applied each capsule membrane, to create additional pattern variations is not something that I've thought about copying with this simple sum and diffence op-amp version.... An idea for the future perhaps?
There are all sorts of 'variations on a theme' of course.
I think the main thanks need to go to Jules, for his suggestion to try the OPA164* series of op-amps as an alterntive to a discrete JFET, as the front end of an impedance converter.
Like many folk, I tended to think that a balanced output automatically included differential audio , but that is not the case.
As long as the line is passively impedance balanced for CMRR purposes, there is no need for differential audio.
And that single sided audio concept is used by - among many others - companies like Rode and Neumann.
Now, whether they do it for lower noise levels - or because it's cheaper - I'll leave you to decide for yourself!
