I have made a schematic and am trying to size the components to fit into the body of a takstar cm60. The hardest part is finding electrolytic capacitors that are not too bulky. Maybe some can be substituted with 35V ones, for example C4 but I am not sure..Connect a (new) 1nF (1000 pF COG/NP0 ceramic or polystyrene) capacitor to point X and to a (new) point Z. Next connect a (new) 1G resistor from Y to Z, and connect the capsule between Z and GND. So you end up having the bias voltage going from Y to capsule and the capacitor AC coupling the capsule from the op amp input X.
C1,C2 and C4 on your schematic should be OK as 35V. C3 and C10 need to be 50V.I have made a schematic and am trying to size the components to fit into the body of a takstar cm60. The hardest part is finding electrolytic capacitors that are not too bulky. Maybe some can be substituted with 35V ones, for example C4 but I am not sure..
C1,C2 and C4 on your schematic should be OK as 35V. C3 and C10 need to be 50V.
I would take R3 to C1 -ve (gnd) and not to C1 +ve, to help optimise the line impedance balance.
It depends on capsule, I think, the 70V or even 60V can be too much for a capsule with thinner spacer (bigger capacitance) designed and built for 48V bias. I got a second hand SCT-800 which couldn't take that about 65V from the PSU (maybe the capsule had gone bad).I did wonder just how much difference using 48v as a polarisation voltage - as opposed to the more conventional 60 or 70 volts -would make, and the answer is - not a lot!
That circuit obviously has some unbalance (thus less CMMR), wonder how much it makes difference in practice, could it be measured and compared to another circuit where the om amp power has been taken from both (+ and -) outputs (phantom source inputs)?I built a version using this concept as a variation of my OPIC mic project. Some notes here: http://www.OPIC.jp137.com/OPIC48.pdf
I can't see why there should be any impedance imbalance that will degrade the CMRR to any real extent?That circuit obviously has some unbalance (thus less CMMR), wonder how much it makes difference in practice, could it be measured and compared to another circuit where the om amp power has been taken from both (+ and -) outputs (phantom source inputs)?
That is the intended 'marketplace' ..... I's only really offered as a very simple introduction to the world of LDC hobby mics....... This scheme only works properly with capacitor coupled mic preamps which have those 6k8 resistors connected to +48 in P48 supply.
I think once there needs to be any additional circuitry for generating a higher polarisation voltage, then a simple voltage multiplier like the one I use on the LDC design on this page: OPIC LDC is probably all that is needed?.... Simple, cheap and easily adjustable .Yes it is fine as it is. I wish there was a simple way to generate the capsule bias voltage. Even simple voltage doubler circuit would be enough if you have some 25-30V to get some 50-60V out depending on topology, if you use diodes these will drop output voltage at least a 0.3V per a (Schottky) diode. That's a single transistor oscillator + two diodes and capacitors. Are there any simple higher voltage switched capacitor charge-pump IC's available, maybe you could construct one from discrete components (MOSFETs)? Most like MAX1682 are low voltage parts for mobile devices.
I think we may be entering the world of 'swings and roundabouts' here? (UK slang term for equal gains and losses)...There are some 60V PMOS and NMOS pairs available from Panjit (https://www.mouser.fi/ProductDetail...user.fi/ProductDetail/Panjit/PJS6839_S1_00001) and others (https://www.mouser.fi/ProductDetail/Diodes-Incorporated/DMN65D8LDW-7) so you could build a discrete voltage doubler quite easily using two P-channel (for Pelliconi cell and two complementary inverter stages) and three N-channel ones (for Pelliconi, inverter and multivibrator which cannot drive the Pelliconi directly). In addition a couple of resistors and the capacitors are needed. Doesn't take much space on PCB. Maybe the Dickson type charge pump using the diode ladder would be simpler though not as good at least on paper.
Edit: Indeed if you build a Dickson type voltage doubler circuit using a complementary MOSFET pair as inverter you will come with a very low parts count circuit.
Complementary MOSFET pair:
https://fi.farnell.com/diodes-inc/bss8402dw-7-f/mosfet-np-ch-60v-sot-363/dp/1713834https://www.tme.eu/en/details/bss8402dwq-13/multi-channel-transistors/diodes-incorporated/Circuit: Fig 2A, just replace the inverter with the complementary pair:
https://gyraf.dk/schematics/Voltage_multipliers_with_CMOS_gates.pdf
If you put a 1nF cap between op amp input and the capsule and use another 1G resistor to bias the capsule you will have the full 60V on the capsule which is obviously connected to the 'real' ground (0V) instead of to the virtual ground (at halfway of op amp supply).Selecting the feed resistors for the op-amp as 2k2 should give you around 30v DC to work with. Doubling that to 60v - but taking off 15v to allow for the input bias to the op-amp - will only give you around 45v as capsule bias. So no real advantage (except for transformer based preamps)
Although we were discussing this almost a year ago!! I've only just got round to addressing the DC imbalance problem, with my simple 'no voltage multiplier' OPIC mic.Impedance balancing is OK but the DC levels at preamp makes my worry at least with all transformer input mic preamps where the current caused by the DC difference goes through the primaries of input transformers, though as the transformer primaries have so low DC resistance the difference will not be that high causing also lower bias voltage. Don't know what this current will cause to transformer cores either, might be harmful. This scheme only works properly with capacitor coupled mic preamps which have those 6k8 resistors connected to +48 in P48 supply.
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