Have you tested this in practice? My findings are the rail voltage droops with increased input signal and current consumption, which is how i came to dedicated PSU solution. Also make sure each opamp has it's own 100nf range cap.
All OpAmp mic design (no FET at first stage)
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at this point just make a dual output mic and make the physical pattern switch an optional box
homero.leal
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No, I haven't tested @kingkorg . At this time it's only a design idea. Maybe a 12V zener "voltage regulator" could be added, as in the OPA Alice, reducing also the values of the supply resistors.
And yes, it's important to have a 100nf for each opamp, thank you!
The updated design, will be like this:
Regards.
You can add a zener, but it still needs to be tested under different conditions. Opamps could still draw more current and drop the voltage. This will of course depend on expected input signal and gain of 1692. I am also not sure if opamps could start fighting for available current due to different current "appetite". This exact situation led me to drop phantom and make a dedicated PSU.
Townsend labs Sphere uses TLC070 followed by 33178 for EQ. I haven't traced the circuit to know how they solved these potential issues.
rogs
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Remind me again why you changed R9 to 47k ? ... In my original circuit IC1A is used as a unity gain difference amp for the figure of 8 pattern. (S1 and A connected)
The idea of adding R11 is to allow any differences (up to c.6dB) between the outputs from the two capsule membranes to be made the same, to help optimise the figure of 8 pattern .
Changing R9 from 1k to 47k will surely simply unbalance the difference amp inputs...... How does that help?
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homero.leal
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In my understanding, if you have this resistor with the value of 1k and you have the switch to fig. 8, this resistor will reduce the signal to 1/2 of the signal on the output of the first OpAmp. As I understand, it will act as a voltage divider only for AC signals, since you have the filtering caps at V/2 (47u and 0.1u).
Does it makes sense?
Regards!
HL
Sorry for double posting, i just shared this in another thread. Thiught it could be relevant to both. Explains somewhat my obsession with stable voltages and potential of these circuits compared to traditional ones.
Here's what a plosive looks like straight into unprotected SDC capsule polarized at about 40v.
22.2Vpp. Add about 8-10% to that due to circuit loss.
Here's what a plosive looks like straight into unprotected SDC capsule polarized at about 40v.
22.2Vpp. Add about 8-10% to that due to circuit loss.
Attachments
rogs
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I decided that I would make all 4 resistor values the same (1k) to create a unity gain differential amplifier, for the figure of 8 configuration.
(The maths involved are about 1/3rd way down THIS PAGE under ther heading 'Differential Amplifier Equation' ).
The minimal adjustable gain in the inverting side was to allow for slight variations in capsule outputs, to try and help maintain an optimum figure of 8 pattern, with matched signal levels effectively obtained from both sides of the capsule.
As I mentioned in my earlier post, I decided that adding gain - either from the following preamp, or digitally in a DAW - was probably a lower noise option, which appealed to me, so I've never been tempted to try and add extra gain within the impedance converter circuitry itself.
Always interesting to see a different approach....
rogs
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My thinking has always been to keep things inside the mic itself simple, and add 'bells and whistles' (extra gain, EQ etc... ) outside the device.
The idea of using three op-amps for a multipattern device was just an idea I thought might be worth a try....
(Two as impedance converters - one for each capsule membrane - and one as a summing/ difference amp for pattern control).
An alternative option might be to use just one op-amp as an impedance converter, and apply an opposite polarity DC to each capsule membrane.
But that makes the voltage multiplier more complex, so not a lot of difference in the component count, I suspect?
Also makes any capsule output balancing slightly more complex....
rogs
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As the need for a stable Lo - Z DC supply only really comes into use with high voltage 'plosive' outputs. might it be worth comparing with phantom power resullts, if the value of the main DC decoupling cap was increased in value?
So, for the LDC OPIC circuit that would mean replacing C1 (currently 47uF 50V) with, say, a 220uF 50v - or maybe even a 470uF 50v - capacitor )
I can see that space might become an issue, so it's just a thought?....
homero.leal
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Hi @kingkorg, I did a quick LTSpice simulation of the circuit with a 6 Vpp signal. If I use a 1k value for the R6 resistor (original OPIC Multi Pattern diagram), the current flowing thru R6 resistor ranges between +/- 1.4mA. VCC for the OPAmps is about 20V.
On the other side, if I use a 47K value for the R6 resistor, the current flowing thru this R6 ranges between +/- 60uA.
So, it seems to me, that having this R6 value to 1k may be the cause of voltage instability, forcing the OPAmp to push a lot more current when an AC signal is applied.
As this R6 is not in series to the signal path, I don't think It could cause significant added noise to the circuit.
Does this makes sense?
Regards!
HL
rogs
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Homero - if you make R6 47k how do you find that affects the symmetry of the figure of 8 pattern?...
Do you still get really effective 90 degree null points?
homero.leal
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Just finished the sim, and certainly not, Rogs. You are totally right, both resistors R6 and R8 need the same value to keep the symmetry of the fig-8 pattern.
So, changing R6 value to 47K won't be a good idea. I thought about this because my suspicion of high current being used by the OPA when a low value resistor (as 1k) is used.
Regards.
So, changing R6 value to 47K won't be a good idea. I thought about this because my suspicion of high current being used by the OPA when a low value resistor (as 1k) is used.
Regards.
R6 or R9? This is referring to the schematic in post #83?
Cqwet Dbdfte
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The transistor count in OP amps is high. The opportunity for IM rises with more active devices.
The OP amp gain is high, but how much is needed? A lot of negative feedback results in a less desirable harmonic structure.
A high voltage device would also allow more dynamic range without clipping.
The fact that very good mikes can use a single active device with no negative feedback and a step down transformer speaks to fact that not much gain is needed, and has been a premium, sometimes only-, solution for a very long time.
There is probably a reason why we do not see too many mikes of this type.
The OP amp gain is high, but how much is needed? A lot of negative feedback results in a less desirable harmonic structure.
A high voltage device would also allow more dynamic range without clipping.
The fact that very good mikes can use a single active device with no negative feedback and a step down transformer speaks to fact that not much gain is needed, and has been a premium, sometimes only-, solution for a very long time.
There is probably a reason why we do not see too many mikes of this type.
rogs
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R6 on the original OPIC schematic........ R9 on the schematic in post #83.
My fault for referring back to the original schamtaic..... Sorry about that.
rogs
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I think that the very top end designs may still prefer to use discrete FETs rather than op- amps. Especially where specific non linearites are deliberately introduced into a design.....
The OPA164* series of op-amps do have extraordinarily low distortion figures, and pretty low noise levels.
I use them as unity gain buffers with no gain applied.
I've found using them very effective, as a simple good quality alternative to discrete component impedance converters .....
And according to the datasheet first page, the input capacitance is very stable with common mode voltage, which will help keep distortion low with high impedance sources. Some older designs have measurable distortion when used with high impedance sources at low gain, even if the low bias current makes them seem appropriate for use with high impedance sources.
