Heya! Quick warning - I'm completely new to both all this audio stuff, as well as circuit design! So much of what I'm doing is complete guess-work. That aside, I'm attempting to make a little circuit that will go into a USB audio adapter off amazon (decided that I'll figure out microcontrollers next time).
Here's my problem (again) - I have no clue what I'm doing. I'm hoping someone could take a quick glance at the schematic to let me know how far off I am (I still don't know how true condensor capsules are meant to be powered). It would be reallyyy helpful if you could!
Here's what I've come up with, and the full schematics are also here for anyone who wants to try spot any stupid mistakes. Thank you for your time!
Note: SW2 is to ground the IN- when SW1 is set in either cardioid or figure-of-8. Andd I'll be using a capsule with 2 diapraghms (aliexpress RK-12)
Here's my problem (again) - I have no clue what I'm doing. I'm hoping someone could take a quick glance at the schematic to let me know how far off I am (I still don't know how true condensor capsules are meant to be powered). It would be reallyyy helpful if you could!
Here's what I've come up with, and the full schematics are also here for anyone who wants to try spot any stupid mistakes. Thank you for your time!
Note: SW2 is to ground the IN- when SW1 is set in either cardioid or figure-of-8. Andd I'll be using a capsule with 2 diapraghms (aliexpress RK-12)
A non-electret condensor mic?
They require a high voltage bias and extremely high impedance circuitry - those 6k8 resistors should more like 1 gigaohm. And lose C33/C34 unless you want to strongly attenuate the output. R17 and R18 will short the signal completely, again 1G is a more plausible value. Your lower opamp at the input has its pins the wrong way round BTW. The OPA1656's input impedance is about 2pF and 6 teraohms, so ought to be OK.
Think of a condensor mike as a few dozen pF with a current source across it. The current depends on the bias voltage and capsule capacitance, and is proportional to both. The load impedance needs to have a time constant with the source capacitance that is large enough to pick up 20Hz, which is typically in the high 100's of megaohms.
The entire capsule and electronics need excellent screening from stray electric fields or all you'll pick up is hum - this makes it tricky to test stuff while probing it with a 'scope!
Don't be put off, this is doable, but its at the exotic end of electronics and you need to understand the requirements. C31 and C32 can be 10x the capsule capacitance, so 1nF is plenty. C35 is effectively shorting the outputs together too, given its low impedance compared to the source.
I'd start by finding some 100M or higher value resistors, you can experiment with series strings of them (and they are cheaper than G values)
They require a high voltage bias and extremely high impedance circuitry - those 6k8 resistors should more like 1 gigaohm. And lose C33/C34 unless you want to strongly attenuate the output. R17 and R18 will short the signal completely, again 1G is a more plausible value. Your lower opamp at the input has its pins the wrong way round BTW. The OPA1656's input impedance is about 2pF and 6 teraohms, so ought to be OK.
Think of a condensor mike as a few dozen pF with a current source across it. The current depends on the bias voltage and capsule capacitance, and is proportional to both. The load impedance needs to have a time constant with the source capacitance that is large enough to pick up 20Hz, which is typically in the high 100's of megaohms.
The entire capsule and electronics need excellent screening from stray electric fields or all you'll pick up is hum - this makes it tricky to test stuff while probing it with a 'scope!
Don't be put off, this is doable, but its at the exotic end of electronics and you need to understand the requirements. C31 and C32 can be 10x the capsule capacitance, so 1nF is plenty. C35 is effectively shorting the outputs together too, given its low impedance compared to the source.
I'd start by finding some 100M or higher value resistors, you can experiment with series strings of them (and they are cheaper than G values)
That was the plan yes! I'll take a look at the resistors... I was looking at schematics from THAT, Analog Devices and Texas Instruments because I haven't found too many that made much sense myself- The lower opamp's output can be switched to both inputs on the INA so that should be okay..
I think I've seen designs using a string of 10x 100M resistors as a 1G one before so I'll see how much those cost. I'll adjust C31 and C32 to 1nF as well.
Honestly I don't remember what C35 is for.. might have been RFI protection? Would that be nessasary? (I think it was on the document from THAT)
I did go into this project knowing that it would be on the hard side as first-time project, but oh well. Why not jump into the deep end!
Thank you for your help - I'll keep working on it!!
I think I've seen designs using a string of 10x 100M resistors as a 1G one before so I'll see how much those cost. I'll adjust C31 and C32 to 1nF as well.
Honestly I don't remember what C35 is for.. might have been RFI protection? Would that be nessasary? (I think it was on the document from THAT)
I did go into this project knowing that it would be on the hard side as first-time project, but oh well. Why not jump into the deep end!
Thank you for your help - I'll keep working on it!!
Quick update with resistor and capacitor values changed (and C33, C34, and C35 removed). The 1G resistors will be 10x 100M resistors. I've kept the op-amps opposite but I might change them later. The rest of the schematic is just for power so it's remained unchanged.
