I am designing a simple 6-channel circuit with an LM317 voltage regulator and RC low-pass filter.
I’m surprised with this solution. Is it commonly used in getting a constant current for microphones?
Microphones should need so little current that using for it 1A ICs looks very unusual.
What is the intended current of these current sources. Isn’t it lower than minimum current needed by LM317 to operate not in saturation?
With audio frequencies layout matters very little if at all, but here the ground trace loops around the entire board and will pick up low frequency magnetic fields.
Think of it as a one-turn inductor, if there is another inductor nearby (from another device) it will act as a transformer and stray magnetic fields will force a current through the ground loop and generate voltage noise on the non-zero resistance of that trace.
Voltage on the ground trace means that input ground terminal will have a slightly different voltage than the output or another input and thus the noise will be added to the signal.
Best way to prevent this is either:
- Break up the loop and have the ground traces converge on one point forming sort of a “tree”, we call that a star configuration.
- Use a solid ground plane on the bottom layer (an even better option).
It’s also possible to inject the 15V supply into all microphones from just one regulator, but that’s a bit outside the scope of this post.
Star grounding used to be recommended in the MIL-STDs for the 50/60 Hz world, but was declared unsuitable for above 100 KHz. As cellphones are everywhere, stick to a solid groundplane unless you have a very good reason.
Thanks! You gave me a great insight, I will indeed use one regulator for all mics. This is such a logical solution, I don’t know why I haven’t thought of it before.
So I can remove the 50Ohm resistor, and use a 33Ohm (instead of 200Ohm one) resistor to get enough current for all six microphones connected in parallel, right?
I’m having trouble deciphering the circuit without a schematic, but something seems wrong. LM317 terminals are as follows 1-Input, 2-Adjust, 3-Output. Here all the outputs are connected together and each input is coming from the RC filter sections. Also the adjust pins have no path to ground or any other reference voltage.
Make sure you’ve read the LM317 datasheet, there are example schematics that show how to use the device:
datasheet
If it’s just a 15 DC voltage then connection is simple: just connect al the resistors into the regulator output.
Edit: I’d also recommend adding a capacitor between the regulator’s output and ground to reduce crosstalk between channels.
I’m not even attempting such things.
If there is a question about the schematic, then post the schematic (And the whole schematic, inclusive microphones and the sort of loads these microphones represent). Simple as that. I’m not spending a second on deciphering info that is readily available to OP but not posted.
In first post you said:
What suggest that they are used in current source configuration. At PCB I see one resistor connected to each LM317 and no connection to GND what is typical for current source configuration so everything seems correct.
Last time I was designing anything with microphone was in 1977 - the circuit to switch on/off light at my desk by clapping. My (dynamic) microphone didn’t need supply but technique always changes so I assumed that you have microphones needing current source as a power.
Now you think about powering them all connected in paralel from one source - in such circuit they will not be powered by current source. So I don’t understand what power your microphones need to get.
It would be easier if you show schematic and say how you expect it is working.
I’m 99% sure that first or second your solution (or both) are wrong.
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