Hi all!
I'm working on a "stomp box" for microphone, not guitar, so the input is a balanced XLR (dynamic mic) and the output should be suitable for a mixer/PA (balanced output).
The circuit is based around a PT2399, so I need to take out a single-ended signal using an INA333. My design relies on op amps (TLV2462) to amplify the signal etc. All these ICs run on 5 V single power supply, so I need to add a 2.5 V DC offset to signal path.
I am currently focusing on the input and output stages, how to hook up to XLR contacts and feed into the INA333. Similarily, on the output, how to connect the balanced signal (which I create using op amps). Please review my design as shown in the images.
Are the coupling caps on the input stage oriented the right way, with cathode towards the mic? Also, I believe I should have a 48K to ground on the output (just before the XLR contacts) but I should not have this ground on the input stage. The 48K to ground on the output is to stabilize the potential at the output, right? On the input, the potential will be stabilized by the "bias" resistors which add the 2.5 V offset to the signal, so I don't need the ground connection (also, it would create an unintended voltage divider) so these should be omitted.
Input stage
Output stage
I guess I also wonder if it makes any difference if the op amps on the output (which create the balanced signal) are in series, or in parallel as in my design?
I'm working on a "stomp box" for microphone, not guitar, so the input is a balanced XLR (dynamic mic) and the output should be suitable for a mixer/PA (balanced output).
The circuit is based around a PT2399, so I need to take out a single-ended signal using an INA333. My design relies on op amps (TLV2462) to amplify the signal etc. All these ICs run on 5 V single power supply, so I need to add a 2.5 V DC offset to signal path.
I am currently focusing on the input and output stages, how to hook up to XLR contacts and feed into the INA333. Similarily, on the output, how to connect the balanced signal (which I create using op amps). Please review my design as shown in the images.
Are the coupling caps on the input stage oriented the right way, with cathode towards the mic? Also, I believe I should have a 48K to ground on the output (just before the XLR contacts) but I should not have this ground on the input stage. The 48K to ground on the output is to stabilize the potential at the output, right? On the input, the potential will be stabilized by the "bias" resistors which add the 2.5 V offset to the signal, so I don't need the ground connection (also, it would create an unintended voltage divider) so these should be omitted.
Input stage
Output stage
I guess I also wonder if it makes any difference if the op amps on the output (which create the balanced signal) are in series, or in parallel as in my design?
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C2 and C6 in the output circuit should be reversed as their left sides are at the positive bias (artificial GND) voltage.
And in the mic input the coupling caps should have GND referencing resistors (100k) on their left sides so they can charge no matter if something is connected to the input and whether it is ground referenced or not (dynamic mics usually have floating voice coils). Basically the same idea as it it used for the output.
And in the mic input the coupling caps should have GND referencing resistors (100k) on their left sides so they can charge no matter if something is connected to the input and whether it is ground referenced or not (dynamic mics usually have floating voice coils). Basically the same idea as it it used for the output.
Is there any risk that either side will ever be accidentally connected to something with 48 V +/- 4 V phantom power? If so, 63 V bipolar electrolytics may be a better idea.
INA333 has a rather high voltage noise when you want to use it as a dynamic microphone preamplifier: 50 nV/√Hz where the thermal noise of the microphone itself is only 2 nV/√Hz if it has a resistance of about 200 ohm.
INA333 has a rather high voltage noise when you want to use it as a dynamic microphone preamplifier: 50 nV/√Hz where the thermal noise of the microphone itself is only 2 nV/√Hz if it has a resistance of about 200 ohm.
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Yes! Phantom power. Sorry, I forgot to mention this. I measure ~40 V on the XLR cable from the mixer.
Thanks for replying!
Thanks for replying!
So I should basically move the 48K to ground to the other side of the coupling caps? Thanks for replying!
But should the caps on the output be oriented the other way around, given the fact that the mixer might have phantom power? If I design for the case when there is phantom power, will the same design work just as well in the case when I use it on a mixer which doesn't have phantom power?
Another thing I forgot to mention, the mics I'm designing for are dynamic (not condenser, so no power in mic) and 600 ohm. Are you saying that the INA333 will cause audible noise? Because I am hearing an annoying hum in the system, which is sort of the stuff I'm focusing on now.
My current prototype is working, I can sing through it and it produces nice echo 😊 But there sound quality can be improved. However! If there is some degradation at the end, I can accept that and it will just be part of the "style" of my custom echo fx 😉 That said, I am doing this to learn about sound and analog electronics, so I want to fix at least the obvious errors.