Boundary Microphone with a AD MAX9814

[bigben83]

I have designed a Boundary Microphone using the Analogue Devices MAX9814 and would like some feedback on the circuit diagram. I see lots of very decent microphone schematics here.

Link to Data sheet https://www.analog.com/media/en/technical-documentation/data-sheets/max9814.pdf

As you can see by the pcb its made to fit onto a neutrik xlr connector.



Here is the PCB and how it goes into the Neutrik connector with a 3D Printed case.

 

[Khron]

1uF output capacitor(s) coupled with the usual 1-3k preamp input impedance will result in a pretty high cutoff frequency (160Hz @ 1k to 50Hz @ 3k) - something to keep in mind.

R1 and R2 could possibly be increased in value, if the zener's only 5V, but the chip draws some 3mA at idle, so it's a coin-flip.

Remains to be seen if the minimum gain is low enough to be useful though, but that'll depend a lot on the electret capsule used...

 

[jp8]

Next to increasing the output capacitor value, you want to insert a 50 Ohm resistor between the "cold" net and C7 to make the output truely impedance balanced. The output impedance of the MAX9814 is 50 Ohm and the "cold" side should have exactly that value included in its path to GND. It improves hum rejection.

I'd also propose to add some surge protection diodes to the output. The MAX9814 seems quite robust against abuse, but it won't harm to add them. One 1N4148 or similar with anode on the output pin and cathode to Vdd and another diode with cathode on the output pin and anode to ground. You could also use a BAV199W double diode to that end.

Jan

 

[ccaudle]

The datasheet does not give enough details of the output stage. Based on the description it is probably a feedback amplifier stage, and those generally can have a problem with oscillation driving a long cable directly. The usual solution for that is a 30 to 60 Ohm resistor between the amplifier and the XLR connector.
Possibly the 50 ohm output impedance mentioned by jp8 is outside the feedback loop, which would mean that it is already incorporated in the package, but I could not tell from my quick read through the datasheet.

What preamp will this be connected to? The device will have a minimum gain of 40dB until the AGC kicks in, which could drop it to 20dB. Make sure that the preamp providing phantom power has a minimum gain low enough to work with that much gain at the mic.
Usually you would use a compressor at the preamp or mixer for that function, since you would have to go on stage to adjust the AGC settings (assuming you bring that out to an external pot; having to disassemble the mic first is even less convenient).

This MAX device appears to be designed to be the entire preamp for a capsule connected to a PC, phone, USB headset or similar. Performance will be quite compromised trying to incorporate into a typical pro audio system.

 

[ccaudle]

I don't see a connection between chassis ground symbol and the circuit reference ground symbol. Without that there is no path for the phantom power current to complete the power circuit.
You do need to be careful of how the connection is implemented physically to avoid injecting shield noise into the audio circuit.
Is the housing conductive? I don't see how pin 1 actually connects to the connector shell. Will that be external to the circuit?

 

[bigben83]

What preamp will this be connected to?

I am using the Panasonic WM61A replacement
https://www.jlielectronics.com/microphone-capsules/jli-61a/
but if someone knows a better electret than this in the same size id be interested.

This boundary mic is used in the same way as the AKG CM562 in the same profile. I am 3D printing the M8 bolt thread with button type head as per https://au.akg.com/speech-spoken-word-microphones/C562CM.html

 

[k brown]

I take it the idea is to have a boundary mic with a line level output?

Remember that the final output will be 6dB higher than the chip gain because of the boundary effect; so minimum 46dB.

 

[jp8]

I am using the Panasonic WM61A replacement

I'm afraid you misinterpreted @ccaudle 's question: he was not asking about the capsule but the preamp, aka mixer connected to the output of your mic.

Jan