Microphone pad

I have breadboarded this mic design:

http://www.mp3forkidz.com/mic/phantxy.html

It uses the linkwitz mod on the capsule so that the internal FET works as a souce follower, this allows the capsule to work without distortion right up to 134dB. However, when I connect this to my mixer, it can only take much lower sound levels as the mixer itself overdrives long before that output level (as there is no pad on the mixer). What I wonder is if there is a way to integrate a 20dB (or just a significant reduction) into the mic design itself? Many existing condensers have pads built in for this reason. I just can't figure out how, or if it is possible to do on this design, would resistors in series with the 1uF caps work, or will that upset the cutoff frequency of the design?

Put in a pot to ajust senetivety

If you add two resistors, each between the mic and the junction of the 2.2Kohm + 1uF (one on each side of the mic) you will get a reduction of level in keeping with the ratio of the two resistor values. However, you would need to keep the value of the new resistor significantly higher than 2.2Kohms so that you don't change the effective source resistance of the signal feeding the 1 uF cap. As long as you don't change the source resistance, which at present seems to be 2.2Kohms, you shouldn't see any change in the lower cutoff frequency. Since you want a reduction of about 20dB, which is a voltage or resistor ratio of 10, the added resistors should be about 22Kohms. However, this radically changes the bias current through the FET, which may change its performance enough to distort. Then again, I'm not sure if that is a problem. Anyone? If this doesn't work, you could add the resistors as suggested, using a value of 2.2K for the new ones, then change the value of the existing shunt resistor from 2.2K to 220 ohms, then increase the value of the series capacitor by 10 (to at least 10 uF) to readjust the cutoff frequency back to where it was.

Thanks, I can try it out, but adding the resistors where you suggested seems like it would significantly alter the bias current, and i'm fairly sure doing that reduces the dynamic range of the capsule (changing the voltage certainly does).

This might work; there are 48v avialable, with the capsule needing 10v. With the current set up, and the capsule needing 0.5ma, there is enough loading to bring the 12v down to 10. If I were to mabye add the resistors, but then alter the zener voltage so as to supply more voltage, mabye it could be balanced so that with the extra resistance, the capsule still gets its 10v at the correct current? I was originally intending resistors to be added here

Ok, well i've been experimenting and firstly tried your idea of adding the 22k resistors. This works and lowers the level, but it did intruduce quite a lot of distortion. Reading the voltages at the capsule it appeared thay had changed quite a lot. I eventually came up with the design attached. I figured the 100k resistors in the original design assigned the transistor buffer input impedance. I then added 1M resistors where the 100k are shown on my design. Using 1M would have been too noise prone, so I lowered the original 100k resistors to 10k and used 100k attenuators, keeping the 10:1 ratio.

So far it has tested quite well, I haven't been able to make it clip. I was trying sine test tones on an old hi-fi, but the hi-fi was far more distorted than the mic. The mic recording sounded similar to how it did in the room (mic was located just 2 inches from the bass driver, so it was actually clearer than in the room). The voltages on the mic read the same as the original. The only issue with this is it can only record louder sounds, as something like a voice level has significant noise, so the dynamics are limited, I might lower the ratio a little. Either way, I have other mics for lower levels.

If anyone sees and serious issues in this alteration, please let me know, i'm not totally sure what i'm doing!