MXL 992 Circuit Mod Kit
The MXL 992 is an older and slightly better-made version of MXL's standard transformerless condenser mic. But it suffers from a few critical problems:
- The JFET has much too high an input capacitance figure for this circuit.
- The JFET is not biased properly to minimize distortion.
In addition, critical signal-path capacitors are no-name film types, or worse, non-NP0 ceramic. The latter can cause microphonic noise in the audio signal.
We have cooked up a "mini mod" that addresses all of these problems. It includes a selected, ideally biased JFET, with 1% tolerance bias resistors. It includes premium Wima film capacitors for the audio signal path. It replaces the generic thick film 1G resistor with one that is lower in noise and better suited to audio.
Circuit Mod Benefits
Expect these benefits from this circuit modification:
- Higher sensitivity
- Lower noise floor
- Lower distortion
- Cleaner transient response
- Extended (deeper) low-frequency response
All the other circuit kits we sell include fantastic documentation, with step-by-step photo illustrations. This kit has something simpler. Because the 992 is no longer made, we expect to sell only a handful of these kits. We've prepared a basic set of text instructions, and added a few color photographs.
Therefore we recommend this kit for experienced DIY enthusiasts only.
This kit does require desoldering parts from the MXL circuit boards; if you do not have relevant experience and the necessary tools to remove through-hole components, please do not purchase this kit.
Mod Service Option
We can mod your 992 for you for $150 plus the cost of this kit; email us to coordinate the work.
Before and After
The image above shows circuit traces (measured electronically, via signal injection) of the MXL 992 before and after the mod. The two dark, overlapping lines at the top of the graph show the circuit frequency response, which is linear. The post-mod trace, in blue, shows less low-frequency rolloff.
The two separate red lines show the 2nd (solid) and 3rd (dashed) harmonic distortion before the mod. The 2nd harmonic line is only 21dB below the signal at 1kHz. This is stupidly high distortion.
The two lowest blue lines show the circuit distortion after the mod. The 2nd Harmonic drops 42dB @ 100Hz (!). The 2nd Harmonic performance after the mod is excellent -- far superior to what you'd see in most transformer coupled microphones.
The 3rd Harmonic distortion drops ~10dB across the spectrum due to the mod.
Results in your mic might not match these, as it depends how bad your mic's JFET is. Our sample mic was in bad shape. MXL does not bias JFETs, so far as we can tell, so you can't know what your mic's distortion performance is until you measure it.