MXL 990 Capacitor Mods
Replacing the capacitors that are involved in the signal path is a fairly straightforward mod. The MXL 990 has a single circuit board - the preamp and the power supply, unlike the v67g, are on the same board. Basically, we replace at least all of the capacitors on this board that are involved in the signal path. It is not a bad idea to also replace the filter capacitors that are involved with the power supply, but we did not do that on this particular mic. There is also one capacitor on the backside of this board which is critical. It is between the capsule and the gate of the FET, and we want a high quality cap here. There are differing opinions on what to use for these, but there seems to be a general consensus that WIMA caps work well for all of the preamp caps.
With this mic, it is probably easiest if you completely remove the board from the mic body, which just involves unsoldering the wires from the capsule, and from the XLR jack.
The capacitor on the backside of the preamp board is shown below. This may be the most important capacitor to replace.
Because the WIMA cap leads are quite short, we cut the old cap off, leaving part of the original leads in place.
Below is the new WIMA soldered in place onto the original capacitor leads.
Back on the top side, we ran into an issue when replacing C4, as R7 was in the way of the larger WIMA capacitor.
The solution was to move the resistor to the bottom side of the board.
We have replaced the signal path capacitors with WIMA caps of the same values as the originals. In the picture below, we can see the red rectangular WIMA caps in place on the circuit board. Most of the caps have been replaced - a few more to go, but some of these are parts for the power supply, and don't really need to be replaced.
Once the capacitors have been replaced, the circuit board cleaned (it is especially important that the area around the FET is very clean), and the wires from the capacitor and XLR jack are resoldered to the board, the mic can be reassembled. Note that if you plan to replace the capsule (next section), you will want to do that first.
This capacitor mod, along with the headbasket mod (see previous section), go a long way towards improving this mic. The next step would be to replace the somewhat bright original capsule (see the next section). The original capsule is modeled after a K67 type capsule, which is somewhat bright on its own. The original Neumann U67 uses a capsule like this, but has internal eq to flatten it down, resulting in reduced self noise at the upper frequencies. The 990 has no such eq, so an additional good mod would is to replace the original capsule with a K47, K12, or even a K-7 type capsule. Again, we have been quite happy with the capsules from microphone-parts.com, and would recomend you check them out.