The Input Pad

I often record using high output mikes placed close to loud sources, so the SX202's stock 15dB pad and 26dB minimum gain was a problem. Reducing the 2015 minimum gain is possible up to a point, but this will reduce the stability margin, and honestly, I don't know if you can safely lower it more than a handful of dB anyway.

A simpler thing is to change the pad from 15dB to 20dB. This can be done in a way that also lowers the effective input noise of the preamp, so it's not such a bad tradeoff for most situations. Finally, the SSM 2015 has less distortion if it is presented with a smaller input signal, so using a pad and higher SSM 2015 gain is often not such a bad idea for medium to low gain situations.

The SX202 pad is actually a pair of unbalanced L-pads, one for each input leg. This is actually a good design, since it will be more effective than a three resistor pi network when used with mikes that have single ended outputs. Some modern transformerless condenser mikes, such as the high output Neumann TLM 103, have single ended outputs, in the sense that only one lead, pin 2 or pin 3, has signal, and the other output lead is unmodulated. Such a mike therefore presents half of its output as a differential signal, and half of its output as a common mode signal. If a pad only attenuates the differential signal, as would a three resistor pad, a large, unattenuated common mode signal is presented to the mike amp's front end, and this can cause problems if the amp's common mode rejection is not high, and the common mode gain is not linear.

The trouble with the dual L-pad design is that the two pairs of resistors used in the unbalanced pad must be matched carefully to preserve the input stage's balance. I used 3K09 and 301 ohm resistors instead of the stock 3K01 and 681 ohm resistors, yielding approximately 21dB of padding when the next stage's 20K load is considered.

These resistors should be closely matched from a supply of quality metal film resistors. Buy 50 or 100 of a particular value and measure them with a quality ohm-meter. These days, the HP 3490A multimeter can be found as surplus for $25-100 and it's hard to beat for this work. It has a 5.5 digit display, provision for 4 wire Kelvin measurement leads, and is sensitive enough to measure the effects of your body heat and the temperature coefficient of a typical resistor when you handle it during the sorting process. Sorting resistors is tedious, but it can help to preserve the input stage's balance, and it's probably cheaper than specifying ultra-precision parts.