Coupling Between the SSM 2015 and the Output Stage

The original rev. C circuit board layout simply connected the 2015 stage's output to the output amplifiers, and this was probably found to cause stability problems. Most SX202s will have a 511 ohm resistor retrofitted in series between the 2015 and the output amplifiers to address this issue.

If your unit does not have this retrofit, it should, or else it will probably suffer from marginal stability and the resulting increase in distortion. In all of the units that I have seen, this resistor is installed as a PC board re-work, but there may be a few early rev. C units that do not have the resistor. If that's the case, you will have to cut some traces and install the resistor yourself. I decided to use 432 ohms because it was handy. In the rev G board, this part was designed into the PCB layout, identified as R78 and R79.

If you want to perform the coupling cap modifications, you need to eliminate any DC component from the output of the 2015 stage. To do this, place a small film coupling cap in series with the resistor to block any DC developed in the 2015 gain stage. If DC is removed here, the coupling caps after the buffer stage, which have to be much larger to handle low impedance loads, can be replaced by jumpers.

This coupling cap is loaded by the 10K input resistors of the summed ch 1+2 output stage, (R37, R38 on rev G boards, R51, R52 on rev C boards), so it must be scaled accordingly to assure proper LF response. I decided to use a .22uF film-foil polypropylene cap, so the stock 10K load would not be appropriate. On some units, I have removed the summing stage entirely (by removing R37 and R38 or R51 and R52) and placed a 150K resistor across pins 3 and 5 of the two remaining output amplifier chips. This yields a single order 4.8Hz high pass filter, which is only .24dB down at 20Hz.

On other units, I changed the ch 1+2 summing stage to use 100K summing and feedback resistors, and while this increases the noise in the summed output, it's not an issue for my purposes. Note that if this approach is taken, the offset voltage of the summed ch1 + ch2 output stage amplifier determines the output DC sit point of the ch1 and ch2 outputs. A quality, low offset JFET amplifier like the OPA2132P will do nicely there.

In addition to the series resistor and coupling cap, a small capacitor to ground should be added across the input of the output amplifier to further reduce transmission gain at extremely high frequencies. A 120pF capacitor added across pins 3 and 5 of the output amplifier chips U3 and U5 will accomplish this. This capacitor should be an NP0 ceramic capacitor with short leads added to the non-component side of the PC board, right at the pins of the IC socket. With this component installed, the SX202 is only a fraction of a dB down at 100KHz; no useful bandwidth has been lost, only the potential for distortion causing instability and slewing.