Using a step up transformer of 1:20 will give you an output from the cartridge of 5.2mV, about the same as that of a MM cartridge. The PH5 overloads at 70mV and typically if you allow for about 10x the input voltage for peak overloads, you have enough headroom. I would not recommend using an input of over 7mV into it. If you use a step up of 1:13, as you have described, the effective output from the SUT would be 3.5mV, less than if you used a MM cartridge. So, something is not right here.
Audio Research PH5 Phono Preamplifier Input Clipping Problem
I have an ARC PH5 phono preamplifier and most of the time it sounds fine. But on a few of my favorite LP's with wide dynamic range, it will clip and distort on loud sections, especially those with lots of bass content. This only happens with use of a step-up transformer, without the SUT in line, there is no clipping. I do prefer to use the SUT, as without it I have to turn the volume setting on my ARC REF 3 preamp to a very high level. I have to conclude that the PH5 is distorting because of high input signal levels, due to the gain of the SUT. I have tried two SUT's, one with 13:1 and the other at 7:1, both exhibit the same problem (although the lower gain SUT is much better). The cartridge is Ortofon A90 with .27 mv output. Given this, would you expect clipping or do you think there is something wrong with the PH5? I have talked to ARC about this issue, and they seem to think that there is a chance the PH5's maximum input of 70 mv is being reached on these loud sections. Would appreciate opinions from those that have experience with a similar setup. Thanks.