It's there a simple and cheep way to measure the frequency response of a cartridge? I'd like to know what mine is.
You need a test LP with an
accurate frequency response sweep or set of individual frequencies, such as the Decca Frequency Response LP, or you can use white or pink noise, but again, this must be accurate. My experience of using several test LPs is that those available to the 'public' are poor in terms of accuracy, as they're meant to be evaluated by listening. Laboratory standard LPs, such as the Decca, JVC or Ortofon I don't think are available any more as new.
For accuracy, the Decca test LP specified that it was guaranteed only for 5 plays each side, and whilst that's probably excessively pessimistic, LPs do wear at HF, so a test LP that's been used many time is unlikely still to be accurate.
Then there's the issue of how good your RIAA stage is. Assuming it's within 0.5dB, then errors in the cartridge and LP are likely to swamp that, but with certain phono stages that may not be the case. You can measure directly off the back of the cartridge, but noise gets in the way, so you have to pay close attention to capacitance and screening of any interconnect cables. MM cartridges are very sensitive to capacitance and resistance termination, so if you're measuring a MM cartridge, it's pretty much necessary to treat the cartridge/arm cabling/phono stage as one entity. MC cartridges are much more tolerant, but the signal levels are a lot lower, so making noise-free measurements more difficult.
After all this, in answer to your question, is there a simple and cheap way to measure the frequency response of a cartridge, I have to say, no.
S