Last week I started with my JFET RIAA stage. This stage is designed for my current MM cartridge (Ortofon 3e) which has a relatively high output at about 4mV. The stage is a variation of the Werner Ogiers’ circuit, inspired by the John Curl Vendetta. Rod Coleman helped me out here to get this design optimised, so thanks Rod again for the contribution.
A two stage RIAA stage with only about 42dB gain is not ideal. I’m looking at replacing my cartridge soon and hopefully buying an DL-103 MC cartridge. I will then easily tweak the gain and RIAA network to use the Denon one instead.
I built a breadboard of one channel and tested it:
Here is a first test of the RIAA response:
Output is normalised to DC gain. Level @ 1kHz is 5% close to the -19.9dB expected at this frequency. I can trim the resistors to improve RIAA accuracy due to capacitor value being 98nF and not 100nF (2% variance). I’m using Mundorf MCap EVO Aluminium which are rated 3%. This is a further improvement to do next…
Let’s see what the stage noise looks like. 50Hz noise is really low.
Now looking at the response to a 1kHz test signal of about 4mV rms (close to maximum input level) the output stage is about 344mV @ 0.2% THD. The gain is roughly 38dB-40dB as can’t measure properly the test signal at 4mV input due to HF noise in my workbench. Either way is close to the 42dB expected, again within the 5% range.
It sounds really nice. I’m very surprised with the stage and how quiet is. Given is operated from two 9V batteries, noise is at minimum. Listening to one channel is not very objective, but it gave me a good first impression of this stage. I’m keen to build second channel board for a proper listening test.
Stay tuned…
