Alternative approach to RIAA Phono

[Hans Polak]

I've been taking a look at an alternative approach to RIAA equalisation that has been patented in the last few years (https://patents.google.com/patent/US20170126186A1/en) by Graham Slee. This basically equalises the pickup's constant velocity frequency response by putting an integrator as the first stage providing the 3180us pole (50Hz). This effectively provides a constant loop gain for that stage. The remaining 318us (500Hz) and 75us (2122Hz) time constants can easily be dealt with by a simple passive filter. My take of an MM phono using this approach is attached. I've include a secondary LP filter to address the non-inverting amp's roll-off at unity gain, a 20 Hz rumble filter and a simple flat gain stage with potential gain adjustments to allow for differing cartridge output levels. Overload margin is in excess of 30dB for a 5mV MM cartridge.

I've modelled the input stage with an OPA1656, a very low current noise/low distortion FET op-amp. Other suitable choices would be OPA1641 / OPA1642 or similar. As an aside, the use of a FET device as the input stage will also provide good RFI proofing.

This has been built and is my current phono amp in use on a regular basis. On a subjective basis, its clean, precise and provides an open sound stage. As far as measurements go, I've run a quick and dirty frequency response check using an HP3312A function generator, inverse-RIAA network and an analogue 'scope. This confirms the flat response across the 20Hz to 20KHz range. I also confirmed the overload margin.

It seems that you can almost apply a patent when switching the Left and the Right channel when using the most wooly explanation.
It took almost 8 full pages to explain the idea behind the patent, that could have been done in less then one page.
But not a word about noise penalty or even comparing it to a split Riaa correction over a two amp solution, where in his patent a second opamp only serves as a buffer.

Hans

 

[Chris Hornbeck]

This basically equalises the pickup's constant velocity frequency response by putting an integrator as the first stage providing the 3180us pole (50Hz). This effectively provides a constant loop gain for that stage.

Putting all of the RIAA in that stage provides very close to the same constant loop gain, but without the reduced overload penalty. Also optimizes amplifier generated noise. Food for thought.

All good fortune,
Chris