Yep, it's my understanding as well that the MixPre-D will not provided a useable 24-bit optical signal to the D50, as the D50 wants spdif and the MixPre-D sends AES. It should work at 16-bit.
I owned an optimod V3 for years and used it with my D50, works fine, including at 24bit. Tonedeaf is right that the V3 may need a replacement chip, though technically this is a firmware change and not a hardware upgrade. At any rate, Grace will do it for free for you if you send it in, or will provide the chip if you want to do it yourself -- which is quite easy. (As an aside, this is a firmware upgrade for the V3 since Grace advertises the V3 as having a spdif out and when they found out they weren't properly formatting a spdif signal, they provided a free firmware upgrade. Many other A/D manufacturers, including very high end ones, will market their A/D's as having spdif out, when they in fact they know full well they only provide AES-formatted outputs. Good luck getting a refund when you find out your AES A/D doesn't work with your spdif recorder, or getting them to address the problem like Grace did. Grace is an A-number-1 class act all the way.)
I recently purchased a SD USBPre 2, and it also works fine with the optical input of the D50, even at 24bits. The USBPre 2 also has an optical output, so you wouldn't need a format converter. If the D50 is to be the main or only recorder you use, you might consider selling the MixPre and picking up a USBPre 2.
Topic: Sony PCM-D50 Optical Options (Read 7613 times)