MC Phono preamp gains

Almarg, Re your comments on the Rhea. I own a Janus, recently inherited from the estate of a friend. I was poking around in it, and it seems that the output from its phono section is a 6992 used as a plate-follower, rather than as a cathode follower, as one might otherwise have expected. This topology would indeed give it a rather high-ish output impedance, but in the Janus, the phono section only has to drive the built-in line stage. Does the Rhea output stage also consist of a plate-follower? Thanks.

Lew, sorry to hear of the loss of your friend.

My relevant knowledge of the Aesthetix products is limited to what can be inferred from JA's measurements and from info at the Aesthetix site. Both the Rhea and the Janus are specified as having output impedances of 1K single-ended and 600 ohms balanced. The recommended minimum load for the Janus is specified as 10K single-ended and 20K balanced. A load recommendation of 10K min is specified for the Rhea, with no indication of which output it pertains to. In view of JA's measurements of the Rhea, I suspect that all of those minimum load recommendations are aggressive rather than conservative.

What is particularly noteworthy is that it can be inferred from JA's measurements that different topologies are likely to be used for the Rhea's single-ended and balanced outputs, and given the similarity between the specifications of the Rhea and the Janus I suspect that the same holds true for the Janus.

For the Rhea's balanced outputs JA measured 110 and 156 ohms at 1 kHz and 20 kHz respectively, which is suggestive of a cathode follower, and 3846 ohms at 20 Hz, which is suggestive of a somewhat low value coupling capacitor connected between the cathode follower and the output.

For the Rhea's single-ended outputs he measured 2K throughout most of the audio band, and 3.05K at 20 Hz. That would seem to be suggestive of the output signal being derived from a plate circuit, again with a somewhat low value coupling capacitor in series.

As I say, the similarity of the specifications suggests that the main outputs of the Janus are probably similar in those respects. In which case the concern I mentioned earlier about cable capacitance, with respect to the Rhea's single-ended outputs (but not with respect to its balanced outputs, since the balanced output impedance is low at high frequencies, where cable capacitance may come into play), seems likely to be applicable to the Janus as well. And the concern about the minimum input impedance of the component that is being driven, with JA's measurements suggesting more stringent guidelines than are recommended in the specs, seems likely to be applicable to both outputs of both components.

Best regards,
-- Al

Al,
My preamp's input impedance is 50K ohm, so I guess it would be fine?

Yes, 50K is fine, balanced or unbalanced.

Regards,
-- Al

Did not know about JA's measurement. Thanks. I have not poked around in the line stage section of the Janus, as mine works fine. On the other hand, I did something to the L channel of the phono stage that I cannot seem to fix, which is why I was trying to figure out its topology without a schematic, just by measuring voltages. However, you've given me some food for thought. I had intended to use the Janus to drive my Beveridge direct-drive amplifiers, which are mounted inside the speakers. The Bev amplifiers use a solid state input and driver stage, so there is a chance that the Janus with its high-ish output Z might have a problem driving them, although I do not know exactly what is the input Z of the Bev amplifiers. I will need to figure that out.

Thank you for your remark about the loss of my friend. He was by far my closest "audio" buddy; we knew each other for more than 40 years, and I do miss him.