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RE: High compliance ceramic cartridges

Posted by coffee-phil (A) on March 8, 2016 at 12:25:25

In Reply to: RE: High compliance ceramic cartridges posted by maxhifi on March 8, 2016 at 10:35:34:

Hi Maxhifi,

Records with RIAA EQ are constant amplitude from the bass shelf at 50 Hz to 500 Hz at which point there is a point of inflection and they are constant velocity up to 2120 Hz where there is another point of inflection and they become again constant amplitude. This matches a ceramic cartridge other than for the two octaves from 500 Hz to 2120 Hz. This doesn't sound all that bad and gets buy for inexpensive systems.

I have read than there is mechanical EQ in some of the better ceramics to deal with the points of inflection at 500 and 2120 Hz. That stretches credibility to me as I find it hard to imagine a mechanical network on the cantilever of a cartridge to do something at those low frequencies. Unless the cartridge is loaded into some sort of electrometer input I think the low frequency response will provide all the bass shelf you would want.

I wouldn't fret too much over Miller with a medium triode such as a 12AU7 or 6SN7. You will be in the tens of pF input C and you will have more gain than you need to proceed an AUX input. A ceramic cartridge can be modeled as a voltage source in series with a capacitor (~1000 pF). The input capacitance of the input stage will form a voltage divider but not roll off the highs. Do worry about the input resistance as it sets the low frequency with the cartridge (plus cable and input stage) capacitance. With cathode self bias a one meg grid leak resistor should be OK with those tubes.

There is a schemo of a ceramic phono stage in the RCA RC-30 (and I suppose other editions) tube manual. It uses a pentode input. It uses the 7199 triode/pentode which I think is the unobtainable tube used in the Dyna 70 amp.
I haven't modeled it so I can't say if it deals with the points of inflection at 500 Hz and 2120 Hz.

Phil





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