RIAA preamps matching to cartridge

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wh7qq
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Joined: Tue Jun 27, 2017 4:07 pm

RIAA preamps matching to cartridge

Post by wh7qq » Mon Jan 14, 2019 6:01 am

Rather than tack on to an old thread, the last posting on the thread of last year, viewtopic.php?f=10&t=3166#p6838 made a reference to a guess and by golly effort to match his preamp schematic to the loading characteristics of the cartridge in use. Many of the better cartdridge manufacturers, even the now defunct Shure cartridge operation, have specifications listed for their cartridges that provide the optimum resistive and capacitive loading for their cartridges. This information, which is probably correct, plus a separate measurement of the lines between the cartridge and the preamp input (which have considerable capacitance and are part of the "load") gives a good value to use on the preamp input stage. Better DMMs have capacitance measurement capability but you should subtract the DMM lead capacitance from the measured value.

Using a TC750LC, I found it to sound better to my ears than the built in stages on my Onkyo and Yamaha receivers but on internal inspection, a compromise value of 220 pf had been used by the designer, probably expecting a cartridge value of 450 pf or more. My Shure M-97ed was specified at a standard 45 K ohms resistance but only 220 pf input capacitance. When I checked my connecting cables, they measured about 200 pf on their own, so I lifted the input side of the TC750 input capacitor's legs (desoldered). The change brought the M-97 highs to life with no trace of harshness or distortion...a marked improvement.

@mictester: You were squeamish about posting your preamp schematic here but I would be very interested in seeing it if you still visit this forum. A PM would do that if it is not appropriate here. Thank you.

I have no idea of the layout of the newer model, the TC760LC. The old one used through-hole components and a single-sided PC board and the recent upgrade may well have gone to surface mount devices so I am not sure if this would work with the 760. In any case, if it performs as well as the 750, I can heartily recommend it as a lower cost preamp with high performance characteristics.

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