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In Reply to: RE: Phono preamp and/or step up device posted by semuta on September 20, 2016 at 16:27:18
Making the most of mc cartridges requires solving 3 parts of the equation - "gain" and as well as "impedance loading" and "capacitance loading".A stepup trans only resolves the gain issue - increasing the signal level into your phonostage so that it has enough levels to work with.
But the mm only phonostage's impedance is most probably loaded at 47kohms. Not suitable for a lot of low output mc cartridges.
The most commonly required impedance loading is between 100 to 300 ohms.
The last issue is capacitance loading. Most mm only phono probably have around 150 to 300 pico farad setting range. Thats too high for mc cartridges.
See link below for explanation what is impedance and capacitance loading
Edits: 09/20/16 09/20/16Follow Ups:
If the OP is using a SUT, then the 47k will be reduced by the turns ratio of the transformer so the effective load resistance seen by the cartridge will be reduced by a factor of at least 100 (for a 1:10 ratio which would be a typical if not minimum value).
Due to the low coil inductance for an MC, load capacitance is a non-issue as the electrical bandwidth will be well above the tip resonance of the moving assembly. Capacitance of the order of 150pF to 300pF that you quoted is only an issue for MM cartridges due to the higher coil inductance and the need to match the electrical bandwidth with the mechanical resonance to achieve the flattest possible response.
For EMI immunity, the input stage for a MC head amp will often have capacitance added over and above the capacitance of the cables. A couple of designs I am familiar with have 470pF or even higher.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
I don't think that loading is much of an issue. The recommendation is a rough guide and such additional loading is primarily useful in preventing ultrasonic frequencies from overloading a phonostage that is not capable of handling such signals. The inherent bandwidth limitations of the transformer and tube electronics involved here will take care of that. Besides the 470 ohms is a reasonable load for most cartridges anyway.
If the buyer wants to experiment with loading, that can be done by using loading plugs for use on the primary side or secondary side of the transformer. I tried that out of curiosity and ended up with not using any additional loading for either my Transfiguration Orpheus or Lyra Titan cartridge. In theory, the Orpheus should need some additional loading because it has a source impedance of 1 ohm.
A somewhat complicated but interesting read on loading SUT can be found here (the overall conclusion is to focus on gain and forget about additional loading:
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