Capacitance loading and fixed coil cartridges


Hi all.  I am looking at all the various models of Soundsmith cartridges and seeing three different output levels, 2.12 mV, 1.1 mV and 0.4mV). I happen to have a low output version. I was wondering if anyone knew if the ideal amount of capacitance applied by the phono preamp (and phono cable) was dependent on the cartridge output?  In other words, higher output MM and MI cartridges require more capacitance and lower output cartridges less capacitance. Is this on track? How does inductance play a role here?  

-Karl
128x128karl_desch
@karl_desch , Here's how it works:

With a low output cartridge you don't add any capacitance!

The issue of loading (which is what you are asking about) works like this: The cartridge has an inductance which interacts with any capacitance present (usually the tone arm cable) to form a Radio Frequency tuned circuit. This RF circuit can resonate due to energy from the cartridge, resulting in bursts of RF energy at the input of the phono preamp.

If the phono preamp is unstable, you'll hear the RFInterference affecting the sound of the preamp. If the preamp is stable with RFI, it will be unaffected by the RFI and sound fine. In the case of the former, loading will affect the sound, in the case of the latter it won't.

If you were actually to add a capacitance into the mix, it would reduce the resonant frequency of the RF tuned circuit. It might make it more of an ultrasonic issue than RF but it certainly would not help!!

Please note that there is also no way of forecasting the loading value of the resistor (despite the loading value being specified by cartridge manufacturers in some cases). This is because the manufacturer can't predict what cable you are going to have with your tone arm, and that will affect the loading resistor value.

IMO/IME you are better off with a phono section that does not care what RFI is being injected into its inputs, then you don't have to play this game; its just plug and play.
Thanks lewm and Ralph.  Your posts are very helpful.  I particularly enjoy the units of inductance as milli Henries.

I have a low capacitance phonocable and have the preamp set at the lowest capacitance, 100pF.  I am resistive loading the cartridge with 1000 Ohms.  Currently have about ten hours on this guy and it is sounding sublime.  
-Karl
BTW,  I have Ralph to thank for suggesting that in my system, that is a true balanced circuit from phono pre to power amp, that the interconnect is probably not a factor in its sound quality.  I spent a whopping 60 dollars on a pair of 20 foot Canare star quad cables and moved my entire rig away from my speakers and power amp.  It has never sounded better.