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
Thanks doncito101. I'm not asking if capacitance on the phono pre/cable affects output of the cartridge, but rather, if low output MI cartridges, like the SoundSmith Zephyr Star, benefit from capacitance loading like some higher output MI/MM cartridges? This is assuming the cartridge is getting the right amount of resistive loading and is on a compatible tonearm. 
The dealer recommended capacitance loading around 400pf total FWIW.
I doubt that there is any difference whether it is a low or high output. Do you have any way to change it? If you do give it a try and see if you can hear it.  
The need or lack of need for capacitance loading has nothing to do with cartridge output. However, MM cartridges sometimes do sound best (flattest frequency response) with some added capacitance between the signal carrying wire and ground, and MM cartridges do tend to be higher in voltage output than MC or MI types. (One of those TRUE, TRUE, UNRELATED situations.) But MM cartridges benefit because they have much higher inductance than do MC cartridges, typically 1000X more, not per se because of the signal voltage they put out.  Typical MM cartridges have inductance that measures in the hundreds of milli-Henries, whereas MC cartridges have inductance that measures in tens of micro-Henries.  Conversely, MC cartridges are more tolerant of variations in capacitance, up or down.  Adding capacitance to the load seen by an MM cartridge moves the resonant frequency up out of the audio frequency range, sometimes.  Whether to do it and how much to add is also a function of the load resistance you choose to use.  For example, 47K vs 100K vs something less than 47K.  You have to take this on a case by case basis using data supplied by the cartridge maker. You need to know: load resistor and cartridge inductance, to calculate optimal C.

But to answer the basic question: The need or lack thereof for capacitative loading has nothing directly to do with cartridge output.

@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