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
karl_desch
Its a matter of power. If you have a low output cart, you need a more powerful phono preamp, or a SUT. Most phono preamps that are rated for MC use, shouldn't have any problems driving a low output cart. For the other settings, just set them to whatever they recommend in the manual for the cart.
Thanks sfall, my question is not about how to deal with low output from a gain perspective but rather if low output fixed coil cartridges need a little capacitance unlike low output MC.  I use a phono pre that has no problem with gain or resistive loading (Pass XP-15) but not sure what to set for capacitance.  Here is the relevant section from the cartridge manual which doesn't really answer the question, but of course I understand that it all comes down to listening preferences.

"The new Zephyr MIMC Star requires a MINIMUM RESISTIVE LOAD of 470 Ohms, with best loading determined by your system and your listening preferences. Usually, this is between 470 and 1000 Ohms. There is no harm in using a 47K typical MM load, but you may experience a peaked high end. Capacitive loading may reduce that effect somewhat to a satisfactory degree. Again, loading requirements will be determined by your system, and your personal listening preferences. PLEASE NOTE that loading well below 470 Ohms will result in loss of high frequencies."

Does this mean at proper resistive loading, I don't need to add capacitance? Just trying to learn more about cartridge electronics, otherwise I would just email Soundsmith.

-Karl
@karl_desch
I’m pretty sure Soundsmith is like Grado. They are non sensitive to capacitive load. Give them a call just to make sure that this is correct!
Karl,

For MC, as low a capacitance as is possible is preferred. See below:
http://www.extremephono.com/Loading.htm
http://sound.whsites.net/articles/cartridge-loading.html

I would suppose that your tonearm wire and interconnect would produce about 100pf shunt capacitance, so maybe set the Pass at the minimum or no additional capacitance and adjust loading to suit.

Best / Paul.
Thanks Paul but what about MI cartridges with low output?
Also, these are great links.
Capacitance does not effect the output on a MI or MM cartridge, including low or high output!
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
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.