MM cartridges and capacitance

Load and cable capacitance will interact with the inductance of the cartridge's coils to produce a resonant peak in frequency response, at upper treble or ultrasonic frequencies. Increased capacitance will raise the amplitude and lower the frequency of that peak.

MM cartridges often count on that peak to compensate for what would otherwise be an upper treble rolloff, at frequencies that are within the audible (<20 kHz) part of the spectrum.

BTW, that is the exact opposite of the effect of capacitance in line-level interconnects, where too much capacitance will roll off the upper treble. In that situation, the cable capacitance is interacting with the output impedance of the component driving the cable, which is essentially resistive and not inductive.

Best regards,
-- Al

Al,
From the things I have read, and the cart designer I have talked to on the subject of that resonant peak in frequency response, that peak is waaaaaaay outside the audible band. However, the problem is that it is so large, that just like a really big earthquake, it can cause problems a long distance away, and so can stress phono stages at levels within their reach (which would still be outside the audible band), which causes performance issues inside the phono stage.

The numbers I have seen of that resonant peak would suggest that it is like no earthly peak in comparison but magnitudes higher in comparison from the sea level starting point of the audible band. As I understand what you are saying, cart designers are effectively moving to the foothills of Mordor (and raising it ever higher) in order to make their badly insulated houses warmer.

Then again, I may be misunderstanding this...

From the things I have read, and the cart designer I have talked to on the subject of that resonant peak in frequency response, that peak is waaaaaaay outside the audible band.

T_bone,

That's true in the case of MC's, because of their much lower inductance (which results primarily from the smaller number of turns in their coils). But in the case of MM's, with more turns and much higher inductance, the peak occurs at much lower frequencies. See the following:

http://www.hagtech.com/loading.html

Your comments about the possible effects on phono stages of the peak that MC's can produce, at very high ultrasonic frequencies, are quite correct btw.

Best regards,
-- Al

Al,
Thanks for the link to the Hagerman calculator. I'd actually read that page a few times but until now had never tried the calculator myself, AND more importantly I had mistaken the range of resonant frequency peak by at least one power of ten on MM carts. If the inductance estimates are correct, it means one wants to have capacitance as low as possible in order to preserve the harmonics 'air' which extend beyond the audible band. It would be difficult to imagine why one would want to bring that mountain closer to the audible frequency.

As an aside, I am looking at the formula just above the MM FrR calculator and I don't understand how a high SQRT(LxC) in the denominator would lower any ratio with a 1 in the numerator. Could you explain ResonantFrequencyCalculations for Dummies?

If the inductance estimates are correct, it means one wants to have capacitance as low as possible in order to preserve the harmonics 'air' which extend beyond the audible band. It would be difficult to imagine why one would want to bring that mountain closer to the audible frequency.

No, not for MM's. As I indicated in my previous post, if the frequency of that mountain is brought down such that it occurs just a little above where the cartridge would otherwise be rolling off, it will compensate for that rolloff within the audible range, and have the effect of extending the overall treble response. Lower capacitance will move the peak to a point where it will no longer provide that compensation, and where the response will already have rolled off significantly, resulting in a less extended treble.

As an aside, I am looking at the formula just above the MM FrR calculator and I don't understand how a high SQRT(LxC) in the denominator would lower any ratio with a 1 in the numerator.

As with any fraction, if the denominator increases and the numerator stays the same, the resultant value goes down. The presence of the square root function simply slows down the rate of decrease as LC increases.

Best regards,
-- Al