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RE: Question about capacitance loading...eq-ing the sound of your cartridge? How does this work?

Posted by dconsmack (A) on June 18, 2012 at 16:24:33

In Reply to: Question about capacitance loading...eq-ing the sound of your cartridge? How does this work? posted by Mr Blue Sky on June 18, 2012 at 15:01:59:

I my experience, it's necessary to adjust capacitance AND impedance loading using a test record such as The Ultimate Analogue Test LP and recording software. Many moving magnet cartridges such as yours will have a steady rise in frequency response around 10kHz with a peak around 15kHz when the impedance is loaded at the standard 47kΩ. I've seen a treble peaks at 15kHz with MM carts at +7dB! This is where the problem is. That peak is caused by two things, 1: an ELECTRICAL resonance (peak) that can be tamed by impedance loading, and 2: a MECHANICAL resonance from the cantilever mass who's treble frequency peak is determined by the total capacitance (measured in pF) of the tonearm wiring, phono cable, and the amount of capacitance at the input of the phono preamp (this spec is often not listed in the phono preamp's manual.)
So, with impedance loading you can tame the amplitude of the peak, and the amount of capacitance an influence WHERE that peak is. The less capacitance, the higher the mechanical resonance peak. Both the electrical and mechanical resonances are likely in the upper treble above 10kHz.
The way a test can be done with the test LP mentioned above is to record the 3 high frequency tests into recording software and look at the waveform. The first test (1kHz test tone) is the reference level. The second test is a 10kHz tone and should match the 1kHz level. The third test is a frequency sweep from 1kHz to 20kHz. Here, you can see where the peak is. Lower the impedance value (example: 47kΩ to 33kΩ or 22kΩ or 15kΩ) to tame the frequency peak and/or the 10kHz tone to as close to the reference 1kHz as possible. When this happens, it will probably induce a frequency dip from 5kHz-9kHz. This is where capacitance comes in. By INCREASING that capacitance, you can lower the mechanical resonance to compensate for the dip in the 5kHz-9kHz range. It might take a lot of capacitance to get that low, and if you get down to that range, it'll likely cause a frequency roll off before 20kHz.
It's a balancing act of compromise, and you just have to put in the time and do that test over and over with different values to get what is a near flat frequency response. It is not possible to get a cart to test perfectly flat in REAL WORLD conditions (you can with math equations though), but it is realistic to get a cart to test within ±1.5dB (which is excellent). If you can do this, I'll be willing to bet your cart won't sound lean in the bass. The exaggerated treble in almost every type of cart has this influence. Unfortunately, not many phono preamps have adjustable impedance, especially ones that go low enough (such as 15kΩ). I have the EAR 324 phono preamp and has the adjustability to neutralize a wide range of MM carts.
Some examples of loading for different carts to measure ±1.5dB are as follows: Shure V15VxMR: 200pF/47kΩ. Shure V15Vx with Jico SAS stylus: 570pF/22kΩ. Audio Technica AT150MLX: 570pF/15kΩ. The pF values listed here is 100pF of tonearm wire/phono cable with the balance from the input of the phono preamp. These carts loaded at these values sound almost identical and sound great. But, it's not necessarily the recommended values specified by the manufacturer!
I hope you find this helpful.

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