Cartridge Loading- Low output M/C
I took a look at the manual, which can be found at the Plinius site. On page 11 (pdf page 13) it can be seen that the capacitance switch is to the left of the resistive loading switches, and appears to be identified as switch "1", and it should be set to the "Up" position for 100 pf. Enjoy! Regards, -- Al |
As the Ortofon is a fairly low output MC cartridge there are generally two components to the frequency response which interact. 1. The electrical LCR response. Contrary to what has been said, for LOMC the capacitance, unless it is quite large (in the order of .1uF), is essentially irrelevant and the objective is to set the response to look like a single pole LR system. See the MC cartridge section in this article. http://www.hagtech.com/loading.html In most cases a resistance close to 100 ohms is fine- largely because the coil DC resistance is often a proxy for the inductance (most cartridges have a similar internal magnetic structure) and turns out to be close to 5 ohms and c.0.5mH, (There are exceptions- Miyajima cartridges are a good example of this) and as a result a 100 ohm load gives you a well damped electrical system (no peaking) with a bandwidth of c.35kHz. This results in c. 0.3dB attenuation at 10kHz and c. 1.1dB at 20kHz. You could try to flatten the response out a bit by greatly increasing the cap, but it’s tricky and usually impossible because of the tip/cantilever/suspension resonance. To illustrate this point, use the Hagtech calculator and set the capacitance to 200pF and the inductance to 0.5mH- the resonance is at 500kHz and into a 47k load the Q is c.30! To reduce the resonance to 50kHz the cap must be increased to 20nF and the Q (R/inductive impedance) becomes 300! If the load R is reduced to 100 then the Q is 0.06 for the 200pF cap and 0.6 for the 20nF cap. So, clearly a low load R reduces the electrical system to a simple, single pole RL form. 2. Tip/cantilever/suspension resonance. Typically this looks like an electrical equivalent LCR resonance plus an additional HF pole. I wont go into the math, but it’s not unusual for this to have a resonant peak in the high sonic/supersonic range- 18-40kHz, and with an amplitude of 2-6dB at the peak. This is NOT the electrical resonance and cannot be corrected for by changing the loading of the cartridge. It can be modelled as an LCR plus an RC followed by a unity gain buffer or as a mismatched transmission line. Essentially the cantilever flexes and resonates when stimulated by the movement of the stylus tip and the response is damped by the suspension/coil dampers. So, the bottom line is- you’ll never get it perfect. You can either listen and decide what you like, or use a test record to check the sonic response. A couple of other things, the RIAA deemphasis of your amp comes into play, and it’s not unusual for that to be off c. 0.5dB or so over some frequency range, and most amps have restricted frequency responses to reduce the infrasonic and ultrasonic signals. Also, your room/speaker response is probably poor with errors at least as large as any from the above sources, so unless you’ve characterized and corrected that then listening is probably your best bet. |
