Ohm Loads for cartridges

Actually, the first part is wrong. The RIAA curve cuts the high frequencies before it goes to the cutting amplifier during proeduction, and the highs get reboosted on playback of the end product thru equalization in the phono stage. This is widely known. The stylus could not track the groove if the hf was as loud as the lf.

If any of you guys remember J. Peter Moncrieff's International Audio Review ( IAR ), he spent quite a bit of time in a few issues charting and documenting what worked best with dozens upon dozens of cartridges. While this was quite a while ago and he obviously couldn't have covered the current models of today, his findings on many of those was that what worked optimally in terms of frequency response and noise characteristics RARELY matched the manufacturers suggested loading characteristics. He produced charts showing the both of these specs with the factory specs and with a few others for you to draw your own conclusions from. Sean >

You got that backwards Carl. In the RIAA system, records are cut with a constant amplitude up to 500Hz, a constant velocity between 500Hz and 2120 Hz, and a constant amplitude again above 2120Hz. A phono cartridge is a velocity transducer, not an amplitude transducer. So for a flat response the records would be cut with a constant velocity at all frequencies. But that would result in very large groove excursions at low frequencies, which (amongst other things) would necessitate a very large spacing between the grooves. By attenuating the bass frequencies (in the constant amplitude region), the grooves can be cut closer together. By boosting the high frequencies (in the constant amplitude region again) the signal to noise ration is improved. Brian

Okay everyone, I have been reading all the words regarding cartridge loading on this msg. board but I am still a bit confused. I keep getting conflicting information from different sources regarding "cartridge loading" so I hope someone can clear my misconceptions up. Correct me if I'm wrong but I thought that a resistor with a higher value, ie. 10K ohms reduced more current than a 500 ohm resistor. If so, does it not follow that placing a 10K ohm resistor to "load" a cartridge would alter the signal from the cartridge, ie., reduce the bandwidth and lower the high frequency signals, etc., more so than a 500 ohm resistor. If this is so than how can a resistor with a lower value, say 500 ohms be said to offer more "loading" on a cartridge than a 10K ohm resistor? This is what Mr. Fremer suggests and yet I have dealers telling me this is wrong. I am confused.

You are right. The terminology is confusing. Also frequently people get it backwards. The term load, when used relating to resistance, comes from a mechanical analogy: a SMALLER resistance generally means more current and hence a LARGER load. Conversely a LARGER resistance is often referred to as a SMALLER load. Confusing - I agree.

In any case with MM cartridges the issue is not current or voltage, the issue is to match the coil inductance and phono stage input capacitance with the correct resistance. MM cartridges have a very high series inductance, typically 0.5 Henries. The combination of the three values behaves like an RLC circuit, which under the wrong conditions will give a highly curved response (in fact an RLC circuit can act as a very effective band pass filter). The loading resistor is selected to dampen the response so that it is almost flat within the audio spectrum. Though there is always a residual hump in the response at around 10-12KHz. Therefore the load is critical, and not only that, the input capacitance of the phono stage is also critical.

As a general pointer, the input capacitance is typically around 250pF, larger values will reduce the frequency of the hump, but will decrease the damping, so should be matched with a slightly lower input resistance (maybe 40K Ohm for 330pF capacitance) I imagine that some sonic improvements could be achieved by carefully matching these values. So trying values of between 30K Ohms and 75K Ohms is a good idea if you have the option.

A benefit of using an MC cartridge is that the coil inductance is very low (around 20 micro Henries as far as I remember, I measured it for a couple of MCs recently, but I don't have the result to hand). So in the case of an MC cartridge changing the load / resistance does not significantly alter the frequency response within the audio spectrum.

Ironically though, you will see more discussion on the MC question. I believe that is because the people who spend $2000 on an expensive MC, are more likely to be serial tweakers, whereas Joe Public, who spend $200 on an MM, just wants to set it up and be done with it!

Hope this helps.

Brian