Ohm Loads for cartridges

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

Brian,
Thanks for the help. Unfortunatly answering one questions leads to more questions, so it is said. (Especially among people who know more about the biological world than the electronic world.)
Maybe you can address this problem as it relates to cartridge loading. I was fortunate, (or unfortunate as the case may be) in one swoop replace my table, arm and cartridge. The problem is is that I now have much more emphasis in the HF spectrum than before and the surface noise on LP's is accented quite a bit more than in the past.
Since the first cartridge failed after 6 months I hoped the second one would do better. Surface noise is still too prominent. Even on virgin LP's.
So, in talking to Benz I am told that my phon stage, which is permanently set at 560 ohms, is too low and should be at 10K-20K. And that this is a major cause for my problems. The 2 phono stages I have tried have allowed me to either agjust to a 10K or are set at 20K. In either case the HF emphasis was reduced with more emphasis placed in the med-low range and surface noise was improved a fair amount. So, was it the loading charecteristics of the phonostages or the phonosatges as a whole which made this improvement?
Also, sorry for asking what does "RLC" refer to as you mentioned?
Thanks!

Hi Gerard,

Which Benz do you have? I have never used one so I hesitate to comment.

All of the Benz cartridges are MC, and generally speaking, MC cartridges are not as sensitive to the load as MM cartridges (due to the low coil inductance). However connecting an MC to a low resistance load (such as 100 Ohms) results in a larger current, and this mechanically dampens the cantilever. This may be the more rolled off kind of sound you are looking for.

However, returning to your original observations, the two phono stages you tried gave a preferable sound (ie less HF emphasis), even when the input impedance was set high (10K, or 20K). Since your phono stage is already set to a lowish impedance, and you think you still have too much HF emphasis, then I can't see how you can cure the problem with your existing phono stage. Increasing the resistance will make it worse if anything.

Best regards,

Brian

PS An RLC circuit is a circuit containing a resistor (R), an inductor (L), and a Capacitor (C).