mc cartridge loading


I currently load an Audio Technica ART9 at 90 ohms, the load 'inherited' from another cartridge, am pleased with the sound but recognize the manufacturer's recommendation that at least 100 ohms be used.  Changing load requires opening the phono-amplifier case, somewhat tedious.   Is increasing the resistance (decreasing the load) worth the effort?
seventies
Not worth the effort if you are talking about the difference between 100R and 90R.  However, when the manufacturer says "at least", that implies that higher resistances (less of a load on the cartridge) might sound even better than 100R. Whether you would think so, if you tried it, depends upon your personal taste, the reactance of your phono cable and phono stage input, even the topology of your phono stage.  MC phono cartridges are typically loaded down in order to tame a rise in response at very high frequencies that can occur when the aforementioned factors are in a certain alignment.  But in some cases, MC cartridges can sound surprisingly great into a standard 47K load resistance, a la that used for MM cartridges.
to lewm
Thanks for your response.
I might try reverting to 200 ohms, which is what is pre-installed in the phono amp, so involves removing the resistor I added in parallel without replacing it.

Right now, on two different turntables, I have set up a Koetsu Urushi, in one case, and an Ortofon MC7500, in the other, both looking at a 47K load.  Sounds really good, but not necessarily better than into, say, 1000R or even 100R.  (I do think I like them better into 1000R or 47K vs into 100R but would need more listening time to be sure.)  This is using an Atma-sphere MP1 preamplifier.
I'm with you lewn.....Most of my cartridges operate at 1000
The loading only needs attention if the phono section is sensitive to RFI at its input. The RFI is generated by the cartridge itself in part due to the afore-mentioned resonance that is set up by the inductance of the cartridge and the capacitance of the interconnect cable from the tone arm. If that is kept low and if the preamp is stable the loading will have very little effect.

What is happening here is Audio Technica is essentially stating that the preamp they use isn't stable. Just because that is so does not mean that it is that way with you. If you do hear a difference (usually an improvement) with loading, it suggests that the phono preamp is unstable at ultrasonic or RF frequencies.

Bottom line: loading is about the phono section and not the cartridge.
In all the years I have been reading and talking to engineers who are involved with step up trans and carts this is the first time I ever heard this.

You can argue that the best impedance match on paper might not be the best sounding one.  But there Is a difference in sound.  It sounds like he may not want to go through the effort to experiment but it may be well worth the effort in his system.

You might look to some other forums where there are a lot of comment on this subject.

Enjoy the ride
Tom

While loading is always the question i want to remind about phono preamps with so called "Automatic Impedance Matching" for any cartridge. One of them is WLM Phonata from WLM Acoustics. I believe it was probably made by Trafomatic Audio for WLM Acoustics (Austrian-Serbian collaboration) and i think it’s out of production now. Anyway, that’s what i use, not sure if i miss something with loading, but with this device i can forget about loading at all - right? "This unique preamp has automatic impedance matching so that the input impedance for the cartridge is perfect each and every time."

p.s. I've posted WLM Phonata's specs here on audiogon before.


There is a kind of rule which state '' 10x the cart impedance value''.

I like such kinds of rules because they provide a kind of orientation.

I don't like quessing with my ears because one is never sure while

this uncertainty lead away from our primary goal: listen to the

music.

Jonathan Carr, the designer of Lyra cartridges says much the same thing as Atmasphere--loading is only needed with some phonostage which overload when presented with RFI. 

I have found that with most high quality phonostages, not much more loading than the typical, built-in 47k ohm loading is needed.  I once helped with a setup involving a Hovland preamp.  There was obvious RFI interference (noise) even when the tonearm was sitting on its rest and not playing (external RFI).  It turned out that the "default" loading on the Hovland was 100k and that just switching to 47k cured the problem.

The problem with a simple rule like 10X is that it will simply be wrong under certain circumstances.  I have a Transfiguration Orpheus cartridge which has an extremely low source impedance of 1 ohm; the 10X rule would mean an extreme amount of loading at 10 ohms.  Because it is a quite low output cartridge, that amount of loading would throw away a significant amount of signal and would mean, in my system, not enough gain.  Also, a lot of loading does affect tonal balance--it substantially reduces the top end; I paid a lot of money for the kind of open and extended top end that good MC deliver and that would go to waste with this kind of loading.  This rule is also not very useful when it is applied in the case of using a step up transformer where one has to consider where the loading is applied--on the primary side of the transformer or the secondary side?

The best approach is to try different loading to see what sounds best.  If one is not so inclined, stick with a modest amount of loading that would work reasonably well under most circumstances, something in the range of 100-150 ohms--that would cure any RFI issues and is not so much loading that high frequencies would be severely attenuated.

The rules in audio are like the rules in economics.  Everyone likes to believe they have the answers but the outcome varies because we Do Not know all the variables.

In regards to your ears not being the ultimate test well that is exactly what they are.  You have to listen to it.  We can argue who can hear and who can not.  We can also argue with tradeoffs involved where are your priorities but these goals will only be met by using your ears.

Never looked at a sine wave on my scope and started dancing.  The one size fits all in audio is Well optimistic!!!  I build all my own gear so I am lucky. I can change everything around a million different ways.  I do not have to show the bean counters where the profit is or explain why I went down this dead end road.

So the moral of the story for the original post is the Only way he is going to know in his system is to experiment with different resistor types and values.  Then listen.  If that 2 dollar resistor puts a smile on his face who cares if it is real or in his head.

Enjoy the ride
Tom
The 10X Rule is important mostly for achieving a flat frequency response within the audio frequency range (up to 20kHz), when two amplifying elements (e.g., preamplifier and amplifier, CDP and preamplifier line input) are in tandem; the input impedance of the driven stage ought to be AT LEAST 10X the value of the output Z of the preceding stage.  But this is only a rule of thumb. Even at a 10X ratio, the math tells us that there would be a small loss of signal voltage at frequencies nearing and beyond 20kHz.  In the case where the driver is a phono cartridge, which is also a transducer subject to inherent nonlinear behavior, the situation becomes more complex.  So, I agree that probably one should not go lower than a 10X ratio between the internal resistance of a phono cartridge and the input resistance of the phono stage, but a higher ratio can be better and usually is, at least where phono cartridges are concerned.  However, I have read on-line many times that a ratio lower than 10X is subjectively preferred by some of us, with some cartridges.  This seems to be true of the Denon DL103, for one example.  (I don't own one.)  

My preamplifier is an Atma-sphere MP1, and now that I have set it up so as to be able to switch easily between 100R, 1000R, and 47K, I find that I most of the time prefer 47K.  This gives a more "open" and richer treble, for want of a better vocabulary to describe it, but the difference between 47K and 1000R is subtle.

Do you listen to the music? Are you crazy? Have you any idea

about variables involved? I am , of course, still adjusting ...

I use the same cartridge as the OP, in conjunction with a Herron VTPH-2 phono stage.  After briefly trying 1K and 47K loading I settled on no loading, meaning the nearly infinite input impedance of the Herron's FET-based input stage.  Keith Herron recommends that with his particular phono stage that nearly infinite impedance will be optimal with many cartridges.

Also, FWIW, I would characterize the differences I noted between the three loading conditions as being recording dependent, relatively minor, and hard to pin down.  I suspect that the difference between the lowest value I tried (1K) and 100 ohms would have been much more profound, and probably not for the better in conjunction with the VTPH-2.

Regards,
-- Al
 
In all the years I have been reading and talking to engineers who are involved with step up trans and carts this is the first time I ever heard this.
Apparently they don't know the physics of phono sections.

In the case of an SUT you have a different set of variables! The SUT is the major player in terms of loading and you have to satisfy its requirements for the best sound. That will vary according to the cartridge and interconnect used, as transformers transform impedance, so if your cartridge x is lower impedance than cartridge y, the loading required to prevent the transformer from ringing will be lower too.  Go too low and the transformer will start to roll off the highs.

My comments precluded the use of an SUT, which will usually block RF interference due to a lack of bandwidth. If you are using an SUT you most definitely have to pay attention to loading! You also have to be careful about the interconnect cable, as the inductance of the SUT is part of the resonance equation. Keeping the interconnect cable capacitance down is key.

If we are simply talking about loading of the cartridge (no SUT), sometimes it is helpful to use test equipment and ears at the same time! Turns out that when the cartridge is loaded  properly (called critical damping) that is also the value at which it sounds best as it is not ringing and not rolled off in the highs. The only thing is, **that really does not apply to low output moving coil cartridges** (instead applies to higher output moving magnet types). You can load LOMCs or not, and when you 'ring' them with a square wave, all you get in response is a square wave. In essence, the loading is only affecting them at RF frequencies and any engineer worth his salt can tell you why: the inductance of the cartridge is very low so the interaction with the loading resistor is occurring at ultrasonic or radio frequencies.

If you search this and other forums you will find this has been discussed quite a lot. 

To summarize:

1) Loading of a LOMC cartridge generally does not affect the cartridge. There will be audible effects if the phono section is unstable with RFI at its input.
  a) for unstable phono sections, the 'loading' resistor detunes the resonance caused by the inductance of the cartridge in parallel with the capacitance of the interconnect cable. Therefore the 'loading' value will change with the cable. We can see from this that the 'correct' value is not likely found in manufacturer's recommendations.

2) In the case of a LOMC cartridge and an SUT, loading of the output of the transformer is paramount for proper performance. Often this will include a capacitive value as well as a resistive component.

3) In all cases the results can be measured and readily correlate with what is heard, meaning that if you know what you are doing you can often get pretty close to what is correct in your system by ear. examples:
  a) if the phono section is stable and no SUT used, the loading will have no electrical effect on a LOMC cartridge. The extra energy asked of the cartridge by loading it may cause the tracking characteristics of the cartridge to change- IOW it may track better without loading. This will be found to be an interaction with the effective mass of the arm/cartridge combination as the loading essentially affects compliance.  If a value of 10x or more of the cartridge impedance is used, the effect on tracking will be minimal. If the phono section is not stable then the loading resistor will be needed to detune the tank circuit at the input of the phono section (cartridge and cable). The best loading value will the highest one that halts the stability problems in the phono section.
  b) if an SUT is used with a LOMC cartridge, the best sound will occur when the transformer is critically damped.
  c) If a high output moving magnet cartridge is used, the best sound will be when the cartridge/cable combination is critically damped.
  d) in all cases it will be found that a low capacitance cable makes things go easier.


Thanks Ralph (Atmasphere).  That all makes sense to me.

I would just add that the generally unspecified input capacitance of the phono stage will add to the capacitance of the cable, and may also be significant in some cases.  In fact one phono stage that I recall reading about (the AcousTech PH-1) has a specified input capacitance in LOMC mode of 10,000 pf!  And probably not coincidentally a fixed input resistance of 100 ohms.  Why one would design a phono stage in that manner is beyond me.

Best regards,
-- Al
 
One of the most commonly used phono input tubes is the 12AX7 which has a very large Miller capacitance, given its high mu factor, albeit nowhere near to 10,000pF!  How did AcousTech manage to "achieve" that?
The PH-1 is solid state, Lew, and according to Stereophile's review (which was favorable and made no mention of the high input capacitance) "the circuit itself appears to be based on Burr-Brown OPA2134 op-amp chips." I suppose that the high capacitance reflects a capacitor being placed across the input, for whatever reason. Perhaps (just guessing) to apply such a heavy load at radio frequencies that RF energy would be essentially kept out of the circuitry altogether.

Best regards,
-- Al