Cartridge Loading.....Part II


I read last night the below noted discussion with great interest.  It's a long post but worth the effort and I found it interesting.

It started me thinking about the amount of loading on my moving coil cartridges.  Years ago I purchased my first MC Cart, a very nice Benz Micro Glider, medium output of 0.5 mV as I recall.  At that time I inquired about loading here on Audiogon.  I was convinced, via discussion, by another member, that 300 Ohms was the magic number, so I thought.

Time moved onward and my second MC Cart is currently a Lyra Delos, again medium output 0.6mV.  Both carts had Boron cantilevers', 6 nines oxygen free copper coils and line contact diamond stylis.  When I set up the Delos I did not change or even consider 'loading' changes.  That was a grand mistake.....

Well, thanks to this specific thread I started to second guess myself . (you can do this when retired and more time is on your hands....)

My take from this recent thread is as follows.  Load at 100 Ohms or at 47K Ohms with a quality MC cartridge.  I opened up my Conrad Johnson EF1 Phono Stage this afternoon.  Found it set at 500 Ohms loading.  100 Ohms is not an available setting.  Damn...All these years I've been running the wrong loading, and on two carts, back to back...  I don't recall why I set the loading at 500 Ohms.  Faulty logic.

I reset the loading to 47K, buttoned things up and called the wife in for a listening session.  Sure as heck both of us noticed the highs were crisper and more 'apparent' than in the recent past.  Not a huge difference, but yes, a difference..  Hard lesson learned!

So, you smarter folks on this site might banter amongst yourselves, but in reality there are those of us, behind the curtains, reading and listening!  I just wish I hadn't wasted all those years listening to the incorrect load setting!

Ending with a sincere thank you very much!!

Lou

 

quincy

Showing 18 responses by intactaudio

I do not think the value of the loads chosen invalidates the test.  Sure it may over-exaggerate things in the traditional sense in the voltage realm. When you consider the proliferation of "current mode" phono inputs in vogue the 5Ω value isn't so strange.

dave

If not loaded the transformer can 'ring' with excess harmonics- its making distortion. Your ears will respond to that as sensing it as loudness and yes, it might even hurt if the volume is up a bit.

That is not the case in this instance.  The measured frequency response of both cases is within 0.1dB within the audio band and the 1:40 has a 2dB low Q resonance at 90kHZ and the 1:8 has a 6dB peak @ 235kHz.  To isolate the contribution of the SUT I left the 1:40 unloaded to reflect 187Ω and placed a 200Ω resistor across the primary of the 1:8.  I then level matched them and am hard pressed to tell the difference between the two.  This gives me confidence in my belief that the load on the cartridge is responsible for the sonic changes that I reported above.

dave

 

The lower value 200 ohm vs. 47k ohms means more loading which tends to attenuate higher frequencies.

If this were the case it would be easily measured yet I have yet to see this actually documented.  

Another way to attenuate highs is to lower the VTA/SRA

This is another case of something widely accepted, easily measured yet completely undocumented.

I want to be clear that I am not doubting there is a change in sound from varying load or changing SRA, I am just suspect about the "change in frequency response" explanation that is widely accepted as the cause. 

Peter Moncrief in IAR 5 makes a compelling case with measured confirmation that loading of an MC cartridge does not materially effect the measured frequency response of a MC cartridge.  He then goes on to show how loading does measurably change the amount of IMD created from the trackability tones on the Shure TTR103.  He then finally proposes that the excess IMD caused by a lightly loaded MC cart is often perceived as high frequency detail that many have grown accustomed to and the "dark" sound of heavily loaded cartridge can be due to a system being tuned for the typical lightly loaded case.

dave

 

Ralph

The loading has no effect on the cartridge other than making the cantilever harder to move.

Isn't that essentially suggesting that compliance has no effect on the sound of a cartridge?

did you ever get a chance to look at the copy of IAR #5 I sent you?  I find it entirely plausible that making the cantilever easier / harder to move would have a sonic impact on the sound of a cartridge.  Add to this that this effect will be dynamic based on the musical content and things get interesting.  If you trust the measurements made by Moncrief, he clearly shows a reduction of IMD and it isn't a huge leap of faith to believe that the degree of measured IMD change he shows would have an audible result.

 

dave

@rauliruegas

What Jcarr and in parallel Ralph are saying is completely different than what is presented by moncrief.  They all agree that loading has no effect on the measured frequency response of a cartridge but any similarities end there.  Ralph and Jcarr suggest the unloaded "harshness" is caused by the MC cartridge inductance resonating with cable/input capacitance resulting in input overload of a phono stage that is not "RFI stable".  Moncrief suggests (and shows) that for MC carts, IMD distortion levels are directly related to applied cartridge load.

dave

So, if PM did not shows from where comes the IMD only JC is rigth?

I would say no since the two loadings  PM shows for his IMD are 5Ω and 100Ω both of which will surely damp any possible LC resonance which would then eliminate the whole RFI aspect.   I will note that the cartridge used has a 30Ω internal impedance so the 5Ω and 100Ω values are odd but that is secondary to demonstrating the effect.

 

I didn’t mention a dead short and was only referring to the two terminal impedance the coil of a cartridge sees. Any reference to ground be it real or virtual does not factor into the load seen by the coils. Surely there has to be an actual input impedance for a current amp and it has to be low otherwise the coils will not generate any current to amplify. I am not trying to get into an in depth analysis of how various transimpedance amplifiers work but address the general first order behavior of the system and believe that a cartridge operating into a ideal voltage amp behaves differently than the same cartridge into an ideal current amplifier primarily due to the fact that the loads are at opposite ends of the spectrum.

Try placing a 5 Ohm resistor across the output of any LOMC cartridge and see what happens 😁

that depends on what load the input of the following stage gives.

If it is a voltage amplifier with 47kΩ, a 30Ω cartridge loaded by 5Ω it will be 17dB down.  When you replace that 30Ω cart with a 2Ω cart the output will only be down 3dB. If it is a current amplifier with a 1Ω input impedance a 5Ω parallel load will lower the current into the 1Ω input node by1.6dB with both the 2Ω and the 30Ω cartridge. The absolute currents will be different for the 2Ω and 30Ω carts but the relationship of how the 5Ω load affects a 1Ω input impedance stays the same.

dave

Ralph,

The two statements you quoted from me seem to align well with each other and I was unaware that there was a different versions of ohms law for static vs dynamic loads

At any rate, Moncreif threw off his numbers by using a 5 Ohm load

I agree it is an interesting choice but his measured results clearly show signal and if  if anything a lower noise floor so I fail to see where the issue is or how it invalidates his results.

dave

 

The topic of this thread is cartridge loading and it only makes sense to look at it from the two terminal perspective of the cartridge without concern of the "technology" used to provide that load.   There are two basic extremes of operation of a cartridge.  It can operate as a current generator where the load value is << the cartridge internal impedance or it can operate as a voltage generator where the load value is >> than the cartridge internal impedance.  There is also a fairly grey area inbetween these two extremes where the load value is ≈ the cartridge impedance.  For this basic discussion I think the two extremes need to conceptually be looked at from the ideal with respect to how the cartridge converts a mechanical movement into an electrical signal.  Since it is the source to load relationship that dictates whether a cartridge generates current or voltage it becomes important to determine if changing the load causes any mechanical or electrical change to the behavior of a cartridge.  

 

When operating as a voltage generator the load can easily be modified over a fairly wide range and still maintain the basic principles of operation.  I think most will agree that loads  of 10X the cartridge impedance and up have a cartridge operating squarely in the voltage realm and people will start to cry foul as your load approaches  the cartridge internal Z.  This doesn't have anything to do with actual cartridge behavior and everything to do with the type of amplification that follows.  For voltage amplification the unique case where Rsource=Rload nets a 6dB voltage loss and in the case of microvolt level signals that is huge.  Going to the case where Rload is 1/4 that of Rsource the voltage loss will be 18dB which immediately disqualifies that as an option for many.  The problem with that categoric disqualification is that you are trying to make a cartridge operate as a source of current into a voltage amplifier. The problem has nothing to do with the actual load and everything to do with using the wrong tool for the job.  If the goal is to actually load the cartridge with 1/4 the internal impedance then that load should simply be provided by a current amplifier.  If we want to discuss the effects of loading on the behavior of the cartridge we have to assume that the appropriate type of amplification is used.

The first question that needs to be addressed is in a perfect world with ideal amplification, will a 40Ω cartridge sound the same into a 5Ω load as a 47kΩ load? 

 

The question cannot be answered at all if current amplification is used.

Why not?  Consider the case to be an ideal current amp with the appropriate series resistance added so the input impedance is 5Ω.   In this gedanken world the ideal voltage amp and the ideal current amp sound identical.

If we can agree on this then we can get to the question I am really curious about and that is....  How much of the sound of current amps vs. voltage amps is simply due to the radically different load the cartridge sees? 

 

A simple experiment I have been contemplating playing with is to compare the loading extremes is to use a 1:10 sut loaded with 300kΩ to load an 8Ω cartridge with  3K.   Then take the same cartridge and feed a 1:20 and apply an 8Ω resistor as a load directly to the cartridge.  The extra 6dB of gain from the increased turns ratio will be offset by the ~6dB loss of the cartridge being loaded with its internal impedance.  One could even take it a step further and try a 1:40 with a 4Ω parallel load to see the sonic effects of the extremes.  

dave

 

OK... lets try a different tact since i do not think ground (be it virtual or real) matters in this situation.  I am simply relying on Ohm and Kirchoff for this ideal case.

Take a 30Ω cartridge and hook it up to an ideal voltage amp and then another one and hook it up to an ideal current amp.  Now take two AC microamp clamps and monitor the current output of each cartridge.  Will the currents be the same for each cartridge?

dave

@dover  This needs to be distilled to the most basic level to get a solid foundation that everyone can build upon equally.  It is not about any particular design and more about cartridge loading as a whole.  My belief is that the nature of current amplification vs. voltage amplification (namely how they load a cartridge with low vs high impedance) has a dramatic effect on the mechanical behavior of the cartridge and may be partially responsible for the differences heard between the two types.

I am not suggesting that all current amps are the same but if we can separate them conceptually from their voltage amp brethren then a discussion about how each loads a cartridge can ensue.  

 

dave

Whereas, Dave said that any device with an input impedance much lower than that of the cartridge internal resistance can act as a current driven stage. Is that correct?

I am not suggesting that if the load is a fraction of the cartridge impedance the stage must be a current amplifier.... I am suggest  as a whole current amplification stages will typically load a cartridge with less than its internal impedance.  

A good example of this is the situation I outlined above where I used an 8Ω cartridge through a 1:8 SUT and then directly compared it to a 1:40 with a 2.2Ω resistor across the cartridge directly.  The 2.2Ω resistor was chosen so that the output of the SUT in both cases was identical as was the frequency response from 10Hz to 100kHz.  (±1dB).  In this case even though the cartridge load was 1/4 the internal impedance when using the 1:40,  I would still consider it a voltage gain stage. 

The result of this experiment was interesting.  Even thought the gains of both situations were the same, the 4.7kΩ load through the 1:8 sounded a good 2dB louder.  When the gain of the 2.2Ω load was bumped 2dB suddenly it was preferred and then going back to the 4.7kΩ 1:8 at the same +2dB level started to hurt my ears. 

dave 

Who came up with the thought that cartridge loading affected tracking? That is rather silly.

A guy named Lenz observed and documented the underlying concept and I'd hesitate to call his work silly.  Now extrapolating his work to cartridge loading is interesting.  Moncrief showed decidedly different IMD results between the same cartridge lightly and heavily loaded.  While I have not been able to replicate his results perfectly, I have seen several areas where loading effects the measured behavior of a MC cartridge. 

Stepping back and looking at the big picture One has to ask "What causes IMD sidebands from the record groove needle interface?"  The only answer I can come up with is mistracking.  A number of tracking ability tests exist.   Tests like the Audio Obstacle Course use recored music with increasing levels of one selected instrument and the user can judge where mistracking happens.  This is the closest we can come to a real world example but it is important to understand the results are subjective which makes the results unique and hard to relate to others.  The other approach as used on the Shure TTR103 and the CBS STR110-112 is more scientific and requires the use of two tones typically 400hz and 4kHz  and an intermodulation distortion measurement.  Here is the blurb from CBS:

The two things that jump out at me in the above are the use of the phrases "Instantaneous force" and "tracking distortion" both of which seem to be directly related to the topic at hand.  The last bit about other system nonlinearities causing IMD is valid but outside the realm of the variable being tested here. (tracking force)  If we agree that the load will as ralph states above:

cause the cantilever to be harder to move (stiffer)

It seems fair to follow up with the premise that this can also effect tracking ability.  It is important to note that the extreme and clear results given by Moncrief do represent the two extremes where Rcart<<Rload and Rcart>>Rload which just happens to be the exact case of the ideal current mode vs. voltage mode input stage. 

dave

All cartridges no matter what has a mistracking issues always and in every LP recording.

If we accept this to be the case (and I do) then the next logical step is to agree that we are simply discussing varying degrees of mistracking.  Treating mistracking as black or white serves little purpose in this discussion.

Wyn proved that loading does not affects the tracking cartridge abilities

Where is this proof?

you will find that the parameter that needs to be changed is the compliance

In your stack of 20 test records I assume you also have CBS STR-100 which has a test for compliance.  

It is interesting to note that the telltale here is also mistracking.  Again to repeat myself... If you accept that according to Lenz that loading a cartridge will stiffen the cantilever, how can that not effect the compliance?

dave

 

Cutting and pasting text without any reference to context is not proof.

Where was it ever proposed that heavy loading causes additional mistracking?  It has been my contention that exactly the opposite may be occurring. Everything you mention specifically refers to it being impossible for heavy loading to cause mistracking with which I concur.  However the possibility that heavy loading may actually reduce mistracking hasn't been mentioned or covered by anyone beyond Moncrief that I know of.

dave

Mijo,

I agree that there is a big difference between loading @ 47kΩ and loading approaching zero.  These two extremes very closely represent the "ideals" of current vs. voltage amplification.  It may very well be conjecture but discussion of this on the most basic level is the only way to get a common solid foundation for everyone to build upon.

 I have seen distortion measurements on the same cartridge run voltage mode vs current mode and distortion is certainly lower in current mode.

Interesting.  Now the question becomes what was the cause of the distortion change?  I see two possible options:

  1:  The distortion of the two modes of operation were different.

  2: The effect of the radically different loads altered the behavior of the cartridge resulting in different measured distortions.  

Chances are the difference is some combination of the above two and simply removing the cartridge from the tests and using a sound generator should illuminate the differences between the two modes of operation. Repeating the measurements with a number of different cartridges should also illuminate if there is a specific pattern happening.  

 

dave

I suspect that the mention of Lenz's Law with respect to back EMF may be a bit misplaced here.  The simple story of back EMF when it comes to a speaker is the inertia forces the cone to continue to move after the signal tells it to 'stop and go the other way'.  This 'undriven motion' will generate an additional voltage which will then appear at the amplifier.  Lenz comes into play here when the low output impedance of the amplifier appears as a near short allowing a 'large' current to be generated which following lenz will create an opposing current which acts as a 'brake' of sorts.

The Key difference between back EMF in a driver and a cartridge is the speaker is electrically driven to create a mechanical sound and the cartridge is mechanically driven to create an electrical signal.  In a driver inertia causes an additional mechanical movement that is electrically 'damped' by Lenz.  Unlike the speaker where the heavy lifting is done by the electrical signal, the cartridge has the groove as the guiding force.   A lateral cut groove drives the diamond left and right and an important  difference is after a peak in the left direction the groove wall forces the tip back in the other direction and the inertia causes instantaneous pressure on one wall to increase as the pressure of the opposite wall decreases.  There is inertia but no real 'overshoot' to invoke Lenz like in the case of a speaker.   

When you look at a stereo cut where there is a vertical component in addition to the lateral things change.  On the downhill path to a valley a similar thing happens to the lateral situation.  The tip hits the low point and reverses direction up momentarily increasing the downforce.  It is when it reaches the following peak where I see inertia coming into play since there is nothing beyond gravity to push things back down to the next valley.  After the peak,  inertia will keep the tip going in the same direction resulting in the situation where downforce approaches (or reaches) 0.   If you look above at the screen grab I posted from CBS STR112 it states that the dynamic groove wall force can vary from 2x the static force to nearly 0.  The simple solution to mistracking is to add VTF until you are kept a 'safe distance' from 0.  I think it is generally accepted that too much tracking force leads to record and diamond wear and too little force leads to mistracking so it is the 'approaching 0' aspect that needs to be looked at.  It is my belief that Loading can effect the behavior on the vertical uphill peaks causing a dynamic brake if you will which results in better dynamic tracking ability. 

An interesting test of this on my list of things to try is to compare the results of loading with stereo vs. mono records.  For what I say above to hold water, I would expect the results of loading a stereo record to be greater than that of a mono record due to the addition of the vertical component. I should get time to try this experiment around 2024 :-)

dave