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Cartridge dynamic behaviour
Cartridge dynamic behaviour
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Old 13th March 2018, 01:50 AM   #1
billshurv is offline billshurv  United Kingdom
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Cartridge dynamic behaviour
Default Cartridge dynamic behaviour

I've started this thread to explore some issues that came up on another thread recently that were starting to drag that thread in the wrong direction. Hopefully I'll get time to pull in stuff on some other discussions here, but the drive here is to discuss things that would cause non-linear behaviour in cartridges, how to measure that and if anything can be done about it.

The starter for 10 is the conjecture that cartridge frequency response is not independent of program level. This is a contentious issue, not least because there is almost no discussion of this in the literature from the day one way or another.

More to follow...
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Old 13th March 2018, 03:11 PM   #2
luckythedog is offline luckythedog  United Kingdom
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At any given playback level, a cartridge's overall 'f-response' comprises a composite of several separate component parts. Electromagnetic generator (typically hf loss/roll-off), a mechanical 'top' resonant system (centred 8kHz-30kHz) that sometimes props up generator hf roll-off, an electrical resonant LCR impedance system (10-20kHz) that also sometimes props up generator roll-off system in MM/MI carts, and a lf spring-mass elastomer system. There's also a contribution from geometric thd and mistracing distortion that can influence f-response somewhat. And other contributing elements besides.

When combined, the result is the well-known f response curve: lf peak, mid-dip, resonant peak and hf roll-off. Nominally +3dB/-1dB at normal test levels. However, each of the component parts has losses and non-linearities which are level sensitive, and many of them slew-rate sensitive as to losses. It's slew-rate, rather than 'frequency' per se, which is mostly at issue.

I first encountered significant level-sensitive parameters about 6 years ago in collaboration with David Laloum trying to 'perfectly' terminate MM carts, where cartridge inductive behaviour, and hence the generator (not least LCR resonant f and Q), apparently changed significantly dependant upon test level for some cartridges. Especially for very small signals. I still have the charts from that work, and stand by that it is very probably real and a part of vinyl sound.

Exploration of theoretical loss mechanisms in each of the contributing elements to f-response reveals many and various level-sensitive non-linearities. Some are slew-rate specific, or displacement specific. Some increase with level, some decrease.

One might surmise the overall result is probably a level-dependent series of contours of f-response, typically within a bracket of +/- 3dB at the mid-dip, over the decades of normal playback level range where the most audible effects happen. AFAIK there is no single test record to confirm.

IMO the audible result is probably then a big part of vinyl sound - certainly something one doesn't get from digital sources, and only to an extent from tape.

LD
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Old 13th March 2018, 03:30 PM   #3
davidsrsb is offline davidsrsb  Malaysia
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Interesting. Commonly used Permalloy has low but non-zero coercivity, so there is scope for a low level non-linearity there
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Old 13th March 2018, 03:41 PM   #4
billshurv is offline billshurv  United Kingdom
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Cartridge dynamic behaviour
Although hard to find the measurements from the parts of that work that were published are still up at vinylEngine. As per in the mechanical resonance thread I won't post them over here unless LD requests as some of the thinking has perhaps been overtaken, but I will pick some choice FR response plots to look at to trying to unpick these.

And just in case anyone asks 'why MM'. MC carts will have a number of the same issues, but I've started looking at MMs in a new light when I realised that, in many respects I took a step backwards when I dumped my P77 for my first MC. MM offers a number of possibly advantages, not least that you can easily change stylus and this ability to chop and change and experiment makes (for me) a lot of fun. It doesn't actually worry me if I end up discovering I prefer my MCs but I hope to learn a lot.
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Old 14th March 2018, 02:18 AM   #5
Pano is offline Pano  United States
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Cartridge dynamic behaviour
Do we even know if cutter heads have level independent frequency response? Some of what affects phono carts may also affect cutter heads. And cutter heads are likely to have their own sets of problems.
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Old 14th March 2018, 02:26 AM   #6
billshurv is offline billshurv  United Kingdom
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Cartridge dynamic behaviour
They have feedback windings!
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Old 14th March 2018, 02:28 AM   #7
billshurv is offline billshurv  United Kingdom
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Cartridge dynamic behaviour
I should note at this point that, as my system is more or less DIY end to end I don't see why I should have to follow things like 47k loading for a cartridge. I can do what I want if the end result is a more accurate reproduction. Just so you are aware

Moving on: Up for discussion is the attached snip from an ortofon brochure for the 520 series from around 40 years ago. This was the first appearance of the split pin generator that Ortofon still use to this day. The claim is eddy current reduction. There is a comparion between solid and split pins. Ortofon use B&K plotters set at 50dB scale for all their other FR measurements, so safe to assume 1dB per division. This does show the classic mid-range dip that some cartridges have. Why this is interesting is that you can still get new cartridges with both pole pin constructions in the OM and superOM ranges and the stylii are interchangeable allowing some testing to be done. And testing is needed as we immediately run into the first problem which was discussed at length in the mechanical resonance thread* here mechanical resonance in MMs, which came to a grinding halt mainly due to my inability to complete a test board. However George did build a unit and came up with some interesting data using a Shure M97.

If you look at the work of Hans van Maanen and the very interesting Steven Van Raalte paper from Linear Audio volume 3 you can see that it is possible to assemble a combined electrical/mechanical resonance response that produces the response curve that ortofon show. However I immediately see something I don't like as this requires a 13dB lift from mechanical resonance at 21kHz. 3-4dB I could believe, but 13 is a lot. The 13dB figure you can see is the same between the VR paper and the ortofon graphs. If you look here mechanical resonance in MMs, there is the transmission line model LD has worked on that does show the possibility of this sort of level of enhancement. I still don't like it as that infers an exceedingly undamped setup.

So we need some measurements that try to isolate cartridge response from cantilever. mechanical resonance in MMs shows a measurement of the ortofon solid pin generator using a transimpedance circuit LD designed and build and I have a variant waiting for me to get off the internet and build. With a loading scheme that puts cartridge electrical resonance out the picture you can see that you get a flat response within a gnatfart across the audio band**. There is a rising response indicating something, possibly mechanical, but it's not anywhere near that you would expect cw vR or ortofon.

Overall I am not sure the generally accepted cartridge electrical model is correct! But I have probably missed something as usual . If we are to explore losses in the generator, we need to make sure we have a half decent generator model to start with?

*I will x-ref that thread but don't want to copy wholesale chunks out of context. Sorry if that means you have to have several tabs open. It's worth it.

**LD will need to explain if he had any additional corrections in place to get this response.
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File Type: jpg svr response.JPG (33.6 KB, 497 views)
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Old 14th March 2018, 09:32 PM   #8
jaddie is offline jaddie
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Still no data, guys? Graphs with no scales? No test conditions, specific groove velocities (there had to be more than one), cartridge and load used?

I'm not doubting the theory here, but how much of a problem is it?

@billshurv - I discovered 40 years ago that all carts don't all work well with 47K/200pf termination. At the studio where I worked then we build up cart termination boxes with pots to alter the R and variable capacitors to vary C to flatten off cart response. Used a paper-based tracking response plotter and a test record with recorded 20-20K log sweep recorded without RIAA. That was in the days of preamps with reactive inputs and carts like the Shure V15-III. It was kind of a mess, but the term box fixed a lot of it. We moved to a preamp with completely non-reactive input and an purely resistive load with and adjustable purely C load. Used to use a C meter on the cabling so we know what it was.
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Old 14th March 2018, 10:49 PM   #9
billshurv is offline billshurv  United Kingdom
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Cartridge dynamic behaviour
We're just starting out on this thread and setting the scene. Sorry if it's not going fast enough for you but I have a full time job and a baby who isn't sleeping well due to a chest infection.

Personally I don't yet know the scale, that's what we need to find out. One step at a time.
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Old 14th March 2018, 10:54 PM   #10
jaddie is offline jaddie
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Ok, fine, take whatever time you need. But I would also advise anyone against drawing conclusions of any kind at this point.

Too bad conclusions have already been drawn sans supporting data.
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