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Old 6th February 2017, 10:41 PM   #1
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Default mechanical resonance in MMs

Slightly off the wall one here but been reading a very interesting article from the mists of linear audio volume 3 by Steven van Raalte and comparing with the Bob Cordell vinyltrak article. In it he discussed an inverse resonance filter for dealing with the resonances that MM manufacturers build in to prop up the high end response, the end result usually being a bump in the response somewhere between 12 and 15 KHz then a 4th order roll off. Whilst this is
not a horrendous problem for us oldies who can't hear up there anyway it is predicted that this causes some unwanted time domain issues that the inverse filter addresses.

Now what would be nice is for there to not be any resonances at all in the audio band. Low inductance cartridges don't need the help so don't have the resonance. Therefore there is something about the design of the cantilever and/or the suspension that causes this. Once we know what it is then we stand a fighting chance of working out how to remove it. If it can be addressed then, with some changes to cartridge loading as discussed by both Cordell and van Raalte we can get a nice 1st order roll off to well above 20KHz and put the MM back on a level footing with MC models. Looking at this more deeply it appears this issue is one of those that was well known about in 1975 and many have forgotten.

So first step is to get a grip on the potential problem. Now I have 3 cartridge types I am interested in (as in own)
1. AT 150MLx 2K 450mH, 100-200pF recommended load
2. Ortofon super OM 1K 580mH 200-500pF recommended load
3. A&R x77 series (aka Garrott, aka Sumiko Pearl, AKA shelter 201 etc). 580 580mH

The last one is more of a 'project' as I have 4 cartridge bodies to perform evil experiments on and there are a wide range of replacement stylus types that will fit including the sapphire Jico SAS.

Now immediately I have run into some problems as both the Cordell and van Raalte cartridge models are over simplistic. Digging into this further shows the model is more complex and difficult to measure (Elliot). So we need to get empirical. More research shows that the best way to do this is to find a test record with square waves on and measure the response. Bother I don't have one of those, so 40 will have to go Ortofon's way as willing to bet CBS STR-112 costs $$$ these days. Once I have Jan's permission will pop in a diagram from the van Raalte article to show how the inverse filter cleans up the ringing on square waves. If I am right (unlikely but we all get lucky sometime) then you can record the output of the preamp and use audacity to mess around to find centre frequency and Q for the filter. When that is known we can see how much of a resonance is designed in.

Note: I am expecting to find that the ortofon and AT are not using too much in the way of mechanical cheating, but suspect the A&R does. Certainly both ortofon and AT have -3dB points at 30kHz which suggests the main cantilever resonance is way out the audio band.

Links. There are a couple of JAES papers that would help in this, but at $33 each I'll bumble around being empirical a bit longer.
https://linearaudio.net/sites/linear...lte_vol3_1.pdf
Magnetic Phono Pickup Cartridges
http://www.kallhovde.com/advent/phono-pre-research.pdf
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Old 7th February 2017, 02:27 PM   #2
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Here is the diagram I was talking about, thanks to Jan and (c) Linear Audio. This figure shows simulated cartridge square wave responses into a 47k load with 275pF total capacitance.

Trace A is the resonance superimposed on the signal. Trace B shows this output damped by the preamp loading. Trace D is the signal fed through an inverse resonance filter. There is some overshoot left, and the article explains how to deal with that, but I'm not giving it all away as its only 3.50 to get the whole article.
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File Type: pdf Figure 8.pdf (11.9 KB, 143 views)
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Old 7th February 2017, 03:19 PM   #3
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Quote:
Originally Posted by billshurv View Post
Here is the diagram I was talking about, thanks to Jan and (c) Linear Audio. This figure shows simulated cartridge square wave responses into a 47k load with 275pF total capacitance.

Trace A is the resonance superimposed on the signal. Trace B shows this output damped by the preamp loading. Trace D is the signal fed through an inverse resonance filter. There is some overshoot left, and the article explains how to deal with that, but I'm not giving it all away as its only €3.50 to get the whole article.
I have an STR-112 but no MM carts these days. Frankly I don't see using it for anything, you could have it on indefinite load if it would help.

EDIT - Sorry it seems to have gone missing in the move, I have to open more boxes.
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Last edited by scott wurcer; 7th February 2017 at 03:22 PM.
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Old 7th February 2017, 03:29 PM   #4
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Scott: No rush as will be a while before I have space made to start testing. Took me 2 years to get my miniDSP into a box!
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Old 8th February 2017, 10:29 AM   #5
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Quote:
Originally Posted by billshurv View Post
In it he discussed an inverse resonance filter for dealing with the resonances that MM manufacturers build in to prop up the high end response, the end result usually being a bump in the response somewhere between 12 and 15 KHz.......
Interesting. Such resonance also often crops up in MC carts, presumably because the generator has relatively high inertia and the cantilever has spring, and is surprisingly often found in the audioband. Paul Miller's website catalogs numerous measured f response of MC carts, many of which show such features IIRC.

Not all MM carts have this resonance in the audioband: some manufacturers design for it to be above the audioband, and only have minor artifacts ( a bit of lift) at hf audioband which few of us here can hear anyway, let's face it.

Top end MM Ortofons and some AT MM carts have it centred above the audioband IME, and I settled on using these carts a few years ago because I use an unconventional pre-amp which avoids LCR electrical resonance, and I enjoy the sound of the results.

I think this is a better solution than inverse filters. Use a cart which has an ultrasonic resonance, be it MM or MC, and deal with the electrical interface so as to avoid an LCR resonance as necessary.

Because of the way effective tip mass is 'measured', carts with low spec effective tip mass have high resonant frequencies, IME. It correlates because measured mechanical resonant f is typically used to derive effective tip mass; wrongly so IMO, but that's another matter.

To avoid LCR resonance in MMs, I load the cart with a very low input impedance, near zero, and deal with the LR pole being wrong. This doesn't necessarily work well for all MM carts, but when it does IME it levels the playing field between MM and MC.

An upside of MM is then that a quality stylus is generally replaceable and cheaper.

Bill, the British Library subscribes to JAES, so if it's easy for you in London and you have a few articles to browse, that can saves fees/subscription.

Quote:
Originally Posted by billshurv View Post
More research shows that the best way to do this is to find a test record with square waves on and measure the response.
Sort of, except programme material of a square wave does not produce a square wave shaped groove at all, of course !

A tiny sharp physical scratch or click, being impulse like, serves well IME. So long as one doesn't overdo it and rip the stylus off

LD
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Old 8th February 2017, 10:59 AM   #6
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LD, sounds like you also follow the Cordell/Van Raalte idea of loading the cart hard and getting the RIAA HF pole for free at least conceptually. I also agree that picking a cart which doesn't need audioband mechanical uplifts. But given its easier than ever to measure these things a 21st century approach is needed. I also have (in bits) a very low input C phono stage which should help.

Good point on the scratch. B&K used to to that, although with a saw!

https://www.ortofon.com/media/15182/...re-page-1.jpeg shows in glorious unlabelled Axes the change in reducing eddy currents and therefore the self inductance.

What is becoming clear to be is that the usual 47k plus 100-400pF of most MM stages is just plain wrong as regards getting the best response. (with usual caveat about smug Grado owners).
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Old 8th February 2017, 01:13 PM   #7
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Originally Posted by billshurv View Post
LD, sounds like you also follow the Cordell/Van Raalte idea of loading the cart hard and getting the RIAA HF pole for free at least conceptually.
Yes, probably to the max. My current MM pre-amp ('scuse pun ) effectively presents a short circuit to the coils and is effectively current driven. Then external C loading is irrelevant. Yes, the 75uS pole is then 'wrong' and needs fixing, but I found a very tidy circuit way of doing that.


Quote:
Originally Posted by billshurv View Post
https://www.ortofon.com/media/15182/...re-page-1.jpeg shows in glorious unlabelled Axes the change in reducing eddy currents and therefore the self inductance.
Yes, cartridge coils are effectively one half of a transformer when one thinks about it, and the same principles of audio performance and losses can apply. Ortofon's MM armature is a very light hollow tube, and IIRC they have a patent for an eddy current reducing split in that which crops up on the higher end styli. IMO Ortofon's VMS generator is really MI.

Without opening Pandora's box, IIRC there might be some complication with Grado's MI coil arrangement as to empirical loading sensitivity despite measuring as low self-inductance. Or was it a dream ?

LD
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Old 8th February 2017, 02:11 PM   #8
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I've never studied the Grado MI designs in detail. I know they have a tendency to act as aerials. Certainly VMS as described by Ortofon IS MI, although they call them MM. Any idea which Stylii have the split?

For those who haven't seen it https://www.ortofon.com/vms-cartridges-p-614 has some nice pics of the principle.

Calvin also has a nice cutaway diagram on his pages https://calvins-audio-page.jimdo.com/projects/phono-vinyl/vinyl-genera

EDIT: VinylEngine does have the 1975 patent as a download https://vinylengine.com/ve_downloads...atent_1974.pdf

Last edited by billshurv; 8th February 2017 at 02:17 PM.
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Old 8th February 2017, 04:02 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by luckythedog
A tiny sharp physical scratch or click, being impulse like, serves well IME.
An impulse in the groove will not have had RIAA pre-emphasis applied, so it will not produce an impulse out after RIAA equalisation. If you tune for an impulse out then you may have ensured that your equalisation is wrong?
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Old 8th February 2017, 06:10 PM   #10
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Originally Posted by DF96 View Post
An impulse in the groove will not have had RIAA pre-emphasis applied, so it will not produce an impulse out after RIAA equalisation. If you tune for an impulse out then you may have ensured that your equalisation is wrong?
I assume the idea is to compare the response to a mechanical impulse to that of an idea transducer, no equalization on either side.
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