Moving magnet phase shift

0 Members and 1 Guest are viewing this topic. Read 29316 times.

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Moving magnet phase shift
« on: 4 Mar 2011, 01:33 pm »
A funny thing happened on my way to the Circle the other day. I stopped off at the Asylum. There was a thread about a guy who got a SL-1200 and a $60 Grado. He said that it smoked his Rega Planet CD, and he is looking for a sub $200 cart for a MM gain stage. Somebody got on and said how a LOMC was better, blah, blah. I told the OP that I thought he should consider the Silver 1, DL-110, AT-7V, etc.

To make a long story short, the discussion got into loading considerations of a MM vs a MC. Hagermann loading calculator was used to plot electrical high frequency resonance of a Shure V15VXMR. Apparently this cart is 1K ohms and 425mH. This results (loaded at 47K/250pF) in a high frequency electrical resonance at 15.44KHz. It was then postulated that this results in a low pass filter that causes a phase shift in the output (even at the preamp out) and it is 120 degrees out of phase at 20K. This doesn't seem right to me. 180 degrees is opposite phase. 360 degrees is back in phase.

I'm not looking for a particular outcome of this question, I happen to prefer MCs usually. I think this analysis is an oversimplification. The effect of a filter on phase depends on the type of filter. Because there are electronics in the signal path after the shunt resistors/cap, I would speculate that this is a design consideration for the preamp. Nobody on the thread seems to have the equipment or program to analyze the output of a phono preamp.

Any thoughts?
http://www.audioasylum.com/cgi/vt.mpl?f=vinyl&m=953118
neo

BaMorin

  • Full Member
  • Posts: 407
  • AR turntable rebuilder/modifyer
Re: Moving magnet phase shift
« Reply #1 on: 4 Mar 2011, 09:10 pm »
A funny thing happened on my way to the Circle the other day. I stopped off at the Asylum. There was a thread about a guy who got a SL-1200 and a $60 Grado. He said that it smoked his Rega Planet CD, and he is looking for a sub $200 cart for a MM gain stage. Somebody got on and said how a LOMC was better, blah, blah. I told the OP that I thought he should consider the Silver 1, DL-110, AT-7V, etc.

To make a long story short, the discussion got into loading considerations of a MM vs a MC. Hagermann loading calculator was used to plot electrical high frequency resonance of a Shure V15VXMR. Apparently this cart is 1K ohms and 425mH. This results (loaded at 47K/250pF) in a high frequency electrical resonance at 15.44KHz. It was then postulated that this results in a low pass filter that causes a phase shift in the output (even at the preamp out) and it is 120 degrees out of phase at 20K. This doesn't seem right to me. 180 degrees is opposite phase. 360 degrees is back in phase.

I'm not looking for a particular outcome of this question, I happen to prefer MCs usually. I think this analysis is an oversimplification. The effect of a filter on phase depends on the type of filter. Because there are electronics in the signal path after the shunt resistors/cap, I would speculate that this is a design consideration for the preamp. Nobody on the thread seems to have the equipment or program to analyze the output of a phono preamp.

Any thoughts?
http://www.audioasylum.com/cgi/vt.mpl?f=vinyl&m=953118
neo


I read the whole thread........some of what one was posting made no sense to me, but I'm certainly not an EE.  Most of the other stuff posted seemed only there to pick an argument.  I'm thinking along similar lines as you Neo, that the topology of the preamp comes into play here.

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Re: Moving magnet phase shift
« Reply #2 on: 4 Mar 2011, 10:17 pm »
Aside from the abusive nature of some posters, I think it's an interesting question. My thinking is that the designer would probably analyze the output of the phono stage as part of the design process. The electrical high frequency resonance does not show up on the measured output. This was said to be because the load resistor damps it. Makes sense. But a V15VXMR is spec to 25K and has response to at least 30K. I saw a 4-ch forum where they were discussing using it for that, successfully. I wonder if a near out of phase signal can be used to grab the rear channel info at 30K? I have no idea about that, maybe it can.

Maybe I should pose the question to some preamp designers. It would be interesting to have Hagerman's thoughts about this. There are a couple of other manufacturers that have circles here, including Frank Van Alstine.

Yo Wayner, any info on this?

Is that part of the magic of a Grado or HOMCs, low inductance = no phase shift? What's the resistance and inductance of a Grado anyway?

neo

Wayner

Re: Moving magnet phase shift
« Reply #3 on: 4 Mar 2011, 10:50 pm »
Grado coils have a resistance of 440 ohms, and the induction is rated at 45 mh. Phase shifting can be caused by high capacitance ICs, phono preamps and even the RIAA EQ curve the preamp MFGR uses can cause it. 30K is something only my dog could hear, I wouldn't get too worked up ion it, unless it's in the 12-15k region. I haven't read the article yet, but is on my list.

Wayner

*Scotty*

Re: Moving magnet phase shift
« Reply #4 on: 4 Mar 2011, 10:53 pm »
CD-4 discrete 4 channel vinyl records had the rear channel information present as an FM subcarrier signal at 30kHz. See wiki link http://en.wikipedia.org/wiki/Quadraphonic_sound#CD-4_.28Compatible_Discrete_4.29_.2F_Quadradisc
  The MM phono cartridge has phase-shift present in both channels equally due to the inductance from the coils that are part signal generation mechanism. This phase shift clear down into the mid-band is what mucks up sound of a MM compared to a MC which has much lower inductance and hence much lower phase shift in the audio band. The inductance of a MM cartridge can be canceled out by splitting the signal from cartridge and connecting an identical cartridge body to ground. You are left with a  signal level of 2.5mv in the case of a 5mv output cartridge but there is zero phase shift and no electrical resonances. The only limits to the cartridges high frequency extension are mechanical in nature not electrical. I run a modified AT 440ML this way. 
Scotty
« Last Edit: 5 Mar 2011, 01:37 am by *Scotty* »

Wayner

Re: Moving magnet phase shift
« Reply #5 on: 5 Mar 2011, 12:10 am »
And in the end the CD-4 format was a failure, because few, if any cartridges could respond to 30k signals (at least at reasonable distortion levels), and while it sounded good on paper, I think the format went as quickly as it came. Remember, the tiny stylus has to vibrates 30,000 times per second to make a 30,000 cycle wave, and was (and still is) way beyond may cartridge capabilities.

Wayner

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Re: Moving magnet phase shift
« Reply #6 on: 5 Mar 2011, 01:25 am »
Gents, there's an excellent article on this subject at VE written by an Ortofon rep.

Although it's nearly three decades old, it should be considered required reading.   

http://www.vinylengine.com/phpBB2/viewtopic.php?t=33679

Hi Otis,
Thanks for the link. Unfortunately I can't access VE. People were saying something about new filters. Anyway, what is the gist of the article? I assume there is a phase shift. Maybe I'll be able to read it on another PC using a different server.
neo

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Re: Moving magnet phase shift
« Reply #7 on: 5 Mar 2011, 01:52 am »
Grado coils have a resistance of 440 ohms, and the induction is rated at 45 mh. Phase shifting can be caused by high capacitance ICs, phono preamps and even the RIAA EQ curve the preamp MFGR uses can cause it. 30K is something only my dog could hear, I wouldn't get too worked up ion it, unless it's in the 12-15k region. I haven't read the article yet, but is on my list.

Wayner

Wayner,
I figured that electronics inside a preamp would effect the results of any filter. RIAA EQ is basically a filter(s), I would think. It isn't the frequency response that was of concern. It was the phase shift.

I think the 4-ch thing never really took off. Many people were fed up with formats coming and going, 8 track, El cassette, etc. But we all benefited from it. The shibata stylus was developed for 4-ch to track 30K rear ch info. Soon after, the line trace came along, and then the micro ridge. A guy on another forum has a collection of 4-ch records. He said the rear ch info is easily damaged and they can only be played a few times successfully. I remember Empire ads for 4-ch speakers. They were shaped like cylinders, and looked like end tables. Some had marble tops.
Thanks for your input,
neo

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Re: Moving magnet phase shift
« Reply #8 on: 5 Mar 2011, 02:18 am »
CD-4 discrete 4 channel vinyl records had the rear channel information present as an FM subcarrier signal at 30kHz. See wiki link http://en.wikipedia.org/wiki/Quadraphonic_sound#CD-4_.28Compatible_Discrete_4.29_.2F_Quadradisc
  The MM phono cartridge has phase-shift present in both channels equally due to the inductance from the coils that are part signal generation mechanism. This phase shift clear down into the mid-band is what mucks up sound of a MM compared to a MC which has much lower inductance and hence much lower phase shift in the audio band. The inductance of a MM cartridge can be canceled out by splitting the signal from cartridge and connecting an identical cartridge body to ground. You are left with a  signal level of 2.5mv in the case of a 5mv output cartridge but there is zero phase shift and no electrical resonances. The only limits to the cartridges high frequency extension are mechanical in nature not electrical. I run a modified AT 440ML this way. 
Scotty

Hi Scotty,
Apparently the phase concern with the V15V goes all the way down to 2K. I still wonder what the phase looks like coming out of a well designed preamp. I would think this would be something that was taken into consideration. They say that phase isn't readily heard by most people, but you would think that when it gets severe enough, info would start dropping out or as you say, muck things up.

That really is interesting about a 2nd cartridge connected to ground. You're turning a 12dB filter into a 24dB filter? I'd guess you have it hooked up at the end of the wires coming out of the arm. Come to think of it, you're running a linear tracker with short wires? I'm sure you've got every one's interest, I'd like to hear more about the physical hook-up and what it does to the sound. How is the 440 modified?
neo

*Scotty*

Re: Moving magnet phase shift
« Reply #9 on: 5 Mar 2011, 03:05 am »
  This phase canceling trick requires that your phono stage have an intrinsically high input impedance exclusive of the ubiquitous 47k input network. Tube and FET input designs have this characteristic. The voltage dividing network replaces the existing input network and you have 10k to ground through the coils in the cartridge body.
 My modifications to the AT 440 consist of removing the flip down stylus guard and cutting away the side wings. The remainder of stylus assembly is super glued to the cartridge body. Three layers of 3M viscoelastically damped stainless steel sheets are attached to each side of the cartridge body. Both of these treatments are done to try to render the cartridge as free from spurious resonances as possible. The stainless steel sheets also add some mass to the cartridge as well,I think it was almost 3grams worth.
Movement of the entire stylus assembly relative to the cartridge body is a big problem in most MM cartridges and this problem does not even exist in a good moving coil, ie (no user replaceable stylus).
 
 
 
 
 Audio Technica and Grado are the cartridges best suited for this type of application. These two brands show the greatest similarity to moving coils in their sound-staging and image focus when they have their inductance canceled.
  They also have a vivid and dynamic presentation which betters that of some MC cartridges. The secret to the results that can be had from Grados and ATs is in the way spurious motion of the stylus cantilever is controlled by the suspension design used in these two brands. In the case of the Audio Technical cartridges, the cantilever has the same piano wire tie back at the end of the cantilever to inhibit its longitudinal motion that all moving coil designs have.  Grado cartridges also control undesirable longitudinal motion quite effectively.
  Wayner, I used a JVC CD-4 4DD-5 Disc Demodulator to play my CD-4 records and never noticed any distortion problems in the rear channels. Admittedly the Harmon Kardon quad receiver I was using back in the 70s was probably not state of the art playback but the sound was satisfying to listen to. I think all of the Quad formats failed due to the increased cost and lack of good recordings. Then as now neither multi-channel classical music nor quad encoded prog-rock drives the market place.
Scotty
« Last Edit: 5 Mar 2011, 04:35 am by *Scotty* »

*Scotty*

Re: Moving magnet phase shift
« Reply #10 on: 5 Mar 2011, 03:25 am »
neobop,that cartridge body is back in the phono preamp. It is connected to the circuit board by about two inches of OFC 24ga. The cartridge loading is purely resistive there is no roll-off from any filter function due to the network. The high frequency response of the cartridge is now limited only by the the mechanical system involved. The tone arm wiring isn't any shorter than a conventional tonearm.
  The arm is a precursor to the ET 1 designed by Bruce Thigpen. He was project engineer at the company that would become Mapelnoll in a later incarnation. The best way to understand what goes wrong due to inductive phase shift is to listen to a record played back with a good low output MC cartridge and then listen to it played back via a MM cartridge. It's kind of like you know right when you hear it, the MC sounds much more correct due to it's vastly lower inductance which results in much lower phase shift.
Scotty

dlaloum

  • Full Member
  • Posts: 683
Re: Moving magnet phase shift
« Reply #11 on: 5 Mar 2011, 11:00 am »
This sounds very similar to what I have been working on with LD... on the "Near Zero Capacitance..." thread on VE:
http://www.vinylengine.com/phpBB2/viewtopic.php?t=33929

We are using a spare cartridge body as an inductor combined with a resistor in line.

This has 2 effects - 1st it can reduce the resonance peaks caused by the reaction of high inductance MM's (compared to MC's) with cable capacitance, and 2nd F/R and Phase are intrinsically linked - the result is hugely improved phase linearity.

To assist with the process LD wrote a spreadsheet model that allows one to plug in various resistor, cable, and cartridge values to model the resulting frequency response.

If you get rid of the resonance - the phase ends up intact....  (that's the theory anyway)

One of the outcomes of the Ortofon article/research was that it demonstrated how phase starts to go off in concert with resonances - and their solution was and has been since to move the resonant peak as high up as possible to provide phase linearity within the main audio range.

The experiment that LD and I have been working through tries to achieve linear phase and extended Frequency Response ina "standard" MM phono stage with standard cabling (ie 47K + 220pf in the phono stage and then 150pf+ in the cables)

Similar superb results can be achieved with most MM/MI's by dropping total capacitance to below 100pf, and using custom resistive loading (mostly loading between 47k and 100k).
The trouble is most phono stages don't have the facility to drop the capacitive loading on MM setting, and where they have adjustable Resistive loading it is adjustable downwards from 47k, and very very rarely upwards.

If your phono stage can be set to a load of 100k or higher (1M is great!) - you can then adjust the loading to anything you want by using loading plugs (resistors inside a blank RCA plug) plugged into RCA double adapters.

To achieve total capacitance below 100pf usually requires custom wiring - all the way into the turntable/arm. The cable needs to be very low C - and it also needs to be SHORT.

I got my JVC QL-Y5F down to 51pf the impact on almost all MM / MI cartridges is dramatic - once I get my new Phono stage (JLTI with some custom configuration bits added...) I will publish some comparative tracks so people can actually hear the consequence of this type of setup.

In the days of CD-4 Quad - for CD-4 to work required not only a frequency response out to 40k+ but also minimal phase variation within that range.
The CD-4 cartridges achieved this successfully - and the standard parameters were total capacitance 100pf with resistive loading of either 100k or 47k (depending on the cartridge).

The cartridges that achieved this normally had special styli and cantilevers as well - they later became legendary stereo cartridges.... (AT15/20, Empire 1000z/4000D)

Also worthy of note - a low inductance cartridge, has a more extended F/R and reduced phase non linearity for any particular capacitance. - The Quad cartridges were mostly around 200mH to 350mH. Post Quad cartridges raised the inductance as it is "less sensitive to capacitance" - but it also becomes more peaky and F/R is more limited.

The V15V has inductance of 330mH - the post Quad V15Vx is 425mH - guess which one sounds better !

One more thing - people rarely stop to note the parameters manufacturers used when measuring their cartridges for their frequency response charts/specs - a lot of the classic cartridges were measured at 100pf.

And I think these are the very issues which ended up causing the demise of MM/MI from the high end mainstream. If you set up a top MM right (low C, custom R) - it will beat any MC within twice its price.... and a lot of MC's that cost even more.
But a MM requires some custom setup - it usually won't give you that kind out sound straight out of the box on a "standard" setup - where a lot of MC's will give you most of the improvements straight into a "standard" MC setup. (with scope for custom improvements of course)
But this ease of use comes at a cost.... higher VTF, lower compliance, higher noise (caused by increased gain required of the phono stage), and the additional cost of the more expensive phono stage.

I'll step off my soapbox now..... :)

bye for now

David

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Re: Moving magnet phase shift
« Reply #12 on: 5 Mar 2011, 06:53 pm »
There are lots of fascinating aspects to all this that are interrelated. Makes it more difficult to find answers. What's doing what? Like, what's that fuzzy sound coming from, stylus not being firmly planted or phase shift in the treble? I think the reality of a phase shift might be very dependant on preamp topology, but maybe I'm looking at it in the wrong way. I guess I should try to get some answers from preamp designers to see what their take is. Actual info on phase at the output of some phono stages would go a long way to clearing things up.

Perhaps the use of the term filter isn't giving the right approach for the solution. I tend to see it as phase consequences in a loudspeaker crossover or electronic dividing network. Maybe it's more straightforward than that. The plot of the electronic spike looks like a big resonance spike on a woofer impedance curve. Just like the severe consequences of physical resonance in woofers, this is the equivalent for cartridge phase?

I notice a dramatic difference with HO cart performance between my various phono stages. But that too has other aspects of performance that are very different. My AHT has maybe a 50pF shunt capacitor, if anything. But loading any MM at 100K is enough to make my ears bleed. I got best results with a stock 440ML with 1' tonearm cables going straight in and 32K load. Of course I didn't have the cart going to ground with another body. That's really wild. But maybe I would have wound up with a 22K load, LOL.
neo


Wayner

Re: Moving magnet phase shift
« Reply #13 on: 5 Mar 2011, 07:13 pm »
I'm not going to the "creating problems" territory, but the whole scope of vinyl playback is in the ears of the beholder. The program material itself is filled with phase shifting. I suspect, by the answers and the questions, that the problem (if it really is a problem) is filled with many still unanswered questions and solutions.

And I ask myself, is phase shifting in the mid to upper frequencies a problem anyway? Certainly it can foul up bass response, but that doesn't' seem to be the target area for the problem.

I was listening to Queen - A Day at the Races last night from a new pressing from Hollywood records (who I'm not really fond of), and the recording was marvolous. Cymbal crash sounded real, Freddy's voice was epic and the performance of the album was surreal. What more could I ask for with any table.

I'm not against further investigation here, but are we splitting hairs again?

Wayner

neobop

  • Full Member
  • Posts: 3448
  • BIRD LIVES
Re: Moving magnet phase shift
« Reply #14 on: 5 Mar 2011, 08:20 pm »
Can't argue with that Wayner. You could look at it as part of the old MM vs MC debate. That certainly came up in the Asylum thread as a big reason that MCs sound better. I'm no so sure, and I'm curious about the implications of this. I tend to agree about recordings as well. Sometimes I can hear that an instrument is out of phase on a recording. I guess with mixing boards and instrument amps you could wind up with all kinds of crazy phase angles. How many pots and trimmers can you use and wind up with a phase coherent recording? None, I would think.

I'm not one of those guys that has a handful of audiophile recordings. I go for musical content. Monk is playing Trinkle, Tinkle right now in the background on Pandora Radio. It sounds nice. Kind of distracting when I'm typing, maybe I'll stop.  8)
neo

Wayner

Re: Moving magnet phase shift
« Reply #15 on: 5 Mar 2011, 09:23 pm »
Thanks neo.

I'm not trying to take away "scotties" thunder, but I like the way my vinyl sounds on any one of my 8 turntables. Not that 8 is a magic number, and it doesn't make me any TT "god", but from owning lots of tables, I have learned on thing. They sound more similar then different.

Good measures to keep things in balance are using low pf ICs and the other usual stuff. I think this thread will go on for quiet a while, and it should because the topic does offer new and interesting directions to improve vinyl playback. Perhaps we will get it so dialed in that CDs will never have a chance!

Wayner  :D

felixscerri

Re: Moving magnet phase shift
« Reply #16 on: 5 Mar 2011, 09:51 pm »
G'day all, an interesting thread.  For what it's worth, as many people here will know, I've long been an advocate of phono stage input resistance optimisation (specifically associated with the Shure M97xE with the 'supplied' stylus), running my DIY phono stages at 62 k and minimal input capacitance from all input sources, so none of this latest new research is 'new' to me! 

I personally query whether all the implied complexity is really necessary in practice though.  Yes 'physics is physics', however in my personal observation things are not 'that' critical, beyond a certain point.  I'm very happy with the performance of my Shure M97xE's running at 62 k through my DIY phono stages and now all I do is listen to music.  After all isn't that the ultimate aim?  Fair enough?  Regards, Felix.

Wayner

Re: Moving magnet phase shift
« Reply #17 on: 5 Mar 2011, 10:33 pm »
No offense Felix, but the Shure M97xe is without a doubt, the most boring, unmusical cartridge I have ever heard.

Wayner

felixscerri

Re: Moving magnet phase shift
« Reply #18 on: 6 Mar 2011, 12:02 am »
G'day mate, at 'standard' 47 k loading I might agree with you, although it's a matter of personal musical taste.  However at 62 k it's fantastic.  Try it at 62 k sometime, I think you'll be rather pleasantly surprised.  Regards, Felix.

dlaloum

  • Full Member
  • Posts: 683
Re: Moving magnet phase shift
« Reply #19 on: 6 Mar 2011, 01:47 am »
Hi Felix,

the 62k is important (get's rid of some of the "dullness" - but I think the biggest improvement is the short low C cable and absence of shunt C....

I'm not against further investigation here, but are we splitting hairs again?

Now Now Wayner - you know how important this is...

For the next thread I am proposing a discussion of how many angels can fit on the head of a cartridge screw - and their impact on Effective Mass.

Other variables to be considered - The impact of High Frequencies on angels, effective mass and tonearm resonances... (My Superstitious Aunt does not allow people to whistle in her house.... it apparently chases visiting angels away...)

I think this opens up a whole new avenue of investigation..... there's only so far you can go with hairs!