Not sure how to read this, but the self resonance frequency has nothing to do with what's on the record. What's on the record can or cannot excite that particular resonance frequency, but does not determine it. But maybe that was what you meant?
Jan
I was having a look at the easiest to manufacture method to test centering on my turntable. there are 4 or 5 of varying levels of faff to adjust. My test for tonight will be using a micrometer to see if that has the right clearance to fit between the inner spindle and edge.
Still need to find a record where this is a problem though.
Still need to find a record where this is a problem though.
@Tom: you are mixing the self resonance (Eigenfrequenz) of the arm tube with that of the pickup-arm assembly. The tonearm might resonate in itself, like a tubular bell or marimba tube, or a tiny aluminium tube of a windchime. Obviously you have taken all measures to make your arm dead in this respect.
However, the whole system has an unavoidable self resonance, as others pointed out. The resonance frequency is calculated form the compliance of the pickup (in essence determined by the tiny rubber disc that the coil is glued on), and the effective mass of the arm. The effective mass can't be easily calculated, and providing the static mass of each individual element does not help either. It is the hypothetical mass of the pickup+arm+counterweight+whatever projected in a single point (to the stylus tip). In itself this resonant sytem would have infinite Q. In a pivoted arm there is no other damping than the rubber element inside the cartridge. In your floating construction you are providing a heavy damping at the other end. So your Q might be well below 1. If you study a 2nd order low-pass electric filter, you don't want to attenuate the passband, and you probably want no resonance peak. So a Q of about 0.5 to 0.7 but below 1 is desirable. Too low is not good either, it will attenuate the passband. You could measure the resonance frequency in your construction by removing the water and hanging up the arm on a thread (or something similar, just for the test).
However, the whole system has an unavoidable self resonance, as others pointed out. The resonance frequency is calculated form the compliance of the pickup (in essence determined by the tiny rubber disc that the coil is glued on), and the effective mass of the arm. The effective mass can't be easily calculated, and providing the static mass of each individual element does not help either. It is the hypothetical mass of the pickup+arm+counterweight+whatever projected in a single point (to the stylus tip). In itself this resonant sytem would have infinite Q. In a pivoted arm there is no other damping than the rubber element inside the cartridge. In your floating construction you are providing a heavy damping at the other end. So your Q might be well below 1. If you study a 2nd order low-pass electric filter, you don't want to attenuate the passband, and you probably want no resonance peak. So a Q of about 0.5 to 0.7 but below 1 is desirable. Too low is not good either, it will attenuate the passband. You could measure the resonance frequency in your construction by removing the water and hanging up the arm on a thread (or something similar, just for the test).
A very simple experiment, take a cheap piezo-buzzer and remove the cover. Inside there will be a metal disk with a round dab of piezo-ceramic, whats left of the assembly is just about as thick as an LP. Simply put the stylus down on the center of the disk and flex the disk by applying a DC (10V or so) pulse, you then can observe the step response of the arm cart system independently.
Your setup is probably overdamping the primary resonance for the entire range of carts you have tried.
Jan - The mutual coupling of the horizontal and vertical resonances will tend to cause a fairly chaotic relationship of the phase/amplitude between the two. To think the .55Hz appears only in the horizontal would be folly.
Your setup is probably overdamping the primary resonance for the entire range of carts you have tried.
Jan - The mutual coupling of the horizontal and vertical resonances will tend to cause a fairly chaotic relationship of the phase/amplitude between the two. To think the .55Hz appears only in the horizontal would be folly.
Scott I wasn't thinking of the resonances; rather of a way to dynamically compensate for the record (hole) being eccentric.
I have a test record with a very low freq sweep that beautifully uncovers the resonances, both the cart/arm in the 8-12 Hz region, as well as the arm assembly higher up.
I posted some graphs earlier in this thread.
Jan
Too bad you never saw Jim Williams demo of coupled resonators. If you look at all the physics going on, control of only one axis is an insufficient number of degrees of freedom and if you are not careful you could end up with the cart bobbing up and down at .55Hz by slowly coupling the H to V modes.
Furthermore I would opine that if damped enormously a warp that exceeds the limit of mechanical motion would collapse the cantilever.
well the caliper idea won't work, but I now know my inner spindle is 4.3mm. And I appear to have a trashable record to test on that 'may' have some eccentricity. I say may because the arm movement isn't entirely consistent with what I would expect on mk1 eyeballing.
I found that out the hard way in the '70s when I foolishly put an ADC XLM into a Weathers arm.
@jan.didden (#121),
I will look it up. But this is what they told me, long ago.
Normally, self resonance cannot be excited by adding energy to a macroscopic system (e.g. by heat). The source must generate frequencies. When the source is a couple of different frequencies, the self resonance will be one of these frequencies (or a harmonic) or a modulation of a couple of regular altering continuous frequencies (like a running mechanical machine). In spite of this the other frequencies are "pumping" energy into the object (molecular vibrations), so the object can easily get into self resonance.
When the source is only 1 frequency, the excited self resonance is mostly the same frequency or a harmonic. So there must be a direct causal relation between the source and the object that’s in self resonance.
When the tonearm starts intensely vibrating at a harmonic frequency of the source, the cantilever will not “lost control”. Therefore, I think the self resonance of these tonearms was equal to the source frequency (by the way, it was a new Ortofon MM Bronze cartridge).
@billshurv (#122),
“Still need to find a record where this is a problem though”.
I became a bit anxious about your post (had I made a mistake and is the influence of eccentricity smaller?). So I decided to delete the spreadsheet to calculate the frequencies in relation to the eccentricity of a record. Only to start a new one from a different point of view (half a rotation). So I discovered that I made a mistake in de spreadsheet before... Even worse, the influence of the eccentricity on the stability of the frequencies shows to be nearly double. Below the new diagram.
So when I looked again to the diagram of the Nakamichi self centring turntable (post #45), the remaining eccentricity appeared to be nearly 0,1 mm (4,67 Hz for a 3150 Hz source frequency). An accuracy that’s in line with the technical expectations.
The average deviation with an eccentricity of 0,1 mm of the record hole at a source frequency of 440 Hz (a’ tune) is 0,65 Hz. So I took a music instrument and a frequency analyser (for music instruments) to detect the audible deviation when I alter a tone of 440 Hz a bit. I haven’t the best ears at the West side of the Chinese Wall, so I was a bit surprised I can detect a deviation of 0,2/0,3 Hz (439,75 - 440,25 Hz). It sounds a bit out of tune.
Is it a problem? Well, I enjoy music so it doesn’t matter while I am listening a recording from a vinyl record. Nevertheless, one have to reduce the eccentricity to nearly 0,035 mm to put the deviation at the end of the record groove below audibility (source frequency 440 Hz). This is totally new for me. I didn’t expect so much influence by eccentricity...
I will look it up. But this is what they told me, long ago.
Normally, self resonance cannot be excited by adding energy to a macroscopic system (e.g. by heat). The source must generate frequencies. When the source is a couple of different frequencies, the self resonance will be one of these frequencies (or a harmonic) or a modulation of a couple of regular altering continuous frequencies (like a running mechanical machine). In spite of this the other frequencies are "pumping" energy into the object (molecular vibrations), so the object can easily get into self resonance.
When the source is only 1 frequency, the excited self resonance is mostly the same frequency or a harmonic. So there must be a direct causal relation between the source and the object that’s in self resonance.
When the tonearm starts intensely vibrating at a harmonic frequency of the source, the cantilever will not “lost control”. Therefore, I think the self resonance of these tonearms was equal to the source frequency (by the way, it was a new Ortofon MM Bronze cartridge).
@billshurv (#122),
“Still need to find a record where this is a problem though”.
I became a bit anxious about your post (had I made a mistake and is the influence of eccentricity smaller?). So I decided to delete the spreadsheet to calculate the frequencies in relation to the eccentricity of a record. Only to start a new one from a different point of view (half a rotation). So I discovered that I made a mistake in de spreadsheet before... Even worse, the influence of the eccentricity on the stability of the frequencies shows to be nearly double. Below the new diagram.
An externally hosted image should be here but it was not working when we last tested it.
So when I looked again to the diagram of the Nakamichi self centring turntable (post #45), the remaining eccentricity appeared to be nearly 0,1 mm (4,67 Hz for a 3150 Hz source frequency). An accuracy that’s in line with the technical expectations.
The average deviation with an eccentricity of 0,1 mm of the record hole at a source frequency of 440 Hz (a’ tune) is 0,65 Hz. So I took a music instrument and a frequency analyser (for music instruments) to detect the audible deviation when I alter a tone of 440 Hz a bit. I haven’t the best ears at the West side of the Chinese Wall, so I was a bit surprised I can detect a deviation of 0,2/0,3 Hz (439,75 - 440,25 Hz). It sounds a bit out of tune.
Is it a problem? Well, I enjoy music so it doesn’t matter while I am listening a recording from a vinyl record. Nevertheless, one have to reduce the eccentricity to nearly 0,035 mm to put the deviation at the end of the record groove below audibility (source frequency 440 Hz). This is totally new for me. I didn’t expect so much influence by eccentricity...
Last edited:
@oshifis (#123),
I agree. I didn't notice that I was suggesting that the carbon tonearm has no self resonance. Of course it has, but it cannot start because the energy is dispatching to the water. That's why I wanted to design a "floating" tonearm: eleminating unwanted vibrations.
I agree. I didn't notice that I was suggesting that the carbon tonearm has no self resonance. Of course it has, but it cannot start because the energy is dispatching to the water. That's why I wanted to design a "floating" tonearm: eleminating unwanted vibrations.
so a rummage in the toolchest and all I had of any use was my laser level. Now this has a wide beam (around 2mm), but putting it on the edge of the xerxes puts the beam across the surface of the LP. aiming it at the inner run off groove illuminates an arc. If there is any eccentricity the arc lengthens and contracts so an approximate centering is possible. Not done the maths yet to work out how approximate this is.
Now on replaying the record there is still some 'nodding' of the arm, but it's not what I would expect from an eccentricity. But I need to think what to expect. On the inner groove the movement is certainly less than 0.1mm so either my anti skating is all wrong or its the way the record was cut.
Will retry with a less trashable record with a bit more modulation (crystal clear records direct cut recording of the Widor toccata on the last track!). Oh and before anyone jumps up I like organ music. I did try to play the organ as a teenager and failed.
(for organ geeks, I know Virgil Fox was all about speed and Widor himself played it a LOT more slowly, but its fun)
Now on replaying the record there is still some 'nodding' of the arm, but it's not what I would expect from an eccentricity. But I need to think what to expect. On the inner groove the movement is certainly less than 0.1mm so either my anti skating is all wrong or its the way the record was cut.
Will retry with a less trashable record with a bit more modulation (crystal clear records direct cut recording of the Widor toccata on the last track!). Oh and before anyone jumps up I like organ music. I did try to play the organ as a teenager and failed.
(for organ geeks, I know Virgil Fox was all about speed and Widor himself played it a LOT more slowly, but its fun)
well turns out the laser across the disk only works with filthy vinyl. But looks like the hole is DEAD centre. Still, it sounds good 🙂
Back to the scrap record (lionel richie, so deserves to die)
Back to the scrap record (lionel richie, so deserves to die)
this is embarassing. I have 2 copies of said scrap record. the second copy is a lot better centred, but still has some nodding. I measured the frequency of the arm movements and they are (within my measurement error) 1.8 seconds so is due to eccentricity.
oddly the worse copy has .020 written in the dead wax. Anyone know what that means?
so I have proven that you do get eccentricity of the order of 0.1 to .3mm on some records. Buggered if I can hear anything on pop music tho.
oddly the worse copy has .020 written in the dead wax. Anyone know what that means?
so I have proven that you do get eccentricity of the order of 0.1 to .3mm on some records. Buggered if I can hear anything on pop music tho.
Tom either you need a new "they" or possibly you fit what they said into a framework that you were comfortable with. There are lots of physics pages that deal with the damped resonance problem. I think if you stick with mass, compliance, moment of inertia, damping factor etc. you would find better explanations for what you observe.
- Status
- Not open for further replies.