@super10018 I'm a little confused by your graphs.
Firstly the -70dB was it referenced to? remember dB is a ratio between 2 signals.
What was the difference in the test setup between the 2nd and 4th graphs?
When I test a tonearm I always measure the cartridge output tracking either a 1Khz or 300Hz tone this way I have a reference and know how far below that reference the noise is. It's the output of the cartridge that's important, if the cartridge is reproducing the 3.5Khz resonance you'll see it in the output.
Firstly the -70dB was it referenced to? remember dB is a ratio between 2 signals.
What was the difference in the test setup between the 2nd and 4th graphs?
When I test a tonearm I always measure the cartridge output tracking either a 1Khz or 300Hz tone this way I have a reference and know how far below that reference the noise is. It's the output of the cartridge that's important, if the cartridge is reproducing the 3.5Khz resonance you'll see it in the output.
Previously with LC prism I couldn't make the middle X (prism) shape accurate enough,
Had to use 10g weight to avoid the carriage swinging back and force.
Also there's 0.3g+ tracking force difference in inner/outer track.
With corkscrew2, it's more stable,
Initially I had the rolling balls touching all the walls (4 points) caused high friction.
It helped a lot moving the rails bit apart to have less points.
Now I need to figure out how to have less friction as cutting the weight of carriage is even harder.
LC prism,
corkScrew2,
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Niffy, warrjon
The purpose of the first two graphs is to show if the air film may shift the resonant frequency or reduce the level of the resonant frequency.
I laid two graphs together. You can see once a piece of metal hit the air-bearing, it excited the resonant frequency of the air-bearing. A contact mic attached to the end of the air-bearing shaft would pick up the signal. The red line indicates the signal with the air-bearing sitting on the shaft without any air supply. The green indicates the signal with the air-bearing filled with 70 psi compressed air. In both cases, the resonant frequencies are both about 3.5kHz. The air film makes no big difference. But if you look at the left and right sides of 3.5kHz, the amplitudes of the red line are larger than the amplitudes of the green line. The air-bearing without air takes longer time to calm down than the air-bearing with air. In other words, the air-bearing without air has more ringing.
The purpose of the next test is to see if a signal picked by the cartridge will be transmitted to other parts of the tonearm and to the turntable. If so, how strong the signal will be transmitted?
I also laid the next three graphs together. Before I started the test, I held the wire of a contact mic so the contact mic didn't touch anything. I recorded the signal from the contact mic. It is the green line in the graph. There was no external signal at all. Next, I attached the contact mic to the end of the bearing shaft and started to play a tack 30Hz to 30kHz. This is the yellow line in the graph. We can see there is a small peak at 3.5 kHz. The rest is almost identical to the green line. It means that only the signal at the resonant frequency was enforced and transmitted to the end of the shaft. Its amplitude is -70 dB. The rest signals at different frequencies are so low that they can't be recorded since they are lower than the noise level of the contact mic. I moved the contact mic to position 2, which is on the top of the VTA tower. The red line indicates the recorded signals at position 2. The red line is almost identical to the yellow line except there is a smaller peak at 3.5 kHz, too. Although I changed the position of the contact mic, the resonant frequency is still the same but with lower amplitude. It is understandable because the contact mic was moved further away from the cartridge.
From these tests, I can conclude that there is no or very little signal transmitted to the rest of the tonearm or the turntable except at the resonant frequency. Even at the resonant frequency, the amplitudes are low.
I don't have the stethoscope yet. Once I get it, I will try to listen, too.
Jim
The purpose of the first two graphs is to show if the air film may shift the resonant frequency or reduce the level of the resonant frequency.
I laid two graphs together. You can see once a piece of metal hit the air-bearing, it excited the resonant frequency of the air-bearing. A contact mic attached to the end of the air-bearing shaft would pick up the signal. The red line indicates the signal with the air-bearing sitting on the shaft without any air supply. The green indicates the signal with the air-bearing filled with 70 psi compressed air. In both cases, the resonant frequencies are both about 3.5kHz. The air film makes no big difference. But if you look at the left and right sides of 3.5kHz, the amplitudes of the red line are larger than the amplitudes of the green line. The air-bearing without air takes longer time to calm down than the air-bearing with air. In other words, the air-bearing without air has more ringing.
The purpose of the next test is to see if a signal picked by the cartridge will be transmitted to other parts of the tonearm and to the turntable. If so, how strong the signal will be transmitted?
I also laid the next three graphs together. Before I started the test, I held the wire of a contact mic so the contact mic didn't touch anything. I recorded the signal from the contact mic. It is the green line in the graph. There was no external signal at all. Next, I attached the contact mic to the end of the bearing shaft and started to play a tack 30Hz to 30kHz. This is the yellow line in the graph. We can see there is a small peak at 3.5 kHz. The rest is almost identical to the green line. It means that only the signal at the resonant frequency was enforced and transmitted to the end of the shaft. Its amplitude is -70 dB. The rest signals at different frequencies are so low that they can't be recorded since they are lower than the noise level of the contact mic. I moved the contact mic to position 2, which is on the top of the VTA tower. The red line indicates the recorded signals at position 2. The red line is almost identical to the yellow line except there is a smaller peak at 3.5 kHz, too. Although I changed the position of the contact mic, the resonant frequency is still the same but with lower amplitude. It is understandable because the contact mic was moved further away from the cartridge.
From these tests, I can conclude that there is no or very little signal transmitted to the rest of the tonearm or the turntable except at the resonant frequency. Even at the resonant frequency, the amplitudes are low.
I don't have the stethoscope yet. Once I get it, I will try to listen, too.
Jim
The reason that I run a 30 Hz to 30 kHz track is to see which frequency is easy to penetrate through a particular tonearm structure.
Can you please explain the position of contact mic 1? i think i see it on the fixed part of the air bearing, not the moving part is that correct please.
I am enthusiastic to know the difference in level between the cartridge side and the fixed side, are they well coupled by the air bearing?
M
Yes. The contact mic at position #1 is on the end of the fixed shaft. I can't put the contact mic on the moving air bearing since the wire of the contact mic will interfere with the movements of the air bearing.
Thanks for that, it was as i had understood it, i also have that problem with the stethoscope, the key is to understand the difference across the bearing, from cartridge side to fixed rail side, i wonder if tiny mics are available to do that?
M
I thought about that as well. The contact mic may not be good enough to pick up very weak signals. This is the noise level of the contact mic. I recorded the noise level while I was holding the wire of the contact mic. The mic didn't touch anything.
You can see the noise level from 10 Hz to 20 kHz is under -65 dB. I ask myself. If a signal from the cartridge is transmitted through the bearing shaft and reaches the end of the shaft, its level is under -65 dB. In other words, its level is lower than the contact mic noise level. How much its impact can be?
The only signal transmitted from the cartridge that may play a significant role is 3.5 kHz. Please see the graph below. 3.5 kHz is also the resonant frequency of the air bearing with this particular structure. However, even for the 3.5 kHz signal at the end of the bearing shaft, its level is -70 dB. I would ask the same question. How much its impact can be at -70 dB?
You can see the noise level from 10 Hz to 20 kHz is under -65 dB. I ask myself. If a signal from the cartridge is transmitted through the bearing shaft and reaches the end of the shaft, its level is under -65 dB. In other words, its level is lower than the contact mic noise level. How much its impact can be?
The only signal transmitted from the cartridge that may play a significant role is 3.5 kHz. Please see the graph below. 3.5 kHz is also the resonant frequency of the air bearing with this particular structure. However, even for the 3.5 kHz signal at the end of the bearing shaft, its level is -70 dB. I would ask the same question. How much its impact can be at -70 dB?
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#4858 - Congratulations 2a3set. hope you haven't sent me too many curses since building an acceptable flat parallelogram is not that easy.
I understand your difficulties with the LC prism X cart, I had too. Without a milling machine it took also blueing, sanding and many comparator checks to get it squared. But to me the LC still sounds cleaner; CS suffers from the usual LTAs disease, i.e. excess of Hor eff mass.
ciao - carlo
If you write to me on private message I can send you some construction hints that nobody is interested in here.
I understand your difficulties with the LC prism X cart, I had too. Without a milling machine it took also blueing, sanding and many comparator checks to get it squared. But to me the LC still sounds cleaner; CS suffers from the usual LTAs disease, i.e. excess of Hor eff mass.
ciao - carlo
If you write to me on private message I can send you some construction hints that nobody is interested in here.
Furthermore, I just realized that air does cause resonant frequency shifts. Please see the graph below. The resonant frequency is 3 kHz without air. The resonant frequency is 3.5 kHz with air.
Niffy,
The resonant frequency is related to the structure of air bearing arm, too. In the test of Drew Devitt of Newway air bearing, he used a vacuum load bearing in order to avoid the influence of the structure. It is understandable that the resonant frequency in my tests is different from your calculation. My tests are consistent with Drew Devitt's test.
Niffy,
The resonant frequency is related to the structure of air bearing arm, too. In the test of Drew Devitt of Newway air bearing, he used a vacuum load bearing in order to avoid the influence of the structure. It is understandable that the resonant frequency in my tests is different from your calculation. My tests are consistent with Drew Devitt's test.
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OK, I took the risk and put the stethoscope on the moving carriage momentarily (<1 second- but sufficient to hear some notes.) I did this several times on an old, scratched record. My lift mechanism gave me a stationary and fixed point to hold and slide the scope end down on the carriage at a point centred between the rails, so it didn't tip. I was surprised I could do this without causing skips, but it worked Ok, obviously you can hear if you cause skips because of the scope in your ears! The record has quite a lot of deep bass, previously i had listened to piano where i hadn't noticed this as much. First observation is that the higher notes come through the scope (wherever it is used) at much higher volume relative to the lower ones. That may simply be because the steth. doesn't pick up bass notes (bass guitar and string bass).
More interesting was the relative volume between the fixed rail side and the moving carriage side. My estimation is that the fixed rail is 2/3-3/4 of the volume of the moving carriage observation. So my rail bearings aren't a perfect couple, but not bad. This is also better than some links further down the chain which show a greater relative reduction, but they are at points where i intend absorption to be happening.
It's not a test i recommend unless you can be sure to hold things still and don't mind about your record. Mine is possibly unusual as well in that i can dump a relatively large load on the carriage without upsetting VTF at all..............I hadn't thought of that until I started writing this!
Does anyone else have an idea how to measure % coupling?
M
What do you think the contact points between mobile and static parts will be without air present? - the levels seem similar, so if the tap was made on the mobile side then the coupling is similar, if the tap was made on the static rail side my comment is irrelevant!!
M
Hi Jim,
It would appear that the air film is completely decoupling the carriage from the rail across the entire audio band. I would have expected to see a big difference between the free microphone and that attached to the rail if there was any form of coupling.
As Warren asked earlier, how did you determine the 0dB level in this test?
It would appear that the air film is completely decoupling the carriage from the rail across the entire audio band. I would have expected to see a big difference between the free microphone and that attached to the rail if there was any form of coupling.
As Warren asked earlier, how did you determine the 0dB level in this test?
I now realise an answer for myself of course the stethoscope has no correction curve applied, that's why there is no bass?
Hi Mike,
Vibrational energy exists in two forms, transverse waves and compression waves.
Transverse waves are transmitted from the stylus to the cart body and arm via the suspension of the cartridge which is compliant in both the lateral and vertical planes. This means that the amount of energy decreases as frequency increases by 12dB per octave.
Compression waves, also known as sound waves, are transmitted by the varying pull on the stylus caused by groove modulation. The cantilever is rigidly coupled to the cartridge along its axis so transmits these compression waves. As riaa equalisation is basically constant amplitude (except the end bits) the amount of energy transmitted by compression waves increases with frequency. I believe that it is by 6dB per octave. The stethoscope is sensitive to sound waves so it picks up the increased energy at high frequency.
The vibrational energy will transform between the two types. For example a tuning fork stores energy as bending waves and produces sound waves in the air.
Niffy
Hi Niffy,
I am mature and not exactly sure what you were asking. The process of my tests is very simple. I used Adobe Audition to record the signal and analyzed the results with Audition as well. The 0 dB level is settled in the software.
Jim
Mike,
The purpose of my tests is to study the damping factor of air film inside the air bearing. So, I need the signal transmits through the air film. All tests are done with parts that are stationary. The air bearing was stationary with or without air. If no air, the air bearing sits on the shaft. With air, the air bearing is actually floating on the shaft. However, the air bearing doesn't move even with air.
Jim
Hi Niffy,
I didn't attach the contact mic to the middle of the shaft because I am afraid the sticky glue on the contact mic may leave residuals on the surface of the shaft. I may clean the sticky film on the contact mic later on and attach the mic with other methods to do the test.
Jim
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Thanks Jim, the thread has taken discussions into an area that I find very interesting (amongst many), hence my questions about some of the details. I have been playing with my version of an RTA a while now and recognise some of it's and my limitations in part and don't know what I don't know in others!
On one hand I can see and hear there are many improvements I can make, on the other, listening and tests suggest it already works OK.
As has been recognised frequently, everything will have its compromises, it's a question of which set to adopt, or adapt or avoid.
It seems to me that your bearing has very low friction, is decoupled and damped. Mine has quite low friction (sufficient that i never get skips anyway, and it tests low as well) is significantly coupled and is largely stiff but undamped. Both those contrasting solutions may produce good results or I may be on the wrong train!
M
When measuring a signal it must be referenced to something, there is no guarantee that the -70dB signal in the software is actually -70dB down from the audio signal, it may actually be a lot less.
I only use the stethoscope to get an idea of how well coupled something is or to look for sources of noise like the platter bearing. Once I have performed the stethoscope test I turn to recording the output of the cartridge as this is the ONLY signal that really matters.
Placing the stethoscope on the bearing housing of my TA I can clearly hear the music (no RIAA of course) there is no audible difference between the stethoscope on the bearing housing and TA mounting base so the arm is well coupled allowing sound energy to pass to the mounting base where it enters the plinth and is then damped.
Pink noise is a good test source as it contains all frequencies, just keep in mind that when vewing on an analyzer the noise drops at 3dB/octave so the trace will be tilted down towards 20kHz. Some analyzers like Virtual Analyzer allow you use a compensation curve which flattens the trace and makes it easier to see peaks and dips.