TEST 1 b
TARGET of this test:
How does the signal strength change with different overlaps between coil and magnet.
The difference to test 1 is the direction of the magnet (NS SN).
Main-Coil Signal: SINUS 1 ppV
RESULT:
The strongest signals can be measured when the middle coil protrudes slightly over the edge of the magnet.
The direction of the magnet has no influence.
The best size of a feedback coil?
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farmersplow
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Re: The best size of a feedback coil?
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farmersplow
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Re: The best size of a feedback coil?
TEST 2
TARGET of this test:
How does the signal strength change with different feedback-coils in the same magnet.
Main-Coil Signal: SINUS 1 ppV
MAGNET M 2 (15 x 8 x 6 mm)
RESULT:
A higher number of turns gives a higher signal
Compare Coil 1 & 3: Coil 3 has 25% lower number of turns but nearly the same results. But both coils have nearly the same length of the wire.
The Coil 4 (series connection) is two times the coil 3. Together they have the double number of turns and length. The signal is (more or less) the double than the signal of coil 3 with better strength in lower frequency.
The Coil 4 (parallel circuit) is also two times the coil 3. Together they have the double number of turns and length. The signal is (more or less) the same than the signal of coil 3 with much better quality of the signal in all frequencies.
In the last of this tests, it is confirmed that the coils in the signal cancel each other out.
TARGET of this test:
How does the signal strength change with different feedback-coils in the same magnet.
Main-Coil Signal: SINUS 1 ppV
MAGNET M 2 (15 x 8 x 6 mm)
RESULT:
A higher number of turns gives a higher signal
Compare Coil 1 & 3: Coil 3 has 25% lower number of turns but nearly the same results. But both coils have nearly the same length of the wire.
The Coil 4 (series connection) is two times the coil 3. Together they have the double number of turns and length. The signal is (more or less) the double than the signal of coil 3 with better strength in lower frequency.
The Coil 4 (parallel circuit) is also two times the coil 3. Together they have the double number of turns and length. The signal is (more or less) the same than the signal of coil 3 with much better quality of the signal in all frequencies.
In the last of this tests, it is confirmed that the coils in the signal cancel each other out.
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farmersplow
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Re: The best size of a feedback coil?
TEST 3
TARGET of this test:
How does the signal strength change with different magnets with the same feedback coil.
Main-Coil Signal: SINUS 1 ppV
RESULT:
The size and strength of the magnet has no influence on the signal strength!??!
In school (35 years ago) I think I learned it differently. I thought that a stronger magnetic field also induces stronger ?! But presumably it has no influence in this constellation.
Perhaps one of you has an explanation.
RESUME:
Take the magnet that is available and fits into the cutterhead?
TARGET of this test:
How does the signal strength change with different magnets with the same feedback coil.
Main-Coil Signal: SINUS 1 ppV
RESULT:
The size and strength of the magnet has no influence on the signal strength!??!
In school (35 years ago) I think I learned it differently. I thought that a stronger magnetic field also induces stronger ?! But presumably it has no influence in this constellation.
Perhaps one of you has an explanation.
RESUME:
Take the magnet that is available and fits into the cutterhead?
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farmersplow
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Re: The best size of a feedback coil?
TEST 4
TARGET of this test:
How does the signal strength change with different washers and with the same magnets & feedback coil. (
Main-Coil Signal: SINUS 1 ppV
RESULT:
The example with the “Small washer” shows that the signal strength increase up to 50% in the low frequency area (up to 200Hz). At frequencies higher 1000Hz it is more or less the same. The air gap between the coil and the washer is only 0,05mm. A small move of the coil out of the axis will destroy the wire!
The example with the “Big washer” with an air gap of 0,53mm shows that the signal strength increase only at frequencies lower 100Hz. Over that frequency the washer is not helpful.
RESUME:
Based on the fact of destroying the coil it would be better to do without a washer and increase the number of turns.
TARGET of this test:
How does the signal strength change with different washers and with the same magnets & feedback coil. (
Main-Coil Signal: SINUS 1 ppV
RESULT:
The example with the “Small washer” shows that the signal strength increase up to 50% in the low frequency area (up to 200Hz). At frequencies higher 1000Hz it is more or less the same. The air gap between the coil and the washer is only 0,05mm. A small move of the coil out of the axis will destroy the wire!
The example with the “Big washer” with an air gap of 0,53mm shows that the signal strength increase only at frequencies lower 100Hz. Over that frequency the washer is not helpful.
RESUME:
Based on the fact of destroying the coil it would be better to do without a washer and increase the number of turns.
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farmersplow
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Re: The best size of a feedback coil?
TEST 5
TARGET of this test:
How does the signal strength change with magnetic field shielding
with the same coil and the same magnet.
Main-Coil Signal: SINUS 1 ppV
RESULT:
Copper shielding can cause a slight improvement in the signals.
RESUME:
The use of the copper foil here has more of a spaceship look than a real benefit
The results can be different when there is less space between the components?! I don't know
TARGET of this test:
How does the signal strength change with magnetic field shielding
with the same coil and the same magnet.
Main-Coil Signal: SINUS 1 ppV
RESULT:
Copper shielding can cause a slight improvement in the signals.
RESUME:
The use of the copper foil here has more of a spaceship look than a real benefit
The results can be different when there is less space between the components?! I don't know
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farmersplow
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Re: The best size of a feedback coil?
And now the question:
If I want to use a feedback coil and connect it to the CARUSO amplifier,
How high must the signal strength be?
I think the answer is a number of wire-turns?
Is there a maximum of Ohm's that the coil is allowed to have to be useful?
I would be very grateful for a helpful answer.
Nice greetings from Vienna
Thomas
If I want to use a feedback coil and connect it to the CARUSO amplifier,
How high must the signal strength be?
I think the answer is a number of wire-turns?
Is there a maximum of Ohm's that the coil is allowed to have to be useful?
I would be very grateful for a helpful answer.
Nice greetings from Vienna
Thomas
Re: The best size of a feedback coil?
This is great, well done. This is something I am really interested in right now, i have been reading about feedback and trying to work out a system.
I’m guessing your best result with the coil half in and half out of the magnet would be due to the magnetic field being strongest around the edge of the magnet.
I’m guessing your best result with the coil half in and half out of the magnet would be due to the magnetic field being strongest around the edge of the magnet.
Re: The best size of a feedback coil?
Hi,
My suggestion is to build and perfect a non-feedback head first so you understand how to control the physical properties of your design such that you get a clean open loop response that does not have any nasty secondary resonances. It is hard to close the servo loop in the presence of these unwanted resonances. You will also need to make sure the head has good stereo separation across the frequency spectrum. Also important, is the drive sensitivity of your design so that you can cut at decent levels at reasonable amplifier power (tip: keep moving mass low as possible). Once you have this down, then you should look to add feedback. One issue I see is that you may not be actually measuring the mechanical motion of the driver. You have to make sure that the magnetic field from the drive coil does not swamp the small signal from your feedback coil via transformer coupling. One way to check this is to make your measurements with the feedback coil magnet removed. You will then see how much leakage you are getting from the driver since the coil will not respond to any motion. This effect becomes larger at higher frequencies.
Hope this helps,
Mark
My suggestion is to build and perfect a non-feedback head first so you understand how to control the physical properties of your design such that you get a clean open loop response that does not have any nasty secondary resonances. It is hard to close the servo loop in the presence of these unwanted resonances. You will also need to make sure the head has good stereo separation across the frequency spectrum. Also important, is the drive sensitivity of your design so that you can cut at decent levels at reasonable amplifier power (tip: keep moving mass low as possible). Once you have this down, then you should look to add feedback. One issue I see is that you may not be actually measuring the mechanical motion of the driver. You have to make sure that the magnetic field from the drive coil does not swamp the small signal from your feedback coil via transformer coupling. One way to check this is to make your measurements with the feedback coil magnet removed. You will then see how much leakage you are getting from the driver since the coil will not respond to any motion. This effect becomes larger at higher frequencies.
Hope this helps,
Mark
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farmersplow
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Re: The best size of a feedback coil?
markrob wrote: ↑Wed Sep 08, 2021 10:23 amHi,
My suggestion is to build and perfect a non-feedback head first so you understand how to control the physical properties of your design such that you get a clean open loop response that does not have any nasty secondary resonances. It is hard to close the servo loop in the presence of these unwanted resonances. You will also need to make sure the head has good stereo separation across the frequency spectrum. Also important, is the drive sensitivity of your design so that you can cut at decent levels at reasonable amplifier power (tip: keep moving mass low as possible). Once you have this down, then you should look to add feedback. One issue I see is that you may not be actually measuring the mechanical motion of the driver. You have to make sure that the magnetic field from the drive coil does not swamp the small signal from your feedback coil via transformer coupling. One way to check this is to make your measurements with the feedback coil magnet removed. You will then see how much leakage you are getting from the driver since the coil will not respond to any motion. This effect becomes larger at higher frequencies.
Hope this helps,
Mark
Hi Markus,
thank you very much for the support. I think it's great that an expert is interested in the concerns of the "beginners" and supports them.
I have already recognized the problem of weights to be moved and I am currently working on weight reduction with carbon parts. During this weight reduction, I ran into the problem that a feedback coil, due to its mass and shape, has negative properties (which makes up around 10 to 20% of the moving mass). That's why I wanted to include this component right away. My first thought was to mount the feedback coil but not to connect it yet. This means that I can see the resulting resonances in advance. I can also see the correct acceleration values at high frequencies. Is that a bad train of thought?
Regarding the influence of the magnetic field of the drive coil on the feedback coil, I will carry out another measurement. To do this, I have to build a small amplifier for the feedback coil in order to particularly recognize the influences at high frequencies. The parts are ordered and I will measure it in the coming week.
Thank you also for this hint.
I will of course post the results here.
Nice greetings from Vienna
Thomas
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farmersplow
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Re: The best size of a feedback coil?
I couldn't wait until next week.
I just have to test how strong the effect of the drive magnets on the feedback coil is.
Is Mark right? (Of course he's right).
The following test 6 should show how right Mark is.
To do this, the experimental setup has to be modified a bit, as it needs a strong feedback coil signal. Therefore I connected the feedback coil to the phone input of my amplifier and amplified the signal accordingly to 5 volts (5ppV). The feedback coil magnet overlaps the coil by 50%.
When the magnet was removed, the signals were measured at different frequencies.
TEST SETUP:
TARGET of this test:
How does the signal strength change with and without feedback magnet.
In special at high frequencies!
(same coil and the same magnet).
THE RESULTS:
RESUME:
At low frequencies, the influence of the drive coil magnet is relatively small. It is 0.2% (1 kHz).
At frequencies around 10kHz the influence increases to 20% and at 16kHz to a whopping 50%!
It can therefore be seen that the influence of the drive coil magnet increases proportionally with increasing frequencies.
The attempt to shield the magnetic fields can be quite successful (as can be seen in test 6). I've gotten pretty good results with copper here.
Could MU metal achieve even better shielding at high frequencies?
P.S.: Mark was right!
Greetings from sunny Vienna!
Thomas
I just have to test how strong the effect of the drive magnets on the feedback coil is.
Is Mark right? (Of course he's right).
The following test 6 should show how right Mark is.
To do this, the experimental setup has to be modified a bit, as it needs a strong feedback coil signal. Therefore I connected the feedback coil to the phone input of my amplifier and amplified the signal accordingly to 5 volts (5ppV). The feedback coil magnet overlaps the coil by 50%.
When the magnet was removed, the signals were measured at different frequencies.
TEST SETUP:
TARGET of this test:
How does the signal strength change with and without feedback magnet.
In special at high frequencies!
(same coil and the same magnet).
THE RESULTS:
RESUME:
At low frequencies, the influence of the drive coil magnet is relatively small. It is 0.2% (1 kHz).
At frequencies around 10kHz the influence increases to 20% and at 16kHz to a whopping 50%!
It can therefore be seen that the influence of the drive coil magnet increases proportionally with increasing frequencies.
The attempt to shield the magnetic fields can be quite successful (as can be seen in test 6). I've gotten pretty good results with copper here.
Could MU metal achieve even better shielding at high frequencies?
P.S.: Mark was right!
Greetings from sunny Vienna!
Thomas
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farmersplow
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Re: The best size of a feedback coil?
P.S .:
The direction (alignment) of the Feedback Coil Magnet (NS or SN)
I forgot to mention that I also tested the north-south orientation of the magnet in both directions (NS and SN).
Different results were found.
The difference was particularly evident at high frequencies (10 kHz).
It can be seen from this that the magnetic direction of the magnet plays a role.
It can counteract the magnetic field of the drive coil magnet!
That can lead to improved results!
The results in test 6 (with magnet) were carried out with a position in which the magnetic fields counteract.
Nice day
Thomas
The direction (alignment) of the Feedback Coil Magnet (NS or SN)
I forgot to mention that I also tested the north-south orientation of the magnet in both directions (NS and SN).
Different results were found.
The difference was particularly evident at high frequencies (10 kHz).
It can be seen from this that the magnetic direction of the magnet plays a role.
It can counteract the magnetic field of the drive coil magnet!
That can lead to improved results!
The results in test 6 (with magnet) were carried out with a position in which the magnetic fields counteract.
Nice day
Thomas