Wow...it's been a while!
The benefits of lowering inductance would depend on your desired passband--more benefit the higher you want to go. However, this is a subwoofer driver after all.
Re: conductive voice coil former--is it aluminum? Usually this is for thermal management and most likely cut so it doesn't form an efficient shorting ring. If it wasn't cut, its movement in the magnetic gap would be greatly damped by eddy currents.
The benefits of lowering inductance would depend on your desired passband--more benefit the higher you want to go. However, this is a subwoofer driver after all.
Re: conductive voice coil former--is it aluminum? Usually this is for thermal management and most likely cut so it doesn't form an efficient shorting ring. If it wasn't cut, its movement in the magnetic gap would be greatly damped by eddy currents.
Hi Bill
Yes the former is aluminium (for thermal management).
Just wondering if the slit really makes it the same as a nonconductive former - electromagnetically speaking.
Perhaps it is the perfect driver with which to test the theory.
I have used FEMM to simulate magnet structures but don't yet know how to do the coil simulations. I had better learn.
Regards,
John
(PS: Some of us have twisted fantasies involving low inductance subwoofers.)
Yes the former is aluminium (for thermal management).
Just wondering if the slit really makes it the same as a nonconductive former - electromagnetically speaking.
Perhaps it is the perfect driver with which to test the theory.
I have used FEMM to simulate magnet structures but don't yet know how to do the coil simulations. I had better learn.
Regards,
John
(PS: Some of us have twisted fantasies involving low inductance subwoofers.)
A conductive former without a slit would act as an electromechanical brake, like a shorted turn does. Because of the slit the effect is much more minimal, and doesn't do a whole lot in terms of controlling inductance or flux modulation.
For a subwoofer driver, shorting rings are of substantially lower value as they do much less at LF due to the scale of the field vs the size of the rings. They still help, and do tend to help thermal management, but pole geometry seems far more useful for keeping LF distortion low (and enough drivers that you rarely, if ever, "see" them move.... but that's a topic for another day).
Modifying the gap/coil/former in a driver is not an easy task, I'd rather get one made to spec or build my own from scratch (or find an existing model that does the job). For subs, you're better focusing on getting the right system design- enclosures minimizing excursion, push-pull mountings, enough woofer, etc.
For a subwoofer driver, shorting rings are of substantially lower value as they do much less at LF due to the scale of the field vs the size of the rings. They still help, and do tend to help thermal management, but pole geometry seems far more useful for keeping LF distortion low (and enough drivers that you rarely, if ever, "see" them move.... but that's a topic for another day).
Modifying the gap/coil/former in a driver is not an easy task, I'd rather get one made to spec or build my own from scratch (or find an existing model that does the job). For subs, you're better focusing on getting the right system design- enclosures minimizing excursion, push-pull mountings, enough woofer, etc.
I also received email, had subscribed to the thread that time.
It is already covered by others posted, just to reiterate, this driver in question 830452 is a subwoofer, and it has a nasty breakup at ~600Hz, so not to be used much high in frequency. A faraday ring's effectiveness at lower frequencies is limited by its internal resistance. So a thin layer of copper sleeve in the gap for this driver, for its working frequencies won't be effective. You need thick rings which would have very low internal resistance to induced current flow, which is what they are using in this motor. It seems to be a very good driver, as the way it is and for how it is meant to be used.
The only thing I can see as benefit, you can add another ring on the top plate, outside of the coil, like the one inside the motor. But this depends if the basket and spider has space for it, and whether it will hinder movement of the spider as driver strokes inside. It is possible that the basket is already covering that section, which means you have already a shorting ring there, the basket itself.
It is already covered by others posted, just to reiterate, this driver in question 830452 is a subwoofer, and it has a nasty breakup at ~600Hz, so not to be used much high in frequency. A faraday ring's effectiveness at lower frequencies is limited by its internal resistance. So a thin layer of copper sleeve in the gap for this driver, for its working frequencies won't be effective. You need thick rings which would have very low internal resistance to induced current flow, which is what they are using in this motor. It seems to be a very good driver, as the way it is and for how it is meant to be used.
The only thing I can see as benefit, you can add another ring on the top plate, outside of the coil, like the one inside the motor. But this depends if the basket and spider has space for it, and whether it will hinder movement of the spider as driver strokes inside. It is possible that the basket is already covering that section, which means you have already a shorting ring there, the basket itself.
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Thanks, Feyz. Your posts are extremely helpful and it is clear that you understand this subject very well.
I realise there are issues such as cone breakup etc. but I am interested in the technology that has been discussed in this thread.
I suppose the Faraday ring has to work much harder because the Peerless voice coil has so many turns. (thinking of it as a transformer with primary winding consisting of many turns and the secondary having one shorted turn).
What effect would an active coil wound around the base of the pole piece have on inductance in the lower midrange if any?
Or perhaps I should talk about phase shift rather than inductance.
Also... Does the conductive former (not shorted) have any effect on inductance and or the interaction between voice coil and Faraday rings?
Does anyone have that AES paper by Birt?
I realise I may seem obsessed with inductance only... please humour me.
I realise there are issues such as cone breakup etc. but I am interested in the technology that has been discussed in this thread.
I suppose the Faraday ring has to work much harder because the Peerless voice coil has so many turns. (thinking of it as a transformer with primary winding consisting of many turns and the secondary having one shorted turn).
What effect would an active coil wound around the base of the pole piece have on inductance in the lower midrange if any?
Or perhaps I should talk about phase shift rather than inductance.
Also... Does the conductive former (not shorted) have any effect on inductance and or the interaction between voice coil and Faraday rings?
Does anyone have that AES paper by Birt?
I realise I may seem obsessed with inductance only... please humour me.
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You got tired of your triple+ layers of XBL2 motor plates, eh?I'm not, really. I was summoned from the netherworld by a notification of a new post to this ancient thread.
If you're Obsessed with reducing inductance, you'd want to start by sticking with two layer coils. The trade-off then is reduced coil volume for a given impedance, which reduces power handling. Just take a look at the Dayton RSS390HF-4 versus RSS390HO-4 for a glance at what happens when going between 2 and 4 layers with everything thing else being fairly equal.
Also, the slit in the conductive former doesn't change eddy loss much (edit: yes it does, but doesn't eliminate it), just shorted turn loss, which would be much greater. There are technically still eddies generated by the induced voltage in the material moving past the gap returning through the portion of conductive former that is not in the gap. The effect might look like a slight reduction in mechanical Q, since the damping is still there with the coil open..
Also, the slit in the conductive former doesn't change eddy loss much (edit: yes it does, but doesn't eliminate it), just shorted turn loss, which would be much greater. There are technically still eddies generated by the induced voltage in the material moving past the gap returning through the portion of conductive former that is not in the gap. The effect might look like a slight reduction in mechanical Q, since the damping is still there with the coil open..
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Thanks Badman. I know the stuff you are saying and I agree with you.
I am actually considering a motor design that I will post shortly as it is relevant to this thread.
I believe the questions in my previous post are still valid.
I will probably close up the Peerless but it is a nice driver to experiment because one can remove and replace the magnet structure easily. It is attached to the basket with 4 screws.
The only thing I would try with the Peerless is a active coil wound round the base of the pole piece. I would appreciate if someone could tell me if it would decrease phase lag above resonance.
And, thanks Andrew. I did consider making a new VC for the former with 2 layers which would reduce inductance and make space for extra sleeves. But I think I'll leave the Peerless except for maybe trying the coil idea I mentioned above.
Your words about the conductive former are consistent with what I learnt at university and what you have said is exactly what I thought. The only question remaining there is whether the conductive former hinders the interaction between voice coil and Faraday sleeve.
I am actually considering a motor design that I will post shortly as it is relevant to this thread.
I believe the questions in my previous post are still valid.
I will probably close up the Peerless but it is a nice driver to experiment because one can remove and replace the magnet structure easily. It is attached to the basket with 4 screws.
The only thing I would try with the Peerless is a active coil wound round the base of the pole piece. I would appreciate if someone could tell me if it would decrease phase lag above resonance.
And, thanks Andrew. I did consider making a new VC for the former with 2 layers which would reduce inductance and make space for extra sleeves. But I think I'll leave the Peerless except for maybe trying the coil idea I mentioned above.
Your words about the conductive former are consistent with what I learnt at university and what you have said is exactly what I thought. The only question remaining there is whether the conductive former hinders the interaction between voice coil and Faraday sleeve.
Here is the design I would like to try.
Differential drive with long Faraday sleeve(s).
I still need to get my head around a few things:
1. Does it make a difference if there is one long sleeve or if there is a sleeve for each voice coil with a gap between the two sleeves?
2. If I understand correctly, the sleeve(s) won't work at low frequencies. Does it need to (does differential drive do the trick?) Or would there be benefit in fitting an active coil in the region shown in red?
Differential drive with long Faraday sleeve(s).
I still need to get my head around a few things:
1. Does it make a difference if there is one long sleeve or if there is a sleeve for each voice coil with a gap between the two sleeves?
2. If I understand correctly, the sleeve(s) won't work at low frequencies. Does it need to (does differential drive do the trick?) Or would there be benefit in fitting an active coil in the region shown in red?
Attachments
If done optimum it will reduce the inductance. I still don't know why you are obsessed about inductance. Inductance itself is an indicator of distortion in the motor, it is not the cause of the distortion. Cause of the distortion is modulation of the otherwise supposed to be stable constant magnetic field generated by the magnet. (There is also Le(x) but it is somewhat other story....) High Le is an indicator that these distortions are expected to be high.... So it appears to me you are trying to catch and reduce the indicator, rather than the source.
Which phase shift? On the electrical side, i.e. phase shift on the impedance of the driver? Then not only Le but other things also dictate it, like the whole rest of the mechanical system of the driver, suspension and all... Try to find an electrical model of loudspeaker driver, you will see. The phase shift on the acoustic side will also be the result of all these, plus the box etc etc...... The acoustic phase at low frequencies will be easily determined by the LF alignment of the driver plus box (or dipole etc etc...) You may find that for the frequency range you are looking at, Le doesn't have much effect on phase as other things. Just guessing....
Yes to the former, it would reduce the inductance, but just a little. Luckily I know of a case to compare easily, 18W8545 has aluminum former, 18W8545K has Kapton former, the rest is same between the two driver from what the documentation says. Looking at their impedance curves, at 2Khz 18W8545 is at 8 ohms, 18W84545-K is a bit higher than that. So a small effect from aluminum former. Any eddy current induced around the voice coil will tend to lower the inductance of the voice coil (Lenz law dictates that)
For later part of your question, no it doesn't other than helping the Faraday ring. In theory you can place all of the air inside and around the motor with high conductive material, then you would have the most effective faraday rings effect. I don't know any gaseous conductive material (save plasma jokes please
)I answered this one already, the more conductive material stuffed around the better. The more eddy current the better, for reducing flux modulation that is which is the root cause of BL(I) and Le(I) distortions and causing higher Le, which increases Le(x) distortion.
Differential drive may do the trick for the low frequencies, depends on how well they are coupled in the magnetic system. Need to do simulations etc. Copper rings will still be helpful. It has been sometime I had looked at JBL's patent and docs on their differential drive motor, but from memory they still had a shorting ring (one or more?) in the driver.
Active coil will work at low frequencies. The thing with it, it needs be well optimized, because if it is not it can even work completely the opposite and increase the distortion. Assume the active coil was wound too many turns and is generating twice the more magnetic field as the voice coil itself. then the active coil instead of just reducing the magnetic field modulation from the voice coil, it will be modulating the field itself. The other thing, it won't be fully coupled with the voice coil(s). So it won't be able to cancel all the modulation from the voice coil. Other thing, voice coil inductance and so modulation of the field is position dependent, but the active coil is stationary. What happens than, needs to be either analysed or simulated.
I remember 18 Sound used to (or still uses ? ) active coil in some of their drivers. I think they were placing the "active" coil in series with the voice coil. Birt (the one with the AES paper mentioned here) was using a nifty feedback device attached to the driver motor, and driving the active coil based on the feedback received from this device. In his useage, the voice coil and active coil were separate electrically. This paper is in the Loudspeaker Anthologies, sorry I can't scan and send to you, nor I have time to find it and reread it
I had written some online document/white paper on shorting rings and their effect on impedance. I don't know if you have seen it, you can find it if you google. I think I was able to explain what I know and how these things are working in there in a good way,which would answer some of your questions, probably most of it
Took a look at 18 Sounds website and found something looks very interesting:
http://www.eighteensound.com/staticContent/technologies/products/aic.pdf
Haven't read it yet, just looked over the charts, it seems impressive on the midrange. But they have the active coil inside the voice coil, not at the base.
http://www.eighteensound.com/staticContent/technologies/products/aic.pdf
Haven't read it yet, just looked over the charts, it seems impressive on the midrange. But they have the active coil inside the voice coil, not at the base.
Thank you Feyz.
One question and then I'll stay quiet until I have studied your paper.
Suppose I have a dual voice-coil woofer with copper sleeve.
I can connect the voice coils in parallel or series.
But in which case will the copper sleeve be more effective?
For example, let's look at it without the copper sleeve first.
Connected in parallel:
Re = 4Ω
Le = 1mH
BL = 4 T.m
Connected in series:
Re = 16Ω
Le = 4mH
BL = 8T.m
According to my calculations, the phase shift will be the same in both configurations.
Adding the copper sleeve will reduce Le in both cases.
But if, for arguments sake, the sleeve reduces 4Ω inductance by a factor of 10 to 0.1mH, would it also reduce the 16Ω inductance by a factor of ten to 0.4mH?
One question and then I'll stay quiet until I have studied your paper.
Suppose I have a dual voice-coil woofer with copper sleeve.
I can connect the voice coils in parallel or series.
But in which case will the copper sleeve be more effective?
For example, let's look at it without the copper sleeve first.
Connected in parallel:
Re = 4Ω
Le = 1mH
BL = 4 T.m
Connected in series:
Re = 16Ω
Le = 4mH
BL = 8T.m
According to my calculations, the phase shift will be the same in both configurations.
Adding the copper sleeve will reduce Le in both cases.
But if, for arguments sake, the sleeve reduces 4Ω inductance by a factor of 10 to 0.1mH, would it also reduce the 16Ω inductance by a factor of ten to 0.4mH?
Connected in parallel, Le will be 2mH still (or very close to it), not 1mH. Because the windings between the two coils are highly coupled. You can think of paralleling as if you made the same number of windings as one coil but with a thicker wire.
As to impedance phase with two cases, there is a copy of my doc here
http://www.diy-audio.narod.ru/litr/FaradayRingsVoiceCoilImpedance.pdf
Using the formulas 20a and 21a, then you will get the answer you are looking for. They are for a very simplified model (pure transformer) but good enough for your question.
Or use the SPICE example I gave there in Fig 7, you can use LTSPice to run it and see how it behaves.
As to impedance phase with two cases, there is a copy of my doc here
http://www.diy-audio.narod.ru/litr/FaradayRingsVoiceCoilImpedance.pdf
Using the formulas 20a and 21a, then you will get the answer you are looking for. They are for a very simplified model (pure transformer) but good enough for your question.
Or use the SPICE example I gave there in Fig 7, you can use LTSPice to run it and see how it behaves.
I'm a bit puzzled with this ^^.
In parallel, you have half the number of turns as in series, but it is like using thicker wire (as you said). But I thought Le was proportional to number of turns squared.
Therefore, halving the number of turns would quarter the inductance?
I feel bad asking questions before reading your paper thoroughly. Is's not my usual style.
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