Current drive for Loudspeakers

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Hörnli

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Joined 2007
41 pages about current and not a single measurement of it?
20230522 1297,67 weiss pegel spez lauter vs dir -10.44 leiser 30-5000 80%.png

Background: https://www.diyaudio.com/community/threads/drive-current-distortion-measurement.402566/#post-7434990
 
Baffless

Baffless

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Joined 2012
baffless.com
CharlieLaub said:
I have always been a proponent and a user of DSP for my loudspeaker projects. Who needs those silly passive components, I thought. Along comes a tech paper by Purifi that recently caught my attention:
https://purifi-audio.com/wp-content/uploads/2022/03/220211_R05-Notchfilter.pdf
In the scenario presented, the driver (the PTT8.0X) has some 10dB tall breakup peaks in its response. The distortion of the driver has "echoes" of these peaks at lower frequencies that are N times lower where N is the order of distortion. This predominantly occurs with 3rd order distortion products, so at a frequency 3 times lower than the breakup peak, which puts the distortion peak within the passband. Bad!

I was able to chat with Lars Risbo of Purifi about the tech note until I finally wrapped my brain around the claims therein. According to Lars, distortion (broadly in the mid passband and above) is caused by modulation of the voice coil inductance due to various mechanisms. The inductance modulation within a magnetic field causes a new "distortion voltage" to be generated. This is generating various order of distortion all across the passband. This is also mentioned in this article in the section "Indefinite EMF generation due to mechanical non-idealities":
https://www.edn.com/loudspeaker-ope...y-of-current-drive-over-voltage-drive-part-2/

Also a factor is the breakup peak of the driver, which is sort of like a small frequency region of enhanced driver efficiency where more output is produced by the drive current compared to the rest of the passband. So, what is happening is that the "distortion voltage" finds a closed loop path over which it causes current flow, and this path includes the driver itself. When the distortion product falls on the breakup peak frequency, the distortion peaks just like the profile of the breakup peak following the increase in efficiency there. The way to prevent this is to reduce or block the current flow induced by the distortion voltage. In the Purifi tech note, this is accomplished via a passive (notch) network placed in series with the driver and immediately next to it without any other network in between. The series network notches out the breakup peak via a peak in impedance put in series with the driver. It is this series impedance at the breakup peak frequency that is "in the way" of the distortion voltage induced current, and so very little can flow. As a result the peak in 3rd order distortion occurring at 1/3 the frequency of the breakup peak is also reduced.

This was all new to me, and I now see some benefits from using series impedance shaping networks and filters as a way to reduce 3rd order distortion within the passband. Very interesting.

Thereis a way to do this without any passive elements, and that is to use an amplifier with high output impedance, which is what current drive produces. So I thought I would post about it here after reading Joe's post above that touches on the subject. It is something that I really did not appreciate previously. Unfortunately I believe that current drive is not possible unless the loudspeaker return is at ground potential, making amplifiers operating in bridge mode (such as many class-D amplifiers) unable to implement current drive.
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don't get the technical idea of this long post. is it possible in two sentences to say what to what is related? :) yes current drive mostly reduces 3rd harmonic and above usually. But how it relates to breakups i don't get. Red is driver output with voltage drive, black with current drive.
current(black) overlayed over voltage.jpg
 
A

abstract

Member
Joined 2014
Re: current drive and break-up modes. As I understand it, there are several different effects, mostly unrelated but with possible overlap.
-- The impedance gets modulated by signal levels, leading to inter-modulation distortion.

-- Bending waves ripple out across surface of the cone, hit reflections like the surround and travel back to the middle. Then the voice coil gets pushed by these echoes, which generates some mix of voltage and current. If the impedance across the speaker is high at the frequency in question, that will block current from flowing and it will 'transduce' that into a voltage. Alternatively, if the impedance is low (say, if a directly connected amplifier clamps the voltage to a set level), current will flow. The key difference is that current loads the vibrations down with friction. AND that friction is subject to the exact same problems of variability described above. (E.g. the VC oscillates +/- a few mm at some bass frequency, which modulates the electrical friction near the break-up modes @ 1~5kHz, changing the ringing behaviour.)
 
AllenB

AllenB

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Baffless said:
But how it relates to breakups i don't get.
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Perhaps because breakup doesn't produce distortion.

However the interesting thing is that breakup changes the polar response, which has a side effect because the normal response is lower/higher at different angles. This can vary the level of the harmonic itself, unrelated to distortion. So if lower frequency distortion leaves a harmonic at a breakup frequency, the distortion you measure will seem lower (or higher, if you sit right on axis). It's not really a change in distortion, just a change in frequency response.
 
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Baffless

Baffless

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Joined 2012
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abstract said:
Re: current drive and break-up modes. As I understand it, there are several different effects, mostly unrelated but with possible overlap.
-- The impedance gets modulated by signal levels, leading to inter-modulation distortion.

-- Bending waves ripple out across surface of the cone, hit reflections like the surround and travel back to the middle. Then the voice coil gets pushed by these echoes, which generates some mix of voltage and current. If the impedance across the speaker is high at the frequency in question, that will block current from flowing and it will 'transduce' that into a voltage. Alternatively, if the impedance is low (say, if a directly connected amplifier clamps the voltage to a set level), current will flow. The key difference is that current loads the vibrations down with friction. AND that friction is subject to the exact same problems of variability described above. (E.g. the VC oscillates +/- a few mm at some bass frequency, which modulates the electrical friction near the break-up modes @ 1~5kHz, changing the ringing behaviour.)
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Yes will be different responce to cone ripples. Current drive will ignore electricaly the incoming forces from outside. Just i think it will be such complex behavior that will not be ralated to any particular distortion order.
AllenB said:
Perhaps because breakup doesn't produce distortion.

However the interesting thing is that breakup changes the polar response, which has a side effect because the normal response is lower/higher at different angles. This can vary the level of the harmonic itself, unrelated to distortion. So if lower frequency distortion leaves a harmonic at a breakup frequency, the distortion you measure will seem lower (or higher, if you sit right on axis). It's not really a change in distortion, just a change in frequency response.
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This is a very good and interesting point. Need to think more on this :)
 
AllenB

AllenB

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lrisbo said:
(in addition to amplifying distortion from the motor)
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What do you base this on?

lrisbo said:
What do you base this conclusion on?
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Primarily, breakup is not a non-linear phenomenon. In any case, a speaker is not going to be used there due to polar irregularities.

jan.didden said:
Breakup produces vibrations in partial areas of the cone which produce frequencies that are not in the original signal.
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Do you have evidence of such a harmonic relation?
 
L

lrisbo

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Joined 2021
purifi-audio.com
AllenB: A breakup peak amplifies distortion from the motor as we have discussed in several threads. see https://purifi-audio.com/blog/app-notes-2/low-distortion-filter-for-ptt6-5x04-naa-11

Speakers are primarily linear but produce distortion nevertheless. Same goes for the speaker at breakup. Distortion is typical increased here because there is more displacement. Most paper cone drivers are used past their first breakup. But yes, best to avoid pumping energy into the driver here.
 
H

Hörnli

Member
Joined 2007
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K

kgrlee

Member
Joined 2011
jan.didden said:
Breakup produces vibrations in partial areas of the cone which produce frequencies that are not in the original signal.
I'd call that distortion.
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ALL cone speakers 'break up' in a LARGE part of their useful frequency range. Exactly how they break up determines whether this leads to 'distortion' or not.
A large part of designing a good cone is controlling shape, thickness, material bla bla so it 'breaks up' the way you want it to. Spent a large part of my previous life doing this with Laser Doppler Interferometry, FEA, black magic, animal entrails bla bla
I used to have a Laser Doppler Interferometer animation from da previous Millenium which shows how a Good sounding Plastic cone 'breaks up'. There must be people who have done similar stuff today as the tools are much more readily available (Klippel ?). Also a Good Sounding Paper cone breaks up in a different manner to a plastic cone :eek:
 
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K

kgrlee

Member
Joined 2011
lrisbo said:
Speakers are primarily linear but produce distortion nevertheless.
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:LOL: :LOL: :LOL: Speakers are definitely NOT Linear Time Invariant (LTI). Us horny handed speaker designers just find it convenient to assume they are :)
Today, with Prof. Angelo Farina's method to measure response & THD quickly, you can easily check this out .. but you could do that in da previous Millenium if you knew what you were looking for.
Getting a speaker more TI is good as you can usually hear that. Surprisingly, the Linear part isn't so 'important' (audible). You need to be doing DBLTs to confirm or disprove my heretical claim :cool:
 
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