I had not seen this answer before.
I think that clarifies the panorama ( to me ) about what is being discussed .....
I say well if what it is about is to achieve a speaker that responds linearly controlling its variations of impedance and phases by means of servos? Is that.?
Servo-controlled subwoofers already exist, for example:
https://www.psaudio.com/pauls-posts/servo-bass/
https://www.eetimes.com/document.asp?doc_id=1274742
I do not know what Joe's speaker is, could you give me a link?
Thank you
Last edited:
Servo bass designs are quite bandwidth limited, and if you also use linear control, the problem will still exist.
Currently I rely of FIR filters for the final correction. The near field measurement for FIR filter basis actually does a pretty good job similar to a servo, but you can linearize phase.
Dr. Geddes, could give advise and explain a bit on which type of drive (voltage or current) to use for a single amp driving a single driver (or a multi amp system through a line level crossover)?
Physicists don't necessarily make good EE's in fact last years Nobel winner asked me to come to his office and discuss noise issues (please don't accuse me of appeal to authority this is just a personal anecdote). I've read your friends stuff some of it is just plain wrong, and some relates to 70's level PA design just not relevant anymore.
Scott, I have no beef with you, but it seems you have with me? I doubt if I quoted any authority that you would recognise, that is why I have not quoted his name and he is far more than a mere physicist and also has a long track history in audio. In fact I have two more physicists more who are actually interested and they also have an interest in things audio. I shall only mention their names in private conversations, it should be obvious why.
Also, I am at ETF18 in Belleme, France right now. There is a growing interest in the subject F=Bli and that current drive may not be the only solution that is raised by that equation. But the equation stands, we need to take it seriously other than just lip service.
Cheers, Joe
It's really a matter of choice, but voltage amps are easier to work with as they would require less EQ to use than a current amp. Since a current amp will response to the impedance by increasing the voltage the shape of the impedance will be reflected through to the sound output, which must then be corrected if a "flat" response is desired. With a voltage amp this won't happen so you don;t have to correct it.
I have to be honest and say I got bored banging my head on the desk whilst reading through this thread, so I have may have skipped a few worthwhile contributions - my apologies! But here is my contribution to a debate that I have contributed to in several other threads on here in the past...
Thermal distortion is not audible in loudspeaker drivers in isolation. Where it becomes relevant is in multi-way systems, where different bands can have differently varying sensitivities. In high power studio installations, for example, there is often a noticeable softening of the bass as long-term rms power input increases. In domestic environments it is much less of a problem. (I suspect most domestic hi-fi drivers would fall-apart with such high rms power inputs in any case).
If a good method is employed to implement current drive (I prefer a modified version of David Birt's self-balancing bridge), then current drive can eliminate these thermal issues altogether (although notably it does not change the propensity for less well-engineered drivers to destruct). In my opinion, in maintaining the overall frequency balance, current driver is worth the extra effort, however.
But... current drive offers several other means to reduce distortion, the most audibly effective of which in my experience is the removal of magnetic non-linearities from the transfer function. Eddy currents impart at least a component of "semi-inductive" behaviour, which for the real part, adds a component of resistance that rises proportionally with the square root of frequency.
The effects of magnetic non-linearities manifest themselves therefore at higher frequencies and introduce non-linear artefacts into the impedance (nominally third-order distortion for say eddys in pole pieces flanking the coil). Whilst dependent on the driver motor design, these distortions are audibly absent for current drive and, in my humble opinion once again, makes the extra effort of using current drive even more worthwhile.
Thermal distortion is not audible in loudspeaker drivers in isolation. Where it becomes relevant is in multi-way systems, where different bands can have differently varying sensitivities. In high power studio installations, for example, there is often a noticeable softening of the bass as long-term rms power input increases. In domestic environments it is much less of a problem. (I suspect most domestic hi-fi drivers would fall-apart with such high rms power inputs in any case).
If a good method is employed to implement current drive (I prefer a modified version of David Birt's self-balancing bridge), then current drive can eliminate these thermal issues altogether (although notably it does not change the propensity for less well-engineered drivers to destruct). In my opinion, in maintaining the overall frequency balance, current driver is worth the extra effort, however.
But... current drive offers several other means to reduce distortion, the most audibly effective of which in my experience is the removal of magnetic non-linearities from the transfer function. Eddy currents impart at least a component of "semi-inductive" behaviour, which for the real part, adds a component of resistance that rises proportionally with the square root of frequency.
The effects of magnetic non-linearities manifest themselves therefore at higher frequencies and introduce non-linear artefacts into the impedance (nominally third-order distortion for say eddys in pole pieces flanking the coil). Whilst dependent on the driver motor design, these distortions are audibly absent for current drive and, in my humble opinion once again, makes the extra effort of using current drive even more worthwhile.
send me a complete system as you have described & i will give you an impartial evaluation of it. or set one up within 100 miles of my abode for me to hear. but this stuff is nothing but esoteric (fanciful) crap if it doesn't improve the sound. and i'll be a monkey's uncle or nephew if it does improve the sound one iota!
I did not make a "crap" statement and nothing I stated was fallacious.
It is proven that thermal heating can lead to significant rises in coil temperature and a reduction in sensitivity in the order of 3-6dB: Birt's elegant solution is well known and not unique to me.
At mid-frequencies, I have examples of third-order distortion reduction of over 40dB that even people with such closed minds cannot fail to notice.
You have every opportunity to prove the advantages of current drive to yourself and you can find many of the tehniques that I have exploited described amongst my other posts in this forum.
Please refrain from your ignorant and objectionable contributions.
It is proven that thermal heating can lead to significant rises in coil temperature and a reduction in sensitivity in the order of 3-6dB: Birt's elegant solution is well known and not unique to me.
At mid-frequencies, I have examples of third-order distortion reduction of over 40dB that even people with such closed minds cannot fail to notice.
You have every opportunity to prove the advantages of current drive to yourself and you can find many of the tehniques that I have exploited described amongst my other posts in this forum.
Please refrain from your ignorant and objectionable contributions.
I would be interested in reading about this. Can you point me to the proper threads? Or is there a good way to search for it?
Hopefully I will, and to a satisfying extent. Since I am in the process of driver design.
I would agree that in systems where the drivers suffer from large heat changes this statement CAN be true, I have measured this myself. SO I would agree with :
except to say that "can" and "do" are not the same things. In my speakers, I was not able to measure this same effect, hence it is, as in most all things, a matter of design. And depending on the design "Birt's solution" may or may not have any effect.
But you would have to show me the examples of:
where once again, I am sure it CAN happen, but again, in my speakers it would not happen. It is a very different things to set up an example of something that can be audible in an extreme case, but if one simply designs it out then it is NOT a factor. And what you are talking about can easily be designed out and hence inaudible.
It may well be that speakers that suffer from poor designs such that they have the problems that you state, could be improved with current drive as it does lower these kinds of distortions, but if they don't exist in a given system then it no longer is a benefit.
You would have to prove to me that this effect is nonlinear. Just going as the square root of frequency does not make it nonlinear. This was shown by John Vanderkooy many decades ago as I remember it he did not claim the effect to be nonlinear. I need some supporting theory to show this, not just some claim. An AES paper?
There is a form of the inductance that is nonlinear in cone excursion, and it CAN BE quite audible, but again a shorting coil removes almost all of this effect, once again making a benefit somewhat inconsequential.
And nuconz - chill out man!
except to say that "can" and "do" are not the same things. In my speakers, I was not able to measure this same effect, hence it is, as in most all things, a matter of design. And depending on the design "Birt's solution" may or may not have any effect.
But you would have to show me the examples of:
where once again, I am sure it CAN happen, but again, in my speakers it would not happen. It is a very different things to set up an example of something that can be audible in an extreme case, but if one simply designs it out then it is NOT a factor. And what you are talking about can easily be designed out and hence inaudible.
It may well be that speakers that suffer from poor designs such that they have the problems that you state, could be improved with current drive as it does lower these kinds of distortions, but if they don't exist in a given system then it no longer is a benefit.
You would have to prove to me that this effect is nonlinear. Just going as the square root of frequency does not make it nonlinear. This was shown by John Vanderkooy many decades ago as I remember it he did not claim the effect to be nonlinear. I need some supporting theory to show this, not just some claim. An AES paper?
There is a form of the inductance that is nonlinear in cone excursion, and it CAN BE quite audible, but again a shorting coil removes almost all of this effect, once again making a benefit somewhat inconsequential.
And nuconz - chill out man!
Last edited:
this is a 'crap' statement: [With over 30 years industry experience and an audio system of my own that implements all of the above, I do not feel the need to answer to such an imbecilic (and erroneous) comment.]
i can tell when someone is a grifter by the response to a challenge. i challenged you to prove that this type of system is worth "...the extra effort of using current drive even more worthwhile." and you call me names.
ok. again, prove it. or will you call me more names?
i see you are in the UK. how is the immigration going there?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.