Surely the definition of a voltage source is that it does not 'control' current? If you want the current to be controlled, then you don't want a voltage source.Joe Rasmussen said:
Current drive and voltage drive are two opposite ends of a spectrum; in between is drive from a specified impedance. It happens to be the case that for most of the life of audio electronics it has been found that the best compromise is to make amplifiers to act as voltage sources and speakers to respond best to voltage drive. This despite the fact that everyone knows that it is current which actually moves the voice coil.
'Controlling' current from a voltage source is just another way of setting a specific output impedance i.e. not a voltage source.
This discussion reminds me of the issue of back-EMF, which crops up from time to time. People talk about it as though it is something wholly unconnected with impedance.
One of the recurring features of audio electronics (and radio antenna design) is that people keep thinking that a failure to grasp one or more basic points of relevant physics is in fact a deeper understanding of physics. They think they are pushing back the boundaries of knowledge, when in fact they have yet to reach the boundary and probably can't even see the boundary from where they are standing.
I am very happy that once again you want to talk about audio design. I have been pondering on issues like this for quite awhile John. Could you kindly elaborate a little bit more on those unmeasured distortions you have discovered so some of us may continue to work on? They say that it is very difficult to improve on anything without being able to verify by measurement. Perhaps some instruments already show you some sign of the problem.
Unmeasured distortions are often mentioned as a way of avoiding the known fact that many people prefer a little measurable distortion. It is postulated that these people do not actually prefer the measurable distortion, but dislike the unmeasured distortion which is allegedly made by other circuits with less measured distortion.
Maybe it is difficult to see them distortions, but perhaps you can hear them.
But seriously, there are some noted aspects in audio left unmeasured like the contributing factors of coupling capacitors with respect to direct coupling and interstage transformers that affect perceived difference as noted by Bud Purvine, Lynn Olson, JC Morrison and others. I quoted a post below as an example.
People might like to ponder why interstage transformers were used in the past, then were replaced by coupling capacitors. There is a difference in performance, but it all points to capacitors being better (unless you intend driving the next stage into grid current).indra1 said:
The most perfect components we have are bits of wire, then come resistors, then capacitors, then last are wound items and active devices. Curious that there is a modern fad to maximise use of wound items.
The fact that a 'guru' has woffled in print about something does not mean that the issue is 'noted', or even that the issue exists.
I try to keep things simple for myself: if current drive or any other non-zero amp output impedance (or even an impedance profile, meaning Zout being complex and frequency dependant) gives a perceived and measured benefit for the actual driver, I use it if it is feasible to implement (single ended AB amps need only a few additional components). Some drivers benefit more than others, typically lesser quality driver seem to profit the most, and the wider their operating frequency range the more so. A nice application is active speakers with a fullrange driver.
Yes, the only application that might benefit from the current drive is a full range speaker or active speakers. However, in both cases we need DSP's to equalize for flat frequency response. It makes the thing complicated and it is questionable if several 0.1% in distortion improvement are worth bothering with more work.
Thanks Joe for putting up with your critics all by yourself apparently.
It has always bothered me that SERVOS which usually are motors that need to be controlled, use CURRENT DRIVE, because it removes the inductance of the motor wiring from the primary equations that make the servo do its thing. What is the difference between a loudspeaker and a 'motor'? Is not a loudspeaker a form of motor? Then WHY is voltage drive specified, rather than current drive?
Of course, it goes back to Rice and Kellog (sp) back in 1925 or so. THEY apparently INVENTED the direct radiator speaker, or at least were one of the first researchers to show it in a mathematical model. They showed that other loudspeaker types could also be made, effectively. This lead to the Manger loudspeaker driver, which derives from one of the early models in the Rice ... paper in 1925 (or so), the horn loaded speaker, and who knows what else. Just take the 'dot notation' equation that they show and MANIPULATE IT for best performance of what you want. Current drive, or finite resistive drive, as well as voltage drive is possible, and something outside of what we normally think as important, MIGHT actually be useful. How can we know, if we don't look and share experience?
It has always bothered me that SERVOS which usually are motors that need to be controlled, use CURRENT DRIVE, because it removes the inductance of the motor wiring from the primary equations that make the servo do its thing. What is the difference between a loudspeaker and a 'motor'? Is not a loudspeaker a form of motor? Then WHY is voltage drive specified, rather than current drive?
Of course, it goes back to Rice and Kellog (sp) back in 1925 or so. THEY apparently INVENTED the direct radiator speaker, or at least were one of the first researchers to show it in a mathematical model. They showed that other loudspeaker types could also be made, effectively. This lead to the Manger loudspeaker driver, which derives from one of the early models in the Rice ... paper in 1925 (or so), the horn loaded speaker, and who knows what else. Just take the 'dot notation' equation that they show and MANIPULATE IT for best performance of what you want. Current drive, or finite resistive drive, as well as voltage drive is possible, and something outside of what we normally think as important, MIGHT actually be useful. How can we know, if we don't look and share experience?
Trasnformers can have level dependent frequency response especially at low frequencies. Easily measured and heard.
Progress of technology has made available direct coupling on various topologies, expensive and heavy wound parts are last choice to most designer. To be honest, I can hear the effect he attributed to coupling caps on the post I quoted. However, due to unavailable access, I can not verify that the effect is not present on a system free of any coupling cap.
John look at Joe's schematics he is not talking about current drive, he even says so frequently.
I had made an LM3886 guitar amplifier for my younger daughter.
It showed such unstable behaviour around the zero crossing. Excellent acoustic effect.
Never understood why it behaved like that.
After some time, I reduced the gain and the instability stopped.
So John please, link to the paper.
George
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