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Old 16th June 2012, 03:34 PM   #1
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Default Lets talk about transconductance amplifiers

Why current drive?
http://www.essex.ac.uk/csee/research...nt%20drive.pdf
Look at the massive reduction in distortion!

A disadvantage of current drive is a loss of damping at the resonant frequency of the speaker. Nelson Pass solution to this was a parallel network but if you think about this it's dumb. As the impedance of the source becomes low at resonance therefore making it a voltage output amplifier at resonance loosing the distortion advantage in the most critical region. I would like to dismiss all discussion of parallel networks as they result in only a high output impedance voltage amplifier. The better way appears to be motional feedback which has good results in the above paper. I have also found another paper where more practical circuits are explored:

http://www.essex.ac.uk/csee/research...0amplifier.pdf

Any thoughts on using duel voice coil woofers to avoid having to take woofers apart to get sensing coil?

Anyway I am wondering if anyone has built or is operating a system with transconductance power amplifiers and would like to utilise the community to search out as much info as possible on the subject as they seem quite exciting!
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Old 16th June 2012, 10:51 PM   #2
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I did some more reading about the lack of damping a resonance issue and it appears that the primary problems are parameter shifts in drivers causing equalisation without feedback to have errors due to parameter shift hence the need for a closed loop. This is also Another reason not to use a parallel network; look at the shifts in fs and q of the system in these tests:
Zaph|Audio

It seems the best way of measuring driver position would be some kind of laser measurement device. My idea for this would to be measure the velocity of the cone via the Doppler shift on a modulating square wave on the laser. I don't know anything about lasers though or how fast photo detectors can go...
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Old 16th June 2012, 11:48 PM   #3
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Hi Kipman,

A couple of points on current drive:

1) look at fig. 14 a. and b. of hawksfords paper and see how current drive wreaks heavoc with the frequency response of a driver;

2) somewhere in the paper hawksford refers to the problem voltage drive may have with xovers. True, there are interactions, but try to design an xover for a current drive amp! A perfect one current drive amp would have an infinite output impedance, which would become part of the xover. So where do you go from there?

And on motional feedback in any way shape or form:

Imho it is overdoing it, because the compensation which has to be made on the low end (where mfb works) is pretty straightforward. The Linkwitz transform, which compensates for the predictable 12dB downslope of a sealed enclosure below F-3dB, therefore does the trick just as well, with much less complexity.

vac

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Old 17th June 2012, 06:42 AM   #4
hahfran is offline hahfran  Germany
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From the mechanical-electrical analogy it is immediately obvious that electrodynamic speakers are current driven. Using a current source amplifier as drive requires a compensation of mass compliance ... i.e. of all mechanical properties of the speaker.
The attachment shows a circuit patented by AES for vented woofers, which is however
applicable to midrange and tweeter as well. The elements at the output of the power amp are the electrical representation of a vented woofer.
Thus current driving makes only sense for active speakers.
The attachment has a circuit of an inverting power amp.
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File Type: jpg inverting mihai.jpg (83.7 KB, 238 views)
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Old 17th June 2012, 07:07 AM   #5
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The proper solution to flat FR is to build a speaker with either flat impedance, or a speaker with an impedance curve that is complmentary to its FR.

An aperiodic box or a very heavily stuffed sealed box with a low Qm driver. Passive XOs should also be avoided.

EQ in the amp can also deal with some of the problem.

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Old 17th June 2012, 10:27 AM   #6
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Quote:
Originally Posted by vacuphile View Post
Hi Kipman,

A couple of points on current drive:

1) look at fig. 14 a. and b. of hawksfords paper and see how current drive wreaks heavoc with the frequency response of a driver;

2) somewhere in the paper hawksford refers to the problem voltage drive may have with xovers. True, there are interactions, but try to design an xover for a current drive amp! A perfect one current drive amp would have an infinite output impedance, which would become part of the xover. So where do you go from there?

And on motional feedback in any way shape or form:

Imho it is overdoing it, because the compensation which has to be made on the low end (where mfb works) is pretty straightforward. The Linkwitz transform, which compensates for the predictable 12dB downslope of a sealed enclosure below F-3dB, therefore does the trick just as well, with much less complexity.

vac

vac

The motional feedback is to modify the parameters of the speaker such that the high Q resonance (that causes frequency response problems) is damped by the motional feedback. Simple EQ of this peak is subject to error due to parameter shift of the driver with time/temp. In the Hawksford paper their Monte Carlo sim of this estimates 2dB of error at resonance using EQ. Simple EQ is defiantly the way to initially approach the problem but not a final solution.

As for crossovers and EQ; I run fully active systems almost exclusively using DSP crossovers. I have a mini-dsp on my bedroom system and use KX-project drivers on the communal TV computer. I'm also a pretty good at digital hardware and have taken many advanced DSP courses so would probably approach this problem by doing all the processing in the digital domain.
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Old 17th June 2012, 10:34 AM   #7
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Quote:
Originally Posted by planet10 View Post
The proper solution to flat FR is to build a speaker with either flat impedance, or a speaker with an impedance curve that is complmentary to its FR.

An aperiodic box or a very heavily stuffed sealed box with a low Qm driver. Passive XOs should also be avoided.

EQ in the amp can also deal with some of the problem.

dave

yup... certainly a valid approach. I like sealed boxes however and that usually means a high Q driver. Motional feedback should allow the usage of pretty much any bass driver with a given cone area and excursion capability to work in any sealed box as long as the power is kept under the limit of the driver.
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Old 17th June 2012, 10:39 AM   #8
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Quote:
Originally Posted by hahfran View Post
From the mechanical-electrical analogy it is immediately obvious that electrodynamic speakers are current driven. Using a current source amplifier as drive requires a compensation of mass compliance ... i.e. of all mechanical properties of the speaker.
The attachment shows a circuit patented by AES for vented woofers, which is however
applicable to midrange and tweeter as well. The elements at the output of the power amp are the electrical representation of a vented woofer.
Thus current driving makes only sense for active speakers.
The attachment has a circuit of an inverting power amp.
At the moment I quite like the look of the topology with current mirrors driving the load. I will have to think about a sensible way of making a current multiplying mirror out of discreet parts though. Some kind of opamp based Vgs control on the 2nd mirror transistors probably. If I was only making it on a chip it would be so easy as I could just make the transistors bigger!
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Old 17th June 2012, 11:32 AM   #9
hahfran is offline hahfran  Germany
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It is almost impossible to have current mirrors symmetric w respect to dynamic properties.
Requires a lot of selection from big quantities of BJTs. Not recommendable imo for diy.
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Old 17th June 2012, 12:02 PM   #10
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It's all very well discussing ways to drive a loudspeaker in such a way that distortion is reduced - but would the improvement be worth the added complexity? Would it even be audible?

Loudspeaker distortion is fairly benign...
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