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Old 8th July 2003, 06:00 AM   #1
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Question output stage with high impedance

I am interesting in problem of create an amplifier as a current source.
In a short abstract I have read that a negative current’s feedback
is not enough for success. It is necessary to create output stage with high impedance for the discontinuance spurious feedback.

Mills P.G.L. , Hawksford M.O. Transconductance Power Amplifier System for Current-Driven Loudspeakers. JAES,
Vol. 37, 1989, ¹10 p. 809-822.

Please, send me copy of this article. I can’t get it in Moscow
in central technics libraries.

PS Where is further development of this idea?
Why is not more publications and devises with this solution?
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Old 8th July 2003, 09:45 AM   #2
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Quote:
Why is not more publications and devises with this solution?
Are you asking why more is not discussed about current-driven loudspeakers? The accepted interface standard between amplifiers and speakers is voltage. I imagine this is because it was historically easier to convert a voltage into a cone displacement than a current, and still is for magnetic, voice-coil designs.

What is your interest in a current source amplifier?
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Old 8th July 2003, 09:53 AM   #3
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Quote:
I imagine this is because it was historically easier to convert a voltage into a cone displacement than a current, and still is for magnetic, voice-coil designs.
This is definitely NOT true: The force generated by the voice-coil is proportional to the current and not the voltage.

The (only) adavantage of voltage-drive is that taming of the driver's fundamental resonance is achieved more easily (i.e. cheaply).

Regards

Charles
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Old 8th July 2003, 10:27 AM   #4
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Quote:
This is definitely NOT true:
I never said it was. You misquoted me.
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Old 8th July 2003, 10:33 AM   #5
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sorry

Charles
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Old 8th July 2003, 12:18 PM   #6
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if you are concerned about sending a current to a speaker, there is an easier way of doing that. Put a sampling resistor between the speaker the and ground. and move the feedback pickup line from top of the speaker to top of the resistor (or between the speaker and the resistor). This way, the feedback will be proportional to the voltage drop on the sampling resistor which in turn is proportional to the current doing through the resistor (and the speaker).

This has been done for many years, going back to the vaccum amplifier days. It is said to improve the damping factor and control of the coil. My personal experience with this topology is that it does clearify bass a little but otherwise has no audioable difference.
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Old 8th July 2003, 12:35 PM   #7
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It depends heavily on the polarity of the feedback you take from the current sensing resistor. With positive feedback you make your amp's output impedance negative, increasing damping (i.e. decreasing Qes).
With negative feedback you will increase the output impedance of your amp, lowering damping (i.e. increasing Qes).

With frequency-dependant feedback you will get a frequency dependant output impedance, allowing more parameters to be altered artificially than just Qes.

But the current-sensing resistor is definitely the most common approach to make an amp behave as a current source.



Regards

Charles
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Old 8th July 2003, 12:39 PM   #8
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I have read abstract of article:

Mills P. G. L., Hawksford M. 0. J. Distortion Reduction in Moving-Coil
Loudspeaker Systems Using Current-Drive Technology. — JAES, vol. 37, ¹3,1989, March,
p. 129—148.

In this paper authors improved decrease of non-linear distortion.
Effect proportionality force of moving-coil to current (not voltage) is additional and independent.
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Old 8th July 2003, 02:15 PM   #9
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I read the Mills and Hawksford paper. By using current drive, they indeed obtained reduced distortion and a reduction of the compression due to voice coil heating and temperature coefficient. However, they had to apply motional feedback with a special second voice coil to get a reasonable damping of the loudspeaker resonance. Perhaps you can apply this to loudspeakers with double voice coils intended for subwoofers, using one coil to drive the loudspeaker and the other for motional feedback.
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Old 8th July 2003, 07:53 PM   #10
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It is very easy to make an amp with current output (nearly infinite output impedance) instead of voltage output - just find a principle of V/I converter with opamp. But does it make any sense? Every standard electrodynamic speaker must be supplied from a voltage output (zero output impedance) to achieve the best transfer and transient characteristics. Electrodynamic speaker is not designed to be driven from current output.
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