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Old 26th March 2011, 10:18 AM   #1
hahfran is offline hahfran  Germany
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Default Power amp with high out impedance

The amp described in thread old surprisingly good amp was thd measured with actual 2 1/2 way passive filtered speakers. This thd is hardly ever published
although power audio amps always drive reactive loads.
Interestingly topologies with voltage feedback perform worse in terms of thd with reactive load than those with current feedback. CF allows to set amplification and bandwidth independent of each other, not so voltage feedback.
I made another test: the feedback is proportional to the load current when taken from a additional resistor in series with the speaker load to ground.
Then the amp becomes a current source, with hi output impedance.
This can only work above the speaker's mechanical resonant frequency.
Very amazing, with this feedback reactive distortion almost disappears.
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Old 26th March 2011, 01:21 PM   #2
WuYit is offline WuYit  Sweden
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Do you mean like this?
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Old 26th March 2011, 01:34 PM   #3
hahfran is offline hahfran  Germany
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yes exactly. I tested with an old Jordan JX53 broadband speaker.
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Old 26th March 2011, 01:52 PM   #4
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It's incredibly effective all the way down to the driver resonance too. I use it to raise the output impedance to a couple ohms on a paralleled set of Eminence Beta12A's. This causes the Qts on open baffle to be around .75 instead of .46 as it would be with a negligible output impedance amp.

One of the intended "side effects" of doing this allows the use of normal to oversized magnet assemblies in open baffle rather than the puny, easily overloaded, magnets specified for such high natural Q.

The downside of current feedback comes into play in drivers driven wide-band when there are major impedance changes. Particularly if the driver has a response peak at an impedance peak, the current feedback amplifier will make the response even worse by stuffing more voltage into it. Power level impedance EQ networks could be used to correct.
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Old 26th March 2011, 02:05 PM   #5
AndrewT is offline AndrewT  Scotland
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How similar is the circuit that creates negative output impedance? or is it completely different?
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regards Andrew T.
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Old 26th March 2011, 02:14 PM   #6
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Not completely different, just opposite. Connect the current feedback to the amp + in.

Variable Amplifier Impedance

I never tried it though.
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Old 26th March 2011, 02:14 PM   #7
WuYit is offline WuYit  Sweden
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No negative impedance, just ordinary high (ideally infinite) impedance.
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Old 26th March 2011, 02:15 PM   #8
hahfran is offline hahfran  Germany
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That is different. Negative output impedance is prerequisite for an oscillator, so it is positive feedback.
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Old 26th March 2011, 03:07 PM   #9
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I have one of those small Yamaha clock radios with "YST technology", should be negative impedance. A 4" speaker in a very small BR box, tuned very low. It plays surprisingly loud and deeper than my KEF 8" subwoofer (glad I got that peace of ---- for peanuts). I think it's an interesting concept for computer speakers etc.
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