Amplifier Feedback

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Date: August 02, 2003 at 09:34:29
From: djk, [tc077.cedar-rapids.net]
Subject: Amplifier Feedback

99.999999999% of amplifiers driving subs have feedback.

When the amplifier clips this can destroy the woofer. It has to do with the time constants in the power supply and feedback loop of the amplifier. Watch the motion of the subwoofer cone at cliping, it will generally have a 1~2 hz component to it.

It is this 1~2 hz component at rail-to-rail output (cliping) that causes the destruction of the driver.

An interesting case:

JBL B380 subwoofer, 2235 driver, a 6th order equalized system tuned to about 23hz.

While this sub can handle in excess of 1KW un-distorted power on music, the 100W Adcom GFA545 will slam the spider into the top plate of the magnet when the amplifier clips (due to the 1hz~2hz component).

Crown builds a circuit into their top-of-the-line VZ5000 to prevent this, they call it a 'loudspeaker off-set integrator'. It's complicated, and an integral part of the amplifier design.

Speaker companies are aware of the problem, but don't really know how to deal with it. They generally just recommend using a 'good' amp rated at about double the RMS power of the loudspeaker and hope it doesn't clip too hard or too often.

The real answer is to make sure the feedback loop of the amplifier never clips.

A 'hard limiter', think DBX, is a common solution.

A hard input clipper is better.

If you hard clip the input of the amplifier at a level just before the amplifier would clip, then the feedback loop of the amplifier will always be under control and not do anything nasty.

There is a brand of amplifiers that have switchable diodes on the input of the amplifier to hard clip the input signal.

NAD

Except the call it 'soft cliping'.

(Would anybody use it if it was called 'hard cliping'?)

I don't think anyone at NAD really understands what this circuit does for the loudspeaker, nor is it common knowledge elsewhere.

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Follow Ups:

Right on! Adrian Mack 10-Aug-2003 00:06:29 (6)

isn't he just that good !!!!!!!!!!!!!!!!!!!!!!!!!(NT) norman bates 11-Aug-2003 20:59:22 (0)

Rane Corp. white paper on amplifiers and clipping Richard Greene 11-Aug-2003 12:16:02 (2)

Average Power Increase djk 12-Aug-2003 04:15:13 (0)

Re: Rane Corp. white paper on amplifiers and clipping Adrian Mack 11-Aug-2003 19:16:14 (0)

Adrian!! Derek 10-Aug-2003 00:23:47 (1)

Have not forgot about you Adrian Mack 10-Aug-2003 04:52:13 (0)

Any examples for DIY subwoofers used in homes? Richard Greene 05-Aug-2003 10:22:46 (8)

JBL B380/BX63 djk 08-Aug-2003 03:58:12 (0)

Adire Audio comment on low power amp NOT damaging their sub driver Richard Greene 07-Aug-2003 14:28:01 (6)

Tempest clipping of small and large power amps Adrian Mack 07-Aug-2003 19:27:11 (5)

"You won't protect your speakers by using a larger amplifier" Richard Greene 08-Aug-2003 08:10:19 (4)

Small vs Large powered amplifiers Adrian Mack 09-Aug-2003 22:19:05 (3)

Clipping harmonics not that dangerous Richard Greene 11-Aug-2003 11:12:54 (0)

About DC components in clipping Adrian Mack 10-Aug-2003 00:02:33 (1)

The cone does not stop due to clipping - there is no DC component Richard Greene 11-Aug-2003 11:14:10 (0)

Re: Amplifier Feedback Brian 03-Aug-2003 20:48:15 (1)

Re: Amplifier Feedback djk 03-Aug-2003 21:24:13 (0)

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