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Question about amplifier DC voltage
JFS
post Apr 18 2009, 22:36
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I read an article about DC volatage at:
http://www.allegrosound.com/Fusing_AllegroSound.html
If an amplifier is leaking DC volatge, would this be a big problem to fix?

I am asking because I plan on buying an amplifier, and am considering
buying something used.

Thanks.
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pdq
post Apr 19 2009, 04:43
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This article seems to say that 50 mV bias in an amplifier's output is unacceptable (acceptable to some manufacturers but not good enough for the author.

The first reason given is that there will be power dissipated in the voice coil due to the 50 mV bias. This is true, but the amount of power dissipated into 8 ohms is 0.3 milliwatts, i.e. this is negligible.

The second reason is that this will bias the voice coil off center, so the maximum wattage will be decreased due to limited travel of the voice coil. Again this is true, but if the speaker is rated at 100 watts peak with no bias, 50 mV of bias will decrease the maximum wattage to 99.6 watts, again negligible.

My conclusion is that this guy is spreading fear, uncertainty and doubt so as to sell somethong.
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uncajesse
post Apr 19 2009, 11:55
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I agree. Fear mongering. Nicely articulated and backed up with the boring facts. wink.gif j/k


And btw - voice coil biasing used on purpose (which isn't what he's talking about) has some really awesome advantages... like being able to control excursion (and know exactly where the driver should physically be based on modeling) without the distortion of a servo driver... and also increasing the frequency response of the driver.

A father/son team from Florida (True Dimensional Sound) invented a technology that does just that, and it's now being licensed by a few companies, such as Harmon International (aka JBL) and available on the market.

Here's the patent for it...
http://www.google.com/patents?id=wpQGAAAAE...amp;as_maxy_is=

very very cool technology, imo.



(they also invented the technology behind several BBE circuits/algorithms)
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JFS
post Apr 21 2009, 23:08
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Thanks for replies.

So then, DC voltage leakage would not be something to be concerned with.

Thanks.
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Glenn Gundlach
post Apr 22 2009, 06:23
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QUOTE (JFS @ Apr 21 2009, 14:08) *
Thanks for replies.

So then, DC voltage leakage would not be something to be concerned with.

Thanks.


I used to repair consumer stereo WAY back and even then with the comparatively primitive gear, only once did I run into an amp with a DC offset problem. In that case it was a damaged transistor that behaved as if it had a low value resistor across the emitter-collector terminals. Took a curve trace to pin that one down. Bottom line, its's a very rare problem. I've bought several old Technics receivers on eBay that are working correctly and have no qualms about used gear.

G
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JFS
post Apr 22 2009, 17:15
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Can DC voltage damage the speakers, or does this just affect sound?

Thanks.

This post has been edited by JFS: Apr 22 2009, 17:15
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pdq
post Apr 22 2009, 17:27
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QUOTE (JFS @ Apr 22 2009, 12:15) *
Can DC voltage damage the speakers, or does this just affect sound?

Thanks.

High enough DC voltage can do both, but we are not talking about millivolts or even a volt or two. I once damaged (destroyed) a speaker, but I accidentally applied 28 volts to it.

The only way an amplifier can damage a speaker or affect the sound is if it is broken.
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