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Whats Wrong with the DC300A answer

Hi to all the terminally puzzled,

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THE *problem* with the Crown DC300A is simple, but virtually unknown in the industry.

Any amplifier that has response to DC is similarly questionable as being su itable or safe to use driving loudspeakers or transformers.

Such amplifiers produce a large DC component when driven into clipping with an unsymmetrical wave (eg speech or singing). Peaks of one polarity passin g undistorted while peaks of the other become clipped off = DC offset cre ated.

With a high powered audio amp, the DC component can reach *25 volts* or mor e with speech or music. Enough to bottom a woofer cone in its frame and pu sh the voice coil almost completely out of the magnetic gap. Bad news for t he speaker and sound quality - results in severe compression.

Most amplifiers avoid the issue simply by having a low frequency pole in th e feedback network, usually an 100uF or so electro cap to ground - so the a mp exhibits only unity gain at DC. Asymmetrical clipping then only causes a small sub sonic signal to appear that tracks the clipping. See this page by my colleague Rod Elliot, who carefully simulated the situa tion after I explained the problem to him 16 years ago.

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FYI: the above plays havoc with speaker muting systems whose drive circuits detect the DC component and open the relay.

.... Phil

Reply to
Phil Allison
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That was the first answer you received.

RL

Reply to
legg

the industry.

suitable or safe to use driving loudspeakers or transformers.

ith an unsymmetrical wave (eg speech or singing). Peaks of one polarity pas sing undistorted while peaks of the other become clipped off = DC offset created.

more with speech or music. Enough to bottom a woofer cone in its frame and push the voice coil almost completely out of the magnetic gap. Bad news fo r the speaker and sound quality - results in severe compression.

the feedback network, usually an 100uF or so electro cap to ground - so th e amp exhibits only unity gain at DC. Asymmetrical clipping then only cause s a small sub sonic signal to appear that tracks the clipping.

tuation after I explained the problem to him 16 years ago.

Not exactly. I referred to the DC coupling as causing DC to damage the spe akers, but I didn't consider that this could be the result of clipping an a symmetric signal. Although it seems to me even if the amp were AC coupled at the input or between various stages, if the output stage suffered hard c lipping from an asymmetric signal it could still drive DC into the speakers . This does not need to be a result of the entire amp being DC coupled, ju st the output stage.

--

  Rick C. 

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Reply to
Rick C

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peakers, but I didn't consider that this could be the result of clipping an asymmetric signal.

** Yep.

Although it seems to me even if the amp were AC coupled at the input or bet ween various stages, if the output stage suffered hard clipping from an asy mmetric signal it could still drive DC into the speakers. This does not ne ed to be a result of the entire amp being DC coupled, just the output stage .

** Nope.

Any low frequency pole inside the amp is enough to stop the effect.

Modern Class D amps without a DC servo loop have the same issue.

It's an elephant in the room kinda problem.

Win should put a chapter in his AofE book about it - and credit me please.

..... Phil

Reply to
Phil Allison

speakers, but I didn't consider that this could be the result of clipping an asymmetric signal.

etween various stages, if the output stage suffered hard clipping from an a symmetric signal it could still drive DC into the speakers. This does not need to be a result of the entire amp being DC coupled, just the output sta ge.

isn't potentially worse with some class-D?

with a half bridge on a split supply and the inductor in the out filter it can pump energy from one rail to the other, potentially increasing the v oltage to unsafe levels

Reply to
Lasse Langwadt Christensen

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