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DC offset for audio circuits

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Joined: 07 May 2005
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PostPosted: Thu Jun 02, 2005 8:32 am    Post subject: DC offset for audio circuits Reply with quote

What is DC offset?

Why is DC offset Bad?

What is Good DC offset?

What is Bad DC offset?

AC riding on DC offset or DC?

DC offset applies to both upswing (positive) and downswing (negative) waveforms
what does this mean?
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Jean-Bernard Lemaire

PostPosted: Tue Jun 07, 2005 12:04 pm    Post subject: Reply with quote


The DC offset is a marginal voltage at the ouput of an electronic stage. In theory, there shouldn't be any, but in real world, parts have tolerances and drift. The problem is with nowadays electronics (mainly audio preamplifiers and amplifiers) that can "pass" DC voltage... and amplify it all along with the musical signal.

As a net effect in such a case, a loudspeaker bobin is continuously misplaced with this voltage, and heats a lot even if there is no music coming... In severe cases, the bobin will burn is seconds !

To prevent that, manufacturers use to place a small capacitor in series with the input (let pass AC, but blocks DC voltage). It is the most economical way to cure the problem, although a most costly one is to have better matched parts (difficult to achieve), or use a "servo" amplifier, that sense any DC imbalance between "O volt" rail and the real output dc voltage... If any umbalance, a contrary signal is fed to the input in order to restore a balance state of operations.

In a few words, a couple millivolts DC offset or so can be usually neglected if there is any input capacitor on the later stage input, but zero or 2 mV would be better...

As you can understand it, either a positive or a negative DC offset is BAD, because it shortens the signal excursion on the path the DC offset is And the smaller the audio signal is, the more noticeable and audible the effect will be.

Usually, you just have to short circuit the input of an amplifier and to look if there is any DC voltage at the output of the amplifier (between the output hot point (usually the "red" one), and the center of the capacitors bank...). Your mutilmeter should be very close to zero. Don't care for some mVolts here...

Hope this serves

Jean-Bernard Lemaire
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PostPosted: Sun Jul 10, 2005 6:56 am    Post subject: Reply with quote

A transistor takes time to turn on so if you play music into it you will miss the quitt sounds and the music will sound broken. You put a little offset voltage into the base of a transistor so it starts to turn on smoothly and you get your good sound back

if you look at the Hfe curve for your transistor, the start and end of the curve are different from the middle which is near a straight line. You want the audio transistor to operate in the straight line portion. You increase your offset, ot bias until the transistor is just in the straight line part.

happy listening.
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