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OFFSET NULL ADJUSTMENT


walid
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Guest SM2GXN

Hi walid!

Let's say you want to amplify a sine wave using the 741 and this so called dc offset makes the output going minus or plus asuming dual supply feeding the 741 with plus/Vcc and minus/Vee.
If the ouput goes to much positive clipping of positive part of the sinus will occur and for negative going output the negative part of the sinus will be clipped.
The headroom is not used effectively if the output isn't zero, lets say that you feed your 741 with +/- 10 volts theoretically you got 20 volts headroom without clipping any portion of the waveform but this assume that the output is zero.
The purpose of the dcoffset in this circuit is to adjust the output to zero volts dc.
Todays audio amplifiers usually don't have any dcoffset adjustment.
Hopefully this will be clarified further by some of the other members of the forum having deeper knowledge in dcoffset adjustment.

Bjorn

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Hi Bjorn,
Audio amplifiers without an offset adjustment use a balanced differential input with an input offset voltage of a very small amount. They use a capacitor on their negative feedback to ground voltage divider so that the amplifier has a gain of only 1 at DC, and the input offset voltage isn't amplified.

Audio amps don't care about the efect that a DC offset has on clipping headroom because they are usually operated at levels far below clipping. The main concern for having a high DC offset is that it causes a speaker's voice coil to be away from its linear center position, creating distortion. ;D

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Guest SM2GXN

Hi audioguru!

Thank's for clarifying some of the mysterys about dc-offset :)
I repaired a Samson 17" rack mounted stereo amplifier where one channel was dead, one of the chipamps STK**** got some of the legs bent off close to the body. I bought a new one soldered it back but the dc-protection said no no!! The output of the chip was several volts minus. Started to measure arround and found that the trace for the capacitor in the feedback was routed acrosss the legs of the capacitor, in other words the capacitor was effectively shorted. Opened the track with a knife and the output went down to no more than 20mV.
Afterwards I've been thinking, did they do this on purpose :)
I think I understand that the dc-offset does not play to big role when it comes to the headroom of the amplifier, as you say they are usually operated at levels far below clipping. A hot or burned voice coil unless the amplifier has got an capacitor on the ouput and voice coil moved either in or out from its center position, right?
What about the dc-offset for the 741, what i wrote does it make sense?

Bjorn


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Hi Bjorn,
A 741 could have an input offset voltage as high as 10mV. If it is used as a DC amplifier with a gain of 1000 for a sensor or something then its output voltage could be 10V without a signal. You could null the offset voltage with a pot or you could replace the old 741 with a modern laser-trimmed opamp having a very low offset voltage.

I don't think about a volt of offset voltage across an 8 ohm speaker's voice coil will burn it. It is only 1/8W and even a fragile tweeter wouldn't be hurt. But it certainly moves the voice coil away from its linear center position. Touch a new D-cell 1.5V battery across a big speaker and see how far it moves. ;D

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Guest SM2GXN

Hi audioguru!

I wasn't thinking of an op-amp when I talked about burned voice coil  ;D I meant a high power amplifier, how could you know I never told you :)
Got to concentrate more on my writing I'm like the wind suddenly changing direction without telling where  ;D

Bjorn

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