DC offset in microphone output? Really? Why? - Page 1
Forum section: electronics / beginners

author: analogo
author_rank: Regular Contributor
timestamp: November 06, 2017, 08:10:06 am
content: In this video by GreatScotthe shows a simple amplifier circuit made with an opamp. What I do not understand is: why does he say that applying a 2.5v offset to the non-inverting input of the opamp will create an output offset of 2.5v around which the waveform will oscillate around? Direct link:https://youtu.be/ts-JqEVzvDo?t=6m59s.IIUC, the 2.5v input offset will _remove_ 2.5v from the inverting input, making a 0 -- 5v input voltage into a -Vrail -- +Vrail output waveform.And even if he is right and there is a 2.5v offset in the output, isn't that a bad thing? I thought that any DC offset in bad in an audio setting.

author: mikerj
author_rank: Super Contributor
timestamp: November 06, 2017, 08:52:17 am
content: The op-amp in question is running from a single supply rail, so the output can not swing below ground. In order to cope with an audio signal in this case you need to impose a DC offset, and for a rail-rail output amp running from 5v, you'd want this offset to be close to half the supply rail to maximise the output swing.Since the inverting input to the op-amp is AC coupled, the DC gain at the non-inverting input is 1. Therefore applying 2.5v to the non-inverting input gives 2.5v on the output.

author: ebclr
author_rank: Super Contributor
timestamp: November 06, 2017, 09:08:07 am
content: Nice sitehttps://www.elv.de/journal

author: tooki
author_rank: Super Contributor
timestamp: November 06, 2017, 12:06:53 pm
content: Quote from: mikerj on November 06, 2017, 08:52:17 amThat makes sense. But I thought that a DC offset at the output is bad (for the input of the computer or mixer, etc.)? Wouldn't you then want a DC-blocking capacitor at the output (after the op-amp, but before the signal reaches the input of the other device)?Quote from: ebclr on November 06, 2017, 09:08:07 amIt is (especially old issues), but it's only available in German, which doesn't help most people here, and what does this have to do with this thread?

author: rhb
author_rank: Super Contributor
timestamp: November 06, 2017, 04:13:54 pm
content: Quote from: tooki on November 06, 2017, 12:06:53 pmProbably. It depends upon what comes next. Lots of stuff is AC coupled, but some isn't. This can be a great source of confusion.

author: analogo
author_rank: Regular Contributor
timestamp: November 07, 2017, 08:21:30 am
content: Quote from: rhb on November 06, 2017, 04:13:54 pmWhat comes next is a standard PC audio chipset. Aren't there specs that tell you "The audio signal should swing between -X volts and +Y volts, the maximum tollerable DC offset is Z volts"?For example, the output of this amp is never going to go under 0, is this OK as an input for an audio card?

author: ogden
author_rank: Super Contributor
timestamp: November 07, 2017, 08:48:13 am
content: Quote from: analogo on November 07, 2017, 08:21:30 amSpecs of audio chipset indeed tell maximum signal swing, but they also say that decoupling of line-in using capacitors must be implemented. Try to google (pictures) "ac97 codec reference design circuit" and you will see - they all have AC decoupling. Yes - circuit shown in the video is not quite correct, but it will work with most PC's/mixers/amps, if not all. Only seemingly bad thing that can happen - jack detection may not work.