coupling caps

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I'm Painting Again
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coupling caps

Post by I'm Painting Again » Sat Apr 01, 2017 4:57 am

is there an easy way to ID them on a schematic ?

this is in tangent to my racked audix 35102 channels thread if you need a reference on what problems I'm trying my hand at solving..

the circuit in question

http://www.twin-x.com/groupdiy/albums/u ... /35102.jpg

http://www.twin-x.com/groupdiy/albums/u ... amps~0.jpg

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Drone
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Post by Drone » Sat Apr 01, 2017 7:35 am

Coupling caps are the ones that join signal stages together. The signal passes through them.

De-coupling caps are the ones on the power line, or sometimes signal lines to ground, basically they couple the unwanted signal to death.

So in that diagram, things like C191 and C252 are coupling caps.
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Post by I'm Painting Again » Sat Apr 01, 2017 11:56 am

would they be anything outside of the blocks?

or only where two or more blocks are connected?

the problem is probably outside the EQ portion since it happens when the EQ is switched out

de-coupling ones filter the DC ?

what are examples of those in this diagram ?

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Post by The Scum » Sun Apr 02, 2017 1:31 pm

So it's a little more subtle than it seems you're thinking.

C191 and C252 are actually elements of the high pass filter...see the little synopsis circuits at the bottom of those sections.

But technically, AC-coupling caps are high pass filters themselves. DC is considered to be a frequency of 0 Hz. To keep DC from passing between stages of a circuit, the stages get capacitors between them, forming a series of highpass filters...typically just outside of the audio band...some designs might target 20 Hz, better designs will target 2 Hz or even lower.

Cuircuits from the era of that Audix design use a single 24V power rail...so the audio signal needs to ride on a ~12V bias, and that bias needs to be added on as signal enters a stage, and stripped off as it leaves.

A bunch of the components in those amp stages are there to create the bias voltage, stabilize it, or remove it when it's done. More modern circuits use bipolar power supplies, and keep the signal centered around 0V, taking fewer components because the bias coupling & decoupling isn't required.

So the coupling caps are usually at the input or output nodes of the amps, but it can take some careful analysis to identify every cap in the circuit that could be clobbering the low end...for instance, C13 and C14 are possible culprits, though they're nested inside the amp stage.
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Post by I'm Painting Again » Mon Apr 03, 2017 9:36 am

:hammer:

fascinating and frustrating to be learning about this stuff

thanks for pitching in to help out y'all

much appreciated

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