are output decoupling caps really necessary in symmetrically powered amps? - diyAudio
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Old 23rd February 2013, 03:46 PM   #1
tsiros is offline tsiros  Greece
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Default are output decoupling caps really necessary in symmetrically powered amps?

if the amp is using symmetric power supply, the output right at the opamp is idling at 0 V. Or, maybe, a couple tens of mV. So i don't see the reason (i suspect there might be).

i tried reading up on it, but i haven't got a satisfactory explanation.

to be honest, i didn't search the forum, mostly because i don't know how to create a query for such a specific question
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Old 23rd February 2013, 03:55 PM   #2
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For one thing you usually need them for stability reasons.
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Old 23rd February 2013, 04:41 PM   #3
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When you use multiple stages of opamps for additional gain, a tiny offset at the output of the first opamp will be amplified. This will result in a much greater offset voltage in later stages. A coupling capacitor will prevent the offset from passing to the next gain stage.
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Old 23rd February 2013, 05:34 PM   #4
blu_glo is online now blu_glo  England
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If the op-amp stage is unity gain you don't strictly need coupling caps on the output. However, I always prefer to include them regardless. Also I would not leave coupling caps both on INPUT and OUTPUT of a volume control. POTs are not very forgiving of DC flowing through them, and scratchyness results and is worse without.

However input switching they are recommeded along with bleeder resistors each side to prevent switching thumps with different devices/DC offsets connected.
Also the feedback capacitor (normally to ground/0V in a noninverting configuration) should not be missed out as if it is, the input offset is then multiplied by the gain of the amp. on a power amp this can lead to significant constant DC offset into the speaker especially when using higher gains (x30 or more)

Last edited by blu_glo; 23rd February 2013 at 05:37 PM.
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Old 23rd February 2013, 06:46 PM   #5
tsiros is offline tsiros  Greece
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looking here,

Click the image to open in full size.

i see that national's schematic does not use output decoupling caps. the pcb i got, from hifidiy, has some caps on the output, but i am not sure what they are for.

Also, not exactly related: i am thinking of inserting relays, one for each speaker, that connects the speakers after the amp has been powered ( 1 s delay, or something like that) . Is it enough? I think it is, i'm just looking for the OK from you guys...
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Old 23rd February 2013, 07:00 PM   #6
Mooly is offline Mooly  United Kingdom
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Are you confusing coupling caps and de-coupling caps and zobel networks (the small cap usually around 0.1uf and in series with a 10 ohm) that connects from amplifier output to ground and which is used for stability.
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Old 23rd February 2013, 07:08 PM   #7
tsiros is offline tsiros  Greece
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was that a question.
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Old 23rd February 2013, 07:42 PM   #8
blu_glo is online now blu_glo  England
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In the schematic there appears to be an omission - there should be 20K resistor from both op-amp +inputs to ground, these along with the presence of Ci1 and Ci2 will keep output offset to minimum
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Old 24th February 2013, 06:17 AM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by tsiros View Post
was that a question.
In a way Decoupling (the thread tiltle) refers to caps on and across supply rails. Coupling refers to caps that connect different stages together. They carry the signal between stages. The zobel network is the series C and R that is often fitted at a power amp output to maintain stability (and that seemed the "best fit" to how you phrased the original question).


Quote:
Originally Posted by blu_glo View Post
In the schematic there appears to be an omission - there should be 20K resistor from both op-amp +inputs to ground, these along with the presence of Ci1 and Ci2 will keep output offset to minimum
There seem to be many omissions. It looks more like a basic diagram rather than a fully worked example.
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Old 24th February 2013, 07:05 AM   #10
blu_glo is online now blu_glo  England
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Look at the TI datasheet....

http://www.ti.com/lit/ds/symlink/lm4780.pdf

P. 22 "Reference PCB schematic" shows the circuit I'd expect to use of such an IC. I'd still recommend that Rin1 and 2 (shown as 15K) should actually be = Rf (20K)

In response to your earlier question about output relays, if its just switch on and switch off thumps are you're worried about; they recommend a soft start circuit P.15.
If that's not good enough or you think you are going to use and abuse the gainclone probably better to get a seperate speaker protection PCB with built-in delay.

Last edited by blu_glo; 24th February 2013 at 07:14 AM.
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