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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

electrolytic coupling cap - polarity
electrolytic coupling cap - polarity
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Old 26th May 2019, 08:11 PM   #1
emosms is offline emosms  Bulgaria
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Default electrolytic coupling cap - polarity

Very lame question..

We have a bipolar supply Opamp module.
The signal through the opamp is audio signal.
The opamp output signal is AC, both negative and positive in regards with the ground.

Could I use electrolytic cap at the output?
If it is 50-100% more voltage rated than the signal peaks?

f.ex. R.C4() on the circuit diagram.

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Old 26th May 2019, 08:17 PM   #2
MarcelvdG is online now MarcelvdG  Netherlands
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Normal polarized aluminium electrolytic capacitors work at reverse voltages up to 1.5 V ... 2 V, so if you are sure the reverse voltage across the capacitor (not to be confused with the output voltage) will stay below that, you can use a normal electrolytic capacitor. In fact that's done quite often in consumer electronics.
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Old 26th May 2019, 08:32 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by emosms View Post
Could I use electrolytic cap at the output?
Not at all lame, use a bipolar (non-polar) type, rated at more than the supply (25V or higher).
They also have lower distortion than a polar type.

Add a large (100k or so) resistor from the output to ground to avoid noises when connecting.
https://hfc-fs.s3-eu-west-1.amazonaw.../nich_es_0.pdf

Last edited by rayma; 26th May 2019 at 09:01 PM.
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Old 26th May 2019, 08:33 PM   #4
sSound is offline sSound
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You can use 2 back to back electrolytics to make a non polar one. Althou the cap in your schematic is very low to be an electrolytic. Whenever use polypropilene if possible
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Old 26th May 2019, 08:45 PM   #5
emosms is offline emosms  Bulgaria
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What is the difference between:
- the voltage across the cap and
- the voltage at the output of opamp (lets use the circuit as a ref.)?

Isn't it the same ?

------------

I am literally putting up an Instrumental amp (bass guitar) out of diffetent modules.
The signal peaks could easily be over 2 volts.
There is an EQ module which could amplify certain frequencies even more (+12 db...).

Moreover, I will try to keep the line signal as high as possible to get better signal to noise ratio.
The supply voltage is +-15.

At the very end, just before the power amp, I will attenuate/ limit to avoid cliping and protect the speaker.
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Old 26th May 2019, 08:59 PM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by emosms View Post
What is the difference between:
- the voltage across the cap and
- the voltage at the output of opamp (lets use the circuit as a ref.)?
Isn't it the same ?
The DC voltage across the capacitor is 0VDC, if the op amp has a bipolar power supply.
The DC voltage across the capacitor is half the power supply, if the op amp has a single polarity power supply.

The AC voltage across the capacitor is 0VAC (within the design bandwidth).

The DC voltage at the output of the op amp wrt ground is 0VDC, if the op amp has a bipolar power supply.
The DC voltage at the output of the op amp wrt ground is half the power supply voltage, if the op amp
has a single polarity power supply.

The AC voltage at the output of the op amp wrt ground is the input voltage times the gain.
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Old 26th May 2019, 09:09 PM   #7
MarcelvdG is online now MarcelvdG  Netherlands
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If the signal frequency is greater than the corner frequency of the capacitor with the load resistance, most of the signal will drop across the load rather than the coupling capacitor.

Example: 10 uF coupling cap, 100 kohm from output to ground, next stage has 10 kohm input resistance:

Total resistance 10 kohm in parallel with 100 kohm -> 9.090909... kohm

Corner frequency 1/(2 pi R C) ~= 1.75 Hz

Example signal frequency: 20 Hz -> only about 8.72 % drops across the capacitor.

For professional use, I would advise you to use a capacitor that can handle at least 52 V, either a bipolar capacitor or a polarized capacitor with its positive side connected to the output connector. The capacitor then sort of protects your circuit if someone should accidentally connect the output to a microphone input with phantom supply.
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Old 26th May 2019, 09:48 PM   #8
ticknpop is offline ticknpop  Canada
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electrolytic coupling cap - polarity
Listen to Rayma hes right
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Old 27th May 2019, 05:42 AM   #9
emosms is offline emosms  Bulgaria
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Thank you for all replies and the corner frequency/voltage across cap calc !!!

Anyway, even if I could (eventually) use a single polar cap, it behaves differently when subjected to reverse voltage - there is a different current flow.
So, the distortions would be significantly more.

I will use bipolar electrolytics or a pair of reverse polarity in series.

---

Still another question.
In general, when we have dual rail opamp, the DC offset at the output should be 0 volts.
In reality, there would be some DC offset at the output.

Most of the coupling cap slots left on the PCB's (as in the circuit above), are to DE-couple the opamp stages from one another.

Should I worry about the opamp's DC offset at the output (dual rail supply) ?

What if we have, let's say 1 mV DC offset at the output?
- If next stage is unity gain (lots of opamps there are just buffers)
- If the next stage has some gain? (If the DC offset at the input is a problem and if the problem is dependent on the voltage gain).

In both cases I am not concern about adding up DC offset - it might be filtered out at just one place.
I am concerned about S/N ratio.
The DC offset would cause some current flow - f.ex. through the input to ground resistors.
Wouldn't there be a current flow noise? Or thermal/other noise?
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Old 27th May 2019, 08:29 AM   #10
Zung is offline Zung  Switzerland
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Quote:
Originally Posted by emosms View Post
... Should I worry about the opamp's DC offset at the output (dual rail supply) ?
...
Yes, that's why you should use a non-polar cap. I prefer 10uF polypropylene, but the Nichicon Muse could be an alternative. Or you can use a servo to null out the offset.
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