polarized cap in audio circuit?

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panfilero - 09 Mar 2010 02:04 GMT

is it ok to use an electrolytic polarized cap to block dc signals in
an audio circuit?

Basically I have this kit

http://www.electronickits.com/kit/complete/meas/ck102.pdf

and I wanted to remove the DC offset at the outputs before sending the
signal to an audio amplifier

thank you

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krw@att.bizzzzzzzzzzzz - 09 Mar 2010 02:09 GMT

>is it ok to use an electrolytic polarized cap to block dc signals in
>an audio circuit?

Sure. Overrate the cap by at least 50% (16V in this case).

>Basically I have this kit
>
>http://www.electronickits.com/kit/complete/meas/ck102.pdf
>
>and I wanted to remove the DC offset at the outputs before sending the
>signal to an audio amplifier

>thank you

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pimpom - 09 Mar 2010 09:35 GMT

> is it ok to use an electrolytic polarized cap to block dc
> signals in
[quoted text clipped - 9 lines]
>
> thank you

Electrolytics are fine except where high fidelity and very low
leakages are important.

Just make sure you observe the correct polarity and that the
capacitance is adequate for the lowest frequency yuo want to use.
I suggest you put a resistor from the output side of the cap to
ground. Thius will prevent getting a loud 'thump' if you connect
the signal generator to the amp after both units are powered on.

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panfilero - 09 Mar 2010 16:06 GMT

> > is it ok to use an electrolytic polarized cap to block dc
> > signals in
[quoted text clipped - 18 lines]
> ground. Thius will prevent getting a loud 'thump' if you connect
> the signal generator to the amp after both units are powered on.

Thanks!

What would the correct polarity be in my case? Does the output get
the cathode or the anode?

I tried putting a resistor on the output to ground but it attenuates
my low frequencies... am I correct in thinking that putting a resistor
to ground on the ouotput would make a high pass filter and is
therefore killing my lower frequencies?

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pimpom - 09 Mar 2010 17:19 GMT

>>> is it ok to use an electrolytic polarized cap to block dc
>>> signals in
[quoted text clipped - 28 lines]
> get
> the cathode or the anode?

The anode of the capacitor should go to the output *from* the
kit, e.g. pin 7 of the opamp if you're going to use the sine
output. The cathode should go to the audio amplifier. The
"anti-thump" resistor should have one end connected to the cap's
cathode, and the other end to ground.

> I tried putting a resistor on the output to ground but it
> attenuates
> my low frequencies... am I correct in thinking that putting a
> resistor
> to ground on the ouotput would make a high pass filter and is
> therefore killing my lower frequencies?

Yes and no. The combination of output cap and resistor to ground
does make a high-pass filter. But if you choose a proper
combination of capacitance and resistance, the roll-off frequency
(the frequency where lower frequencies start to be noticeably
attenuated) can be placed well below the lowest frequency you
want to use.

For example, if we choose C = 4.7uF and R = 10k, the roll-off
freq is about 3.4Hz. The high-pass action will have only a small
attenuating effect at 5Hz, and negligible effect at higher
frequencies.

Note, however, that the 10k resistor is in parallel with the
input impedance of the amplifier. This lowers the effective
resistance and raises the roll-off frequency. If the amp has a
high input resistance compared to 10k, then the effect will be
minor. If the input impedance is comparable to or lower than 10k,
then it will have a significant effect on the roll-off frequency.

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Electronics Forum / Basics / March 2010

polarized cap in audio circuit?