What value input filtering capacitor to use in your amp?

In most amplifier designs you will be tasked with choosing an input filtering(coupling) capacitor value. This is the capacitor that goes on the input stage of an amp and filters DC bias from the audio signal. It forms what is known as an “RC” (resistor capacitor) filter. Depending on the design you may have a resistor in series with the capacitor or the resistance part of the network may be internal to the input pins of the amp. If the latter is true, the internal resistance will usually be determined by the setting of the gain and found on the data sheet of the IC. This network creates a high pass filter and the value of the capacitor you choose(and resistor if applicable) determines what frequencies are fully passed from one side of the filter and into the amp.

With this basic high pass filter there will be a very gradual roll off. It’s not like multi-stage filters which can have a much more abrupt or steep drop off such as Chebychev, butters worth, or Bessel filters etc..(Topic of another article). The -3db point is what we call the “pole” frequency and will let that frequency pass at about 70% strength. Due to the gradual rolloff you will want to make sure this is set low enough that you don’t start to get attenuation at frequencies that are important to pass at full strength.

So there is a formula you can use to determine what capacitor value you need for your desired pole frequency in your basic RC filter, and it is

1

C= ———————————
Pi(2) x R x F

For those who want to see nice and neat logarithmic graphs and simulations I highly recommend using this filter tool to simulate http://sim.okawa-denshi.jp/en/CRhikeisan.htm You just input your values and it will output nice bode diagrams.


So what should you set this at?

Ultimately it depends what your goals are. For very large speaker drivers or subwoofers where you want to pass the full signal through without losing anything you may want to set the pole frequency at about 5hz or less. That is -3db at 5hz or less since due to the rolloff it will start to slowly drop off gradually at a higher frequency than this.

On the other hand if the amp channel you are using is for a tweeter you may opt to set this much higher so as to not let low frequencies through. Perhaps 1khz for instance.

Since small speakers often have little or no ability to reproduce certain low frequencies anyway, you may decide to cut them out so as to not waste amplification power. This is especially useful in portable or battery powered designs where efficiency is paramount. In such cases setting the f-3db at roughly 20hz may make sense since the rolloff will start gradually below 100hz.

All in all it depends what your goals are and what you are desiring to achieve with the overall design. For instance in some cases the small gradual rolloff the RC filter provides could be complimented with a DSP and work synergistically together very well to “fine tune” things.

In regards to what type of capacitor you should choose for this purpose since it is directly in the audio signal path, generally film type(polypropylene film being best) or class 1 dielectric ceramics (C0G, NP0, U2J) will provide the best quality and lowest distortion measurements possible. For more info on choosing capacitor types you can check out –https://givemebass.com/what-capacitor-type-you-should-use-in-your-design/

As an additional reference In regards to clear compiled distortion measurement data, I recommend sound articles from Cyril Bateman who used to design capacitors and work for linear technologies.

He built very specific measurement equipment and tested different types of dielectrics for audio coupling purposes and documented his findings. https://linearaudio.nl/cyril-batemans-capacitor-sound-articles

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