Large I/O cap values?

[the_floyd]

Looking at the schematic for the black Eternity, I can see it has an input cap of 56nf and an output cap of 22uf. The response of the respective RC filters extend well below the guitar range, even audio range - so what is the purpose of using such large values? Is there another principle at work here?

Help would be much appreciated.

[amptramp]

I always use a low frequency turnover for an input stage because the input noise is determined by the resistance to ground in parallel with the resistance of the device that is driving it.  But this works where the capacitive reactance is is low compared to the resistance, meaning the turnover should be set for a very low frequency.  In general, if you are trying to control the frequency response, this should be done within the pedal between active stages where you have complete control over the resistance seen by the frequency determining components via isolation from the input and output.  A low frequency turnover also limits the effect of preceding stages in determining frequency response.

The same consideration applies to the output, but noise is usually not a factor here.  In this case, a low input impedance in the subsequent item in the signal chain will raise the turnover, so it is best to keep the turnover frequency low in order to get consistent performance with any subsequent stage.  Also, electrolytic capacitors tend to lose capacitance over time, so what starts out as adequate response may deteriorate with age, with the low frequency turnover rising with age.

An individual pedal may seem to have a relatively wide frequency response, but if you are using ten of them in series and they each lose 0.5 db at the minimum frequency, you have an overall loss of 5 db which is noticeable.

[amptramp]

I always use a low frequency turnover for an input stage because the input noise is determined by the resistance to ground in parallel with the resistance of the device that is driving it.  But this works where the capacitive reactance is is low compared to the resistance, meaning the turnover should be set for a very low frequency.  In general, if you are trying to control the frequency response, this should be done within the pedal between active stages where you have complete control over the resistance seen by the frequency determining components via isolation from the input and output.  A low frequency turnover also limits the effect of preceding stages in determining frequency response.

The same consideration applies to the output, but noise is usually not a factor here.  In this case, a low input impedance in the subsequent item in the signal chain will raise the turnover, so it is best to keep the turnover frequency low in order to get consistent performance with any subsequent stage.  Also, electrolytic capacitors tend to lose capacitance over time, so what starts out as adequate response may deteriorate with age, with the low frequency turnover rising with age.

An individual pedal may seem to have a relatively wide frequency response, but if you are using ten of them in series and they each lose 0.5 db at the minimum frequency, you have an overall loss of 5 db which is noticeable.