Quote from: amptramp on October 26, 2024, 12:28:25 PMThis is a bit more complicated than just saying you want full audio bandwidth.  If you are making a device for general purpose use, whether it is used alone or with a lot of other stompboxes, you may want full bandwidth and maybe less attenuation at 20 KHz than 0.1 db because they are all going to have some attenuation and if you have a number of tham in series, the attenuation at 20 KHz may be more than 0.1 db.  If you are using this device alone, you may want to cut a lot of guitar energy above 5 KHz.  When Phil Collins put out the "No Jacket Requied" album, everything above 5 KHz was gone.  You don't usually need high frequencies from a guitar.

You wouldn't want to low-pass every stage of a multi-stage device at 5kHz either.   There is a cumulative effect of all the low-pass filters.

These are design choices.  One design choice is not to add any more roll-off.  In the thread I linked earlier bluelagoon wanted to make exactly that choice,
https://www.diystompboxes.com/smfforum/index.php?topic=130663.0

Besides, the point of my example was to show how high impedances promote a stability problem when stray capacitance are considered.   From a stability point of view a 1M pot and a 5pF feedback cap isn't the same as a 100k pot and 50pF cap, even though the cut-off is the same because the stray capacitance part is fixed.

Quote from: Rob Strand on October 26, 2024, 05:19:25 PM

Quote from: amptramp on October 26, 2024, 12:28:25 PMThis is a bit more complicated than just saying you want full audio bandwidth.  If you are making a device for general purpose use, whether it is used alone or with a lot of other stompboxes, you may want full bandwidth and maybe less attenuation at 20 KHz than 0.1 db because they are all going to have some attenuation and if you have a number of them in series, the attenuation at 20 KHz may be more than 0.1 db.

If you are using this device alone, you may want to cut a lot of guitar energy above 5 KHz.  When Phil Collins put out the "No Jacket Requied" album, everything above 5 KHz was gone.  You don't usually need high frequencies from a guitar.

You wouldn't want to low-pass every stage of a multi-stage device at 5kHz either.   There is a cumulative effect of all the low-pass filters.

Yes, the number of db that is cut at the high frequencies add up so that a cut of 0.1 db in each of five stompboxes would result in a cut of 0.5 db at the end of the chain.  If you are only using one stompbox, you can afford that, but if there are more, you might not be able to.

QuoteThese are design choices.  One design choice is not to add any more roll-off.  In the thread I linked earlier bluelagoon wanted to make exactly that choice,
https://www.diystompboxes.com/smfforum/index.php?topic=130663.0

Besides, the point of my example was to show how high impedances promote a stability problem when stray capacitance are considered.   From a stability point of view a 1M pot and a 5pF feedback cap isn't the same as a 100k pot and 50pF cap, even though the cut-off is the same because the stray capacitance part is fixed.

There is also a cumulative effect of highpass filters caused by coupling caps and although devices like the Brian May treble booster exist and have a use, you have to consider noise performance.  The bias resistor of the input stage may have some thermal noise but this is usually swamped by the output resistance of the previous stage.  But if your turnover frequency is too high, there will be noise from each input bias resistor below that frequency because there isn't enough coupling to the previous low-resistance output to limit the noise.  The noise is calculated from the parallel resistance.

If we're talking about electric guitar, 10dB attenuation at 20kHz may be inconsequential  (as long as 10kHz is relatively untouched...).

Thanks for the help everyone!