Changing high pass filter Hz cutoff: better to adjust cap or resistor value?

[bipedal]

I'm tinkering with the high pass filter component values in a TS type of circuit and had a theory-ish question:

Starting from set values for the cap and resistor (for example, 0.1 uF --> 2.2K = 723.8 Hz cutoff), I know that either increasing the cap's value or the resistor's value will lower the filter' cutoff frequency.  If my calculations are right, one could lower the filter's cutoff to approximately 330 Hz by either increasing the cap value to 0.22 uF OR raising the resistor value to 4.8K.

Seems like changing either the cap or the resistor value "works" to change the Hz cutoff, but I'm curious if either path is technically preferable when setting a filter's cutoff.

(Or is it simply about which parts I have on hand?  )

Thanks in advance,

- Jay

[ayayay!]

The tricky thing you'll always find on the TS frequency cutoff isn't so much the 723Hz cutoff (or 330Hz or whatever), it's that the bigger the caps get the more bassy it sounds.   

The rolloff is the easy part.  It's keeping the butt from getting flabby that's tricky.  (<---sounds almost like Quagmire.  Giggidy giggidy giggidy)

[bipedal]

Thanks for your reply Jono -- I'll certainly be monitoring that cutoff value to ensure the sound doesn't get overly flabby or roomy in the lower frequencies as the gain increases...

(330 Hz isn't necessarily the target filter cutoff value for my build; that value was offered as a reference point example for my question.)

My question might drift into arcane/trivial territory, but I'm curious about the method for achieving a specified cutoff value in the filter network:
for example, would a filter consisting of a 0.22 uF cap + 2.2K resistor offer any advantage/disadvantage compared to a filter consisting of a 0.1 uF cap + 4.8K resistor?  (Or, insert alternate component values that would result in other Hz cutoffs.)

Is there a "best practice" on this from a circuit design perspective?  Or are they simply different routes to the same destination?

Best,

- Jay

[R.G.]

Since the rolloff point of an RC is the place where the impedance of the R and the C are the same, you are adjusting the impedance of the network at the rolloff point if you change the cap. This may not matter in the slightest to the R and C, but may make a difference to the source that drives it and the load that follows it. Those probably are what make the choice for you.

As you know, a 10K/1nF filter has the same rolloff frequency as a 1k/10nF, a 100R/100nF, a 10R/1uf and a 1r/10uF. But the impedance at rolloff is the resistor value in each case.

So you can drive a 10K/1nF easily from an opamp, and probably a 1k/10nF. From there on, it gets almost impossible to drive the network from an opamp (using an opamp as an example). You would also like to be able to amplify the result. A 10K/1nF filter should be loaded by no less than 100K input impedance, so while a JFET-input opamp will be fine, you have to do some good design work to get a bipolar transistor to be able to not load it down. However, a 1k/10nF filter can easily drive most bipolar input circuits.

Does that help?

[bipedal]

Since the rolloff point of an RC is the place where the impedance of the R and the C are the same, you are adjusting the impedance of the network at the rolloff point if you change the cap.

Ah, a good nugget of knowledge that hadn't occurred to me before. 

Thanks for setting me on the right track, R.G. -- your post definitely helps.

Best,

- Jay


**Prepares to refresh brain on impedance; points new browser tab to relevant AMZ Lab Notebook article.**