[Russtafarian]

I’ve been experimenting with building simple in-line high pass filters that plug into my power amp to roll off the bass in my main speakers.  The formula for calculating the filter frequency is: F=1/(2PiRC).  Does it make any sonic difference whether one uses higher resistor values and small cap values or vice verse?

For example, say I want a filter frequency of 100Hz and my amp has an input impedance of 100K ohms.  I can get there by using a 19,000 ohm resistor and a 0.1 uF cap or by using a 160 ohm resistor and a 10 uF cap.

I’m inclined to use the larger resistor / smaller cap approach since the small physical size of a 0.1 uF poly cap can be sandwiched between an rca jack and plug to make an in-line filter.  A 10 uF poly cap would be too big to do this.

In this application would a smaller cap sound better and/or have less phase shift than a larger cap of the same type?  Is phase shift determined solely by the cap value or by the overall circuit?  Does the resistor value have a significant effect on sound quality?  Anything else I'm not considering here?

Thanks for your help.

Russ

[amplifierguru]

Hi Russ,

Yes there are issues.

You would do best to use the lowest impedance network that will not load your previous stage. Say it's 10K then use a 11K in parallel with the input 100K and a C that gives the corner frequency you want. The impedance at the input will be raised considerably (~ 5K) at the corner frequency and it will pick up stray hum and supply radiated hash accordingly to the raised input impedance, compared with being connected directly to a low impedance source. As the impedance of the C falls so to does the impedance at the input and the pickup.

In reality you want a good quality C at the input that will not smear the sound , maybe a 1uF WIMA polypropylene so set your R to corner with that.

Cheers,
Greg