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Simple High-Pass Filter

 
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Drone
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Joined: 18 Dec 2009
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Location: Uranus

PostPosted: Thu Jun 08, 2017 10:25 pm    Post subject: Simple High-Pass Filter Reply with quote

If I'm making a simple passive RC HPF, can I consider the input impedance of the next device as R or do I need to define R out of circuit?

This gets confusing as the input is an impedance not a DC resistance, but does it not affect the way the circuit works?
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ubertar
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Joined: 18 Feb 2004
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Location: Brooklyn, NY

PostPosted: Fri Jun 09, 2017 10:51 am    Post subject: Re: Simple High-Pass Filter Reply with quote

Why not use a pot (or trim pot) instead of a fixed resistor, then dial in what sounds best? You could treat Z as R to figure the mid-point of the pot (pot value = 2R) and tweak from there.
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ashcat_lt
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Joined: 04 Jan 2007
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Location: Duluth, MN

PostPosted: Sat Jun 10, 2017 5:22 pm    Post subject: Re: Simple High-Pass Filter Reply with quote

I think ideally you would choose R to be at most 1/10th of the load impedance so that impedance can be effectively ignored. You also want that resistor to be at least 10 times as big as the out-Z of the source.

Usually the input impedance of a device is specified somewhere in the middle of the audio band (like 1K is pretty standarde), but for most active devices we work with, it's actually pretty close to the same until you get out to the extremes. Like, the frequency-dependent component will be the coupling cap at the input - which will not start to get big enough to worry about until you get down below 20Hz - and by cable capacitance which doesn't get big small enough to matter until above 20KHz.

That's in general and for active devices though. Specifics matter. What do you intend to connect on either end of this?
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Last edited by ashcat_lt on Sat Jun 10, 2017 5:23 pm; edited 1 time in total
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ashcat_lt
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Joined: 04 Jan 2007
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PostPosted: Sat Jun 10, 2017 5:22 pm    Post subject: Re: Simple High-Pass Filter Reply with quote

sorry, double post
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Drone
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Joined: 18 Dec 2009
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Location: Uranus

PostPosted: Sat Jun 10, 2017 6:27 pm    Post subject: Re: Simple High-Pass Filter Reply with quote

It's between a mixer output, and a power amp input, so yes active, connected unbalanced, using 1/4" jacks.

Basically, I run my guitar/bass rig thru a mixer, and used to use two regular amps, one with the preamp bypassed, one using the preamp with the bass rolled off, now I want to use one stereo power amp, and I need something to effectively roll all the bass out of one side. So the crossover point isn't too specific. If I can put a cap and a resistor in a jack or a little box or something.
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ashcat_lt
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PostPosted: Sat Jun 17, 2017 4:27 pm    Post subject: Re: Simple High-Pass Filter Reply with quote

IF it's always going to the same input, then you don't even need the resistor. Spec the cap based on the input Z of the amplifier. It'll be plenty close enough.
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