Blocking capacitor as a high pass filter?

NV&H · 14th February 2005, 10:23 PM

I would like to power a single speaker driver with an amp and need to block everything below say 100hz or so.

I'd like to build a power amp for this speaker whose design already includes blocking capacitors at its input terminals. Has anyone out there used blocking capacitors at an amps input as a 1st order high pass filters by changing the caps value?

I'd love to hear about it.

udip · 14th February 2005, 10:34 PM

Hi,

Almost every amplifier has an input capacitor, and this capacitor
together with the input resistor forms a first order HP filter.

To calculate the capcitor value you can use the formula:
freq = 1/(2*pi*R*C).

Hope it helps,

Udi.

NV&H · 14th February 2005, 11:14 PM

Would R in this equation not be the TOTAL resistance seen by the input device?

i.e., the resistance of whatever resistor that's been added to the input path plus the resistance of the first gain device?

Thanks!

udip · 14th February 2005, 11:38 PM

Yes

azira · 15th February 2005, 07:20 PM

Yes but I think you'd be more interested in the node that is tied to the input transistor's base/gate instead of the preamp's output. You should determine R and C based on the total resistance to GND and the total capacitanc to GND from this node. So your typically R would be the Rin of the BJT/FET in parallel with the base/gate biasing resistors. The typical C is the input blocking C. If you have a non-zero or non-incredibly small output resistance from your preamp then things get complicated but it's doubtful.

14th February 2005, 10:23 PM	#1
NV&H diyAudio Member Join Date: Feb 2005 Location: Charlotte, North Carolina	Blocking capacitor as a high pass filter? I would like to power a single speaker driver with an amp and need to block everything below say 100hz or so. I'd like to build a power amp for this speaker whose design already includes blocking capacitors at its input terminals. Has anyone out there used blocking capacitors at an amps input as a 1st order high pass filters by changing the caps value? I'd love to hear about it. __________________ NV&H

14th February 2005, 10:34 PM	#2
udip diyAudio Member Join Date: Oct 2004 Location: Israel	Hi, Almost every amplifier has an input capacitor, and this capacitor together with the input resistor forms a first order HP filter. To calculate the capcitor value you can use the formula: freq = 1/(2piR*C). Hope it helps, Udi. __________________ "Things should be as simple as possible, but no simpler" Albert Einstein

14th February 2005, 11:14 PM	#3
NV&H diyAudio Member Join Date: Feb 2005 Location: Charlotte, North Carolina	Would R in this equation not be the TOTAL resistance seen by the input device? i.e., the resistance of whatever resistor that's been added to the input path plus the resistance of the first gain device? Thanks! __________________ NV&H

14th February 2005, 11:38 PM	#4
udip diyAudio Member Join Date: Oct 2004 Location: Israel	Yes __________________ "Things should be as simple as possible, but no simpler" Albert Einstein

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15th February 2005, 07:20 PM	#5
azira diyAudio Member Join Date: Dec 2003 Location: Near Seattle	Yes but I think you'd be more interested in the node that is tied to the input transistor's base/gate instead of the preamp's output. You should determine R and C based on the total resistance to GND and the total capacitanc to GND from this node. So your typically R would be the Rin of the BJT/FET in parallel with the base/gate biasing resistors. The typical C is the input blocking C. If you have a non-zero or non-incredibly small output resistance from your preamp then things get complicated but it's doubtful.

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