I'm looking for the simplest way to do this. I want use the stereo Aux outputs on my receiver to connect to another amp to power two subwoofers. Effectively I want to put a low pass filter on a 400mv 75ohm signal. What do I need?
First decide on the crossover frequency, and then find out the amplifier input impedance.
The filter will use a series resistor and a shunt capacitor for each channel.
Calculate their values based on:
series resistor ( in ohms ) = Rseries = 0.2 x amplifier input impedance ( in ohms )
shunt capacitor ( in farads ) = 1 / ( 6.28 x frequency x Rseries )
The filter will use a series resistor and a shunt capacitor for each channel.
Calculate their values based on:
series resistor ( in ohms ) = Rseries = 0.2 x amplifier input impedance ( in ohms )
shunt capacitor ( in farads ) = 1 / ( 6.28 x frequency x Rseries )
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Buy a cheap car audio active xo, power it off a wall wart. Could do it for under £20 delivered off ebay.
Or get a plate amp for sub. Good ones have high/low level inputs, variable crossover frequency, phase, left plus right sum...all you need right there.
According to the calculator, 0.47u
http://sim.okawa-denshi.jp/en/CRtool.php
Funny, I cross my sub at 10Hz with 4th order active...
http://sim.okawa-denshi.jp/en/CRtool.php
Funny, I cross my sub at 10Hz with 4th order active...
Always use a consistent system of units, like Ohms, Hz, Farads, to get a good answer.
The 6.28 comes from 2 x Pi ~ 2 x 3.14 ~ 6.28, and has no units, with Pi being a pure number equal to the ratio
of the circumference of a circle to its diameter. The 6.28 converts Hz to Radian, which is needed for the math.
Technically, for the filter calculation you should use a resistance equal to the two resistors in parallel
( 6.4k // 32k) for the proper time constant.
So actually the circuit operates as a filter with a resistance equal to 6.4k // 32k = 5330 Ohms.
This is not a physical resistor, but rather is the Thevenin equivalent resistance for the filter.
Then C (in Farads) = 1 / ( 6.28 x 60 Hertz x 5330 Ohms) = 0.498 x 10^-6 Farads = 0.498 µF
So use something close to that, and 0.47µF is a standard value.
The 6.28 comes from 2 x Pi ~ 2 x 3.14 ~ 6.28, and has no units, with Pi being a pure number equal to the ratio
of the circumference of a circle to its diameter. The 6.28 converts Hz to Radian, which is needed for the math.
Technically, for the filter calculation you should use a resistance equal to the two resistors in parallel
( 6.4k // 32k) for the proper time constant.
So actually the circuit operates as a filter with a resistance equal to 6.4k // 32k = 5330 Ohms.
This is not a physical resistor, but rather is the Thevenin equivalent resistance for the filter.
Then C (in Farads) = 1 / ( 6.28 x 60 Hertz x 5330 Ohms) = 0.498 x 10^-6 Farads = 0.498 µF
So use something close to that, and 0.47µF is a standard value.
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It's my mistake, I missed the -7 on the end to shift the decimal point. 😀 I do apologise..
Attachments
Back in the slide rule era, that's the main kind of mistake we always made,
being a factor of ten (or a million) off. The significant digits were ok, though.
being a factor of ten (or a million) off. The significant digits were ok, though.