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Equations for this bass boost circuit?

Hey all;

I've designed the bass boost circuit below:

.047 uF

|| .----||--. | || | | | | ___ | o---___|-o---o | | | 80k | | | | | | |\\| | .-----|-\\ | | | >o--o----o In o---------------o|+/ | |/| | | | | .-. | | | | 3.9k '-' | | === GND

I started out using a Baxandall filter circuit, then left out the attenuation side since I only want to boost, tried a couple of other things, and came up with this as the best sounding circuit for my application. It works fine, and I understand why it's boosting the bass, but I'm curious to know how to figure out what frequency the boost begins to roll off at (or reach the -3db point), and at what Q it rolls off at. I had thought of using the equations for the Baxandall filter, but the circuit is different enough (most notably lacking a series resistor) that I'm not sure how to go about it.

Can anyone suggest an equation?

Thanks

(created by AACircuit v1.28.6 beta 04/19/05

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Reply to
tempus fugit
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Since the opamp is connected in a non inverting configuration, the amplifier has a minimum gain of 1, even at frequencies so high that the feedback capacitor is essentially a short circuit across the feedback resistor.

Once the frequency falls enough that the feedback divider has a 1:1 division ratio, the gain rises to 2, and climbs in proportion to the period (inverse to frequency, till the capacitor impedance is about the same as the resistance of the feedback resistor.

Below that the gain is fixed at (1 + 80k/3.9k)=21.5

So the frequency below which the gain starts to rise is approximately where Xc=3.9k. From Xc=1/(2*pi*f*C), with Xc = 3.9 k and C=0.047 uF, f=868 Hz. And the gain stops increasing (actually increases a lot more slowly) below about where Xc = 80k, or 42 Hz.

You could simulate this amplifier with LTspice and graph the frequency response with its AC analysis.

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Reply to
John Popelish

It's a high frequency cut circuit actually.

Graham

Reply to
Eeyore

Thanks for that John. You should know that this project was done at your suggestion - a while back, I was making a buffer/preamp for my piezo guitar pickup, and you suggested I make it with a slight bass boost. At the time I barely had time to get the preamp done let alone add the bass boost, but I did get around to it last week, and it sounds much more natural now.

Thanks again

Reply to
tempus fugit

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