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Tone Controls, a use for the frequency response of capacitors.

Treble Cut Tone Control Circuit

Treble Cut Tone Control Circuit

“Daddy sang Bass and Momma sang Tenor”… Maybe today we don’t want to hear Daddy or maybe we only want to hear Daddy.  Tone controls are a nice thing on most stereos and radios and it is a good chance to put all the theory we have been talking about to good use.

The tone controls I am showing in the post are very simple, but it is a good baby-step into more complicated circuits.  At the end of this post I will provide some links to more complicated and more in-depth discussed designs.

Primarily, power circuits are inductor intensive. Motors and other electromechanical devices depend upon coils to generate magnetic fields to do work.   Electronics on the other hand are much more capacitor intensive, because capacitors are cheaper to manufacture and often more useful at the frequencies involved.  Power tends to be at low frequency and relatively low resistance is used in the circuits, so inductors  are used.   Electronics is designed to conserve power and not create heat, so much higher resistances and lower currents are used.  Again this favors capacitors for electronics and inductors in power circuits.

The two circuits talked about tonight are designed around a capacitor.   The circuit in the first picture is a type of tone control called a treble cut control.  It can only attenuate the treble frequencies and does not add extra gain to those signals.  Ro in that diagram can be an extra resistor or simply the input resistance of the next amplifier.

To understand how the circuit works, first imagine Radj and C do not exist.  At this point the circuit is a simple voltage divider.  It would be good at this point if Rin was much smaller than Ro so Rin would have very little effect.  “Much smaller” usually means 1/10 of the value of Ro or less.   Now add C and replace Radj with a wire.  Radj is essentially a wire if it is adjusted to the wiper of the potentiometer is at the top.  Remember, capacitors conduct very well at high frequencies and badly at low frequencies.   If Xc = Rin then the voltage out at that frequency would be much less than the voltage at frequencies below that frequency.   Temporarily neglecting  Ro the voltage out would be 1/√2 of the voltage in.   This is the 1/2 power frequency and is given the special name of the cutoff frequency.   Frequencies above this would be attenuated even more and would be “cutoff”.

I hand waved through all that very quickly because I really don’t want to bore you with all the math.  However, I probably do need to explain how I could say “neglecting Ro”.  At the cutoff frequency we said Xc = Rin and we already said Rin was much less than Ro, so we can neglect it for this quick “rule of thumb” calculation.   Now if we add Radj back into the circuit and the wiper somewhere below the top this resistor is in series with the capacitor and the capacitor will have less effect on the circuit.

A Bass Cut tone circuit.

A Bass Cut tone circuit.

Now we will look at a bass cut circuit.  To analyze this circuit first remove the Radj from the circuit.  Capacitor C will pass the high frequencies, but will attenuate the low frequencies. Again it is acting like a voltage divider.  Now if we add Radj back into the circuit and adjust it fully to the left we have bypassed the capacitor so all frequencies are passed.  As we increase Radj more and more the effects of the capacitor will be felt.

To calculate the cutoff frequency, assume Radj is much greater than Ro.  If Xc = Ro then we will have Vo = 0.707 X Vin and we have our cutoff point.  The big difference is this cuts off the low frequencies while the previous circuit cut off the high frequencies.  This is a “high pass” filter while the treble control was a “low pass” filter.

The cutoff point is also called the -3 db point.  I have not described the calculation of db yet.  Just for now remember that a 3 db attenuation  (- 3 db) is the 1/2 power point.  This is barely perceptible to the most highly trained ears.

Again this post was purposely designed to be easy reading without all the math.  if you want to see better circuits and the math associated with them I suggest you go to the following wikipedia article and follow some of the footnotes at the bottom of it.  Wikipedia tone control article.

If you do go to those articles and feel lost, please contact me via e-mail and I will see if I can be of help.   However, I think you know enough to be able to follow all the math and description at this point.

Gary

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The words “Daddy Sang Bass, Momma Sang Tenor” are from a Johnny Cash song of the same name.


 

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