Audio Tone Controls

I spotted on Amazon an audio tone control circuit with a "mid tone" adjustment setting included. I have often wondered how audio tone controls worked, so I gave it a run on the calculator and came up with the following conclusions.

I found the above circuit within an overall circuit on the web, and began to do some sums.

Starting from basic, consider the below circuit for a bass and treble tone control application circuit.

Let's assume that the "Bass cap" has a capacitive reactance of 1000 ohms at 1KHz, and the "Bass vari" is a 1K ohm variable resistor pot. With the "Bass vari" wiper centre pin tap of the variable resistor, positioned at the top end, the audio 1KHz signal would by-pass the "Bass cap" reactance, thus the audio 1KHz signal would remain un-effected. Now should the centre pin wiper then travel down the variable resistor towards the ground pin, the audio 1KHz signal would be subjected to an attenuation. The signal reduction would be proportional to the wiper position as would it be for any audio volume control implentation circuit. At the midway point, the 1KHz signal would be perhaps half volume, or 6dB down. Now a 100Hz audio signal would have perhaps a 10KHz reactance, as the 100Hz signal is a tenth of the 1KHz signal in frequency. Likewise, the 100Hz signal would be by-pass the "Bass cap" if the "Bass vari" is at the top end, and subsequently the 100Hz signal would be reduced as the "Bass vari" wiper centre pin travels towards ground. Essentially then, the audio frequency cut-off point for the bass tone control would be some where between the 1KHz point to the 100Hz point on the audio spectrum, the cut-off point being to the listeners preference, according to the wiper position of the "Bass vari" position. To that end, the left hand side of the circuit is in essence a "bass cut" tone control, based around a series impedance.

The right hand side of the tone control overall circuit is for the treble adjustment of the audio signal. The treble circuit is based around a signal shunt, that being the audio adjustment now reduces the top end frequency response of the overall circuit to an audio signal. Should the "treble cap" have an capacitive reactance to a 1KHz signal of 1000 ohms, and the "treble vari" is a 1Kohm variable resistor, with the "treble vari" wiper centre point positioned to the top end, then the loading of the 1kHz signal to ground would be 2kohms, after the 1kohm series impedance to the audio signal by the "Bass cap". Should the "treble vari" travel to the bottom end, then the "treble cap" 1Kohm reactance would show a 1kohm series with a 1Kohm shunt, then reducing the 1KHz signal in half or by 6dB down. If the "treble vari " is at the top end position, the a 10KHz signal would have a 100ohms reactance with the "treble cap", the variable resistor "treble vari" then tapering the amount of the 10KHz signal to ground, thus in effect providing again a variable cut-off point for the treble tone control circuit, thus the right hand side circuit a "treble cut".

Using the above equation for the determining the cut-off points, I have arrived at the below circuit for ham radio use.

The circuit functions with its cut-off frequency by virtue of the 50K ohm variable resistor position. The lower and top end cut-off points are listed on the circuit diagram. For some strange reason, the lower components are a 1/10th for resistors, and a 10 times for capacitors. While the resistor and capacitors cut-off point is the same, the lower resistance would mean a lower resistance to ground for the potential divider effect of the volume control principle for the bass and treble tone cut-off point settings.

For a "mid tone" control inclusion, the following circuit could be used.

Do remember that the calculations are just so we say theory, so a bit of twiddling may need to be done. Some tone control designs use a 100K ohm variable resistor, but I have found that any resistor pot above 50K ohms, will start to hum at the mains supply frequency, ( 50Hz in the UK ), and so I used a 47K ohm resistor pot for a volume control with my Bitx40.

The above 3 section tone control, is usually as I have found out is placed on an amplifier feedback circuit. In such cases, I suggest that the tone controls are positioned from left to right, that is bass control on the left, mid tone in the centre with the treble tone control on the right, when installed in a box container. You may perhaps also find that the variable adjustment might find work upside down, that is the lower 96Hz cut may provide a boots, and the 960Hz setting a cut. If this does happen, swap around the two terminal end wires of the variable resistor pot, so as one rotates the control clockwise, the cut goes up in frequency, and as rotates anti-clockwise, the cut adjustment goes down in frequency. The same for the treble tone control adjustment, while the mid tone control would increase its mid tone amplifying effect in the clockwise direction, and decrease the effect in the anti-clockwise direction.

Now two such circuits could be used, one for the microphone, and the other circuit for the speaker or headphones. This would mean two banks of controls, but it would mean that one could set the microphone audio as desired, and also the received audio as required individually, adjustable for each QSO contact when needed.

If have done some more research into designing an audio tone control circuit, and I think I may have now arrived at a design plan with known boost and cut signal gains.

An input circuit buffer op-amp will need to be used in order to provide a low impedance microphone interface to the above active tone control so as to not upset the boost and cut figures. The microphone buffer op-amp will of course provide the required impedance interface to the microphone as well as the electro mic D.C. bias, and A.C. coupling to the above circuit. An output volume control into the radio mic input may be required. Please do not forget the R.F. filtering on the circuit input at the buffer op-amp to isolate any R.F. interference pick-up. If the above current drain is low enough, the D.C. bias voltage from a radio set microphone input that would normally bias the electro mic, could be used to supply the voltage supply to the above active tone control circuit. According to the FT450d circuit diagram manual, the 8Volt bias D.C. supply to the electro mic, is fed from a 8Volt 1 Amp D.C. regulator. This info gave me the idea that the mic bias voltage could be used, as too an external D.C. voltage supply. With the 4 pole socket to twin two pole jack plugs, the D.C. bias voltage for the electro microphone may well be present to use, with the P.C. to RJ45 adaptor cable. A 4 pole socket for the above circuit, also holding the connection for the earphones part also, would then make a direct connection for P.C. microphone headset possible with a 4 pole socket circuit connection. The output of the complete circuit would thus be two 2 pole jacks, one for the headphone the other for the microphone, connecting to the P.C. to RJ45 adaptor cable.

Looking further into the web pages, I found another way to preset or vary the centre tuning point of each audio equaliser band pass. The below circuit some may find interesting, I have calculated some values which may be perhaps useful. If the values are correct, a +/- 15dBV boost or cut will be found, or +/- 6 times signal gain or reduction.

The table below illustrates the 33K ohm variable resistor has on the tuning centre spot frequency.