Tone Control circuits

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There are two types of tone control circuits, passive and active. The passive kind consist of the pots, resistors and capacitors. There is an amplifier tube before the tone circuit and another tube amplifier after the tone circuit but there are no tubes directly involved in the tone control circuit itself. In the active kind there are amplifying devices involved in the tone control circuit. A circuit which looks the same as a passive circuit is placed in the feedback loop of an amplifier stage. This kind of circuit is common place in semiconductor based amplifiers because the amplifying devices take up little space and power. The very first tone controls to be designed into radios were simple passive treble cuts. The basic idea is given in the circuit below. An audio taper pot should be used to prevent all of the effect from being at one end. The pot is wired so the fully clockwise end is maximum resistance. This gives maximum treble. As the pot is turned counter clockwise the treble is cut. The values are for modern high fi speakers and ears. In a radio with a 4 inch speaker the effect would be barely noticeable. The given value of the capacitor would have to be increased by as much as a factor of 10 or more to get that extremely bassy sound that many old radios were famous for. The performance can be considerably improved by adding a 10 k ohm resistor in series with the capacitor and pot as shown below. In the previous version the treble just keeps rolling off as frequency increases. The additional resistor turns the circuit into a shelving control in which the amplitude rolls off to a point and then flattens out again. This circuit gives a true bass boost because the volume control does not need to be adjusted to keep the mid range and highs at a constant level. When the wiper of the pot is set to the bottom, fully counter clockwise, the capacitor is shorted out and the 470 k and 47 k resistors give an approximate 10 to 1 attenuation. As the pot wiper is moved upwards the capacitor is permitted to do its thing and the low frequencies are attenuated less. The boost is a respectable 15 dB. There is no bass cut in this circuit, it is boost only. This one gives about 14 dB of boost. Oddly enough it requires a linear pot rather than the audio taper of the other circuits. Also as shown the pot will work backward unless it is reverse wired. The capacitor can be connected from the wiper to the output terminal without any change in performance. That makes the pot work forward. Cut and Boost in one control. Let's begin with a circuit I used in my old home built tube stereo. Actually I ripped this circuit off from a Harman Kardon A-300. My university roommate had one and I copied the circuit from the instruction manual. The Phono preamp given a couple of articles ago is from the same amplifier. I copied the circuit by hand from the manual into my engineering notebook. Why I didn't take the manual to the nearest Xerox machine, I'll never know. I was coming up through the University of Florida's electrical engineering program as I worked on the design and construction of my tube stereo. The last parts to be constructed are the best. (I use the present tense because everything I built still exists). I made some mistakes early on but something I did get right is this tone control circuit. Thanks to Mr. Harman and Mr. Kardon. Well, since writing the above I ran some simulations on the circuit and maybe misters H and K did make a mistake or maybe they did it on purpose. When the tone controls are set flat there is a rise with a peak at about 200 cycles. This does give the amplifier a strong presents sound which many audio equipment reviewers in the late 50s and 60s liked. For myself I prefer to be in control of things. If I want a peak around 200 cycles I will push up that band on my equalizer. Here for the sake of completeness is the original H-K circuit. This circuit can easily become a bass boost by increasing the value of the capacitor. Because it "shelves", the mid range and treble are cut by the same amount leaving the bass up. As the pot is turned for more mid and high cut the volume must be turned up to give the boost in the bass band. visit page.

The very first tone controls to be designed into radios were simple passive treble cuts. The basic idea is given in the circuit below. An audio taper pot should be used to prevent all of the effect from being at one end. The pot is wired so the fully clockwise end is maximum resistance. This gives maximum treble. As the pot is turned counter clockwise the treble is cut. The values are for modern high fi speakers and ears. In a radio with a 4 inch speaker the effect would be barely noticeable. The given value of the capacitor would have to be increased by as much as a factor of 10 or more to get that extremely bassy sound that many old radios were famous for. The performance can be considerably improved by adding a 10 k ohm resistor in series with the capacitor and pot as shown below. In the previous version the treble just keeps rolling off as frequency increases. The additional resistor turns the circuit into a shelving control in which the amplitude rolls off to a point and then flattens out again. This circuit gives a true bass boost because the volume control does not need to be adjusted to keep the mid range and highs at a constant level. When the wiper of the pot is set to the bottom, fully counter clockwise, the capacitor is shorted out and the 470 k and 47 k resistors give an approximate 10 to 1 attenuation. As the pot wiper is moved upwards the capacitor is permitted to do its thing and the low frequencies are attenuated less. The boost is a respectable 15 dB. There is no bass cut in this circuit, it is boost only. This one gives about 14 dB of boost. Oddly enough it requires a linear pot rather than the audio taper of the other circuits. Also as shown the pot will work backward unless it is reverse wired. The capacitor can be connected from the wiper to the output terminal without any change in performance. That makes the pot work forward. Cut and Boost in one control. Let's begin with a circuit I used in my old home built tube stereo. Actually I ripped this circuit off from a Harman Kardon A-300. My university roommate had one and I copied the circuit from the instruction manual. The Phono preamp given a couple of articles ago is from the same amplifier. I copied the circuit by hand from the manual into my engineering notebook. Why I didn't take the manual to the nearest Xerox machine, I'll never know. I was coming up through the University of Florida's electrical engineering program as I worked on the design and construction of my tube stereo. The last parts to be constructed are the best. (I use the present tense because everything I built still exists). I made some mistakes early on but something I did get right is this tone control circuit. Thanks to Mr. Harman and Mr. Kardon. Well, since writing the above I ran some simulations on the circuit and maybe misters H and K did make a mistake or maybe they did it on purpose. When the tone controls are set flat there is a rise with a peak at about 200 cycles. This does give the amplifier a strong presents sound which many audio equipment reviewers in the late 50s and 60s liked. For myself I prefer to be in control of things. If I want a peak around 200 cycles I will push up that band on my equalizer. Here for the sake of completeness is the original H-K circuit. This circuit can easily become a bass boost by increasing the value of the capacitor. Because it `shelves`, the mid range and treble are cut by the same amount leaving the bass up. As the pot is turned for more mid and high cut the volume must be turned up to give the boost in the bass band. http://www.angelfire.com/electronic/funwithtubes/Amp-Tone.html

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