How tone controls function

[tjgarcia88]

Hello,
I'm in the process of building a few beginner circuits and am having trouble with tone controls in general. My question is about how a tone control works in the first place. My question is specifically related to the tone control in a TS-808. I know tone controls include resistors and capacitors but what role does each one play? Also, what's the significance of using polarized as opposed to np and vice versa? I'v seen some controls with a cap and resistor to ground going to the wiper (like the ts) and others with the signal going to the wiper then cap resistor combos on lug 1 and 3.... I think if I had the basics of how tone controls work, it would make my electronic tinkering life a lot easier now and definitely in the future! Thanks.

[Enzo]

The simplest tone control is like the one in a guitar. A cap and resistor in series, then placed across the output. Make the resistor variable and you can adjust the amount of effect.

Capacitors block DC and pass a signal or varying voltage. Polarized or not is just a matter of what the voltages ar all around the part. If one end is always a few volts more positive than the other, then a polarized cap might be selected. But If one end goes between more3 positive and more negative than the other, then a non-polar cap would be used. Electrolytics are generally polar, the non-polar ones are special, so they are labeled as such. All the film and ceramic and mica ones are natually non-polar.

Caps respond depending upon frequency. Without getting into math, an AC signal will pass through a cap down to some minimal frequency, below which the caps tends to block it. Small caps pass only high frequencies, while larger caps will pass lower ones also. High freqs are treble notes and low ones are bass notes. SO by chosing the cap value you can decide what the cap will pass along. Now a large cap will pass the lows that a small cap will not, but the large cap still passes the highs too.

Think of it this way. You have a bag full of little balls or beads or something, all various sizes. DUmp them into a tray. if you put a small hole in the middle of the tray and shake it around, only the small balls will fall through the hole into your lap. Make a larger hole, the small one still will fall through, but so will some slightly larger one. Make the hole even bigger and all but the largest balls will fall through.

This is useful to make a tone control. If I have a cap that only passes the highs, I can add it to my circuit and only the highs will come through. That would be like looking at the underside of the tray and seeing only small balls falling out through the hole. Now imagine I want to roll the balls across the top side of the tray. If I have the hole there, the small balls will fall through the hole and never make it across, while the larger ones will make it and pass along. SO if I put my cap from the signal path to ground, the frequencies the cap passes will be shunted to ground intead of allowed to pass along. That removes the highs from the signal, simply and easily.

Adding a resistor in series with the cap just makes it more difficult for the cap to pass something - it lessens the effect. Make the resistor variable and you can adjust the amount of effect the cap has.

When you place a resistor and a cap in series - we call that an RC circuit - it doesn;t matter wwhich order they are in. The cap selects the frequencies and the resistor sets the amount. SO on the simple tone control it doesn't matter if the cap is on the grounded end or the resistor is. SO using the control as a simple varaible resistor, you may see it wired to different legs of the control to make it more convenient to whoever is wiring it.

When you get into active EQ circuits, then the circuit is a lot more complex, but the basic frequency dependent reactance of the cap is still the basis for the function of the circuit.

[tjgarcia88]

so enzo,
you said that when you put a cap in the signal to ground it gets rid of the frequencies that that cap passes. this is a filter, no? Also, if i wanted to get rid of highs completely from a circuit, I'd use a low valued capacitor that passes high frequencies to ground correct? My question is how do i get rid of only low frequencies or those between? By your earlier explanation stating that higher value caps will still pass the small marbles but will as well pass the large, it seems that one wouldn't be able to get rid of only the middle sized marbles without also allowing the small and bigger ones to go too.... but I know this isn't true.

[Chuck H]

It might help you to download Duncan TSC (at the Duncan amps website). It's a free SPICE type program that has popular tonestacks programmed in. If you study those circuits and run some simulations, varying cap values, etc., it may help you learn some standard ways of managing bass and mid frequencies.

I will say that in order to manage mid frequencies using only caps you must sacrifice gain. Where a cap can manage a shelf for frequencies above it's knee, to ground for low pass or in series for high pass without loss of gain if you want to control mid independantly with only caps the circuits become more complicated and require more gain to operate and achieve the same unity.

Then there's inductors. Inductors will pass frequencies below their knee much like a cap passes frequencies above it's knee. Inductors are convenient at times but we don't use them much in guitar amps because their construction involves long lengths of wire that can act like an antennea in some amplifier circuits. Picking up noise, hum, radio signals, etc. But there are some uses for them. Active tone controls are one example of where inductors are especially useful so you will probably want to learn about them. Duncan TSC includes at least one tone circuit that uses inductors.

Chuck

[Enzo]

A cap will only do so much, and yes, that would be the simplest of filters. You want to roll off highs? Put a cap to ground. Want to roll off lows? Put a cap in series. Those would be basic low-pass and high-pass filters. Calculate the caps for the freqs you want to deal with. Want just mids? RUn the signal through a low pass filter set for the upper end of your band, then follow that with a high pass filter set for the lowr edge. With just passive components this is exactly what the crossover in a speaker box does.

Or in the marble filter, sort through one hole to eliminate the larger than desired marbles, leaving the size desired plus those too small, now use a second tray with a hole to filter out the too small ones. In the first tray we keep what falls through the holes, in the second we keep what stays in the tray. Just medium sizes left.

The cap as filter doesnt know where it is, if it goes to ground from the signal path it lops off the highs, if it is in series with the signal path it blocks the lows from continuing. All it knows is it is passing freqs higher than X frequency. You decide where to put the cap depending uipon if you want to send the highs to ground or to the next stage of the amp.


Now maybe you want to remove the mids and leave the highs and lows - scooped tone. Send your signal through a Y, and run one branch through a high-pass to cover the upper freqs, then the other branch is used to roll off everything over the bottom end of the blocked band. SO one branch has the highs and the other the lows, and the unwanted middles are gone. Now mix the two back together. Vi-ola.

IN practice you wouldn't likely do that with two caps, you'd make an active stage. But there are more complex circuits called notch filters which knock out a portion in th middle. And bandpass filters work the reverse.

[tjgarcia88]

On this schematic The Free Information Society - Tubescreamer TS808 Electronic Circuit Schematic. The tone control is between 5 and 6 to 7 via 1k ohms resistance. Why would the designer choose that particular place in the circuit and what is unique to he complies and order chosen? Is there another variation or placement that would work also? I'm building this circuit and everytime I get to the tone it just doesn't work right. There's a small increase in volume, and then between 8 and 10 a huge jump. Before this part in the circuit everything sounds fine. Also, what are some equations one would use to find out how much capacitance to use to filter or pass a certain frequency? And what kind of tools do you need to measure the appropriate variables ?

[Enzo]

OK, we discussed very basic simple passive tone controls above. Now you have added to the comoplexity by considering this circuit which is an active circuit. "Active" merely means there is an element of gain involved. The lesson to be taken from that is that caps pass AC or signal and are selective in terms of frequency. Pass means from one end of the cap to the other, not necesarily along through the amplifier. A cap may run signal to ground, but it still is "passing" the signal in doing so.


There is plenty of good published material on how op amps work, look it up. The tube screamer is based upon op amps. The gain or amplification of an op amp is determined by its feedback resistor and input resistor ratio. Others can discuss them better than I. IC1a is the gain of this thing, and R19 is the feedback resistor - which is variable to adjust the gain. Then from there once the overdrive has been created, it goes to the tone control stage. Why there? Because it makes more sense to adjust tone after the distortion is made.

What is unique? beats me, this looks utterly basic and generic to me.

[tjgarcia88]

Does the fact that my circuit is active change how the tone control would act then? And also, is there a basc or beginning formula that you could provide that would give me a general idea of how caps and resistance affect the frequency of an audio signal?

[Enzo]

yes, an active control can boost or cut, while the typical passive tone controls like a zillion Fender amps can only cut. The frequency dependency of the cap and resistor is the same, but like anything else, it is the circuit it is in, not the parts individually.

Download this refernce book, it has a lot of re3levent formulae.

Allied Electronics Data Handbook - 4th Ed 1965