DC-operated=20 Volume control

Abstract

Most Steel Guitar and Guitar players are aware of the problem: When = they back=20 off on the volume using either a foot pedal or the knob on their = guitars, the=20 tone becomes more muffled and lacks presence or brilliance. This is = because the=20 capacitance of the guitar cable and the self-inductance of the pickup = coil, in=20 combination with the resistance of the volume knobs, becomes a =93low = pass filter=94=20 where high frequencies are attenuated. As the volume is decreased the = pass=20 frequency is lowered, so that more of the high frequencies are lost (see = the=20 little appendix below).

This note describes a solution to the problem. Instead of putting a = resistor=20 in the signal line, this circuit uses a voltage controlled attenuator = whose=20 frequency response is not altered by the gain setting. The degree of = attenuation=20 (or gain) is controlled in this circuit by a DC voltage, which can = easily be=20 controlled by a foot pedal or remote volume control.

I am placing this in the public domain for single users. If you are a = manufacturer interested in making this as a for-profit device, some sort = of=20 royalty would be appropriate (honor system for now).

Circuit Notes

The circuit is built around the application notes for the Analog = Devices S= SM2018T=20 voltage controlled amplifier. My primary addition is a means of = controlling=20 the voltage within the specified range while using the sort of = potentiometers=20 found in standard guitar and steel guitar volume pedals. In particular, = the gain=20 of the SSM2018T is specified as =9630 dB/mV, meaning that the gain is = highest at 0=20 volts and decreases as the control voltage is increased. This is a bit=20 inconvenient, as standard foot controllers have one end set to ground = when the=20 pedal is in the low volume position.

I selected a cheap dual op amp (the 1458). The first amp is used as a = high=20 impedance buffer. The voltage divider made up of the 10kΩ resistors = ensures that=20 the control pot presents between 0 and 7.5 Volts at the input to this = amp. When=20 the foot pedal is unplugged, the 7.5 megΩ resistor ensures that the = input sees=20 7.5 volts =96 the maximum gain setting. The second amp is used as an = inverter and=20 has a slight negative bias, so that the actual gain of the circuit is a = couple=20 of dB. When the foot pedal is in the up position, with the control point = grounded, the output of the circuit is about 3 volts, so that the signal = is=20 attenuated by more than 90 dB.

The dotted lines show that you could, if you wished, build this as a = stereo=20 controller with a separate attenuator for each channel. The same control = voltage=20 adjusts each so that the gain is kept the same in each channel.

The SSM2018T can be operated by a single supply, but that eliminates = the=20 possibility of the circuit having any gain, and in order to give some = headroom=20 for the up to 5 V signals that come from our guitars, you will still = need a=20 fairly high voltage supply. I have elected to build a bipolar supply at = plus and=20 minus 15 Volts. I used opamp voltage regulators so that I could grab = power from=20 any convenient source =96 for example the 20 V supply that I found = inside my power=20 amp. You could re-engineer the circuit to run on a pair of 9V batteries = =96 it=20 draws about 20 mA for a single channel version (about 50 hours of = battery time.)=20 The only parts value change you will need is to replace the feedback = resistor=20 (R2) in the second opamp with a 33kΩ part, as indicated in = parentheses (see the=20 appendix for calculations). Obviously, you will eliminate the 7815 and = 7915 and=20 the associated capacitors.

Construction

You will probably want to hit the A= nalog=20 Devices web site to order the SSM2018T as it is not a common item; = try Digikey for the other parts.

You could make this all on perfboard as I did. The two channel = circuit fits=20 comfortably on a 2=94 by 2.5=94 board, including the supply components. = It would be=20 best to put it in a shielded box. I placed it inside of my power amp, so = that I=20 could use the same signal input jacks on the amp. This is pretty exotic = however,=20 and you should be a pretty competent circuit builder before taking = anything like=20 that on. If I get a dozen or so inquiries, I would be more than happy to = create=20 pc boards and sell them at my cost. This could save you some = trouble.

I re-wired my volume pedal to use a single stereo jack. This way, I = have only=20 the signal cable coming from the guitar and a single stereo cable = connecting the=20 volume pedal and the gain control. Since the SSM2018T costs a couple of = dollars,=20 you might want to place it on a socket.

Using It

Appendix

Component Choices

The formula for determining R1, R2 and R3 is this: Let your supply = voltage be=20 Vplus. The maximum attenuation is set when the control voltage from the = pot is=20 at 0. We will want this to be about 3V at the output of the second = opamp. With=20 the pot at its maximum setting, the input to the first opamp is = Vplus/2.

where Vin is the voltage at the output of the pot and Vout is the = control=20 voltage to the 2018. When the pot is in the off position, we would like = Vout to=20 be 3V (100 dB attenuation). In the max position, an output of =960.1V = will give=20 about 3dB of gain.

There is very little concern about noise in the control circuitry. = Thus, you=20 can use cheap cable and connectors to the volume pedal and the = inexpensive (35=20 cents) 1458 dual opamp. The signal circuits surrounding the SSM2018T = should be=20 made from good stuff.

If you want to use a higher supply voltage at the input to the = 7815/7915,=20 consider placing a series resistance. The VCA chips draw up to 15 mA = each, and=20 the 1458 draws about 5.

Here's the math on the audible effects (sorry):

If we use good quality cable (Belden 8410) it has a capacitance of = about 33=20 pF/foot. A 15 foot cable has a total capacitance of about 500 pF=20 (5x10^-10). Most of these foot pedals use a 500 kΩ pot = (for reasons=20 stated below). At say the 50% position, because these are log taper = pots, they=20 add a series resistance of about 400 kΩ.

This sets a rolloff frequency (the frequency at which the output is = down by=20 one-half) at 1/(2πRC), or 800 Hz! Your typical "presence" control = on a guitar=20 amp is set at about 3 kHz and up. This matter is made even worse by the=20 self-inductance of the guitar pickup, which is 3H (or more) in series = with the=20 volume pot. This creates a resonance circuit with a peak in the audio = range.=20 Often the pickups have an additional parallel capacitance to make them = brighter.=20 Click HERE for=20 even more detail.

BTW: this is one of the reasons that commercial microphones = are all=20 low impedance devices. Guitar pickups, however, have higher impedances = for=20 historical reasons. The volume pedals must use high resistances to = minimize the=20 loading on the pickups. If they did not, the relatively high inductance = of the=20 pickups would cause even more severe high frequency attenuation.

If you don't believe all of this (and still believe that this is a = result of=20 the Fletcher-Munson curves for auditory sensitivity) try this:

Set your amp volume to its maximum and use your volume = pedal to=20 play at a low to moderate loudness. =20
Now, bring the volume pedal to its maximum and back off = on the=20 amp volume. You will find that the tone is much clearer.

The reason for using optical pots in some foot controllers is not = tone loss,=20 but relative insensitivity to noise from contamination of the wiper on = the pots=20 themselves. As you probably all know, these things get scratchy - = usually from=20 wear on the internal resistor or from particles getting between wiper = and=20 resistor. A closed pot like the Allen Bradley AJ(?) series avoids the = particle=20 deposition, but cannot be cleaned as it ages. An open pot can be cleaned = with=20 commercial sprays but tends to get full of garbage sooner. My circuit, = by the=20 way, has a short time constant in the DC control circuit to minimize the = problems from scratchy pots.

revised 9/13/01

DC-controlled Volume Control