Most Steel Guitar and Guitar players are aware of the problem: When they back off on the volume using either a foot pedal or the knob on their guitars, the tone becomes more muffled and lacks presence or brilliance. This is because the capacitance of the guitar cable and the self-inductance of the pickup coil, in combination with the resistance of the volume knobs, becomes a “low pass filter” where high frequencies are attenuated. As the volume is decreased the pass frequency is lowered, so that more of the high frequencies are lost (see the little appendix below).
This note describes a solution to the problem. Instead of putting a resistor in the signal line, this circuit uses a voltage controlled attenuator whose frequency response is not altered by the gain setting. The degree of attenuation (or gain) is controlled in this circuit by a DC voltage, which can easily be 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 royalty would be appropriate (honor system for now).
The circuit is built around the application notes for the Analog Devices SSM2018T voltage controlled amplifier. My primary addition is a means of controlling the voltage within the specified range while using the sort of potentiometers found in standard guitar and steel guitar volume pedals. In particular, the gain of the SSM2018T is specified as –30 dB/mV, meaning that the gain is highest at 0 volts and decreases as the control voltage is increased. This is a bit inconvenient, as standard foot controllers have one end set to ground when the pedal is in the low volume position.
I selected a cheap dual op amp (the 1458). The first amp is used as a high impedance buffer. The voltage divider made up of the 10kΩ resistors ensures that the control pot presents between 0 and 7.5 Volts at the input to this amp. When the foot pedal is unplugged, the 7.5 megΩ resistor ensures that the input sees 7.5 volts – the maximum gain setting. The second amp is used as an inverter and has a slight negative bias, so that the actual gain of the circuit is a couple 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 attenuated by more than 90 dB.
The dotted lines show that you could, if you wished, build this as a stereo controller with a separate attenuator for each channel. The same control voltage 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 possibility of the circuit having any gain, and in order to give some headroom for the up to 5 V signals that come from our guitars, you will still need a fairly high voltage supply. I have elected to build a bipolar supply at plus and minus 15 Volts. I used opamp voltage regulators so that I could grab power from any convenient source – for example the 20 V supply that I found inside my power amp. You could re-engineer the circuit to run on a pair of 9V batteries – it draws about 20 mA for a single channel version (about 50 hours of battery time.) The only parts value change you will need is to replace the feedback resistor (R2) in the second opamp with a 33kΩ part, as indicated in parentheses (see the appendix for calculations). Obviously, you will eliminate the 7815 and 7915 and the associated capacitors.
You will probably want to hit the Analog 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 comfortably on a 2” by 2.5” board, including the supply components. It would be best to put it in a shielded box. I placed it inside of my power amp, so that I could use the same signal input jacks on the amp. This is pretty exotic however, and you should be a pretty competent circuit builder before taking anything like that on. If I get a dozen or so inquiries, I would be more than happy to create 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 the signal cable coming from the guitar and a single stereo cable connecting the volume pedal and the gain control. Since the SSM2018T costs a couple of dollars, you might want to place it on a socket.
There is no tune up. You can place this:
The formula for determining R1, R2 and R3 is this: Let your supply voltage be Vplus. The maximum attenuation is set when the control voltage from the pot is at 0. We will want this to be about 3V at the output of the second opamp. With the pot at its maximum setting, the input to the first opamp is Vplus/2.
The output from the conditioning circuit is:
where Vin is the voltage at the output of the pot and Vout is the control voltage to the 2018. When the pot is in the off position, we would like Vout to be 3V (100 dB attenuation). In the max position, an output of –0.1V will give about 3dB of gain.
There is very little concern about noise in the control circuitry. Thus, you can use cheap cable and connectors to the volume pedal and the inexpensive (35 cents) 1458 dual opamp. The signal circuits surrounding the SSM2018T should be made from good stuff.
If you want to use a higher supply voltage at the input to the 7815/7915, consider placing a series resistance. The VCA chips draw up to 15 mA each, and the 1458 draws about 5.
If we use good quality cable (Belden 8410) it has a capacitance of about 33 pF/foot. A 15 foot cable has a total capacitance of about 500 pF (5x10-10). Most of these foot pedals use a 500 kΩ pot (for reasons stated below). At say the 50% position, because these are log taper pots, they add a series resistance of about 400 kΩ.
This sets a rolloff frequency (the frequency at which the output is down by one-half) at 1/(2πRC), or 800 Hz! Your typical "presence" control on a guitar amp is set at about 3 kHz and up. This matter is made even worse by the self-inductance of the guitar pickup, which is 3H (or more) in series with the volume pot. This creates a resonance circuit with a peak in the audio range. Often the pickups have an additional parallel capacitance to make them brighter. Click HERE for even more detail.
BTW: this is one of the reasons that commercial microphones are all low impedance devices. Guitar pickups, however, have higher impedances for historical reasons. The volume pedals must use high resistances to minimize the loading on the pickups. If they did not, the relatively high inductance of the 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 the Fletcher-Munson curves for auditory sensitivity) try this:
The reason for using optical pots in some foot controllers is not tone loss, but relative insensitivity to noise from contamination of the wiper on the pots themselves. As you probably all know, these things get scratchy - usually from wear on the internal resistor or from particles getting between wiper and resistor. A closed pot like the Allen Bradley AJ(?) series avoids the particle deposition, but cannot be cleaned as it ages. An open pot can be cleaned with commercial sprays but tends to get full of garbage sooner. My circuit, by the way, has a short time constant in the DC control circuit to minimize the problems from scratchy pots.
revised