Tabletop Multi Guitar Effect With Compressor, CMOS Overdrive, Parametric EQ. Spring Reverb and Tremolo. Li-Ion Powered

This project actually started forty years ago. At that time I played in a rock group and since I worked with electronics I tought it would be fun to build my own effects. The first schematic is dated April 11 1983, showing the first spring reverb and vibrato design. Since then I have added the CMOS distortion part with the Big Muff tone control, and also the compressor and the parametric tone control. I also had an idea that to reduce hum, the box should be powered by Litium Ion cells. Therefore a charging circuit was needed. Today the unit is totally rebuilt and works very nice. I still play in a small rock group and I use the unit when we record our songs.

The input stage consists of a high performance, very low noise OP amp NE5532 in a standard non-inverting configuration with 550k pickup load. The input filter in the feedback loop has the cross over frequencies of 240 Hz and 880 Hz, where the low frequency gain is 1.22 and the high frequency gain is 5.62.

Compressor

The compressor uses the Analog Cevices SSM2166. Only the compression ratio control is adjustable. All other parameters are set inside by trim potentiometers. The compressor can be bypassed with the panel switch.

The CMOS overdrive part is designed around an unbuffered quad NAND circuit (4011UB). The unbuffered CMOS gate gives a very nice valve sound. Since the gain is only a few hundred compared to an op amp's (maybe) 100000 it will clip the tone in a very nice and soft way, in the same way as when the power stage of a valve amp begins to reach its max output. The effect can be adjusted from "pass thru" to nearly square wave as can be seen in the ascilloscope photos, where the lower curve shows the output at different overdrive settings.

The first stage consists of two gates in series with the gain of both gates is controlled by one single potentiometer, with the maximum gain of around 50.

Gate three and four are used as fixed gain boosters with gains of 3,5 and 4 respectively. The on/bypass switch of gate four (EBK2) also reduces the output level. The 4011 is runs on the +9,7V since the maximum supply voltage is 15V.

 Big Muff tone control

The overdrive tone control is copied from the Big Muff Pi stomp box. Circuit descriptions are available on the internet.The overdrive output level is controlled by the “Dist Volume” control.The Overdrive is bypassed using a relay, which can be controlled from the panel or by a foot switch. I use a Vox foot switch with two switches.

The parametric tone control consists of a frequency pot and a level pot. The frequency can be set from 200 Hz to 3 kHz, and the level can be set from -9 dB to +9dB. Curtesy of  Rod Elliot (Elliot Sound Products).

The spring reverb is a Large size (425mm) Accutronics low impedance reverb. I found the circuit design for the driver and receiver in a Analog Devices book. The important thing is that the driver shall have a frequency curve with +6dB / octave filter (IC3B), and the receiver part (IC3C) shall have the corresponding -6dB/octave filter. To be able to get the high output voltage requiredwith the +/-9.7 Volts available I use two 380 power amps in a bridge circuit.

One of the first parts I built was the vibrato part. At that time, I worked at an electronics company so I was familiar with the XR2206 oscillator and the XR2208 multiplier. I have no idea if they are available now, but the multiplier performs very well as a vibrato circuit. Another solution is to use a Tranconductance amplifier instead of a multiplier.

The multiplier must be  adjusted so that the output is zero when the input is zero regardless of the level of the carrier wave.

The oscillator (function generator) controls the speed and the intensity of the vibrato.

The vibrato can be bypassed by a relay in the same way that the overdrive is bypassed.

During the years I have had many power supply designs, but the current construction seems to work fine. Although maybe a little “overdesigned”. As I wrote in the introduction I had the idea that the box should run on battery only so that hum (50/60Hz) would be non-existent.

The current solution is that the unit can be run on battery only but also with the external 15 V power adapters active. In that case the batterys are working as LARGE capacitors, effectively eliminating all hum. The battery pack consists of 2 x three 2500 mAh Li-Ion accumulators.

The charging circuit uses the LM317 adjustable voltage regulator, set to 12,9 V by the resistors 215 ohm and 2K. The charging current is limited by the transistor BC537B which senses the voltage over the 2,22-ohm resistor. The charging current is then limited to 260 mA. The batteries are protected by 1A fuses. The relays supply the charging power when the power adapters are connected.

The power supply uses the LM2940-5 and the uA79MG as voltage regulators to supply the 9.7V voltage. There is also a low voltage detector which turns off the power supply when the battery voltage is below 10,5V (3.5V / cell), so that the batteries are protected, indicated by a red LED and a summer tone.

Also included is a battery voltage sensor with two sense levels (11V resp. 12V), selectable by the overdrive switch. 

Component choices.

Metal film resistors are recommended, at least in the input stage, The op amp choice is not critical, but low noice types are recommended, not 741 equivalents. The components are soldered on Vero board. I have used "DIY Layout creator" both for the drawings and for the layout as can be seen in the photo.

Here are some photos, the first shows the first design from 1983. The others shows the unit as it looks today. I don't have the ambition to make a step by step build instruction, but to share the design as inspiration. As can be seen in the pictures the electronics reside on five separate circuit boards. The reason is of course that functions have been added one by one during the past years. It is of course possible to build only one or two of the separate parts, and to skip the batteries. The sound of the effects can be heard on the Axis Green songs The Painting and in the intro of Andrea Doria (on Spotify), played on my 78 Telecaster.