Ultra Simple Bass Guitar Compressor

Using a compressor (or to be more correct, a peak limiter) on bass guitar is one sure way to get more apparent volume without distortion.  A good bass compressor will often have a relatively slow attack, so that you get a very solid 'chunky' start to each note, with lovely sustain and equal volume for all notes (speaker box allowing, of course).

The project described here is one that you can just build and have working straight away (although it is entirely possible that you will want to experiment a bit), and requires only a small handful of parts.  In its simplest form, there are no active electronics at all - and this is exactly what is described.

In case you were wondering, it can also be used with guitar, and can give excellent sustain, although this circuit may be too slow to give perfect results.

The way a compressor / limiter works is quite simple.  Once the preset threshold has been reached, the gain of the amplifier is reduced to maintain the output at the preset level.  As the signal decays, the gain is allowed to increase again to compensate, until the amp is at full gain and the signal then dies out naturally.

The unit described here uses a light dependent resistor (LDR) and a small light globe, of the type commonly referred to as a "grain of wheat".  These are very small, and having a small filament, they react quite quickly to an applied signal.  LDRs have a very high resistance when dark, and this falls as more light is received.  Typical LDRs will have a dark resistance of several megohms, and a minimum resistance of about 200 ohms or so.  The distortion introduced is very slight (typically less than 0.5%), especially at low levels.

Now, if the lamp were to be placed across the speaker output of your amp, and its light shines on an LDR, as the light gets brighter, the LDR will have less resistance.  The LDR is arranged in the circuit to form a voltage divider, so that as the resistance decreases, the input level is reduced, and a simple limiter is operational.

The problem with this approach is that the lamp will start to glow brightly enough to reduce the input signal with only a few volts of speaker output, so the level will be very low - with only a few watts of speaker drive.  This is fixed by using a wirewound pot across the speaker terminals, so that the amount of output signal getting to the lamp can be varied.  In this way, the output level is set by the pot, and the amount of compression is set by the amplifier's volume control.  Figure 1 shows the complete circuit.

Figure 1 - Simple Bass Guitar Compressor

The pot will need to be rated at 3W, and with a 500 Ohm pot, it can be used with amplifier powers up to a bit over 150W into 8 Ohms (or 300W into 4 Ohms).  For higher powered amps (or just to limit the range a little), R2 may be used in series.  If R2 is made to be about 470 ohms, the range should be fine for almost any amplifier power.  It is very important that the input section is properly shielded, otherwise the amplifier may oscillate, and the lamp and LDR must not be placed too close together for the same reason.

Ideally, you will use a small piece of clear Perspex rod, with a hole drilled into one end to take the lamp.  The LDR is then glued to the other end using a transparent adhesive (model glue is ideal).  Figure 2 shows the suggested method of assembly, which will ensure that you don't have problems with oscillation from the amp.  Don't glue the lamp in place, as you will probably have to replace it at some time or another.  These little lamps normally will last a long time in this sort of circuit, but they will eventually fail, so keep a spare in the box.  You might want to connect the lamp using a small screw-down terminal block, so that a replacement can be made without having to use a soldering iron.

When the light pipe is completed, wrap the LDR end with aluminium foil, and tightly twist a bare wire around the foil to make good contact.  Tape the assembly firmly so that nothing comes undone.  This acts as a shield, and is connected to the earth (ground) connection on the input jack.  Make sure that the foil does not short circuit the LDR leads, or you will get no signal at all.  Note that one of the LDR leads will be connected to ground anyway - it does not matter which one.

Figure 2 - Assembly Of The Compressor

The complete unit should be housed in a metal box that is completely light proof.  Any ambient light that penetrates the box will affect the LDR, and will either introduce hum or cause greatly reduced performance (or both).  The die-cast aluminium boxes available from many retail electronics suppliers are ideal, as they are very robust, and provide excellent shielding.

Make sure that the speaker connectors are of the insulated type, because some amplifiers do not use earth referenced outputs.  Failure to ensure that these connectors are properly insulated may damage the amp or cause the amp to oscillate.  Also make sure that the speaker leads are kept well away from the input connectors.  If necessary, a shield may be made from thin metal and used to separate the two halves of the circuit.

Plug the bass directly into the input, and another lead from the output to the input on the amp.  Plug a spare speaker lead into the speaker input (or run a lead from the amp to the Speaker In jack, and another from the Speaker Out to the loudspeaker.  The speaker sockets are completely interchangeable, so you can use either for In or Out.  The Input and Output jacks are NOT interchangeable, although the circuit will still seem to work (just not as well, and the tone will go all funny as the LDR loads down the pickups).

Set the 500 Ohm pot fully off, and play a note or three.  Once you are satisfied that all is well, turn the pot to maximum, and increase the volume a little.  When you play a note, there should be a solid attack, and then the level should quickly stabilise, but at a greatly reduced volume.  You will find that you can get really good sustain, and you simply play about with the compressor pot and the amp's volume control to get the sound you want at the volume you need.  The apparent loudness will increase (often by a large margin) because the amp can be consistently driven harder, but will not distort.  You will get some distortion during the attack period, but (surprise) this can often be used as a sound in itself, and is not unpleasant because of the short duration.

So, there it is.  Not much electronics, but more of an exercise in construction.  It works surprisingly well, and I think you will have lots of fun with it.  The attack time is a wee bit long for guitar, but the sound is not unpleasant, and you can also increase the gain (a lot!) and get amazing sustain with minimal distortion.

It's worth noting that many of the most coveted 'antique' valve based studio compressor/limiters use ... a small lamp and an LDR.  In the early valve era, the LDR was about the only usable variable resistance element available, and the characteristics are almost perfect for musical applications.

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