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The 'Basket Case' Guitar Practice Amplifier


This all evolved from a class B amplifier circuit that I'd been experimenting with, based around a TL072 opamp and a couple of low cost transistors. I wanted to see if I could squeeze as much out of a few hundred milliwatts as possible and design a simple guitar practice amp that uses standard readily available parts, and with a very low power consumptio
n. There are many popular guitar amplifier circuits on the internet with a similar power based on the ubiquitous LM386 audio IC, but this is more flexible, with a kind of quirkiness about it!


                


The schematic diagram is shown below. It can be seen that the push pull output transistors have absolutely no bias at all, which is the essence of class B operation. There is no current flow in the output until an audio signal is applied to the input, so the transistors are normally off. The quiescent current is just that of the opamp itself, which in the case of a TL072 is about 3.6mA, making the circuit very eco friendly and able to run for many hours on a battery





The disadvantage of class B is crossover distortion. Most normal push pull amplifiers are biased in class AB mode, meaning the output transistors are already conducting a little, so the 0.6 volts required to turn them on has already been overcome. Not so in this design. It is the audio signal itself that has to turn on the transistors, which takes time, and produces gaps of distortion in the signal. But... If an opamp with a high slew rate is used (meaning its speed can track and cancel out errors in the signal waveform), by incorporating the output transistors within its feedback loop, distortion can be greatly reduced. It's not a perfect solution, but the better the opamp the more effective the cancellation. The TL072 used here is not the fastest opamp these days but it seems to work well in this circuit. Anyway it's primarily a guitar amp so distortion is good right? A veroboard layout is shown below






Both halves of the TL072 are used. One as a high input impedance pre amp to suit a variety of sources, particularly guitar, and the other as a driver for the output transistors. The pre amp section is configured as non inverting, with a fixed gain of 11. The power section is configured as inverting, with a volume control in the feedback loop setting the gain between 0 and 11. This combination gives plenty of sensitivity at the input to cope with pretty much any source you throw at it
. I have also added 'Bass Cut' and 'Treble Boost' controls between the pre and power stages (points A and B). In a circuit like this where power is limited, too much bass will cause overload at higher volume levels. Being able to cut it back means more volume can be achieved before distortion becomes apparent. Treble on the other hand can be boosted, and here it gives more brightness and clarity to the sound. The tone control schematic is shown below





Bass control is achieved by simply using a 250K pot to introduce a larger value capacitor (which passes the full range of audio frequencies) across a smaller value capacitor (which limits the low frequencies) placed in series with the audio signal path. Treble boost is achieved by taking advantage of the opamp principle where gain is calculated by dividing the value of the feedback resistor (in this case the 250K volume pot) by the value of the input resistor (22K). Here, a lower value resistor (4K7) with a relatively low value series capacitor is introduced by another 250K pot across the 22K which effectively inceases the gain, but only at higher frequencies. The tone control components are mounted on the pots themselves just like in the old days. I used a non polarised 2u2 electrolytic capacitor only because it was physically smaller (and cheaper) than a normal type coupling capacitor

The main input socket is a switched 1/4" mono jack, but there is also a 3.5mm stereo jack that, together with some low value resistors, mixes the left and right channels of a stereo source into one, making it possible to connect an iPod or similar device. Placing this before the switched socket allows the resistors to automatically disconnect when the larger plug is inserted, so they don't load the input when used with a high impedance source. When there is nothing plugged in, the input is shorted to ground through a 680R resistor which stops the buzzing normally associated with high impedance circuits. The configuration is shown below





I have not measured the output power but I'm guessing it's about half a watt or so. With such a small amount of power available an efficient loudspeaker is a huge advantage. I shopped around on eBay and found an 8" driver with a 93dB efficiency which works really well. This, combined with careful tone shaping produces a fair amount of sound for such a low powered device





Incidentally, the potentiometer values were specifically chosen so that the type intended for Stratocaster guitars could be used which are readily available along with push fit knobs in various styles that look great. In a strat, there are normally two 250K linear pots for tone and one 250K log for volume, but here I've used the log for the bass and a lin for the volume because it seems to work better that way





Regulated 12V DC from a mains adapter enters via a 2.1mm power connector located at the rear. There is little need for an on/off switch as the quiescent current of the amplifier is so low that a switch is superfluous. The adapter probably dissipates more power as heat! The front baffle was cut from a cheap pine shelf obtained from the local DIY superstore and then stained light oak. The simple 'L' shaped chassis was fashioned out of a small sheet of aluminium purchased on eBay





Finally the enclosure. I wanted something that looked 'Rustic' and different, and I knew I'd found it the moment I came across a rattan basket in a local store. It didn't matter that it would effectively be an open cabinet as there's not much bass anyway. I was really more concerned about how to mount everything in an irregular shaped item of plant life, but there's always a way




The finished amplifier