The preamp circuit is shown in the figure has a few interesting characteristics that separate it from the "normal" assuming that there is such a thing. The circuit is built by TL072 as amp. This is simple design and provides excellent tonal range. The gain structure is designed to provide a huge amount of gain, which is ideal for those guitarists who like to get that fully distorted "fat" sound. The circuit is show in this figure;
However, with a couple of simple changes, the preamp can be tamed to suit just about any style of playing. Likewise, the tone controls as shown have sufficient range to cover almost anything from an electrified violin to a bass guitar. The response can be limited if you wish (by experimenting with the tone control capacitor values).
The preamp uses a dual op amp as its only amplification. The lone transistor is an emitter follower, and maintains low output impedance after the master volume control. As shown in the figure, with a typical guitar input, it is possible to get a very fat overdrive sound by winding up the volume, and then setting the master for a suitable level. The overall frequency response is deliberately limited to prevent extreme low-end waffle, and to cut the extreme highs to help reduce noise and to limit the response to the normal requirements for guitar. If you use the TL072 op amp as shown, you may find that noise is a problem, especially at high gain with lots of treble boost. I strongly suggest that you use an OPA2134 a premium audio op, you will then find this quite possibly the quietest guitar amp you have ever heard. At any gain setting, there is more pickup noise from the guitar than circuit noise and for the prototype pre amp used carbon resistors.
However, with a couple of simple changes, the preamp can be tamed to suit just about any style of playing. Likewise, the tone controls as shown have sufficient range to cover almost anything from an electrified violin to a bass guitar. The response can be limited if you wish (by experimenting with the tone control capacitor values).
The preamp uses a dual op amp as its only amplification. The lone transistor is an emitter follower, and maintains low output impedance after the master volume control. As shown in the figure, with a typical guitar input, it is possible to get a very fat overdrive sound by winding up the volume, and then setting the master for a suitable level. The overall frequency response is deliberately limited to prevent extreme low-end waffle, and to cut the extreme highs to help reduce noise and to limit the response to the normal requirements for guitar. If you use the TL072 op amp as shown, you may find that noise is a problem, especially at high gain with lots of treble boost. I strongly suggest that you use an OPA2134 a premium audio op, you will then find this quite possibly the quietest guitar amp you have ever heard. At any gain setting, there is more pickup noise from the guitar than circuit noise and for the prototype pre amp used carbon resistors.
