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100W Guitar Pre-Amplifier (Rise)


Introduction
Guitar amplifiers are always an fascinating challenge. The tone controls, gain & overload characteristics are individual, & the ideal combination varies from guitarist to the next, & from guitar to the next. There is no amp that satisfies everyone's requirements, & this offering is not expected to be an exception. The preamp is now at Revision-A, & although the whole schematic of the new version is not shown below, the essential characteristics are not changed - it still has the same tone control "stack" & other controls, but now has a second op amp to reduce output impedance & improve gain characteristics.

One major difference from any "store bought" amplifier is that in case you build it yourself, you can alter things to fit your own needs. The ability to experiment is the key to this circuit, which is although introduced in complete form, there is every expectation that builders will make modifications to suit themselves.

The amp is rated at 100W in to a four Ohms load, as this is typical of a "combo" type amp with 8 Ohm speakers in parallel. Alternatively, you can run the amp in to a "quad" box (four x 8 Ohm speakers in series parallel - see Figure five in Project 27b, the original editorial) and will get about 60 Watts. For the adventurous, two quad boxes and the amp head will provide 100W, but will be much louder than the twin. This is a common combination for guitarists, but it does make it hard for the sound man to bring everything else up to the same level.

The Pre-Amplifier
A picture of the Revision-A preamp is shown below. You'll see that there's dual op amps, but the schematic only shows. This is the main part of the Rev-A update - the output section now has gain (which is basically selected), and a better buffered low output impedance. The remainder of the circuit is unchanged.

The preamp circuit is shown in Figure one, and has a few fascinating characteristics that separate it from the "normal" - assuming that there is such a thing. This is simple but elegant design, that provides excellent tonal range. The gain structure is designed to provide a immense amount of gain, which is ideal for those guitarists who like to get that fully distorted "fat" sound.

However, with a couple of simple changes, the preamp can be tamed to suit any style of playing. Likewise, the tone controls as shown have sufficient range to cover very anything from an electrified violin to a bass guitar - The response can be limited in the event you wish (by experimenting with the tone control capacitor values), but I recommend that you try it "as is" before making any changes.

Figure 1 - Guitar Pre-Amplifier

Figure 1 - Guitar Pre-Amplifier

Notes:
  • IC pinouts are industry standard for dual opamps - pin 4 is -ve supply, and pin 8 is +ve supply.
  • Opamp supply pins must be bypassed to earth with 100nF caps (preferably ceramic) as close as possible to the opamp itself.
  • Diodes are 1N4148, 1N914 or similar.
  • Pots should be linear for tone controls, and log for volume and master.
From Figure 1, you can see that the preamp uses a dual opamp as its only amplification. As shown, 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 - not that it helps all that much, because with all that gain, noise is always going to be a problem.

Note:
The schematic has been modified slightly to improve the tone control performance (04 Jan 2002). A new schematic is now on line - the differences are relatively minor, but make the component values for the tone controls a bit cheaper (smaller value caps, and higher value pots). The power amp has been heavily revised, and the new version is also available.

If a really quiet amp is desired, you should substitute a 5532 dual opamp. These are more expensive (and harder to get), but will offer a substantial noise reduction. If you don't need all the gain that is available, simply increase the value of the first 4k7 resistor - for even less noise and gain, increase the second 4k7 as well.

If the bright switch is too bright (too much treble), increase the 1k resistor to tame it down again. Reduce the value to get more bite. The tone control arrangement shown will give zero output if all controls are set to minimum - this is unlikely to be a common requirement in use, but be aware of it when testing.

The diode network at the output is designed to allow the preamp to generate a "soft" clipping characteristic when the volume is turned up. Because of the diode clipping, the power amp needs to have an input sensitivity of about 750mV for full output, otherwise it will not be possible to get full power even with the Master gain control at the maximum setting.

Make sure that the input connectors are isolated from the chassis. The earth isolation components in the power supply help to prevent hum (especially when the amp is connected to other mains powered equipment).

UPDATES: I have had quite a few enquiries about the input connection setup. This is almost an industry standard, and is quite the opposite of what you might think it means. The same basic idea is used on Fender amps, as well as many others. The Hi input is used for normal (relatively low output) guitar pickups, and is "Hi" gain. Lo is 6dB less gain, and is intended for high output pickups so the first amplifier stage does not distort. The switching jack on the Hi input means that when a guitar is connected to the Lo input, it forms a voltage divider because the other input is shorted to earth. I hope this clears up any confusion (it will probably create more!).

I have also had several enquiries about the tone controls, one being that they don't do anything. If the preamp does not work properly, it is because it has been wired incorrectly - period! I know the circuit works, and it works very well, so please don't send e-mails claiming that it does not do what is claimed. For some reason, this project generates more e-mail than just about any other, and in all cases where I have had complaints, wiring errors have eventually been found.

The golden rule here is to check the wiring, then keep on checking it until you find the error, since I can assure you that if it does not work there is at least one mistake, and probably more.
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