Wiring panel

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 everyones 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.

A picture of the Revision-A preamp is shown below. Youll see that theres 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.

From Figure one, you can see that the preamp makes use of a dual op amp as its only amplification. The lone transistor is an emitter follower, & maintains a low output impedance after the master volume control. As shown, with a typical guitar input, it is feasible to receive a fat overdrive sound by winding up the volume, & then setting the master for an appropriate level. The general frequency response is deliberately limited to prevent extreme low-end waffle, & to cut the extreme highs to help reduce noise & to limit the response to the normal requirements for guitar. In case you use the TL072 op amp as shown, you may find that noise is an issue - at high gain with lots of treble boost. I strongly recommend that you use an OPA2134 - a premium audio op amp from Los angels Instruments (Burr-Brown division), you will then find this possibly the quietest guitar amp you have ever heard (or not heard :-). At any gain setting, there is more pickup noise from my guitar than circuit noise - & for the prototype one used carbon resistors!

Notes:
one - IC pin outs are industry standard for dual op amps - pin four is -ve supply, and pin 8 is +ve supply.
two - Op amp supply pins must be bypassed to earth with 100nF caps (preferably ceramic) as close as feasible to the op amp itself.
three - Diodes are 1N4148, 1N914 or similar.
four - Pots ought to be linear for tone controls, & log for volume and master.

The power supply section (bottom left corner) connects directly to the main +/-35V power amp supply. Use one Watt zen-er diodes (D5 and D6), and make positive that the zen-er supply resistors (R18 and R19, 680 ohm one Watt) are kept away from other parts, as they will get warm in operation. Again, the preamp PCB accommodates the supply on the board.

The pin connections shown (either huge dots or "port" symbols) are the pins from the PCB. Normally, all pots would be PCB types, and mounted directly to the board. For a do-it-yourself project, that would limit the layout to that imposed by the board, so all connections use wiring. It may look a bit hard, but is simple and looks fine when the unit is done. Cable ties keep the wiring tidy, and only a single connection to the GND point ought to be used(several are provided, so select that suits your layout. VCC is +35V from the main supply, and VEE is the -35V supply.

In the event you dont require all the gain that is available, basically increase the worth of R6 (the first 4k7 resistor) - for even less noise and gain, increase R11 (the second 4k7) as well. For more gain, decrease R11 - I recommend a maximum of 2k2 here.

If the bright switch is bright ( much treble), increase the 1k resistor (R5) to tame it down again. Reduce the worth to get more bite. The tone control arrangement shown will give zero output if all controls are set to maximum - 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 permit 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 wont be feasible to get full power even with the Master gain control at the maximum setting.

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

If issues are encountered with this circuit, then you have made a wiring mistake .. period. A 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 properly there is at least mistake, & probably more.

The input, effects & output connections are shown in Figure 1B.

The power amp board has remained unchanged since it was first published in 2002. It definitely is not broken, so there is no reason to fix it. The picture below shows a fully assembled board (obtainable as shown as M27). Using TIP35/36C transistors, the output stage is deliberately huge overkill. This ensures reliability under the most arduous stage conditions. No amplifier can be made immune from everything, but this does come close.

The power amp (like the earlier version) is loosely based on the 60 Watt amp historically in the past published (Project 03), but its increased gain to match the preamp. Other modifications include the short circuit protection - the tiny groups of parts next to the bias diodes (D2 and D3). This new version is not massively different from the original, but has adjustable bias, and is designed to provide a "constant current" (i.e. high impedance) output to the speakers - this is achieved using R23 and R26. Note that with this arrangement, the gain will change depending on the load impedance, with lower impedance giving lower power amp gain. This is not a controversy, so may safely be ignored.

Ought to the output be shorted, the constant current output characteristic will provide an preliminary level of protection, but is not foolproof. The short circuit protection will limit the output current to a comparatively safe level, but a sustained short will cause the output transistors to fail if the amp is driven hard. The protection is designed not to operate under normal conditions, but will limit the peak output current to about 8.5 Amps. Under these conditions, the internal fuses (or the output transistors) will probably blow if the short is not detected in time.

Figure two shows the power amp PCB parts - except for R26 which doesnt mount on the board. See Figure 1B to see where this ought to be physically mounted. The bias current is adjustable, & ought to be set for about 25mA dormant current (more on this later). The recommendation for power transistors has been changed to higher power devices. This will give improved reliability under sustained heavy usage.

As shown, the power transistors will have an simple time driving any load down to four ohms. In case you dont use the PCB (or are happy to mount power transistors off the board), you can use TO3 transistors for the output stage. MJ15003/4 transistors are high power, & will run cooler because of the TO-3 casing (lower thermal resistance). Watch out for counterfeits though! Theres plenty of other high power transistors that can be used, & the amp is tolerant of substitutes (as long as their ratings are at least equal to the devices shown). The PCB can accommodate Toshiba or Motorola 150W flat-pack power transistors with relative ease - in case you desired to go that way. TIP3055/2966 or MJE3055/2955 may even be used for light or ordinary duty.

At the input finish (as shown in Figure 1B), there is provision for an auxiliary output, & an input. The latter is switched by the jack, so you can use the "Out" & "In" connections for an outside effects unit. Alternatively, the input jack can be used to connect an outside preamp to the power amp, disconnecting the preamp.

The speaker connections permit up to 8 Ohm speaker cabinets (giving four Ohms). Do not use less than four ohm lots on this amplifier - it is not designed for it, & wont give reliable service!

All the low value (i.e. 0.1 & 0.22 ohm) resistors must be rated at 5W. The 0.22 ohm resistors will get warm, so mount them away from other parts. Needless to say, I recommend using the PCB, as this has been designed for optimum performance, and the amp gives an excellent account of itself. So nice in fact, that it may even be used as a hi-fi amp, and it sounds excellent. In case you were to make use of the amp for hi-fi, the bias current ought to be increased to 50mA. Ideally, you would use better (faster / more linear) output transistors as well, but even with those specified the amp performs well indeed. This is largely because they are run at comparatively low power, and the extreme non-linearity effects would expect with only transistors do not occur because of the parallel output stage.

Make positive that the bias transistor is attached to of the drivers (the PCB is laid out to make this simple to do). A some quantity of heat sink compound as well as a cable tie will do the job well. The diodes are there to protect the amp from catastrophic failure ought to the bias servo be incorrectly wired (or set for maximum current). All diodes ought to be 1N4001 (or 1N400? - anything in the 1N400x range is fine). A heat sink is not needed for any of the driver transistors.

The life of a guitar amp is a hard, and I recommend that you use the largest heat sink you can afford, since it is common to have elevated temperatures on stage (chiefly due to all the lighting), and this reduces the safety margin that normally applies for domestic equipment. The heat sink ought to be rated at 0.5° C/Watt to permit for worst case long term operation at up to 40°C (this is not unusual on stage).

Make sure that the speaker connectors are isolated from the chassis, to keep the integrity of the earth isolation parts in the power supply, & to make sure that the high impedance output is maintained.