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How to build Guitar Amplifier

2015-01-06 17:06  
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10W Old-Style ultra-compact Combo

Two inputs - Overdrive - Treble-enhancement

Circuit diagrams:

Guitar Amplifier-Circuit diagrams


P1 4K7 Linear PotentiometerP2 10K Log. PotentiometerR1,R2 68K 1/4W ResistorsR3 220K 1/4W ResistorR4,R6,R11 4K7 1/4W ResistorsR5 27K 1/4W ResistorR7 1K 1/4W ResistorR8 3K3 1/2W ResistorR9 2K 1/2W Trimmer CermetR10 470R 1/4W ResistorR12 1K5 1/4W ResistorR13 470K 1/4W ResistorR14 33K 1/4W ResistorC1 100pF 63V Ceramic CapacitorC2 100nF 63V Polyester CapacitorC3 470μF 35V Electrolytic CapacitorC4 220nF 63V Polyester Capacitor (Optional, see Notes)C5 47μF 25V Electrolytic Capacitor (Optional, see Notes)C6 1μF 63V Polyester CapacitorC7,C8,C9,C10 47μF 25V Electrolytic CapacitorsC11 47pF 63V Ceramic CapacitorC12 1000μF 35V Electrolytic CapacitorC13 2200μF 35V Electrolytic CapacitorD1 5mm. Red LEDD2,D3 1N4004 400V 1A DiodesQ1,Q2 2N3819 General-purpose N-Channel FETsQ3 BC182 50V 200mA NPN TransistorQ4 BD135 45V 1.5A NPN Transistor (See Notes)Q5 BDX53A 60V 8A NPN Darlington TransistorQ6 BDX54A 60V 8A PNP Darlington TransistorJ1,J2 6.3mm. Mono Jack socketsSW1 1 pole 3 ways rotary switch (Optional, see Notes)SW2 SPST Mains switchF1 1.6A Fuse with socketT1 220V Primary, 48V Center-tapped Secondary 20 to 30VA Mains transformerPL1 Male Mains plugSPKR One or more speakers wired in series or in parallel. Total resulting impedance: 8 or 4 Ohm. Minimum power handling: 20W

Circuit description:

The aim of this design is to reproduce a Combo amplifier of the type very common in the 'sixties and the 'seventies of the past century. It is well suited as a guitar amplifier but it will do a good job with any kind of electronic musical instrument or microphone. 5W power output was a common feature of these widespread devices due to the general adoption of a class A single-tube output stage (see the Vox AC-4 model). Furthermore, nowadays we can do without the old-fashioned Vib-Trem feature frequently included in those designs. The present circuit can deliver 10W of output power when driving an 8 Ohm load, or about 18W @ 4 Ohm. It also features a two-FET preamplifier, two inputs with different sensitivity, a treble-cut control and an optional switch allowing overdrive or powerful treble-enhancement. Technical data are quite impressive for so simple a design:

Sensitivity: 30mV input for 10W outputFrequency response: 40 to 20KHz -1dBTotal harmonic distortion @ 1KHz and 10KHz, 8 Ohm load: below 0.05% @ 1W, 0.08% @ 3.5W, 0.15% at the onset of clipping (about 10W).


SW1 and related capacitors C4 & C5 are optional.When SW1 slider is connected to C5 the overdrive feature is enabled.When SW1 slider is connected to C4 the treble-enhancer is enabled.C4 value can be varied from 100nF to 470nF to suit your treble-enhancement needs.In all cases where Darlington transistors are used as the output devices it is essential that the sensing transistor (Q4) should be in as close thermal contact with the output transistors as possible. Therefore a TO126-case transistor type was chosen for easy bolting on the heatsink, very close to the output pair.To set quiescent current, remove temporarily the Fuse F1 and insert the probes of an Avo-meter in the two leads of the fuse holder.Set the volume control to the minimum and Trimmer R9 to its minimum resistance.Power-on the circuit and adjust R9 to read a current drawing of about 25 to 30mA.Wait about 15 minutes, watch if the current is varying and readjust if necessary.

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