Circuit Schematic learn

Portable Guitar Amplifier Circuit Parts:

R1______________22K 1/4W Resistor

C1______________10µF 25V Electrolytic Capacitor
C2_____________100nF 63V Polyester or Ceramic Capacitor
C3_____________220µF 25V Electrolytic Capacitor

IC1__________TDA7052 Audio power amplifier IC

J1,J2__________6.3mm Stereo Jack sockets (switched)

SPKR___________8 Ohm Loudspeaker (See Notes)

B1________________9V PP3 Battery or
3V Battery (2 x 1.5V AA, AAA Cells in series etc.)

Clip for PP3 Battery or socket for 2 x 1.5V AA or AAA Cells


Portable Guitar Amplifier Technical data:

Max output power: 1.5W @ 9V supply - 8 Ohm load; 60mW @ 3V supply - 8 Ohm load

Frequency response: Flat from 20Hz to 20kHz

Total harmonic distortion @ 100mW output: 0.2%

Max input voltage @ 3V supply: 8mV RMS

Minimum input voltage for Fuzz-box operation: 18mV RMS @ 3V supply

Current consumption @ 400mW and 9V supply: 200mA

Current consumption @ 250mW and 9V supply: 150mA

Current consumption @ 60mW and 3V supply: 80mA

Quiescent current consumption: 6mA @ 9V, 4mA @ 3V supply

Fuzz-box current consumption: 3mA @ 3V supply

Depending on its design an electric guitar may have anything from one to six pickup elements. Classic (acoustic) guitars could also benefit from one or more retro-fitted pickups. Each pickup has a specific sound depending on the type of sensor and the location on the instrument. When a guitar has more than one pickup these can be connected together with or without additional components. However it is preferable for each pickup signal to be buffered individually. These buffered and possibly amplified signals should be level-adjusted in order to produce the desirable effect (or ‘sound’). After that they are mixed and sent to the next stage of the audio processing equipment.

Circuit diagram:

Most guitarists agree that pickup elements cannot drive cables longer than about 6 feet without risking significant signal degradation. Guitar pickups typically require a load resistance above 50 kΩ and sometimes higher than 200 kΩ, hence a preamplifier/buffer is often inserted, whose main function is not high gain but to enable cables between 10 and 30 feet to be connected representing a capacitance between 90 and 180 pF/m. In the circuit shown here, each pickup has its own input buffer with a transistor configured as an emitter follower. Each stage has a gain slightly lower than unity. This is not an issue because most pickups provide significant signal levels, typically well over 200 mVpp.

The input resistance of the first stage exceeds 200 kΩ, which is appropriate for most inductive pickups on the market. If higher input resistance is needed the 1-MΩ resistors marked with asterisks could be omitted, and the 720-kΩ ones may be increased to 1.2 – 1.5 MΩ. This will raise the stage’s input resistance to around 500 kΩ. To ensure the highest possible undistorted signal can be developed at the output of the first stages, the collector-emitter voltage (VCE) of T1–T4 should be about half the supply voltage. It is important for the first transistor in the buffer to have low noise and high DC gain.

The types BC549C and BC550C and the venerable BC109C are perfectly suitable in this respect while the BC546C, BC547C and BC548C may also be considered. The buffered signal from each pickup is adjusted with a potentiometer and sent to the summing circuit of the mixer. The next active element is an audio operational amplifier type NE5534 or NE5534A (IC1), which provides the required amount of signal buffering. The 5534(A) has low noise, low distortion and high gain. It can drive a 600 Ω line when necessary, but the preferred load is above 2 kΩ. Its amplification is adjustable between 3 and 10 with feedback potentiometer P5. At higher values of the gain some limiting and distortion of the output signal is ‘achieved’, which may well be a desirable side effect.

The maximum undistorted amplitude of the output signal depends on the supply voltage. If higher gain is needed the value of P5 may be increased to 470 kΩ. Output K7 has a volume control potentiometer (P6), which could be omitted if not used or required. Both outputs K6 and K7 are capable of driving 600 Ω loads including high-impedance headphones. The circuit is simple to test and adjust, as follows:

1. check that VCE on T1–T4 is approximately half the supply voltage;
2. with no input signal, adjust trimpot P7 for about half the supply voltage at the output of IC1. If precise regulation of the opamp’s output offset is not required P7 may be omitted and R17 connected to the junction of R18 and R19.

The supply voltage is between 12 V and 24 V. It is possible to run the unit off a 9 V power supply but the lower supply voltage will limit the output amplitude and gain. The current consumption from a 9 V battery is typically 10 mA. Two 9 V batteries connected in series is the preferred solution. The undistorted output amplitude is up to 6 Vpp at a 12 V supply with 2 kΩ loads at the outputs. The unit’s frequency band exceeds 20 Hz – 20 kHz. Distortion and noise were found to be negligible in view of the application.

Stand-alone, 9V battery powered unit, Three-level input selector, three-band tone control

This preamplifier was designed as a stand-alone portable unit, useful to control the signals generated by guitar pick-ups, particularly the contact "bug" types applied to acoustic instruments. Obviously it can be used with any type of instrument and pick-up. It features a -10dB, 0dB and +10dB pre-set input selector to adjust input sensitivity, in order to cope with almost any pick-up type and model. A very long battery life is ensured by the incredibly low current consumption of this circuit, i.e. less than 800µA.

Parts:

P1,P2_________100K Linear Potentiometers
P3____________470K Linear Potentiometer
P4_____________10K Log. Potentiometer
R1____________150K 1/4W Resistor
R2____________220K 1/4W Resistor
R3_____________56K 1/4W Resistor
R4____________470K 1/4W Resistor
R5,R6,R7_______12K 1/4W Resistors
R8,R9___________3K9 1/4W Resistors
R10,R11_________1K8 1/4W Resistors
R12,R13________22K 1/4W Resistors
C1____________220nF 63V Polyester Capacitor
C2,C8___________4µ7 63V Electrolytic Capacitors
C3_____________47nF 63V Polyester Capacitor
C4,C6___________4n7 63V Polyester Capacitors
C5_____________22nF 63V Polyester Capacitor
C7,C9_________100µF 25V Electrolytic Capacitors
IC1___________TL062 Low current BIFET Dual Op-Amp
J1,J2__________6.3mm. Mono Jack sockets
SW1______________1 pole 3 ways rotary or slider switch
SW2______________SPST Switch
B1_______________9V PP3 Battery Clip for PP3 Battery

Circuit operation:

IC1A op-amp is wired as an inverting amplifier, having its gain set by a three ways switch inserting different value resistors in parallel to R4. This input stage is followed by an active three-band tone control stage having unity gain when controls are set in their center position and built around IC1B.

Technical data:

Frequency response:
20Hz to 20KHz -0.5dB, controls flat.
Tone control frequency range:
±15dB @ 30Hz; ±19dB @ 1KHz; ±16dB @ 10KHz.
Maximum input voltage (controls flat):
900mV RMS @ +10dB input gain; 7.5V RMS @ -10dB input gain.
Maximum undistorted output voltage:
2.5V RMS.
Total Harmonic Distortion measured @ 2V RMS output:
<0.012% @ 1KHz; <0.03% @ 10KHz.
THD @ 1V RMS output:
less 0.01%
Total current drawing:
less 800µA.