Combo amplifier system is designed to amplify and modify the signal from electric guitars. The signal is processed in two channels that produce clean or edited sound. The channels can be switched manually from the panel or remotely, for example with a footswitch.
Electric Guitar Combo Using correction circuits with parametric centers, the frequency of the transmitted signal can be corrected. The Guitar Combo is equipped with an effects loop for the possible connection of external effect devices and a line output for connection to a mixing console.
All files including measured cabinet, grille, panel drawings, PCB, schematics, others are in the bill of materials. The famous TDA7294 is used in the amplifier board. All components of the amplifier block are mounted on a chassis made of 1.5 mm thick aluminum sheet.
ELECTRIC GUITAR COMBO AMP TECHNICAL DATA
Power supply
Guitar input
- Nominal input voltage: 80~400 mVrms “HI” / 0.4~2 Vrms “LO”
- Input impedance: 300 kOhm
Correction circuits
- Bass control: ±15 dB / 50 Hz, f0 = 0.8 kHz
- Midrange control: ±17.5 dB / selectable from 120 Hz to 6.4 kHz / slope 12 dB/octave.
- Treble control: ±15 dB / 10 kHz, f0 = 0.8 kHz
Effect loop
- Type: serial, monophonic
- Nominal output voltage (SEND): 0.5 Vrms
- Output impedance: 600 Ohms
- Nominal input voltage (RETURN): 0.5 Vrms
- Input impedance: 40 kOhms
Line out
- Type: unbalanced, monophonic
- Nominal output voltage: 0.63 Vrms
- Output impedance: 600 Ohms
Power amplifier
- Nominal output power: 70W rms power / 100W music power
- Speaker: 10 inch, EMINENCE – “Legend” series
Remote control
- Controlled circuits: channel switching
- Controller type: non-return switch
General information
- Dimensions: 330 mm (width) / 345 mm (height) / 245 mm (depth)
- Weight: approx. 8 kg
Electric Guitar Combo Block Diagram
The guitar is connected to the unbalanced monophonic input of the combo using a cable terminated with a standard 1/4″ Jack connector. The impedance of the guitar input is approximately 300 kOhm. The voltage level at the input can be varied over a relatively wide range (approx. 80 mVrms to 2 Vrms), adjustment is made using the IN LO/HI gain switch.
FOOT SWITCH – REMOTE CONTROL The Jack 1/4″ connector is designed for channel selection using the remote control. For correct operation, a foot switch with a switch button function must be used for the remote control. The channel is changed each time the pedal is pressed (i.e. when the button is pressed).
Effect loop: The combo is equipped with monophonic serial effect loop connectors for connecting external effect devices. The output signal for external effects is available at the FX LOOP SEND connector. The nominal signal level on this output is approximately 0.5 Vrms (-6 dBu) and the output impedance is approximately 600 Ohms. The return signal from the external effects is connected to the FX LOOP RETURN connector, the nominal signal level on this input should again be approximately 0.5 Vrms (also -6 dBu, i.e. the external effects device gain is 1), the input impedance is approximately 40 kOhms.
Line output: The unbalanced mono line output LINE OUTPUT is designed to be connected to the input of another amplifier or mixing console, the nominal signal level on this output is approximately 0.63 Vrms (-4 dBu) and the output impedance is approximately 600 Ohms.
The IN LO / HI switch selects the gain of the combo’s input preamp according to the output voltage of the connected guitar. In the “HI” position, the gain of the input preamp is higher (i.e. for guitars with lower output voltage), in the “LO” position, the gain of the input preamp is lower (i.e. for guitars with higher output voltage).
Channel selection: Switching between the clean (CLEAN) and distorted (LEAD) channels is done using the SELECT button. The green LED or the red LED next to the SELECT button indicates whether the clean (CLEAN) or distorted (LEAD) channel is currently active. In addition to manual control from the panel, the channels can be switched remotely, for example using a footswitch (button) connected to the FOOT SWITCH – REMOTE CONTROL connector.
“CLEAN” channel: In this channel, the input signal from the guitar is only amplified without distortion. The desired volume level of the “CLEAN” channel is set using the LEVEL controller.
“LEAD” channel: In this channel, the input signal from the guitar can be distorted. The distorter mixes odd and even harmonics into the guitar signal, so that the resulting sound is similar to the distortion produced in tube amplifiers. The level of distortion is set with the DISTORTION control, the desired volume level of the “LEAD” channel can be set with the LEVEL control.
Correction circuits
The combo is equipped with three-band correction circuits (“PARAMETRIC EQUALIZATION” block) with parametric middle frequencies. The correction circuits are common to both channels.
The low-frequency cut or boost is set with the LOW control, the mid-frequency cut or boost is set with the MIDDLE LEVEL control, the high-frequency cut or boost is set with the HIGH control. The desired middle frequency frequency is selected with the MIDDLE FREQ control. The frequency band control curves are shown in the graphs in the figures below.
source: vyvoj.hw.cz/teorie-a-praxe/konstrukce/profesionalni-kytarove-kombo-vyroba.html
