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It is a keyboard combo, the project of which was assumed to be simple, loud and durable.
The power amplifier is based on well-known N-MOSFET. It uses IRFP240 as power transistors. Quiescent current was set to 55mA. At a voltage of about 2x52V and transformer 330VA, it gains power of approximately 100W/8ohm. Additionally it uses a small heat sink (visible in pictures), although cooling is supported by the fan (without any driver so far, there is a space for it, but it is still to be designed).
Combo is equipped with 4 inputs, two of which have a separate regulation (1, 2), and the next two (3, 4) does not have such a regulation. There is also regulation of total amplification (the sum). Such solution is due to the need for connection of at least two keys with different sources of sound (constant linear input, linear input with adjustable amplification). And of course a common regulation of bass and treble.
Schematic of the circuit:
It is a project inspired by similar project from this website:LINK. Operational amplifiers were changed to TL072.
All the wires of potentiometers (except the potentiometer of treble) are in shielded cables that are connected to the ground.
The device uses a GDS from STX the parameters of which are corresponding to the needs of this project: LINK.
And additionally, connected through a seriial capacitor 2,2uF LINK.
It is an old housing of guitar combo from Ldm.
Front panel was made of 3mm thick transparent plexiglass. Inscriptions are engraved. 2 very bright blue LEDs provide the backlight on the side.
Link to original thread (useful attachment) - Combo klawiszowe by ...:embe:...