25 Watt Power Amplifier
Circuit : Julius Horfilla, Philippines
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Description
This is a 25 Watt power amplifier complete with separate preamp and tone controls. In addition the circuit has a loudness control (to boost bass at low audio power) and options for super bass and super treble enhancement. This is ideal for Ipod's, MP3 or CD players or can be used as a booster amplifier.

Specification
Voltage Supply 12 -14 VDC 3A required for maximum volume
Power Output 25W RMS
Bandwidth 20Hz to 20KHz -3dB
Tone Controls
Loudness Switch
Optional Bass and Treble Enhancements

The original drawing as drawn by Julius can be seen here.

Notes (by the webmaster)
The main power amplifier is the MB3730 IC which directly drives the loudspeaker. The input stage incorporate a Baxendall tone controls and a loudness switch.

From left to right. The first 2SC828 transistor works in emitter follower mode and buffers the input signal. This gives the amplifier a high input impedance. The output if this stage is coupled by a 2.2u capacitor to a Baxendall tone control providing separate bass and treble controls. The tone controls are passive so there will be a small loss of signal. This is overcome by the final 2SC828 stage which provides additional voltage gain.

The emitter of the second stage is connected to a 390 ohm resistor. Tone enhancements affects can be made simply by connecting optional components across points A and on the schematic. For a brilliant tweeter effect connect a 0.1 or 0.2u mylar capacitor to terminals a and B. For a SuperBass effect connect the tapped primary winding of a Blue Captain Micro transformer.

Loudness Switch
The output of the right-most 2SC828 is connected to the volume control via the 4.7u coupling capacitor. Connected to the volume control is another frequency dependant network and switch comprising the loudness control. In the up position, some treble frequencies will be boosted whilst in the lower position bass frequencies will be boosted.

Frequency Response
I have re-drawn the circuit in LTspice, I can now show a simulated tone control response with the loudness switch activated. I do not have the exact transistor models, nor the proper primary inductance for the captain micro transformer, Julius used, so the two diagrams below must be considered as an approximation only.

Loudness Switch On

Loudness Switch Off

Parts List
The MB3730 power amplifier IC is available across Asia and China. I have not been able to find supplies elsewhere. An alternative integrated circuit is the TDA2030, also capable of delivering up to 20 Watts. European equivalent to the transistors will be BC109C or BC549C if you cannot find the alternative parts. An alternative to the Captain Micro (Blue) transformer will be a small choke or inductance. I don't have any specific details for the transformer but estimate an inductance of either 4.7mH or 10mH.
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