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\AUDIO CIRCUITS I have design some power amplifier and many more base on my own basic electronic knowledge. The aim for this circuit is to give DIY fan a chances in making own power amplifier. The circuit originally design by myself therefore some of the result may not satisfied the specifications of HiFi lovers. I use for very cheap material in building the circuit. Please BE CAUTION some of the circuit using our main 230V/110v, please get and advice from an authorize electrician. M25A-25W POWER AMPLIFIER
Fig 1- Full Schematic Diagram For M25A Power Amplifier (Click to get Full View ) Overviews I have a very basic knowledge in design an amplifier, well here is it a my first successful power amplifier the M25A. The power amplifier a good for driving the main loudspeaker that have a subwoofer unit. The bass respond is week, high frequency respond and slew rate are low make this amplifier ideal for doing normal job. The Amp mode is class A. The Circuit The power Amplifier is using a TO264 Power transistor (Q4, Q5) package to make it easy when mounting on a heat-sink. The design of the circuit is base on a simple basic power amplifier and it still run well. This amplifier have problem on the frequency respond time phase and when it run in a high power it seem to produce more heat on the heat-sink, so I suggest to use a really big heat-sink to overcome this problem. I using a 7.5 x 10 cm heat-sink to reduce heat. Using a TL072 IC that have a 2 different output make the circuit more simple. The IC is easily to get at my country. I use a PCB to placing all the component. I think a Vero board still can be use here because only a few component use in the amplifier. The gain of the power amplifier were set by using a R18/R4 and made the amplification to 21.3 x. You can change the gain by changing the value of R18 and R4. An inverter input buffer (IC1b) is to make the input impedance higher and minimize input power loses. A VR1 preset (470R) use to set the quiescent current. Before setting the quiescent current please make sure the value for the VR1 preset is in maximum value and then turn it until there are no humming sound at the speaker. My circuit does not using any protection circuit therefore any mistake here will make the power transistor or the speaker to burn and damage. The circuit is in mono mode for stereo mode you can easily build a pair. The R19 and R20 is from 0.47 Ohm and 0.22 Ohm. I using 3 pieces 1 Ohm in parallel to get totally a 0.33 Ohm. The load for a speaker is varies from 4 Ohm to 8 Ohm and when loading to a lower than 4 Ohm it will pushed the limit of the power transistor and easily to burn. At 4 Ohm load the risk of damaging the power transistor is bigger than driving a 8 Ohm load. Power Supply The M25A use a 200VA (25v-0-25v - 2A) transformers to run the power amplifier in a stereo mode. The circuit is a basic unregulated circuit. You can get the circuit in a power supply section. I suggest the pair of reservoir Capacitor to use is 4,700uF to 13,000uF /50V for regulated the ripple and make the power supply ratio rejection higher (more punch in bass respond). The amplifier is also run for a lower voltage (18V to 40V). When using a 40V the amplifier will reach the maximum power. Most of the components for the power supply are expensive and for the protection you can put a pair of 6A slow blow fuse of each supply rails. The slow blow fuse will blow if a short circuit happen, prevent form the transformers to get blow, omit it this will make you a trouble.
Fig 2 - The Picture Of One Channel M110A Power Amp Prototype. The Result Of My M25A Under Testing The power supply use a 10,000uF/50V reservoir capacitor at each rails (better PSRR) 100VA 25v-0-25v center tap transformers. I have test and burn in process of the amplifier for 12 Hours and the power transistor is run hot, it is normal for the size of the heat-sink. The sound sometimes harsh and the bass respond is weak. When the amplifier driven together with my subwoofer it seems to work well. The sound is detail and sometimes I can hear a sound that it never been heard before (because of the fast switch Power Transistor). I test using Bond-victory and very detail in small and sparking sound. The violin not very natural and the bass is very flat. I then play a songs Celine Dion - Prayer the voice of Celine is not very clarity and sometimes a little bit harshy. I hope someone will help me in modified the a amplifier that have a different amp in the input section to get lower THD. The simplicity of the M25A amplifier is the aiming of this circuit. A simple circuit, low in budget and easy to build. You want a high fidelity sound sorry this is not the amp. Mostly this amp is for driven a subwoofer section and for a PA unit. My prototype unit have burn out in the pre test, therefore please use a old speaker. I hope it will not blown out your precious loudspeaker. GLOSARRY THD-Total Harmonic Distortion (lower the THD the better usually in percentage unit E.G.. 0.0001% at 1Khz) PSRR- Power Supply Ration Rejection ( Term borrowed from Douglas Self) nanhifi 2002
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