HASIB'S ARTICLES AND AUDIO PROJECT REVIEWS
STEREO POWER AMPLIFIER - 120 WATT (4-OHM)
The circuit was based on ESP project (Project 3A) available at ESP Page. I have built this project intended for driving my 10Ē subwoofers. I canít supply the diagram/ construction/technical notes here since itís not mine. I suggest for whom want to build this project go to ESP page. Every detail you needed available there including PCB order (if you want to purchase the PCB). The circuit published is mono. You must build two for stereo version.
Several modifications have been made over the original design including:
On building this project, one of my goals is to keep the cost down as minimum as possible. I did not use recommended output transistors that mentioned above, did not used 1-% metal film resistors/MKT capacitors/torodial power transformer etc. Thatís mean Iím able to save lots of money. But anyone who have an experience building power amplifiers knows that the sound quality will not good as expected if use premium devices. I can accept this and sacrifice the sound quality a bit since I design the power amplifiers to drive subwoofers.
The case built using 3-mm plywood and 11-mm plywood. I did not used metal case or equivalent (aluminum, metal etc) since I donít have enough money/time. 3-mm plywood was used for front, back and upper panel, only sides and bottom panel used 11-mm plywood. The 3-mm plywood is doubled with 11-mm plywood to make it stronger. All cutouts were done using jigsaw and sharp knife. The case measured WxHxD, 43.5-cm x 12-cm x 34-cm. For finishes, firstly I cover all panel joints using masking tape and use flat black spray paint to finish its up. Please take a note that you must shield the case (using aluminum sheet for example) to keep RF disturbance. Marking was made by printer using MS Paint program.
I build every circuit on strip board. Yes I used strip boards for all the circuits. I did not recommend anyone to follow this approach. If anyone does you must separate high/low impedance section separately or the result will be unpredictable. I used hard wires to link the connections from power transistors to main board and keep them short as possible. I mount the power amplifier circuit board to heatsink using long threaded screw. I design it that way because I can keep the connections for output transistors as short as possible. You must isolate the circuit board to heatsink to keep from short circuit. All power transistors isolate from heatsink using Sil-pads.
For heatsinks, I use the big one that I can buy. The heatsink for output transistors for one channel uses big heatsink for good heat transfer/longer component life (1155-cm email@example.com degree/watt). (Drivers transistors didnít need a heatsink. If you really want to do that you can use small ďflagĒ type.) No need extra cooling or forced cooler fan. I use that configuration (heatsink mounting technique) since I donít want them stay inside the case. I know I will push the amplifier hard. Mounting them outside will keep them cooler a bit. The heatsink I used here is actually two heatsinks that joined together (4Ē+8Ē) because my supplier didnít have 12Ē heatsink available.
I design this power amplifier as minimalist design, no fancy features included. Off course this is a power amplifier not a preamplifier or something and nobody will bother with that. The only accessories here is muting circuit (just muting, no DC protection). Be advised (Yeah, I advised myself) that there is no protection available on this power amplifier if the output terminal is shorted to ground. The output transistors will be destroy if do so. The power amplifier produced small clicking sound when switched on and low level thump after five seconds when switched off, so I use speaker muting circuit to avoid this phenomenon. The relay will trigger about 5 second after powering up and close as possible when switched off. There is an LED indicator to monitor the process (blinking when powering up and stay Iid if ready). I use standard type RCA terminal and binding post speaker terminal (banana plug accepted).
For AC input I use IEC power terminal. The panel mounting fuse holder complete the back panel arrangement. The muting circuit I use here is in front of power supply reservoirs capacitors. For anyone who interested on the circuit diagram please go to multichannel power amplifier construction, the complete circuit available there including the new update DC protection circuit.
For power supply I use two 24V 0V 24V 4A power transformers in parallel to obtain 8A. The secondary windings are wired together in parallel. I want to give some advice if you want to do this arrangement, make sure the two transformers has same output voltage and have same output current capability (make sure the transformers have same size and from same manufacturer). If you failed to do this the power transformers will be overheated and ruined (due to large circulating current). I know you want to know why I do this arrangement? The answer is a single 8A power transformer is too big (I mean EI power transformer, torodial transformer will fit nicely) to install and it a little bit pricey too. I use 35A 400V bridge diode for rectification and 2 x 4700-uF electrolytic capacitors for each supply rail (18800-uF in total).
What can I say, it sounded good on driving my 10Ē subwoofer. I also have tested it using full range speakers it also sound good to my ears. This is very flexible power amplifier and you can use it to anything you like but please very caution, do not hook up 2-ohm load, the power amplifier is not designed to do this and will fail.
Completed project pictures
Fig1 - 140W Stereo Power Amplifier (Plan View)
Fig2 - 140W Stereo Power Amplifier (Back View)
Fig3 - 140W Stereo Power Amplifier (Plan View 2)
Fig4 - 140W Stereo Power Amplifier (Front Views)
Fig5 - 140W Stereo Power Amplifier (Side View)
Fig6 - 140W Stereo Power Amplifier (Complete With Cover View)
LINK TO ESP PAGE