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HASIB'S ARTICLES AND AUDIO PROJECT REVIEWS
6-CHANNEL POWER AMPLIFIER ( FOR TRI-AMP SPEAKER SYSTEM) The active crossover (3-way 24dB/octave Linkwitz-Riley) has completed. Now for it power amplifiers. Subwoofer power amplifiers excluded since I built them separately. The Power Amplifiers For tweeter and midrange power amplifiers I use versatile LM1875 IC (25-watt). I know using integrated (power op-amp) is not so good compared to discrete amplifier but… (You know already). Oh, anyone who doesn’t – the cost can be higher. From personal experience, the LM1875 is good for driving tweeter and midrange drivers. The circuit was based from ESP page http://www.sound.westhost.com/. It's actually taken directly from National Semiconductor with some modifications included. I build all four-channel amplifiers on two pieces of strip boards and able assembled the circuit properly (considering the component placements to avoid oscillation). Two channels shared one heatsink. I use Sil-pad to isolate the LM1875 –ve potential to heatsink surface. The heatsink was mounted too close to chassis and I don’t want it short circuit. I’m able to get lower thermal resistance if I mount them directly to it heatsinking. All four channels shared one source power supply. And yes crosstalk will be higher. Total capacitance for power supply is 16000-uF with 144-VA power transformer. The power transformer using two 72VA transformers (18V 0V 18V 2A) run in parallel. I know you may say why bother two transformers. I do this because single 144-VA power transformer is too big to install in case. If you want do so this approach you must determine that the output voltage and current rating for two transformer is identical. I recommend you use two identical power transformers from same manufacturer. Fail to do this the power transformers will be overheated and ruined due to large circulating current. The 144-VA power transformer is quiet enough for four channel 25-watt amps but will do better if higher VA rating selected. There is just one problem appeared when I built the tweeter/midrange power amps. The LM1875s need a fairly hefty heatsink when operate. Please be caution when you want use this IC since the sound quality will be ruined (due to internal built-in protection circuit) if you use small heatsink. National Semiconductor actually recommends using 1-degree Celsius/watt for each device. That’s really a big heatsink. The heatsinks I use here is quiet enough but runs very hot on hot day. Maybe I will consider forced cooling via cooler fan(s) because I cannot make the heatsinks bigger – limited space. For bass channel power amplifiers I use it from my very old power amplifier section. The output power is 65-watt/channel. Some modifications have been done. I higher the power transformer to bigger one (24V 0V 24V 15A 720-VA). I know it overkill but it cheap in my area. I also change conventional power supply diode to bridge and larger filtering caps to 18800-uF. Others minor modifications also made but I don’t want bother you with the details. Also I changed fake power transistors (2N3055 Toshiba model) to genuine (ST Thompson). It really makes me feel better. No I didn’t say Toshiba make fake transistors but other manufacturers that fake Toshiba devices. Every output channel has muting circuit (just muting no DC protection). For tweeter/midrange power amplifiers the muting circuit is on front panel between the heatsinks. For bass power amplifiers the muting circuit (including DC detector is already on it PCB. On tri-amped speaker system care must be taken especially for tweeter since turn power on/off create DC transient may destroy tweeters if using direct approach. This is the function for muting relay. If not you can use 20-uF capacitor in series to tweeter (not bipolar caps but something exotic, i.e. polyprolene or polyester). From personal experience the series capacitor(s) will degrade the sound quality no matter what kind exotic capacitor(s) you use since phase shift still occurs. From the pictures you can see that the case is compact with the electronics. I have several problems to mount the circuits especially for tweeter/midrange power amplifier modules. It’s muting relay circuits also posses a problem. Good plan is needed here. Tweeter/midrange power amplifiers need long input shielded cables because it’s too far from RCA input terminal. For bass power amplifiers it is easier since the RCA jacks mounted directly on it PCB. There are no input buffers available for each channel (gain control). The 3-way active crossover has this facility and not needed here. The Case This is different from other of my projects. The case I use here is from my very old integrated amplifier case. The power amplifier section still in use (65-watt for bass amp) but pre-amplifier and other controller circuits removed (replaced with tweeter/midrange power amplifiers). All control knobs were removed from front panel and cover it with some kind of masking tape and spray them black. The power switch and LED indicator were still there. From the original amp there are two sets of binding post speaker terminals (for speaker A and B). I use speaker A terminal for midrange terminal and speaker B for bass terminal. Additional speaker terminals are needed for tweeter outputs and you can see them from the picture. For input terminals I use it from original terminals (for inputs ie CD, tape, tuner). The terminals that I don’t use I just pull them out and cover its mounting holes. Modify the case is a lot of fun and tricky too. The hardest parts are mounting the tweeter/midrange power amp modules including its power transformer. I spend lots of time to think about the arrangements. It is a little easier when you have a case and modify it to suit your need, here just like me. The markings made by printer, no commercial markings used here. Total cost for this project is around RM500. It’s quiet high but we talking about system do justice here (hi-fi system). Sometime we can’t get rid them off the total cost. I spend about RM250 just for power supply sections and heatsinks. Personally I really love this amplifier. I mean the sound quality is really good (all channels) and it exceed my expectation. Completed Project Pictures
Fig1 - 6 Channel Power Amplifier (Plan View)
Fig2 - 6 Channel Power Amplifier (Rear View)
LINK TO ESP PAGE 1. Project 72 (20-watt/channel power amplifier based on LM1875 chip) Muhammad Hasib bin Md. Nor 2001 Any Comment And Question Email to : hasib_tangkak@yahoo.com
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