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Old 19th April 2008, 02:11 AM   #1
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Default Build a Loud, Simple & Cheap amp for a friend

Hello, the goal of this is to build a car amp as cheaply and easily as possible, while still providing decent power to satisfy the average person.

Approx 150W-300W RMS total in stereo is the goal, although actual wattage isn't a major concern.

A younger friend of mine is interested in me building a car amp for him, but he's not spending huge cash on a system, but still wants good sound that's loud. I also want to make some money on such a project, the other reason to go cheap. I found some large efficient heatsinks for only $20 a piece, so to keep it cheap and simple, I want to build the whole amp & SMPS as a whole on only one heatsink and circuit board

I don't need to build the amp as beefy as a regular overbuilt DIY amp I'd build myself, I just want it to perform with minimal parts, and be easy to build, and also repair if needed.

I had a few ideas:

1. Single pair of MOSFETS for SMPS 75N06 or similar. (75A)
2. Two audio channels rated @ 2 ohm each using a single pair of NJL0281/0302 thermaltrak to reduce usual costs and complexity of using a VBE transistor and mounting to heatsink for temp comp
3. Use SMPS transformer from old recycled computer power supply, and use in reverse as stepup (worked great on an SMPS I just made so far) to get amps voltage rails.
4. Use a few small cheap capacitors in parallel, or just use a single pair of larger caps for the power supply
5. Use TL494 PWM IC with a socket for easy & fast replacement if IC blows, and no voltage regulation, and simple 2N3904/3906 totem poles to drive the mosfets from IC
6. No snubbers on SMPS to save space and cost since they are not necessary for operation

What ideas/components, placement, etc, would you recommend in the building of a cheap DIY amp????
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Old 19th April 2008, 02:29 AM   #2
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Here is an option for a small TL494 supply (if you plan to build your own)......

http://www.electronics-lab.com/projects/automotive/012/
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Old 19th April 2008, 03:23 AM   #3
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Why not find a commercially available amp (used) that's relatively good quality as is and tweak it. You'd have the sink, board, transformer/core and virtually all of the other parts you'd need.

The class B amps (most of the budget amps) have little or no bias compensation so they shouldn't present any problems if you want to use those transistors.
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Old 19th April 2008, 04:12 AM   #4
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Quote:
Originally posted by Perry Babin
Why not find a commercially available amp (used) that's relatively good quality as is and tweak it. You'd have the sink, board, transformer/core and virtually all of the other parts you'd need.

The class B amps (most of the budget amps) have little or no bias compensation so they shouldn't present any problems if you want to use those transistors.
Interesting. I'll look around, but I haven't found any used/broken amps lately. If I could get one cheap and fix it, that's cool too. That would also save a lot of time and effort in getting a cheap amp for him.

I still want to see if the DIY is possible being I have most of the parts, but don't want to use all my good parts because I will build other things too. No paralleling with the transistors, and using single pairs gets a working amp with mimimal complication and parts. I also thought about just using a single pair of 6800uf capactors for the amps power. 2 caps makes things easy.

I remember an old BOSS 250W max amp I fixed a long while back had only one pair of transistors for the SMPS, and each channel, and a simple TL494 with two pull-down PNP's. However, the amp was built with cheap sucky transistors, and weak mosfets.

I figured I could try and build a better cheap design of my own, by using much better transistors, that's even simpler in circuit complexity and more reliable than something like that old cheap BOSS.
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Old 19th April 2008, 06:16 AM   #5
dangus is offline dangus  Canada
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Location: Vancouver Island
Last week I bought a made-in-USA Rockford-Fosgate 4 channel amp for $44. The case was a bit corroded, but the innards look fine and unmolested, and it works. Not the kind of deal I come across regularly, but I was holding out for something "nicer" than average, and that sure qualified.

While I'm sure it would be a great learning experience to build an amp, in terms of the time and money cost you'd be better off trolling pawnshops and yard sales for a good used amp. Or just buying something new off eBay or some online megastore, if you can figure out which ones are worth owning.
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Old 19th April 2008, 04:53 PM   #6
jol50 is offline jol50  United States
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I bought a 2x200/1x500rms at 4ohm insignia for <40 TMD. It was an unused return (new) and tested, even fan cooled. Just happened upon a good auction. It is huge though and unless a friend of mine wants it not sure what I will do with it. I'm sure some people here have old amps around for that. I might have one soon that is 360x1rms insignia with bad outputs, case is beat up some but fan cooled also.
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