Cheapo amp project help pls

downhere

Member
2001-10-03 10:24 am
I have no idea which designs are applicable, but I was thinking of building an amp to drive two mini compo speakers I have lying around with leftover computer parts, e.g an old 230W power supply lying around, and mounting it in an old case.

These speakers are 6ohms and probably can't handle or don't need the 50 watts or so. I'm guessing 10W per satellite maximum.

Are there any amp projects out there that are cheap and easy to build?

Secondly, does anyone know the specs of the Cambridge soundworks satellites? I have 2 leftover without a sub. Wondering if it'll work with the cheapo amp.
 

paulb

Member
2001-06-01 4:53 pm
Calgary
Oh, I just noticed you said you want to use the computer power supply - it may not be suitable, its main outputs are probably 3.3 and 5 volts.
See if it has +12 and -12 volt outputs available, and how much current on each. But they're likely to be a pretty noisy source of DC power.
 

downhere

Member
2001-10-03 10:24 am
PS specs

Thanks, I was considering that, but I have no idea how to obtain the IC... maybe I'll check around the electronic parts shops.

The PS does +12V 12A, -12V 1.0A, +5V 30A, -5V 0.3A, +3.3b 15A.

got those numbers off the HowStuffWorks site, suppose my PSU won't be all that different.
 
hmmm.. computer power supplies usually concentrate all their current capacity on the 3.3V output for the processor core, and also the +5V output. Often, you'll find the -5 and -12V supplies will be very weak...

I'd look for the PSU's specs online somewhere (if you're lucky, you'll find them), or perhaps trying to test their current capability. If you're really stuck on using this surplus PSU because you don't want to spend any $$, then you may have to make the amp a single-supply jobbie, with AC coupled outputs to the speakers.

Personally, i'd say it's not worth the trouble, and just go build a simple linear supply...
 

paulb

Member
2001-06-01 4:53 pm
Calgary
I agree. Computer power supplies are not well suited for what you want, you also need a minimum load or they go wonky.
Downhere, how about (say) buying an old amp or receiver at a garage sale or something, ripping the transformer out of it, and use that? Make sure it's old enough that it uses a linear, not switching, power supply (if it's heavy it probably does).
There's lots of old dead equipment out there because it's cheaper to buy new stuff than to fix old stuff.
 
Power Supplies

A good place to get free power supplies and parts are electronic repair shops. They are always throwing stuff out that is not repairable and will sometimes let you haul it off for free. Ive gotten a lot of transformersand cases for nothing. Just be sincere and explain that you are a diyselfer just starting out learning and most will generally help you out.
 

downhere

Member
2001-10-03 10:24 am
Thanks, in fact I have an old pioneer amp thingy hanging around, and it no longer works... smoke comming out, smells like salted fish or something. It still runs tho, and I have no idea whats wrong with it so maybe I'll just take the transformer out.

I wanted to build the amp because of the speakers I have around, 2 pairs of identical speakers from the Pioneer amp/cd/tape/graphicEQ thingy... hoping to rig up a surround system for my comp.

:) Mebbie when I have time I'll try and hook the graphicEQ up too...

Thanks for the ideas!

BTW, I asked National Semi for samples, hopefully they'll send 4 of those ICs to me in a few days time. Has anyone ever tried their Sample service?
 

downhere

Member
2001-10-03 10:24 am
They shipped my samples, be getting them tomorrow. :)
I live in Singapore(where the National plant is) and its amusing that they advertise a 24hr shipping to the US and a 3-15 day shipping to another part of Singapore. Maybe the US does get priority over us locals after all.

Going shopping for parts tomorrow, here's a list of what I'll buy, hoping for some remarks and tips before spending the cash.

1) I can't afford the 36+ bux it'll take to buy boards from Elliot. It'll probably put me off budget, so I'm thinking of point 2 point wiring, like the Foreplay and Paramour preamps and amps, using 18 gauge solid OFC throughout. Is 18 gauge overkill, cos I'm not very confident at soldering IC leads, or should I use a lower 22 gauge for the ICs themselves?

2) If I do point 2 point, should I use veroboard, or use a board with holes for mounting, but no copper conductor?(2nd option cheaper by a few bux)

3) I have no idea how to mount the parts together, planning on using LOTS of tape. advisable? or should I use some sort of glue instead.

4) ESP doesn't tell me of any test points, how can I test the circuit before running the electricity?

5) How do you heatsink an IC, I have no experience in this. The last time I heatsinked something was my CPU and it had clamps for me.

6) How much do those metal film resistors cost anyway, and is the added 1% accuracy essential to the circuit or just a good thing to have? Whats the difference between metal film resistors and carbon ones anyway.

7) When I remove the transformer from the scrapped amp, how do I discharge the capacitors around the transformer? I'm worried because I've had a screwdriver blown to bits when I discharged a Flash camera cap. I suppose the powersupply cap would be much more dangerous & destructive.

Thanks a million paulb! I'm learning as I go along, and I've learnt much about audio already.
 
Whoa! Let me try to address these:
1. I really recommend a PCB for the LM1875. It can easily oscillate if you try to wire it pt.-to-pt. The leads are fragile, so you can't really solder wires (especially 18 ga.) to the leads. And the leads don't quite match the hole spacing in veroboards, etc.
18 ga. solid wire is hard to work with. I suggest you use it only for power supply and speaker output wiring.
2. But you can do it using a veroboard or just a perforated board. Read the National datasheet carefully for layout precautions, they're important.
3. If you don't go with a board, use your imagination. Tape, glue, stuff hanging in free space...
4. The only way to test it is to power it up, check if the current consumption is what you expect, check for oscillation at the output (if you have a scope), then run audio into it and check the output for sound. If you have access to a current-limited power supply, this is great. Some people wire their transformer in series with a light bulb for the first test.
5. The LM1875 is in a transistor-type package (TO-220). You can bolt it to a chunk of aluminum (use an insulator and heat sink compound) or buy a (fairly large) heatsink.
6. Metal film are supposed to sound better. I don't know if I can tell the difference. You don't need 1%, at least not in this circuit.
7. You can discharge them using a resistor first (100 ohms or so?), then use a screwdriver. The caps in these receivers are fairly low voltage and leaky, so there's not much danger like there is in a TV set (or flash camera). But if you don't like sparks, use a resistor first.
Don't forget to have fun!
 

downhere

Member
2001-10-03 10:24 am
how large is enough for an IC thats only 1.5cm?
I can't imagine how it'll support a pentium class heatsink
thinking of lying it flat on the board and just clamping the heatsink down with suitably sized screws...

Will RAM sinks work? They look the correct size and have thermal epoxy bundled with them
 

downhere

Member
2001-10-03 10:24 am
Progress...

I've been looking at capacitors, resistors being rather clear cut and inexpensive.

Is there a difference between General Purpose resistors and the Plastic film types? I can't seem to find plastic film ones that are non metallized and go high enough to .22 and 2.2uF. Anybody has any good online stores to reccomend, MECI.com only sells them in 100pcs no mixing .. which is 90 more than I need.
 

downhere

Member
2001-10-03 10:24 am
More Progress...
I just went to buy my caps and resistors. Amazed cos the parts cost only 10 bux with board and large caps.. but disaster struck when I asked for a 200VA 10A 18-0-18 transformer. The guy laughed and said that you'd have to order one, and It'll have to be carted to your house.

So. now, what kind of transformer should I get, and I'm sure it isn't 200VA 10A 18-0-18
the most powerful ones I saw were 50VA. would that be enough?
 
A 150 VA transformer should be enough. It's not that big, your sales person seems to be confused.
The "10A" part was wrong, though. 150 VA, 36 V = 4.2A. It will probably be more expensive than the rest of the parts.
You can use a lower voltage supply, getting less power out of it (and needing less VA). The LM1875 is pretty versatile that way. Check the datasheet, you should be able to figure out what you can get.
A 50 VA transformer could probably get you 5 watts per channel or so. You'd be surprised how loud that actually is.
 

downhere

Member
2001-10-03 10:24 am
What about buying seperate 50VA transformers to drive a single channel each? Would that be possible? and can I run such a setup with both transformers connected in parallel?

2nd) should I parallel a few 1000uF caps to get 10,000uf
or would a 5,000uf cap be good enough? I''m planning to build the transformers and caps seperately, and run a power cable to the amps.