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Old 11th October 2001, 07:26 PM   #21
paulb is offline paulb  Canada
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Join Date: Jun 2001
Location: Calgary
1. I would have thought that separate transformers would be more expensive. If they're identical, you could probably parallel their output, but if you're using a number of smaller caps anyway I would just keep things separate.
2. If you parallel transformer outputs, if you wire them wrong you'll blow fuses like crazy. Like wiring batteries in parallel, if you don't hook + to +, they'll burn each other out.

You are planning for fuses, right?

3. Again, it's probably cheaper to buy bigger caps than a number of small ones. I'm assuming you're building the dual-supply version + and - power supplies. If you're building a single-supply version you have the speaker output capacitor to consider as well.
And to repeat: if you want to build a lower-power amplifier to save money on the transformer, you should reduce the voltage of the transformer as well as its VA rating.
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Old 12th October 2001, 03:09 AM   #22
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Power supplies are as clear as mud.

anyway.. 50VA ones are only rated to about 1A I think
I have lots of those around from those power bricks. the 100VA one they're selling is 68 bux, while the 50VA one is only 13 or so...

I don't understand that bit about lower power = lower voltage.

if I'm running 2 channels from 150W, whats the diff with running a channel on 50W?
the rating for the transformers should still remain 18-0-18 right? or should I use a 15V or lower trans?
I'm just planning to split the mains line to run 2 transformers in parallel, which will then connect to each channel seperately.
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Old 12th October 2001, 06:25 AM   #23
paulb is offline paulb  Canada
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Look at the graph on p. 3 of the LM1875 datasheet of "Power Output vs. Supply voltage". This shows what voltage you need for a certain power level.
But more power requires both more voltage and more current. Add the power output to the power dissipation (another graph on the same page), and you have the total power the power supply must provide.
DC voltage from a full-wave bridge is about 1.4 times the AC voltage from the transformer. VA rating for the transformer should be at least double the DC power you need to supply. This plus the graphs should give you enough information.
By all means if you have the transformers or they are way cheaper then use 2 of them. I'd suggest keeping one per channel, each using a separate bridge rectifier and caps.

If the DC voltage is higher than what you need for a given power output, the rest is dissipated in the IC. If you use a +/- 25 VDC supply (from your 18-0-18 transformer), even with no power to the speaker, the IC will be dissipating about 8 watts (see the graph). Never run these ICs without a heat sink attached! It looks like you can get about 8-9 watts per channel; each IC will then be dissipating about 17 watts. 25 watts total, 50 VA transformer.
Notice that if you ran from a +/- 20 volt supply and produce 10 watts per channel, the power dissipation per IC will only be 12 watts. 22 watts total, 44 VA (14-0-14 volt) transformer. You're actually drawing less power from the supply to produce more power into the speaker. Weird, eh?
I hope that made sense; email me if it didn't.
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Old 12th October 2001, 08:33 AM   #24
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Looking at those graphs
it seems that the THD is lowest at slightly more than 10W or so and 10 watts of output power means I need 15V & 60mA into that thing, I need to dissipate 7-13W of heat. Is that all correct?

If I want my 20W then I'll have to get a 23V transformer and 65mA.
Now I'm getting somewhere.. I can probably take out the 15V transformer in my CPU powersupply, and use their bridge rectifier. Caps should also be aplenty inside there.

Alternatively, would you reccomend I use one of those power bricks? I have one thats 15V, and I could just run the DC to a bunch of caps in parallel for smoothing. It'd also provide for easier wiring.

Question. The input on the ESP plans say 25W, with 1A fuses.
and I see no way I can actually reach 1A without blowing the thing. If I use the power bricks method, does it mean that if I buy one of those adjustable bricks, I can just up the voltage for more power?

: ^) thanks alot,
I'll be done with the soldering tonight, leaving the testing for the weekend. Any ideas for boxing up the project? its on a pcb thats 12cm by 8 cm... I'm thinking of mounting it on a wooden cutting board lying around.. easy to drag, and very stable. + the transformers look cool.. with that copper gleam
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Old 12th October 2001, 02:23 PM   #25
paulb is offline paulb  Canada
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To get 10 watts output, with a +/- 15 V power supply, the IC will be dissipating about 6 watts (with an 8-ohm speaker). The power supply will have to provide 16 watts at 30 volts, or 0.53 Amps, per channel.
When I say +/- 15 volt supply, this means a total of 30 volts. If you have a 15 volt "brick" (this is a DC power supply?), you'll need two of them in series to provide the +/- 15 V.
You can run the IC from a single power supply (a different circuit diagram applies), but 15 V is getting pretty low for this IC. National has other parts that will be happier at this lower DC voltage (LM383 comes to mind).
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Old 12th October 2001, 02:29 PM   #26
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I think I''ll just get one of those 50VA transformers at
15-0-15 and run it from there. Is that right?
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Old 12th October 2001, 03:20 PM   #27
paulb is offline paulb  Canada
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Sounds good. If you use one transformer per channel, I think you'll be able to get about 10 watts per channel.
Fuses: 1 amp is probably okay, but you may end up having to increase it depending on how loud you play your music.
Enclosure: whatever turns your crank. It's you that's looking at it. The cutting board idea sounds cool. The SAF (spousal acceptance factor) in my house would inhibit having a bunch of parts on display. Should be lots of ideas in the thread archives at this site.
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Old 13th October 2001, 06:44 AM   #28
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For the ppl following this project, DON"T save money by buying a smaller trans. Its not worth the extra cost.
I made that mistake.. sighz.. now stuck with a 18-0-18 1.1A
think I'll use it for centre channel amplification or something

That's cos the bridge rectifiers and caps can easily go to 13+ dollars for this project. and that's for only one channel. If you wanna do 2 channels then you have to do 13+ Again + the cost of the trans.. for me its 10
so I'll spend 13+ extra

I think I'll get another trans 18-0-18 3A and wire the two in parallel for 4A total... But more work and more expense
so don't do something stupid like this.

I also bought a 10cm by 5cm heatsink,with fins 4 cm tall
hope it'll work well. PaulB, should I lap the LM1875? or should I just leave it be? I have no heatsink compound and 7.50 for a tube seems too much since i just need a small dab.

2nd, Heatshrink tubing.. required for the transformer connections? or should I just use electricians tape or not bother at all.

3)Mistakes. I didn't read the schematics closely enough, so my 100nF and 220nF caps are electrolytic instead of plastic
should I go buy the plastic ones or should I just use the electrolytic.. If I do, which direction should I connect the electrolytics?

4)I'm using bare speaker wire. Should I buy those expensive nakamichi binding posts or should i just use those springy cheapo things? the Red and black thing that traps the wires

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Old 13th October 2001, 02:32 PM   #29
paulb is offline paulb  Canada
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Join Date: Jun 2001
Location: Calgary
Ah, the joy. Don't get discouraged, this is your first one, right?
"should I lap the LM1875?" - don't understand. It NEEDS a heatsink. The metal tab on the IC is connected to one of the pins, so if you don't insulate it from the heatsink, the heatsink will need to be insulated from everything else (including the box - this is a pain). You normally use a mica or other kind of pad to insulate the part from the heatsink; this interferes with heat transfer so you use the compound to fill in the air gaps and help transfer. Radio Shack used to sell a small tube size.
2. electrical tape is okay, heat shrink is expensive and just looks better.
3. I don't think the values of your caps are correct - 100 nF and 220nF are unlikely to be electrolytic. You probably have 100 uF and 220 uF if they are metal and have a + or - sign marked on them. So you'll need to buy the correct ones.
4. I'd stay cheap on the speaker posts, this is a "cheapo" amp, remember.
What did you decide to build it on? You mentioned a PCB in a previous post, did you buy one?
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Old 13th October 2001, 04:07 PM   #30
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Oh I just used perfboard, and joined the leads together with solder... saved me lotsa trouble.

Regarding the heatsinking stuff... lapping means using sandpaper and rubbing it until its flat. I think its a computer term, used to mean sanding till you expose the copper of the heat spreader.

Heatsinks are a pain. I can't get them to stand on the perfboard properly. as in... they're elevated off the PCB and I'm afraid of the stresses on the IC. They're not soldered in yet. as for electrical insulation, I'm thinking of buying those thermal interface pads. but I've heard that they have aluminum powder as a heat conductor, so they're out too. How much do mica sheets cost anyway? They also mean that I need heatsink paste right? : ^(
2 seperate large heatsinks would be much cheaper. Which is probably the path I'll take

Anyone knows why they actually connected the pins to the metal block? isn't that counterintuitive?

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