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Old 13th October 2001, 05:35 PM   #31
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and regarding the caps
no, they're 0.1uF made by rubycon
and they're very much electrolytic
never mind
I think I'll go get the plastic ones
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Old 13th October 2001, 06:23 PM   #32
paulb is offline paulb  Canada
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Okay, no, don't "lap" the heatsinks if they're already finished, unless they're bare metal. The usual black anodizing is okay as-is.
Yes, you'll have to figure out a way to stabilize the heatsink. The ESP PCB mounts the IC right on the edge of the board so the heatsink is not on the PCB.
Hey, if the heatsink is right on the board and you can ensure the heatsink is insulated from everything else, I'd consider skipping the pads. You should still use the compound, but you may be able to get away without it. Try it without - if the IC gets too hot to put your finger on it, that's too hot.
You'll probably end up taking it on and off the heatsink a few times anyway, and the compound gets messy.
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Old 14th October 2001, 12:44 PM   #33
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Gonna get another 50VA 18-0-18 for a total of 18x(4.1A) = 73.8VA And thus 1.48A at 50V, 0.75A per channel, putting me comfortably at the 25V supplies.

Considering getting another 10 dollars worth of caps cos I'm running so close to the limit.

Question. I'm worried about connecting the outputs of the two transformers to one bridge rectifier. Will there be like phase problems or anythign making the bridge rectifier run hotter than rated? or should I just buy another rectifier and run both directly to the resistor arrays in parallel?

Mounting the stuff is still a problem. Think I'll mount the T0-220 the wrong way round, leaving it up in the air where I can solder the leads to wires. Think I'll also hang the 0.1uF cap in the air to prevent oscillation. I'm gonna get Heatsink paste and the mica plates since there's no decent solution to the thing. Using Elliot's heatsink calculator, I'm at 1.28 for that heatsink which is pretty close to the 1C/W I need for disspating 32W of heat.

[Edited by downhere on 10-14-2001 at 07:51 AM]
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Old 14th October 2001, 03:17 PM   #34
paulb is offline paulb  Canada
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If the transformers are identical, you should be able to parallel the secondaries (if the identical leads from each transformer are connected). But it sounds like you want to use two different transformers? The 18-0-18 @ 1.1A (about 40 VA) and a 50 VA unit? It may be difficult to tell how to connect the secondaries together; they have to be "in phase". Also, they may not share the load very well; you may have to add small series resistors to each.
I would use two bridge rectifiers, and combine the outputs after them. It's easy to make sure they're wired correctly: + to +, - to -.
"or should I just buy another rectifier and run both directly to the resistor arrays in parallel?" you mean capacitor arrays, right?
It sounds like you've learned a lot doing this.

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Old 14th October 2001, 05:06 PM   #35
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Yup haha, I've always been a digital kind of guy. Used to do robotics and make cute little LEGO robots. Still not too used to analogue circuits and AC power. VA is measured by max voltage x current right? but if I take power from say +18 to 0 instead of +18 -18 the VA should still be the same right? As in, I'll get half the AC waveform, and half the voltage, and half the current.

I think I'll screw the one of the extra heatsink holes into the side of the chopping block as support for the heatsink and make a clear acrylic box around the whole chopping board, leaving two rectangular holes for the heatsinks. I'm gonna buy the parts and leave the assembly for december or so. Using this forum as a kind of diary of what to buy and how to assemble. I can kinda imagine it now, two transformer blocks, one large one small, a PCB with 20 1000uF caps and another PCB with the circuit and 2 heatsinks. and inputs and outputs + LED + switch on the front side.

Thanks alot PaulB for the help over these few days : ^) Will post pics when I'm finally done.
Next project is the 63W single chip amp, when I get money from my job in December.
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Old 14th October 2001, 05:49 PM   #36
paulb is offline paulb  Canada
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The total VA will be the same, but if you're only using half of the secondary winding (18 - 0), you'll get half the voltage but the full current. Therefore you can only use half the VA. Look at using a full-wave CT instead of a bridge rectifier if you only want to use half the voltage, then you can use the full VA.
You're welcome. I've enjoyed hearing about your progress. Looking forward to pictures.
I'm about to replace my LM1875-based amp with Rod Elliott's Project 3A 60W discrete amp. It's an addictive hobby, and this is a great site to get help and inspiration. Cheers to Jason for putting it together and keeping it running!
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Old 14th October 2001, 06:33 PM   #37
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Hmmz.. How wuz your LM1875 amp? Details.. like how many caps you used for power supply and any custom mods.
pics? : ^)
I'd love to see a completed amp : ^)
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Old 15th October 2001, 04:08 AM   #38
paulb is offline paulb  Canada
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Default My LM1875 amp

My amp is based on an article from Popular Electronics many years ago. It uses LM1875s, each in the feedback loop of an op-amp (AD712), two per channel in a bridged configuration.
The power supply uses a 288 VA transformer, 18-0-18 V, with 12,500 uF per channel.
I did the circuit boards myself, and used a rackmount chassis from a place called Mark V Electronics (not sure if they're still around).
I also built a "standard" LM1875 amp into a desktop speaker enclosure. It uses a 16 VAC "wall-wart" transformer to drive a 40-ohm speaker.
I'll try to post pics when I borrow a digital camera.
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Old 15th October 2001, 12:22 PM   #39
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Sounds interesting
I'll be waiting for pics.
now, anyone has any ideas on an amp case that cannot be drilled?
The tools I have are a lousy handsaw, and no drill. Means no holes watsoever.
I'm thinking of mounting the PCB by slotting it into rubber feet with slits in them.(the taller 1 inch type ones)
the transformer I'll duct tape to a board. the output and input jacks I'll duct tape too
question. I have no chassis. Where do I connect the leads that say chassis ground to?
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Old 15th October 2001, 02:19 PM   #40
paulb is offline paulb  Canada
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Default Chassis ground

The chassis ground connection doesn't apply if you don't have a metal case. But for safety reasons (in case of an accidental short to the power line) it should connect to all exposed metal parts. That way you blow a fuse or circuit breaker instead of electrocuting yourself.
With a metal case, it also provides shielding from radio interference, etc.
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