| bigparsnip |
Well, I finaly finished my PCB layout for this project, and i thought beofre I sent it off to be amde, I'd let you guys have a look and see if you could spot any problems with it.
The project itself is a stereo, non-inverted design, with a single 180VA 18-0-18 transformer, with two sets of rectifier bridges per channel and 2200uF on each rail. It sohuld eventulay all be going into a small cast aluminium housing I have allong with two big pentium II heatsinks to keep it all cool (I might post pictures of this lot soon if I can find my camera).
So, if anyone has any sujestions on any improvements/modifications that they think I should make please let me know as this is my first attempt at building something and I would like it to turn out nice as hopefully it should turn out as a christams present for the missus.
Pic one, the rectifier/fuse board: |
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| bigparsnip |
| Pic two, the amp board: |
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| bigparsnip |
| Pic three: I found my camera, so here's the case (the pot behind it isn't as bad as it looks, it's just my tea from the noodle bar down the road:rolleyes: ): |
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| GregGC |
It looks very neat, but I’m not too crazy about the GND arrangement of the amp PCB. There is no GND-Star. I wouldn’t use GND-Plane that way, as you can’t predict the current distribution. Placing GND plane around the output of the amp increases the amount of capacitance the amplifier has to drive (not very good).
Greg |
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| bigparsnip |
The grounding is one of the main things that I wasn't too sure about, but one thing that I should have made clear is that there will be two wires connecting the power supply ground to the amp board ground for each channel, and these have been placed so that they are as close as possible to the relevent decoupling capactior pins. One thing that I did just think of though, would be to swap the position of the speaker out pins, so that the -ve pole sits next tot eh ground cable input, again giving a short direct return route to the power supply ground. I guess I could go off and fix that now before I forget.
Andrew. |
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| bigparsnip |
| Pic four: amp board rev 1.2: |
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| Asen |
| If I were you I would try a kind of star grounding. |
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| GregGC |
Andrew,
I'll pray for you.
With two separate wires for the gnd doesn't guarantee you the same effect as the GND-Star. The FB resistor to GND and the speaker GND potentials are quite unpredictable in your layout. But if you are willing to risk it, go for it and get the boards build. It may work.
Greg |
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| bigparsnip |
well, I guess if the only thing to improve on is the grounding, I think I will plough ahead with this one how it is, as it's only my first amp, and I am quite ahppy to modify the design for the next version (gives me a goo dreason to build another) where I will probably spend a bit more time and money putting it together (as long as this one come up to scratch).
Andrew. |
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| Faber |
I don't know wich device you will use, but the LM3886 will not work at all with this configuration. Another tip, I normally put a wire from the GND pin to the star, normally it works better than not.
bye, Fabrizio |
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| bigparsnip |
Hi, I guess I should have said, it will use a LM3875 (I only didn't put this in the PCB layout for the component, os I can re-use the package for the different devices if I try any others in the future).
Andrew. |
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| Faber |
| quote: | Originally posted by bigparsnip
, os I can re-use the package for the different devices if I try any others in the future).
Andrew. |
That's why I said to you. If you will try the 3886 you will have to insert the mute resistor if you want to switch the device on and listen to it ;)
Bye! fab |
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| bigparsnip |
| no, I simply meant to re use the PCB footprint, but I would use different pin connections for the various different connections for use in the schematic, so I will end up with two different parts, that share the same PCB layout (as the physical sizes of the 3875 and 3886 are identical). |
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| bigparsnip |
Well, I sent the PCB files off to be made today, and hopefully they should turn up tomorrow so i can start to build hitngs up a bit more. However, I have decided I would like to keep a little log of how things are going here as I have'nt got a web site of my own yet to post the project up on.
So, tonight, with my (VERY) limited tool selection, I started to cut out some of the slots I needed to remove on the back of the case for the IEC mains filter/inlet and the phono sockets. Tomorrow, I might even go out and buy a bigger drill so that I can put the holes in for things like the switches and volume control.
But as of midnight tonight, my little box looks just like the picture below,
Andrew. |
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| Alex S |
| I use diecast aluminium chassis extensively myself. No need to use heaksinks - just bolt the ICs to the chassis (with insulating pads, of course). The case in your pic is more than enough dissipation for gainclone. |
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| bigparsnip |
Hi alex, I did think at first that I could probably get away without the heatsinks, but then I thought that my girlfriend would probably be a bit happier if it didn't get too hot on the outside.
But, I did get my PCB's today as you can see below, but I did have a little trouble with the transformer I was putting together to allow my US soldering iron to run on UK mains (see the pic in the next post) as the socket and plug were a 'little' snug and it all ended up with me thinking it might be better for me to fix this before I start to use the iron a lot. So, perhaps if I get a chance tomorrow, I will start to put it all together.
Andrew. |
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| bigparsnip |
As you can see, when I tried to pull the plug out, the socket came away with it, not the best snap fit I have ever seen :bigeyes: .
edit:
One thing I forgot to ask is: when I sent off the info for the boards to be made, I forgot to include the holes in the copper layout, so now I have no refference of where some of them should be for when drilling the board. Short of printing a coppy of the copper in reverse and putting it over the top of the tracks, is there any good way of getting the holes in the right place? |
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| bigparsnip |
A little it of an update now, as after loosing one of the PCB's (oops) for a few weeks in a stack of CD's, I have finaly finished mounting all of the components on the PCBs. After doing all this I then tried to fit tehm both into my case and supprise, they don't fit (well they do, but there would be no room for a volume controll, or anything else around them) so I am now in the process of trying to figure out the best way to re arange things to see if I can still cramm both sets of rectifiers in there.
But, the picture below, shows what the main amp board looks like with the two LM3875 chips, power supply caps and the heatsinks for both channels (sorry about the poor pic, but teh camer didn't seem to want to focus on anything less than about four foot away) |
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| bigparsnip |
| Here is another shot of the internal layout, which as I mentioned above, may have to change a little to allow me to fit a switch in behind the rectifiers as well as leaving enough clearance for the cables. |
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| dhaen |
| I hope you are going to add some ventilation for airflow through those nice heatsinks:eek: |
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| bigparsnip |
| That is the plan, lots of littel 3 or 4mm holes in rows above and below each of the two heatsinks. The only problem being the fact that to get them all done to a good degree of accuracy, I will probably have to centre punch, drill a small (~1mm) pilot hole and then drill out to full size, so it will take most of an after noon (or at least a good couple of hours) to do. |
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| bigparsnip |
| OK, well, after letting this sit about for quite long enough I pulled it out to do some more work on it the other day, and should be able to get things finished off just after my exams now. Basicaly, since the last time, I have finished drilling the case out and sorted out most of the internal wiring now. In fact, I all I have left to do is to mount a few indicator LED's, wire in the input select switch and conect the transformer primarys so that I can still remove the lid from the case. |
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| bigparsnip |
| And one of the outside, showing about half of the few hundred holes I had to drill (wasn't that fun). |
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| Nuuk |
All's well that ends well!
It's turned out looking very dapper Bigparsnip - well done. Any reviews of the sound yet? ;) |
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| bigparsnip |
| not quite yet, there are still a few important conections to be made as I mentioned above, but if I put off the revision untill this after noon, I may be able to let you know how it sounds then. |
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| Nuuk |
Hey! Don't put off your studies for me :D
"The ox is slow but the earth is patient" ;) |
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