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Old 17th March 2010, 06:25 PM   #1
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Join Date: Apr 2005
Location: South NL
Default One PCB, one complete amp (lm3875)

In my quest for getting more familiar to PCB-making I would like to turn my "ugly" Gainclone into a more aesthetic appealing version.

What does that mean?
I would like to take relevant parts of my existing amp and put them on something nicer looking. Since I am new to etching PCB's I would like to fit everything! on a single side PCB. (just for practice) From rectifying the AC lines with a stabilized PSU to the output of the chips including loudspeaker protection. The maximum size of PCB I can get my hands on is 250mm*250mm, but I think that will be large enough.

My wishes;
AC from the outlet directly to the board
Fuses on the outlet's 220V on board
Rectification of the AC lines on the PCB
Stabilization on board
2 mono LM3875 chips on board
Single stereo audio input on board including simple volume control
Loudspeaker protection on board
And my "wet dream" would be to put the 100mm radius torroid in/on the board

Here is the part when you guys come in;

What are things I should or shouldn't do concerning the fabrication of my nicer version? Keeping certain parts far away from others, keeping other parts as close as possible? Etc.

As you can all read I am not a native English writer. Please correct my foul writing. I am not easily hurt
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Old 17th March 2010, 08:00 PM   #2
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Well , to be true ,I'm not the man who has to tell exactly what to do.
My humble suggestion is to do quite the opposite of your nice proposals
Instead of a big PCB , make little (like the ones-even if they're double sided- made by chipamp or audiosector ),same for the PSU .
Of course you might be amused by the perfection of tidy boards ,but that's a point of engineering that's achieved in a long time . If you have troubles and I hope you won't it will be easier to separate all the parts to find the faults.
Of course you can give a must ! I would keep the toroid away from the signal ...if you already have one amplifier running ,it's easier because by moving the transformer (and sensing also if it vibrates ...)you'll find the optimal position for it.
Bye ! !
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Old 17th March 2010, 09:41 PM   #3
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Join Date: Apr 2005
Location: South NL
I know I might be getting myself into trouble but if it is possible, with several attempts, I want to give it a shot. Keeping the torroid away from the signal path seems logical.
Any more suggestions? What about the PSU? What circuit should I use to stabilize the lines? What loudspeaker protection circuit would you suggest?

Last edited by robertvalk; 17th March 2010 at 09:47 PM.
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Old 17th March 2010, 09:57 PM   #4
Join Date: Jan 2008
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OK, do you have PCB schematic/layout software? I guess you do, this is a lot of trouble without. A laser printer and transparency media or architect's drafting film + a UV lightbox and pre-coated resist PCB are the quickest and most direct technique, although an inkjet can be made to work.

First you need to draw the schematic. The parts need a package outline or foil trace in the PCB layout part of the tool. The pin numbers must correspond with the numbers and functions in the datasheet. You should be able to print the outlines true-size on a laser printer. Check the individual packages against the parts by printing the foil outlines on paper or transparency. Then you can hold the part up to the print and see that the fit is exact. Check also that the package outline is fairly accurate to prevent parts touching unless required.

Many developments start with a specified board area, but not all. The mechanical arrangements for connectors, box and power lead strain relief are important constraints, but are often flexible for DIY. If you can, decide to keep the inputs away from the outputs and both away from the power lead. This is not always possible, in this case try to stop power hum or ioutput signal from getting into the input.

Where the layout is relaxed, typically the signal path might be laid out from left to right, or vice versa, with the power entering at 90 degrees from top or bottom.

Arrange the active devices as they occur from input to output, clustering the associated capacitors and resistors around the pins they connect to. Wire the immediate connections where they connect to the active devices, or each other, and wire the signal from device to device. Treat the incoming power circuits in the same fashion treating the + power rail as the signal, and noting the position of any filtering and dropper resistors. Route the power into the signal processing sections after the signals have been wired, leaving the ground trace to last. Decoupling caps are frequently drawn on a single rail in the schematic, in the case of digital circuits or other circuits with ICs, take care to distribute the associated caps.

Route the ground through the power supply following the elements of the filter as they occur logically in the schematic to the last capacitor. In the case of a dual rail PSU bring the rail grounds together at this point. Now route all the remaining signal ground returns individually to this point. Power supply decoupling caps for ICs are positioned and routed to minimise the loop area enclosed by the IC, the tracks and the cap.

Trace width, separation and thickness can be determined according to voltage and current and board specification from tables (google).

If you choose to use board-mounted connectors accessible thru the enclosure you need to plan these in from the beginning. You have a certain amount of choice about where you bring the LV AC onto the board, you don't need to mount the toroid to the PCB.

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Old 18th March 2010, 03:45 PM   #5
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Join Date: Apr 2005
Location: South NL
Change of plans;
Since I am getting more and more sceptical about this attempt I will start wat an all-on-one PSU. De case I have takes a 100mm*160mm board so that's the maximal space.

I started playing/struggling with eagle, but not quite successfully. Any tips on how to make the following image reality?

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