Bridged/Stereo, w or w/o servo, inverting 3886, inverting buffer PCB - diyAudio
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Old 23rd March 2004, 02:35 PM   #1
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Default Bridged/Stereo, w or w/o servo, inverting 3886, inverting buffer PCB

I have cooked up a pcb that can be stuffed for several diferent versions, all using a pair of 3886's in inverting mode, you can either stuff it as a 100W bridged mono amp with inverting buffer, or as a stereo amp with inverting buffers on both. Bothe versions can be built with or without a dc servo. It is on a standard eurocard and is single sided so it can be made with hobbyist level pcb facilities.

Be aware that some of the traces are quite thin (0.016) although there is enough space on the board still to widen the traces if you have the time and inclination. It is all star earthed with heavy traces for the power connections. There's space for three local capacitors per side but there is no recitifer (but it could be added).

Anyone see any glaring errors before i make one...

Hopefully there will now finally be a working pcb design in the public domain. I should have it built pretty soon. Credit goes to Per-Anders Sjöström for the schematics i worked from.

Alexander Rice
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Old 23rd March 2004, 03:26 PM   #2
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Default hmmn...

i just printed it out and it is less than half a eurocard - about 150*150 mm - no wonder i had such a job routing the darned thing!

It's design purpose is the amplification for some active speakers - a bridged module for the low end and two stereo pairs with active crossovers for the mids and highs. to this end it is designed to 'stack' using standoffs and then all three boards will attatch to the same heatsink and be housed in the subwoofer - hence the 3 caps so that i can use low profile ones

Here's an image for those who haven't got EAGLE
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Old 23rd March 2004, 08:11 PM   #3
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I have reworked the board with 'Chunky Trax' technology ;-) to make it easier to make - most tracks are now 0.032in and smallest clearance is 10mil which should be easy enough for most hobbyists. I'll probably have one made by the end of the week so i'll be able to report some results then.


{Edit} ARRGH GROUND LOOP ALERT - newbies, see if you can spot my mistake... that what comes of ripping up the board and then not referring to the schematic!
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Old 23rd March 2004, 08:30 PM   #4
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Did you get the right answer?
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Old 23rd March 2004, 08:32 PM   #5
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here are the updated eagle files....
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Old 26th March 2004, 01:08 AM   #6
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great layout and design! I like it.What source is ' Chunky Trax ' technology that you say? I want to know the more detailed .

Some suggestion of me, see toplayer placed some jump wire, why mend and consider using two-sided pcb? There can be more space given play.
Do you supply the power chip and servo chip with same power ? I think supply power separately them will be kind. Because most normal working voltage of op amps is + - 15V,Since NE5532 such a high-voltage chip is + - 22V. It may be very tall that the voltage drop above the LM78/79 steady voltage chips.


-digi01
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Old 27th March 2004, 06:01 PM   #7
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Thanks -

It's not a two sided pcb because i am making the pcb myself and soldering some jumper wires takes a lot less time than making a two sided pcb (and i have a load of single sided board handy) but it is true that you get way better layouts with two layers to play with.

You are right about the regulators, i don't think a 7x15 will handle having 35v across it, i'll look for a better part or see if i can figure somehting with a bit of passive regulation before the 7x15's

There is an error in the schematic, i oopsied with the invering and non inverting connections, i'll upload a new version in a few days once i have simulated it all.
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Old 27th March 2004, 06:12 PM   #8
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oh.. 'chunky trax' is just a joke, i just mean that the minimum trace width is quite big which makes it much easier to make for hobbyists esp, if using the toner transfer technique, i make my boards with a UV exposure box and i can happily work with .010 traces but this board is intended as a contributed project so i have made them .032
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Old 27th March 2004, 06:28 PM   #9
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Nice layout, Alex!

About the ground loop, would that really be a problem for a power amplifier?

Taking a trace between the legs of an opamp is a bit risky, considering this is also intended for the toner transfer method. Consider running a wire bridge under the opamp package.

If you want to go single-sided, you have to modify the pad sizes (change the "restring" parameter in the Eagle DRC dialog). The default pads for resistors and capacitors are too small for a single sided board, they require the use of a PTH board.

I might also suggest you don't connect the speaker ground return to the PCB, connect it instead to the power supply common point. That is, of course, if this point is not already on the board. Same goes with the Zobel network at the output, it should be seperately returned to the supply common point. BTW, R5 which you have used for the zobel should be a 1W resistor, it will dissipate quite a bit of power.

The 7815/7915 you are using is probably not going to last very long... you can use a Zener diode pair instead. Or use the 78XX regulators but include a series dropping resistor which will share heat dissipation with the regulator.
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Old 27th March 2004, 06:53 PM   #10
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the power chips sharing a ground return would almost certainly cause oscillation - you are creating a feedback path between the the input and the output. If someone wanted to do it iwth toner transfer they may have to make a few minor modifications, i can make that board easily enough though.

I had misread the resistor value on the zoebel network, thanks - i thought it was kohms! At 20kHz the Xc of the .1uF capacitor is 79Ohms, so peak current with a +/-35v supply is about 0.4A so it has a peak power dissipation of 0.5W so it 'should' be ok most of the time but i'll put a 1w in to be sure.
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