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Old 9th June 2006, 07:32 AM   #1
wxn is offline wxn  Lithuania
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Default Head amp pcb. Please check for errors

Classical topology, nothing fancy. The PCB looks quite tolerable. Separate capacitors for each channel/supply rail (probably an overkill). I'm worried about outputs returning to main ground plane. I just can't seem to find a way to route them to supply pins :/

Gain = 2 because of neat DC matching and because my 40ohm phones won't need more gain when fed from a 2Vrms source anyway.

Any suggestions are very welcome.
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Old 9th June 2006, 07:34 AM   #2
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Default component view

pcb components
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Old 9th June 2006, 07:36 AM   #3
wxn is offline wxn  Lithuania
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tracks
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Old 9th June 2006, 08:30 AM   #4
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Hi wxn,

A few suggestions:
[list=1][*]You can get better output transistors than BD139/140. Try MJE340/350[*]It may reduce distortion at low levels if you add a resistor (say 470R) from the opamp output to the board output.[*]Sharp angles (<90 degrees) are bad on PCBs, as it provides a spot for etchant to get in under the track. You've got nasty angles on your base connections, and some other spots. Try to stick to 45/90 degree tracks wherever possible, as it makes things neater.[*]You could probably go for thinner tracks and larger spaces in a lot of places. Your narrowest space should be equal to your narrowest track. A good number for home-made boards is 0.02" (0.5mm) tracks and spaces. The only spots that need more are the power supply connections and the output nets.[/list=1]

Hope that's been of use.

Cheers,

Suzy
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Old 9th June 2006, 04:16 PM   #5
wxn is offline wxn  Lithuania
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Thanks for such a great reply!
Those BD139/140 are there simply because I know little about power transistors. Thanks for the hint, substituting for MJE340/350 shouldn't cause any problems.
About that distortion, the output stage is runing class A at (almost) all times so there won't be any crossover problems anyway. Am I right?
And the suggestions about angles tracks are valid. I missed them somehow :/
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Old 10th June 2006, 08:17 AM   #6
wxn is offline wxn  Lithuania
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Updated. Is it better now?
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Old 10th June 2006, 10:50 PM   #7
suzyj is offline suzyj  Australia-Aboriginal
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It's an improvement, but you've still got a heap of acute angles between tracks. Don't ever do this. It's asking for trouble.

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Suzy
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Old 10th June 2006, 11:45 PM   #8
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My advice? Rip it all up and start again.

It can take four or five attempts to get a good layout. I know, I've been there. Set up the track routing to only give you 45 degree corners, get a nice smooth signal path, then add the star ground. Resistors, etc. to rails can be really useful for getting you out of trouble, and don't be afraid to move components around to get things neat.
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Old 11th June 2006, 12:56 AM   #9
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hi wxn,

Good start.

The schematic looks like it *may* have an error around the feedback resistor area. Have a look at this link below:

http://www.siliconchip.com.au/cms/A_30588/article.html

If you have a look at the print firendly version of this page it gives a very small picture of the PCB.

The schematic also shows some alternative output transitors (as pointed out by Suzy). If my understanding of headamps is correct, because the cct voltage is low and power output is low, you can afford to select transistors with lower noise values.

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