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Old 28th August 2012, 11:45 AM   #81
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Thanks again.

Would something like this be what I am after:

Leaded Ferrite Beads for noise suppresion etc. Qty 10 New | eBay
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Old 28th August 2012, 12:29 PM   #82
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Yep they look like just the ticket.
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Old 28th August 2012, 03:50 PM   #83
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When you say to connect separate resistors to pin 7 of each chip, would that mean I would need 8 trimmers, or is using a trimmer OTT? I was going to use the trimmer to be able to give me adjustment, but would it be better to just pick a suitable size resistor and then use 8 (1 for each DAC)?
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Old 28th August 2012, 03:57 PM   #84
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I haven't done the math on this but its too much of a stretch to believe you'd need 8 trimmers. The adjustment you'll be making is setting the symmetry (between the positive and negative swings) of the output - this is only really necessary if you're going to aim for maximum swing. I reckon 7 carefully chosen resistors and a single trimmer on the last one should do it, but its just a moderately educated guess
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Old 28th August 2012, 04:41 PM   #85
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I see. So if I use an 8 volt supply, going by what is said on this website that was posted earlier in the thread:

http://myweb.tiscali.co.uk/g8hqp/audio/TDA1543IV.html

I could simply use 1.1K resistors from ground to pin 7 on each DAC. Maybe forget the trimmer altogether eh?
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Old 28th August 2012, 05:34 PM   #86
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OK. Here is a slight alteration to the PCB using ferrite beads (I used resistors on Eagle for the beads as I couldn't find them in the library and couldn't be bothered to make the part). I have also changed the grounding for the DAC chips, it's more of a ground plane than a star ground, but it should be better I think. I still have pins 7 paralleled for now.

Click the image to open in full size.
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Old 28th August 2012, 07:14 PM   #87
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If I were to leave all pin 7s connected in parallel, what's the worst that will happen?

Loads of schematics connect them all together, I understand the tolerance on this pin is 2.2v +/- 0.1, so the voltage on this pin can be anywhere between 2.1 - 2.3. If these are all tied together, would this just mean that the DAC chips will increase in temperature more than if not?

Does the sound deteriorate if these pins are all connected?
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Old 28th August 2012, 11:36 PM   #88
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If you tie the pins 7 together it just means the bias current isn't going to be very predictable and hence you'll probably need the trimmer once again. It won't have any impact on the sound - unlike your new ground plane which in my experience will sound congested on more complex music.
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Old 29th August 2012, 09:00 AM   #89
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That is not a ground plane, it is a thick ground trace. A ground plane is an unbroken contigous plane underneath the signals that allows the return current to flow properly and thus maximise
signal integrity and reduce EMC problems, both of which are critical in getting the best out of a design. For digital signals a ground plane realy is mandatory these days with the ever increasing rise time
of digital signals. Ground planes are also used for analogue to again get the best results. Learning where and when the return current flows will help you picture the signal loop, and having a ground plane minimises the loop area and
thus lowers inductance. Star grounding is realy only for connecting system components such as amps, reciever CD etc, it realy has no place on low level signal PCB's.
The first link by John Wu covers the basics, and is a good start.

http://www.x2y.com/filters/TechDay09...%20_JohnWu.pdf

Grounding of Mixes Signal Systems

http://www.ti.com/lit/ml/slyp167/slyp167.pdf

http://www.analog.com/static/importe...es/AN-1142.pdf
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Old 29th August 2012, 09:09 AM   #90
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But the plan here is not to get the lowest overall inductance - if we'd wanted that we'd not be using ferrite beads (which most certainly add more inductance to the loop than a few cm of copper trace).

I agree though - a thicker ground trace does not constitute a ground plane
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