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Old 27th November 2002, 02:00 PM   #11
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by mhennessy
I'm impressed with your board layout, and very pleased that my site was helpful to you

You've been able to optimise the layout by freeing yourself from the mechanical constraints that I imposed on myself. I wanted to minimise the interconnections by using things like ribbon cable and PCB-mount phono sockets...

Your layout has lots of copper dedicated to earth, which is an excellent plan. One suggestion, though. I notice that there isn't a power supply decoupling capacitor at the input-end of the PCB. It might be worth trying to solder a couple on the back of the board during listening tests. It might not make any difference, but the PGA2310 datasheets made quite specific recommendations about it.

I notice that you've included gain on the input buffer. I added a very small amount of gain there to compensate for the losses in the input filter - this was only for the benefit of the fixed-level listen-source output that I provided, indeed I didn't bother for the surround-channels. So, you'd be able to simplify that end of the board if you wanted...

Out of interest, what do C15 and C25 do? I ask because they appear to be electrolytics! I guess that they are the 1nF caps that I used to set the -3dB point to 100kHz...

Very impressive work! I can't wait to see the assembled version... Which CAD package does your roommate use?
This is my roommate second pcb layout that he has made. He messed up on the first try with this board. I showed him how to use the ground plane in Protel, so that is what he is using. He is using Protel DXP now. I will ask him about the purpose of those caps.

Thanks again. I will get more pictures up after Thanksgiving.

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Old 27th November 2002, 02:29 PM   #12
BrianGT is offline BrianGT  United States
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Well, here is the picture of the assembled board. Notice the reversal of the +/- vcc for the opamps...

Mark,
The caps you were referring to are not electrolytics, he just set the footprint wrong. He will fix it in the next revision. The circuit is straight from your schematics on your website.

Picture at:
http://brian.darg.net/album11/aag
click the image for a hirez version.

I should have shown the bottom, there is a single row of pins sticking out that allow you to stack the board into another. It should work well for a scalable pre-amp.

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Old 27th November 2002, 06:32 PM   #13
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Quote:
Originally posted by mhennessy
Hi Brian,


dwk123: If you are able to control the individual bits of a PC parallel port, then you should have no problem controlling these chips. You just need 3 lines for clock, data and latch (or strobe). You could also control the mute input with another bit - I mute via software. The timing diagrams in the datasheet should tell you everything, but feel free to ask for more info...
Thanks. I reread the datasheet last night, and I can't see anything that would cause a problem. The signal levels should be fine, and the timing is so flexible on these chips that almost anything should be able to drive them.
I'm running Linux, so low-level port control shouldn't be a problem. Looks like it's time to forumlate a plan.
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Old 27th November 2002, 07:49 PM   #14
paulb is offline paulb  Canada
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Quote:
Originally posted by dwk123


Thanks. I reread the datasheet last night, and I can't see anything that would cause a problem. The signal levels should be fine, and the timing is so flexible on these chips that almost anything should be able to drive them.
A suggestion: plan for optoisolators to break any possible ground problems. I'm sure you could clock the bits slow enough so that any old devices would work, but it could save you a lot of grief.
You may also want to fuse the lines to avoid damage to your expensive PC if you have a wiring error. Seen it happen...
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Old 28th November 2002, 11:14 AM   #15
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Hi Brian,

Still impressed with your PCB! Question: what are the resistors you've used? They look expensive

And, how do you think it sounds?

Something else to try - C15 and C25 (the 1nF caps). I notice you've used ceramic caps here - it might be worth trying something better. Let us know if you hear a difference. I used a plastic cap (can't remember the type).

You could also try losing the small ceramics on the input and output. I put them there to protect against ultrasonic noise, but I'd be interested to know if you hear a difference without them (or if they sound better for being non-ceramic)

Unfortunately, I haven't got the time to try these tweaks for myself at the moment Maybe during the Christmas holidays...

I like the stacking idea. Just one question - how are you going to cascade the shift registers in the PGA2310's? If you look at my diagrams, you'll see that I took the data-out from the first PGA2310 and connected it to data-in of the next. This means that I have to send a 6-byte word when changing volume, but it means that I can address them individually. It appears that you are planning to simply parallel-connect them all, and this might work ok (just watch data-out pins), but you have to send the same data to all of them at once. Obviously this is only an issue if you scale up for 6-channel, as you might want to control them individually...


And dwk123 - optocouplers are an excellent plan Forgot to suggest it myself... Should only need 3 for a basic implementation... Good luck with your experiments - let us know how you get on!
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Old 9th December 2002, 12:43 AM   #16
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Default Volume control

I have looked this one up-
The disadvantage is that TI assures us that the voume regulation is done thru a standard Op-amp, and I wouldn't want one more of these.
A competitor is the analoge devices 8400-8402 8404,which insofar as the company reports, is a straight potentiometer output. The control interface, serial data, is more or less the same. I would be happy if you provded more info on this kind of circuitry, as I don't understand anythihg about this level of digital.
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Old 9th December 2002, 06:32 PM   #17
alvaius is offline alvaius  Canada
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It looks like you have used a copper pour for the ground plane. There are advantages and disadvantages to this. From a purely simple noise standpoint, an all-over ground plane is great. However, for the absolute best in analog performance, a poured ground plane is actually not the best way to go. By using a poured ground plane, you have lost all control over where your ground currents are flowing. Even though it is a lot of copper, when you are talking 96db of dynamic range or more, a small difference in ground potential from one side of the board to the other can make a huge difference. Star-Grounds, often applied when wiring, are just as applicable in PCB design.

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