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Old 22nd March 2008, 06:19 AM   #21
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How do you like its performance (either version), and are there some differences between the smd and the larger components?

Its such a pretty project. Thanks for the link to the nifty transfo!
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Old 22nd March 2008, 06:57 PM   #22
dfdye is offline dfdye  United States
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I have no idea yet (I have yet to make the board, much less fire the amp up), but in my experience there isn't any discernible difference between equally spec-ed SMP's and through hole parts for the instrumentation I work with (in my real life I am a chemist heavily involved with instrumentation and data acquisition systems). The latter allow more compact designs and shorter PCB leads which are sometimes important, but the goal here was space savings more than anything.

As for the chip itself, I personally have never built a LM1875/6 based amp, so this is my first go-round with this chip. My expectation is that it will sound just like my LM3886 based amps, just not capable of playing as loudly.
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Old 24th March 2008, 07:13 AM   #23
dfdye is offline dfdye  United States
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Haha! I was going over gerbers before ordering a prototype and my wife asked where the headphone output was! I hadn't even thought that a substantial part of the time I am working on my computer I am listening via headphones.

So, back to the drawing board for a little bit while I drop a TPA6120 in the circuit. I just have to figure out how to get the mute of the LM1876 tied into a headphone jack plug in switch (IE mute the speaker output when you plug in a pair of headphones) I know it can be done, but I've never thought about it.

Any brilliant ideas? I know somebody has solved this before!
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Old 24th March 2008, 08:52 AM   #24
sangram is offline sangram  India
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You can use the LM1876 output as well for headphones. You'll need a series resistor switched in by the headphone jack switch. Relatively simple, and a solution used by integrated amps all the time.

If you still definitely need a separate headphone amp, there are jacks available with SPST switches built in, which you can connect across the mute pin or in series with it (depending on make or break). They're quite freely available in our local market.

As a last effort, you can build a headphone amp which is in the signal path, and the jack can break the signal to the power amp when the plug is inserted. This is also a common method, but degrades the SQ going into the amp. With an LM1876 and desktop computer speakers, I'm not sure that matters.
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Old 29th March 2008, 11:33 PM   #25
dfdye is offline dfdye  United States
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OK, the boards are ordered, but I have a quick question for anyone with experience with this chip; Is there a max voltage for the mute or standby pins? I have pads for a voltage divider on the boards, so no biggie to throw a couple of resistors on there and get +5V from the +12V rail, but if I can avoid having to wasted the (minimal, I know) current, it would definitely be cleaner. I would be using the +12V to bring up the pins. . .

Any experience with this? I would hate to blow up a chip just to see if this is possible.

David

PS I forgot to attach the latest version of the board. It incorporates a +/-12V regulated PSU stage for the buffer and headphone amp and changes out the PSU cap bank a little. The headphone amp will sit on a daughterboard since I ran out of pads on my free version of DipTrace. Plus, I have a couple of small TPA6120 boards that are already populated from testing, so I can put those on rather easily.
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Old 30th March 2008, 08:24 AM   #26
sangram is offline sangram  India
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IIRC there is a current you got to pull out of the mute pin, for the 3886 it's 0.5mA. I use 1mA as a calculation. Should be in the datasheet...

However some user here connected it directly to the -v rail (not the +v rail) and liked the results. I forget where I saw that among the thousand or so other threads.

The 1876 could be different though and I don't have the datasheet. But it's quite likely the internal configuration would be similar and it is normally connected via a small resistor (5K is a good value for a wide range of supply voltages) to the -v rail.
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Old 30th March 2008, 05:35 PM   #27
jaycee is offline jaycee  United Kingdom
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One mod that could be worth making, is generating a 3.3v supply and powering the PCM2707 with that instead of the potentially noisy 5v from the USB bus.

Note that you will have to level shift the VBUS from the USB socket down to 3.3v so that the chip knows that it's connected - but that's a simple matter of a resistor and 3.3v zener diode to ground.
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Old 30th March 2008, 10:50 PM   #28
dfdye is offline dfdye  United States
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Quote:
Originally posted by sangram
IIRC there is a current you got to pull out of the mute pin, for the 3886 it's 0.5mA. I use 1mA as a calculation. Should be in the datasheet...
For the 1876 the data sheet says 2.5V (not current) is the minimum to switch the logic state. I am sure they are assuming you have a +5V digital circuit somewhere that you can pull from easily. That being said, I guess I could use the power from the USB bus, but I don't really like that solution right now. I may change my mind--we'll see if/how it works when the time comes, I guess.

As for the power supply for the USB chip, I had been bouncing back and forth about using the USB power, and finally threw in the towel due to component limitations (I can only put 250 pads on a project until I pony up for a larger version of DipTrace ). I'll definitely have a dedicated power supply for my USB DAC that I will be building for my main system since I'll have to power the DAC chip in addition to the 2707. . .

Still I can't get the card ahead of the horse! Time to finish this one.
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Old 31st March 2008, 05:50 AM   #29
dfdye is offline dfdye  United States
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I finished back annotating the schematic with the changes I made while laying out the PCB. Here is the schematic for what the prototype board will be. There are a few parts that will be p2p with the +/-12V PSU if needed, but I threw them in the schematic since anyone wanting to recreate this would probably want to put them in the circuit.

PS I just noticed that the CK pin is connected in the schematic but not on the board. Oh well, easy fix.
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File Type: pdf dfdye usb amp proto0.9.pdf (63.1 KB, 166 views)
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Old 4th April 2008, 09:23 PM   #30
dfdye is offline dfdye  United States
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Quick BOM.
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File Type: pdf parts list revised.pdf (9.1 KB, 68 views)
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