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dfdye 11th March 2008 10:16 PM

Compact USB DAC/Integrated LM1875 Amp
2 Attachment(s)
I have been toying with the idea of building a compact, moderately powered amp for my "work" computer that would be capable of driving a small bookshelf speakers. This has to be compact due to office space limitations, and it won't need to crank out much power since high volumes will be right out of the equation.

Enter the obvious choice: LM1875's driven by a +24V or +48 wall wart. (And before anyone suggests, I am indeed considering a +/-24, but I am not sure if it will fit in the case I have in mind)

I was originally considering buying one of Peter Daniel's USB DAC's for the front end of this project, but after thinking what I really needed, the size of Peter's unit (and the fact that it was too good for my needs--sorry Peter!) lead me to try and put together a one shot solution that integrated the USB DAC and amplifiers into one board to effectively minimize space.

I have attached the first rough draft of the circuit I am thinking of using for this project. It is essentially duct taping Russ White's PCM2707 USB circuit used in his Opus project in front of a couple of LM1875's using a single ended PSU as per the National data sheet. So far there has been zero optimization of the circuit, and I know there will be some cleaning up between the digital out and the opamp input, but I wanted to get the idea out there in case anyone has any brilliant ideas.

In the mean time, I do have a couple of questions if anyone could offer up some suggestions:

1) What crystal would you recommend for use with the 2707? I sort of got lost in that section of my Digikey catalog and figured someone here would offer up some quick suggestions. Russ?

2) I know the LM3886 and LM3875's work great with unregulated PSUs. How about the LM1875s?

Like I said, if anyone has any brilliant ideas, I would love to hear them.

(Edit: Attached the schematic--that would help!)

dfdye 11th March 2008 10:36 PM

Quick update: the 2X 1875's are getting changed to a 1876 in the interest of saving space. I'll repost the updated schematic later.


ttan98 11th March 2008 11:04 PM

Are you making PCBs for thses combo? I am interested in buying the PCB.

TheMG 11th March 2008 11:32 PM

If you want something compact, you might want to look into class D.

Particularly something like this:

Or this:

You won't have to worry about heat, which is a very nice bonus.

dfdye 12th March 2008 12:56 AM


Originally posted by TheMG
If you want something compact, you might want to look into class D.


That just isn't in the cards this time around, but I really do like the idea! Thanks for the suggestion. For now I am sticking with the opamps, but if a second generation comes around I may have to look into that.


dfdye 12th March 2008 12:59 AM


Originally posted by ttan98
Are you making PCBs for thses combo? I am interested in buying the PCB.
I will absolutely be making boards for this. I typically make prototype boards at home, but I will be posting board layouts for anyone who wants them. If the prototype works OK, I will be happy to look into doing a group buy if there is enough interest.


dfdye 12th March 2008 06:50 AM

2 Attachment(s)
Here's the update. After thinking it over and looking at what wall warts are available, and re-reading the "popping" problems associated with power up and power down, for single ended PSU's with the 1876, I decided to go with a more traditional +/- supply. I'll be using a BCB mounted transformer, and if my back of the envelope math is correct, I should be able to shoe-horn everything on the 10 X 7 cm board with minimal effort.

I have to decide on a transformer, and then I'll start laying out the boards.


sangram 12th March 2008 02:06 PM

Watching with keen interest. I've built up an Alien which uses a PCM2702, and I know you're some way down the road, but the 2702 has excellent sound, is pretty simple to build in the form of the Alien, and is a well-supported design. It specs out quite a bit higher than the 2707, and is SSOP28 which may be easier to manually solder in.

I'm looking at almost exactly the same thing, but I've decided to integrate a couple of things together at the construction level as I don't have the expertise for board creation. I have a couple of LM4766 spare, and I've built those P2P before, and a spare Alien or two. This will run off an inbuilt supply, and power a couple of minimonitors which I'm still dawdling on.

All the best. I like these tiny and quick projects!

Edit: I just noticed your output configuration, and the PCM series is happiest when driving >10K loads - your current config is about 1.5K. The 2707 can handle the load, but performance may be better if the 3.3K resistors are changed to 33K-47K range.

Edit2: Also a typical ECS 12MHz has a load capacitance of 33pf, or at least that's what came with the Aliens. I'm not sure they'll operate properly with a 1uF cap.

Edit 3: The PCM 2702 does not require the output pop/zobel filter. However all these solutions will pop somewhat, so an amp with a built-in mute will help supress the switch-on and switch-off pops with a well-designed mute circuit.

Edit 4: The one big advantage of the 2707 over the 2702 is that you can implement volume control directly on the chip, but you aren't using it...

dfdye 12th March 2008 11:09 PM


Many thanks for the suggestions!

1) I wasn't aware, but I haven't thought about those values much yet. I am looking at changing both those and the gain resistor values to make sure everything is optimized to get the most out of the system, so those values are quite likely to change. Thanks for pointing out the preferred load of the 2707, though!

2) The caps were just place holders while I looked for appropriate crystals. I know the total capacitance around the xtal should be 33pF. Those values will change once I decide on a crystal. If you have specific suggestions, I would love to hear!

3) I'm not really interested in including a mute function for this amp, thus tying the mute pins to ground to keep the amp on all of the time. It is my understanding based on the specs of the LM1876 that the internal circuitry should suppress turn on/off pop pretty well this way as long as there isn't a voltage on the inputs. I'll play with the prototype and see if this needs to be changed.

4) I completely agree that the volume control pins are nice, but I was originally designing this amp to be used from a computer front end. I hadn't considered "on amp" controls since I would have a full control panel on the desktop. I can easily include headers for these pins, though, to allow future use of these pins. The more I think about it, the more I like this suggestion!

Thanks again!


dfdye 13th March 2008 01:26 AM


Originally posted by sangram
Edit: I just noticed your output configuration, and the PCM series is happiest when driving >10K loads - your current config is about 1.5K. The 2707 can handle the load, but performance may be better if the 3.3K resistors are changed to 33K-47K range.
I double checked the data sheet to make sure I didn't miss anything after I first replied to your post, and the output of the 2707 is configured to drive a pair of headphones! Since the data sheet recommends loads greater than 32 ohms, I am rather happy with the 1.5K load presently in the circuit. Fortunately, this can be changed rather easily once the boards are built if need be!

Still, thanks for making note of this since I wanted to double check those values anyway.


EDIT: Woopsie! I finally found the part of the document you were taking about! The noise characteristics, etc. are indeed best driving a load greater than 10K. I stand corrected!

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