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pupDAC Step-by-Step Build Thread - Page 23

post #331 of 338
Thread Starter 
Quote:
Originally Posted by UnsatisfactoryR View Post
 
Quote:
Originally Posted by tomb View Post
 

 

Unnecessary lead length?  Are the resistors flush to the PCB.  For the four you measured above, are they on the bottom of the PCB, next to the opamp?  What about the mica capacitors?  Those have to really be pressed into the PCB to keep from sticking way up over any other part.  The added lead length could be offsetting things.

 

Have you tested the voltage into the opamp?  These would be pins 8 and 4.  See if those are the same voltage.  Pin 8 should be the same as the 2.5V test point coming from U10.  Pin 4 should be the -2.5V test point coming from U8.  See what those voltages are at the pins on the opamp, though - relative to Ground.

 

I can't think of anything else.  I would suggest that 3.3mV is not that much offset in the scheme of things, but you are claiming to hear a sound level difference between channels.  How about the rest of the through-hole resistors?  Can you compare their resistances, left channel to right channel and see if there's something not matched?

Everything is flush to the board, the mica caps do stick up maybe 1-2mm since that was as far as I could get them. Voltage into pins 4 and 8 match up.

 

As for the other resistors, I actually edited my last post with those readings.


Well, some of those resistors are in parallel - so the absolute values are off.  Since the problem is offset, though, we'd be looking for an imbalance somewhere and you're not measuring any.  The only thing left are the rest of the through-hole resistors, R5, R7 & R16, R18.  Try measuring those for an imbalance.

 

You've checked the resistors that control the opamp and you've verified that the opamp is getting the same voltage between the channels.  The only other possibility is that the signal level feeding into the opamp is off before it gets to the opamp.  That would happen at the four resistors mentioned above.  They take the output directly from the DAC chip (IOLP, IOLN & IORP, IORN).

 

If none of that checks out, I don't have an explanation.

post #332 of 338
Quote:
Originally Posted by tomb View Post


Did you solder those top pads for the oscillator (clock chip)? It doesn't show in your pics, but what c an be seen looks awfully bare. Also, one of the two joints showing looks awfully cold. If the clock chip is not working, there's no way to synchronize/hand-shake the USB connection and the PC will not recognize the device.


Added solder and reflowed the clock chip, now it is detected by my pc (for some reason only usb 2.0 ports, usb 3.0 ports shows unrecognized device.)

 

Listening with an iem... low volume as expected but quite noisy... havn't fixed my amp yet.

post #333 of 338
Thread Starter 
Quote:
Originally Posted by Jawmare View Post
 
Quote:
Originally Posted by tomb View Post


Did you solder those top pads for the oscillator (clock chip)? It doesn't show in your pics, but what c an be seen looks awfully bare. Also, one of the two joints showing looks awfully cold. If the clock chip is not working, there's no way to synchronize/hand-shake the USB connection and the PC will not recognize the device.


Added solder and reflowed the clock chip, now it is detected by my pc (for some reason only usb 2.0 ports, usb 3.0 ports shows unrecognized device.)

 

Listening with an iem... low volume as expected but quite noisy... havn't fixed my amp yet.

 

The USB chip (PCM2707) is not compatible with USB 3.0. ;)  That said, I have read and used USB 3.0 sockets that were backward compatible, so I'm not sure why it's doing that.  I'll try to o some testing when I get a chance.

 

The big thing is that you got it working. :)

post #334 of 338
Hi there, im thinking about building a pupDAC myself, the only question i have is, is it possible to output the analogue signal via 3.5mm, as opposed to RCA?
post #335 of 338
Quote:
Originally Posted by HarryOMalley View Post

Hi there, im thinking about building a pupDAC myself, the only question i have is, is it possible to output the analogue signal via 3.5mm, as opposed to RCA?


Yes, you use the 3.5mm jack in the back instead of the RCA jacks

post #336 of 338

Ah ok great! Thanks

post #337 of 338
Which resistors in the schematic (http://www.diyforums.org/PupDAC/schematic/pupDACschematic.pdf) control the gain of the PupDAC?

My amp and speakers are very sensitive and I can't really get past 9 o'clock on the volume dial, is it possible to reduce the vrms output of the Pup somehow?
post #338 of 338
Thread Starter 
Quote:
Originally Posted by lordearl View Post

Which resistors in the schematic (http://www.diyforums.org/PupDAC/schematic/pupDACschematic.pdf) control the gain of the PupDAC?

My amp and speakers are very sensitive and I can't really get past 9 o'clock on the volume dial, is it possible to reduce the vrms output of the Pup somehow?

 

The gain is set from the quotient of R6/R5 (Left) and R17/R16 (Right).  Along with R7 and R18, R5 and R16 establish the voltage reference for the diffierential output of the PCM1794 DAC.  So, you don't want to mess with the four resistors R5, R7, R16, and R18:

 

The gain is actually set by R6 (Left) and R17 (Right).  The BOM specs them at 1.5K.  You might try anything down to 1K and see how things work.  You do not want to use less than 1K or the opamp may lose stability. 

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