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The SkeletonDAC - Page 9

post #121 of 222

Madebynick,

 

Here's an example of what I was talking about up there with the cutoff frequency.  These are from a spreadsheet I developed where I modeled the actual RC high-pass circuit governing equation (not just the cutoff frequency) made by the SkeletonDAC if you attach it directly to headphones.  The first is using 47uf output capacitors, which will fit if in the standard Hammond case if mounted horizontally (bending the leads).  The second graph is using 220uf output capacitors, which I think is a good choice if the standard case is not used.  The "91 ohms" curve was sandwiched in there based on your question, assuming a 75 ohm adapter with 16 ohm IEMs.  There's a 16 ohm curve, too, but I think the capacitor size required gets a little bit unreasonable, whiich is why I was suggesting the 75 ohm adapter.

 

 

47uf.jpg

220uf.jpg



EDIT: In case someone gets the wrong idea, hooking the SkeletonDAC directly up to an amplifier will give a virtually flat response all the way down below 20Hz.  The load represented by most of our headphone amps is somewhere in the range of 10,000 ohms to 100,000 ohms (typically the volume pot).  That makes a huge difference.  The graphs above are intended only in the special instance of connecting the SkeletonDAC directly to headphones.
 


Edited by tomb - 11/21/11 at 9:29am
post #122 of 222

Almost forgot!  I noticed yesterday that the link of 'Step 2' of the construction pages has C5 listed twice.  The silkscreen on the PCB is correct, it's just the image smily_headphones1.gif  http://diyforums.org/SkeletonDAC/SkeletonDACstep2.php

post #123 of 222
Quote:
Originally Posted by torndorff View Post

Almost forgot!  I noticed yesterday that the link of 'Step 2' of the construction pages has C5 listed twice.  The silkscreen on the PCB is correct, it's just the image smily_headphones1.gif  http://diyforums.org/SkeletonDAC/SkeletonDACstep2.php


Good catch!  That's supposed to be C2 down lower by the bottom.  I'll work on getting that fixed and report back when it is. Thanks!
 

 

post #124 of 222

 

Tomb

 

Many thanks for the reply sir. You have set a few things straight in my head which is a big help. On your advice, I took delivery of some 75ohm resistors and will integrate them into the new cable I'm building. I do think I need to do some reading around this subject however as there are many terms thrown around which are still somewhat confusing to me! 

 

Perhaps you could clear up a couple of other things for me too if thats ok? 

 

You mention that the new TI chip you're using in the SkeletonDAC can now be used as a headphone amp and as I understand it, the headphones would simply be connected to the same output one would use when connecting the DAC to an amp (assuming standard setup with flat frequency response). With this in mind, what makes the new TI chip more suitable for driving headphones from this output compared to the original chip? If the answer is outside the scope of this forum, please tell me. I won't mind. 

 

Secondly, and this is probably basic knowledge that I should have already accrued before I venture to these forums, is to do with those frequency response curves you posted. It's taken a lot of web reading to try to explain to myself exactly what these graphs mean and now I think I understand. Please feel free to send a rolling eye emoji and point me towards electrical engineering nursery school if I'm wrong. 

 

If I'm correct, those graphs are showing (very simply put) the volume of the frequency range indicated by the graph. These curves are the direct result of altering the combination of resistor and capacitor values and suggest that with a larger capacitance value, the volume of lower frequencies will be higher and therefore sound output will appear more bassy? (Apologies if I sound like a bass head here. I don't mean to!) What makes me think that I may be wrong however is the shape of the frequency curve around 1300Hz. Is this curvature due to a filter built into the circuits design that ensure higher frequencies cannot be heard? I may guess that the effect of increasing the load applied to the DAC as a result of the 75ohm resistor is the controlling factor dictating where this curve plateaus? Am I correct in thinking that if this flat response was no seen at 1300Hz and above, that the higher frequencies would be clearly heard as a background hiss as I have experienced in the past? 

 

I do hope these questions won't cement my reputation for being an audio electronic novice to the point where my head will be permanently hung in shame.

 

Thanks in advance,

 

Nick 

 

post #125 of 222



 

Quote:
Originally Posted by Madebynick View Post

 

Tomb

 

Many thanks for the reply sir. You have set a few things straight in my head which is a big help. On your advice, I took delivery of some 75ohm resistors and will integrate them into the new cable I'm building. I do think I need to do some reading around this subject however as there are many terms thrown around which are still somewhat confusing to me! 

 

Perhaps you could clear up a couple of other things for me too if thats ok? 

 

You mention that the new TI chip you're using in the SkeletonDAC can now be used as a headphone amp and as I understand it, the headphones would simply be connected to the same output one would use when connecting the DAC to an amp (assuming standard setup with flat frequency response). With this in mind, what makes the new TI chip more suitable for driving headphones from this output compared to the original chip? If the answer is outside the scope of this forum, please tell me. I won't mind. 

 

Secondly, and this is probably basic knowledge that I should have already accrued before I venture to these forums, is to do with those frequency response curves you posted. It's taken a lot of web reading to try to explain to myself exactly what these graphs mean and now I think I understand. Please feel free to send a rolling eye emoji and point me towards electrical engineering nursery school if I'm wrong. 

 

If I'm correct, those graphs are showing (very simply put) the volume of the frequency range indicated by the graph. These curves are the direct result of altering the combination of resistor and capacitor values and suggest that with a larger capacitance value, the volume of lower frequencies will be higher and therefore sound output will appear more bassy? (Apologies if I sound like a bass head here. I don't mean to!) What makes me think that I may be wrong however is the shape of the frequency curve around 1300Hz. Is this curvature due to a filter built into the circuits design that ensure higher frequencies cannot be heard? I may guess that the effect of increasing the load applied to the DAC as a result of the 75ohm resistor is the controlling factor dictating where this curve plateaus? Am I correct in thinking that if this flat response was no seen at 1300Hz and above, that the higher frequencies would be clearly heard as a background hiss as I have experienced in the past? 

 

I do hope these questions won't cement my reputation for being an audio electronic novice to the point where my head will be permanently hung in shame.

 

Thanks in advance,

 

Nick 

 


1. The PCM2704/5/6/7/ chips all have a built-in headphone amplifier.  The chip that it replaced was the PCM2702.  It had no amplifier and the output was taken directly from the Digital-Analog converted output.  In the case of the PCM2702, it was a voltage output, but very small with practically no current output.  IOW, you could actually damage the PCM2702 chip if you attempted to feed headphones directly from the DAC.  With the built-in headphone amplifier of the PCM2704/5 used in the SkeletonDAC, you can actually hook headphones up and generally speaking - not worry about blowing up the DAC chip.

 

All that said, the quality in driving ability of the DAC's amp is not super.  So, the types of headphones, their efficiency, and the resulting high-pass filter circuit resulting from your parts choice will dictate how well it drives a pair of headphones.

 

2. You have this backwards, actually.  The "0 dB" line on those graphs (the X-axis) is the point of normal volume, all things being equal. So, what the curves are telling you is how much the bass is decreased by the resulting RC-circuit high-pass filter.  The RC-circuit high-pass filter is a natural phenomenon of the electronic circuits and loads in play.  You have no choice in the matter on whether it will occur - only that you have a choice in selecting parts that will minimize the bass loss.  With low-impedance phones, a very large value capacitorr (470uf to 1000uf) will result in almost no bass reduction at all.  However, the caps will be very, very big (in most instances).  So, what I'm trying to portray is that you can come up with a compromise between bass lost and inconvenient capacitor size.  That's all.

 

I'm not sure what you mean by the 1300 Hz.  I suppose if you looked with a magnifying glass at the first graph up there that's based on 47uf output capacitors, one might say that the bass attenuation begins at about 1300 Hz with a 16 ohm impedence headphone load - and bass attenuation begins at about 1100Hz with 32 ohm headphones, 1000 Hz with 60 ohm headphones and so on and so forth.  But - that's all you can say about it.  The whole point is that is the theoretical result with 47uf output coupling caps.  As you can see in the second graph, the frequencies at which bass begins to be attenuated is quite a bit lower with 220uf output coupling caps.

 

The 75 ohm resistors were suggested to raise the "R" value in the fundamental relationship of the RC circuit.  R is multiplied times C (and also modified by other factors).  So, if we increase R or increase C - the result is that bass attenuation begins at a much lower frequency.  The difference is that big resistors are much cheaper and easier to obtain and install in the circuit than big capacitors.  At some point, higher value resistors will have a noticeable impact on the audio sound themselves, but if given a choice between increasing impedance at 16 ohms or selecting some outrageously large and expensive capacitors, the 75 ohm resistor is a no-brainer.  This has a long history of precedence, because that's exactly what a 75 ohm adapter is intended to do with IEMs and small PDP's.  Small PDP's cannot afford to include physically large output capacitors, so the resistor adapter was developed, instead (R times C exists with PDP's and their headphone connections).  Output coupling capacitors may not exist with all PDP's, but usually a line-out adapter bypasses everything that was intended to cut down on the DC offset.  So, to protect your headphones, DIY line-out adapters usually involve output coupling capacitors of some type.  This would also occur in the SkeletonDAC (and the Bantam/Alien before it): if output coupling capacitors were not used, your headphones would see 2.5V on output at zero volume.  That's why the capacitors are used, but the physics of the resulting electrical circuit has some drawbacks - mainly that an RC circuit is formed with the "R" of the headphones.  This means that if you are not careful in parts selection with the capacitors (or some resistor adapter), then you will lose bass in the audible frequency band (what the curves in the graphs above show).

 

EDIT/PS - After posting the first response above, I realized that one neat solution is to simply use the 560uf organic polymer caps - used in the GrubDAC and SkeletonDAC as power caps - as the output coupling caps on the Skeleton DAC [for direct-connected headphones].  I haven't tested how they sound, but others seem to think that OsCons (another organic polymer type of capacitor) sounded pretty good in the signal path. The nice thing is that those 560 uf's are very little bigger than the pads already on the PCB - and height is not an issue, even with the little Hammond plastic case (1551HTBU).  
 

 


Edited by tomb - 11/25/11 at 4:37pm
post #126 of 222

torndorff (above) pointed out an error in the construction graphics on the SkeletonDAC website here:

http://www.diyforums.org/SkeletonDAC/SkeletonDACstep2.php

 

The PCB was showing two "C5's". That's now been corrected - one was C2.  The Layout and Board pages were always correct.  For some reason, I just pulled an older background on those construction graphics pages.

 

Anyone built one yet that can share some pics?

 


Edited by tomb - 12/3/11 at 6:30am
post #127 of 222

OK - here's another correction.  I just got a PM awhile ago asking about parts for the USB-to-SPDIF convertor option.  In searching to remember the answer, I noticed that the part was not documented on the website in the BOM.  I've since made that correction so that the part number at Mouser and DigiKey for the transformer (Pulse PE-65612) is shown on the BOM.

 

Please note that this transformer is not included in the Beezar kit, but it is the only part that isn't.  So, if you desire to use the USB-to-SPDIF option, all you need to do is add the transformer.

post #128 of 222

Thanks for all your clarification with this Tomb. Following your recommendations I've finished my Headphone DAC with 1200uf caps (as I happened to have some that I could salvage from an old mixer) and housed everything in a little extruded aluminium case from Hammond. I soldered my USB cable to the board as the pins on the connector was sadly damaged in the post. The DAC powers my homemade cans and IEMS well with as good an example of bass response that I've heard from these headphones. 

 

I'll be sure to upload some pics when the lighting is better.

 

Postage to the UK was fast and the surface mount soldering is surprisingly straight forward (although a flux pen is essential). I would certainly recommend the kit to anyone thinking of building a DAC and I'd be very interested to hear it next to some of these expensive branded units available. 

 

Thanks again to everyone involved in the development of this kit. I don't think I'll buy another piece of ready-assembled audio equipment again! 

post #129 of 222
Quote:
Originally Posted by Madebynick View Post

Thanks for all your clarification with this Tomb. Following your recommendations I've finished my Headphone DAC with 1200uf caps (as I happened to have some that I could salvage from an old mixer) and housed everything in a little extruded aluminium case from Hammond. I soldered my USB cable to the board as the pins on the connector was sadly damaged in the post. The DAC powers my homemade cans and IEMS well with as good an example of bass response that I've heard from these headphones. 

 

I'll be sure to upload some pics when the lighting is better.

 

Postage to the UK was fast and the surface mount soldering is surprisingly straight forward (although a flux pen is essential). I would certainly recommend the kit to anyone thinking of building a DAC and I'd be very interested to hear it next to some of these expensive branded units available. 

 

Thanks again to everyone involved in the development of this kit. I don't think I'll buy another piece of ready-assembled audio equipment again! 


Glad to hear it!!


 

 

post #130 of 222

Im having trouble with the PCM2704 chip. I decided to go with the flood and suck method as the drag and wipe method just wasnt working for me. However, my suction device isnt working that well. I have some solder wick, but it is radioshack stuff and it doesnt wick very well. What should I do? Should I get some higher quality wick? 

post #131 of 222
Quote:
Originally Posted by Galesden View Post

Im having trouble with the PCM2704 chip. I decided to go with the flood and suck method as the drag and wipe method just wasnt working for me. However, my suction device isnt working that well. I have some solder wick, but it is radioshack stuff and it doesnt wick very well. What should I do? Should I get some higher quality wick? 


Do you have some separate flux?  If the wick is not working properly, more flux usually does the trick.  You have to be absolutely certain that the wick is free of solder, too.  Cut it down to where it's only clean and un-contaminated copper wick, then start with that.  Dip the wick into some flux and apply heat with the soldering iron by pressing the wick between the solder joint and the soldering iron tip.

 

Flux should be the non-corrosive type - rosin is best. Or, if you bought a flux pen, use that.  Wicking is somewhat of an art.  With the right heat and flux, it will literally suck up the solder.  A cold iron, no flux, or a wick whose tip is already saturated with solder will all inhibit this process.  You should be continually cutting off the wick with each new application so that only clean wick remains.
 

 

post #132 of 222

Hello,

 

This project looks very interesting to me. I'm just getting done with finals so I'll be going home for a few weeks, but I hope to order a kit and build it when I start next semester. I'm keeping an eye on how that headphone feature turns out. Anyways, I noticed that on the project website, the text in the browser tab (and probably the window, depending on what browser you use) on the page for the "Tweaks" section says "GrubDAC Tweaks" instead of SkeletonDAC tweaks. It's just a little, insignificant thing that I noticed. I think that that is the only page affected. 

 

Thank you, tomb, for all of your work on all of this. I've been pretty quiet on this forum, but I think all of these projects are really cool.

post #133 of 222
Quote:
Originally Posted by nickosha View Post

Hello,

 

This project looks very interesting to me. I'm just getting done with finals so I'll be going home for a few weeks, but I hope to order a kit and build it when I start next semester. I'm keeping an eye on how that headphone feature turns out. Anyways, I noticed that on the project website, the text in the browser tab (and probably the window, depending on what browser you use) on the page for the "Tweaks" section says "GrubDAC Tweaks" instead of SkeletonDAC tweaks. It's just a little, insignificant thing that I noticed. I think that that is the only page affected. 

 

Thank you, tomb, for all of your work on all of this. I've been pretty quiet on this forum, but I think all of these projects are really cool.


Thanks for the kind comment, but remember that cobaltmute is the genius designer.wink.gif  I wouldn't have anything to do if it wasn't for the wonderfully gifted designers around here that let me support and market their designs.

 

Anyway, I'll get that corrected on the web site when I get a chance.  I'm off for the holidays, so maybe I can get some of this done.
 

 

post #134 of 222

My computer does not want to recognize the DAC. Im trying to use it as a USB to SPDIF converter. I plugged it in and the led lights up, but there are not messages showing up recognizing the dac. Also, I have connected the SkeletonDAC to an external dac, which is connected to an amp. If I play music off my computer, sound should come out of my headphones, yes? All that happens right now is sound comes from my laptop speakers. Should I reflow the pins on the dac chip? I soldered one side pretty cleanly, but the other side is slightly off and Im concerned that it is the problem. I hope I am just using it incorrectly and I dont have to reflow the pins.P1010031.JPG

post #135 of 222
Quote:
Originally Posted by Galesden View Post

My computer does not want to recognize the DAC. Im trying to use it as a USB to SPDIF converter. I plugged it in and the led lights up, but there are not messages showing up recognizing the dac. Also, I have connected the SkeletonDAC to an external dac, which is connected to an amp. If I play music off my computer, sound should come out of my headphones, yes? All that happens right now is sound comes from my laptop speakers. Should I reflow the pins on the dac chip? I soldered one side pretty cleanly, but the other side is slightly off and Im concerned that it is the problem. I hope I am just using it incorrectly and I dont have to reflow the pins.P1010031.JPG


Unfortunately, your mis-alignment is a lot worse than you led me to believe in your PM's.  I see several bridges.  Some of the pins are actually in the absolute worst-case position: they are straddling the space between pads and making connections to both.  This is obvious simply by taking a look at your photo.

 

Ordinarily, I'd suggest an X-acto knife to get in there and cut the bridges.  However, with the pins straddling the pads in most cases, you really need to move the chip.  Get some de-soldering wick and try to remove every bit of solder on that side.  Then bend the chip so that the pins are aligned with the pads.  It's OK - the pins on the other side will be fine, but you need to be sure that all solder is removed from this side before you do that.  If we had caught it when you had just soldered the anchor pins on either side, it would be an easy matter.  I do it myself quite oten.  All you need do is melt the solder on one side and the pins on the other side will bend enough to let you align it.

 

In this case, be sure you can lift the PCM chip up off the PCB on this side (very carefully).  If you can do that, then you know you've removed enough solder.  Then, making certain the chip is flat agains the PCB, rotate it so that it's aligned.  The pins on the other side, by virtue of being soldered, will bend to meet the new alignment.  Then solder the pins on this side again.

 

You can't do anything with the SkeletonDAC - either audio output or SPDIF convertor - until you get the PC to recognize the DAC.  The LED is not consequential - it only indicates power from the USB buss.
 

 


Edited by tomb - 1/7/12 at 5:30pm
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