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Old 7th December 2011, 05:45 PM   #1
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Default DAC project

How does this look for a USB DAC? I'm using the Twisted Pear USB receiver and COD DAC, a pair of Jensen step down transformers, and a line stage I got off of eBay. This DAC is going to be only connected to a computer via USB so I don't need other digital inputs. The output of the line stage is powerful enough to drive some headphones, which is a plus for me. The transformers provide I/V conversion and also provide a low source impedance for the line stage. This is the first time I've considered putting together my own DAC.
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Old 8th December 2011, 06:43 AM   #2
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You could always use a tube stage instead of a transformer..
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Old 8th December 2011, 12:43 PM   #3
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Quote:
Originally Posted by beau2317 View Post
You could always use a tube stage instead of a transformer..
Hahaha!!!
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Old 8th December 2011, 12:51 PM   #4
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transformers are usually a pretty bad choice for directly coupling to a current out dac imo, would you not be better off using a discrete or chip SS IV stage and simply using the transformer to buffer the output of it? usually transformers are put to use on voltage out dacs, or current out dacs after a passive resistive or SS IV stage

I wont mention tubes =) i also dont really approve, so i get it =P

Last edited by qusp; 8th December 2011 at 12:54 PM.
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Old 8th December 2011, 01:35 PM   #5
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I thought transformers were the ideal I/V converter? I think Audio Note has them in their DAC. The output of the Twisted Pear COD is a balanced current source, which could be converted to a voltage with a resistor and a capacitor on each side and channel (4 sets total), but the advantages of the transformer are much more than that in my opinion. They provide RFI filtering (so that the line stage doesn't need it), digital noise filtering, ground isolation, common mode noise rejection and a perfect balanced to unbalanced conversion, all in one unit. The transformer I selected has outstanding response as well as providing a really low source impedance for the line stage, which means it's going to be very quiet. The only down side that I can see is that I should keep the power transformers out of the same box as the audio transformers to prevent any possible hum pickup.
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Old 8th December 2011, 02:06 PM   #6
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i'm not saying you shouldnt use the TX, although ive been there done that with sabre and while it was kinda nice and effortless, i just found it made everything sound like that, if you know what i mean, kinda lacking zing when its called for. but regardless yes they do all you mention and in a kinda elegant way (except the IV conversion), but replacing the actual IV resistor is not one of those things imo. it will work, but you are replacing a pretty predictable and linear component with something that is anything but that.

as part of the IV sure, to provide isolation, impedance matching, BAL-SE conversion, rolling off the junk in the HF etc that other than the added distortion and non-linearity it does all that quite well, especially with a decent component like the jensens; but as a resistor they make a pretty good transformer. all imo of course

Last edited by qusp; 8th December 2011 at 02:09 PM.
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Old 8th December 2011, 02:18 PM   #7
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It's a dang good transformer:
http://www.jensen-transformers.com/datashts/10kbd.pdf
THD@ 20Hz = 0.015%
THD@ 1kHz = <0.001%
-3dB @ 0.5Hz & 180kHz
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Old 8th December 2011, 04:59 PM   #8
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but its still a transformer and thus not linear in the slightest, theres no way around that due to all the interwinding capacitance and parasitics.

i'm intrigued to hear a friends urushi painted Feastrex finemet line trannies. a friend of mine bought some for his sabre for a rather insane price, but i still want to hear them. he seems to love painting everything in urushi ha

Last edited by qusp; 8th December 2011 at 05:02 PM.
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Old 8th December 2011, 06:13 PM   #9
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Well, I'm new to designing DAC's so who knows, maybe a transformer isn't a good idea. I'm going to give it a try anyway though. I don't feel they are as nonlinear as you suggest though. The data Jensen provides is extremely good in my opinion, for this purpose.
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Old 13th December 2011, 12:23 PM   #10
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Here's the schematic from Audio Note showing their transformer I/V conversion circuit. They state:
"I/V Transformers are used to maximize the energy transfer during the Current-to-Voltage phase of the conversion; resulting in increased dynamics."
and:
"The output of most resistor ladder DAC chips is in the form of a current rather than a voltage. There are many ways to convert the current into a voltage but the most commonly used system is that of an op-amp connected as an I to V converter. This system requires the use of a high degree of feedback, and as a result there are problems associated with it. One of those is internal slew rate limiting of the op-amp itself. The rate of change of current at the output of even an audio DAC is very fast indeed. Even modern fast op-amps will slew limit internally and that affects sound quality. Some engineers have found that using extremely fast op-amps improves the sound quality, but we have completely sidestepped the issue by using a transformer I/V system. The transformer not only provides an I to V function but the way it is used in our latest DACs transfers maximum energy from the DAC chip itself. This in itself reduces overshoot and ringing and because the system is slightly overdamped the rise time is reduced to an acceptable rate as well."
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Last edited by dirkwright; 13th December 2011 at 12:26 PM.
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