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Old 13th December 2011, 12:47 PM   #11
qusp is offline qusp  Australia
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yes and as you can see, the IV resistor is BEFORE the TX. even so audionote has been rather specifically vague on the performance of their own solution and that of the generalised unnamed competitors product, not really very useful. i'm afraid i dont take on advertising spiels as very meaningful

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Old 13th December 2011, 12:56 PM   #12
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yes and as you can see, the IV resistor is BEFORE the TX. even so audionote has been rather specifically vague on the performance of their own solution and that of the generalised unnamed competitors product, not really very useful. i'm afraid i dont take on advertising spiels as very meaningful
The resistor and capacitor on the primary and secondary of the transformer are for damping, not I/V conversion, as I understand it. Also, I think you misunderstand my intention here. I'm not trying to convince you or anyone else that this is better. I just think it's interesting.
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Old 13th December 2011, 01:03 PM   #13
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The outpit of a Sabre DAC is very different feom the output of PCM1794. As such, the results of using a transformer will be very different. I think this approach will probably make you very happy.
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Old 13th December 2011, 01:13 PM   #14
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The outpit of a Sabre DAC is very different from the output of PCM1794. As such, the results of using a transformer will be very different. I think this approach will probably make you very happy.
Thanks. That's very encouraging. I did build the USB receiver board last night, except that I made a mistake and ordered 4 wrong resistors. So, I have to get the right ones to finish it. This is the first time I've soldered SMD's and it's a challenge, for sure. I hope it works when it's done!

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Old 13th December 2011, 04:46 PM   #15
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ah ok, i just wondered why the back and forth. as i mentioned very early in the thread, i did this myself but with a common gate Fet input and no tubes quite some time ago, it works reasonably well, just not to my taste and not really inline with my end goal IE providing a transparent output with which to experiment with amps, i do not want colour from the dac.

a cap for damping? .... thats a LP filter and it will traditionally be part of IV conversion as well as setting the load line characteristic. the sabre will see this as an impedance across its output and will be putting out primarily voltage. still sounds quite well, just not for my taste, nothing says you have to have my taste though of course
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Old 13th December 2011, 07:02 PM   #16
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Originally Posted by dirkwright View Post
The resistor and capacitor on the primary and secondary of the transformer are for damping, not I/V conversion, as I understand it. Also, I think you misunderstand my intention here. I'm not trying to convince you or anyone else that this is better. I just think it's interesting.
Hi, dirkwright,

I feel that the sense of adventure and discovery which you are showing is one of the greatest things about this hobby. Might I might make a few comments regarding your DAC box plans?

I suppose the first thing is, that the resistor connected from the DAC chip to ground in that Audio Note schematic does, indeed, perform i/v conversion. I've never understood why AN says that it is the transformer is performing the i/v function. I suppose, one could argue that because the transformer primary requires current it is performing i/v conversion, but that would seem a bit too cute for me.

The capacitor in parallel with the i/v resistor forms a low-pass filter in conjunction with the DAC chip's output impedance, which is 1.7k ohms for the AD1865 DAC utilized by Audio Note. The i/v resistor, in parallel with the DAC output impedance, forms the source impedance seen by the transformer's primary. The lower the driving source impedance the lower will be the transformers distortion.

The second thing is, the transformer you've chosen is a 4:1 step down. Given the small signal which will be generated at the primary, you don't want to have the circuit working against itself by having to amplify the signal more than is necessary. So, if you already have possession of this transformer, I suggest that you try reversing the primary and secondary. That is, wire it as a 1:4 step up. You then could reduce the active amplification by a factor of 16! Which doesn't necessarily mean it will produce better sound, but you won't know untill you try .

Most of all, continue to have fun.
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Last edited by Ken Newton; 13th December 2011 at 07:11 PM.
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Old 13th December 2011, 07:10 PM   #17
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I'm not using their DAC chip. The one I'm using has differential outputs. I won't be using that exact circuit anyway. I was merely illustrating one example of using a transformer in I/V conversion. The "gnd" point on their DAC chip does not appear to be connected to ground, or at least not the ground for the part of the circuit on the secondary side of the transformer. They appear to be floating the "gnd" connection on the DAC.

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Old 13th December 2011, 07:14 PM   #18
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If I reverse the transformer, then bad things start to happen, like loss of bandwidth. What Jensen shows is a unity gain circuit with an op amp. The gain of the op amp is about 4 times, so over all it's a wash. It will have nothing, zero, nadda, between the output of the DAC I'm using and the transformer. So, what I'm using isn't exactly like what Audio Note shows.

thanks for your interest and kind remarks.
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Old 13th December 2011, 07:15 PM   #19
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it really doesnt matter, the resistor still forms part of an RC filter across the primary and performs the IV conversion, or rather in the case of sabre the impedance causes the sabre to perform its own. not the transformer in either case 1865 or 9018

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Old 13th December 2011, 07:16 PM   #20
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I'm not using their DAC chip. The one I'm using has differential outputs. I won't be using that exact circuit anyway. I was merely illustrating one example of using a transformer in I/V conversion. The "gnd" point on their DAC chip does not appear to be connected to ground, or at least not the ground for the part of the circuit on the secondary side of the transformer.
Yes, I realized that. The Sabre DAC chips, and nearly all others today, utilize differential outputs which are referenced not to ground but rather, to a D.C. voltage midway between ground and the chip's positive supply rail. The DAC chip industry made this move mostly as a cost saving measure. Only a unipolar power supply is required, rather than the bipolar supply which used to be required.
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