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Old 26th July 2012, 11:46 AM   #1
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Default Which DAC for 1.5 to 2v into tube stage?

I currently have a RAKK DAC which outputs .5v per leg (differential) into my tube stage. The I/V converter resistors are 75 ohms, which doesn't help.

I need more like 2v, but could get by with 1.5v. Interested in both SE and differential outputs. I'd prefer not to use a step-up transformer if possible.

All ideas welcome!

Andy
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Old 26th July 2012, 12:49 PM   #2
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When you say 'which DAC' do you mean you want to modify your design to use another DAC chip? Or you want a DAC pcb which has already been assembled? Or a finished unit which doesn't need modding? Or a suggestion of which chip to start a new design with? Are you a devotee of S-D parts? (the RAKK uses a TI/BB S-D part it seems).
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Old 26th July 2012, 01:17 PM   #3
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Andy,

Like abraxalito, I'm not certain about what you are attempting. If you merely want to increase the output level of your RAKK DAC, then I suggest simply inceasing those I/V resistors from 75R to 133R. This will produce just about 1.5VRMS differentially at the passive I/V. The only question is whether distortion would increase by a significant amount. It may not, as I believe that the Neko D100 DAC utilizes a pair of mono configured PCM1794A chips (into a passive I/V) as does the RAKK DAC. In the Neko product, I believe they are using 100R I/V resistors without problem. There are dual concerns regarding the value of the I/V resistor. One is the peak-peak signal voltage swing seen at the current outputs, but another is the quiescent (resting) D.C. Voltage level produced by the output bias current of a mono configured PCM1794A, which I recall as being around 12.4mA. 133R resistors will place the D.C. level at about +1.65V, giving a peak positive signal level of about +2.7V.

I would try 133R I/V resistors and compare the THD versus with the 75R resistors if you have access to an THD analyzer, or simply listen and judge subjectively.
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Last edited by Ken Newton; 26th July 2012 at 01:29 PM.
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Old 26th July 2012, 03:15 PM   #4
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Hello guys

Very useful. Thanks Ken. I can try that resistor increase as you say.

I doubt if I could build a whole DAC from parts, but maybe not impossible. I'd really be looking for an assembled board. I can do the PSU, case etc.

So if there were anything on the market that was a step up from my PCM1794A RAKK DAC (v.1) and gave me 2v out I'd be interested in buying and trying it.

I'd be wanting to feed the output straight into the grids of a tube stage, either 26 or 4P1L. Could be SE or differential.

Andy
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Old 26th July 2012, 08:52 PM   #5
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Hi Andy,

Maybe you could try removing the I/V resistor, replacing it with a valve I/V converter.

The schematics we have tried
Explanations on tubecad

The very good points are that the output voltage depends on Ra so you can output 2vp easily and the input impedance is 0,1Ω over all the sonic bandwidth. The downside of this converter is his high output impedance (approx = Ra) if that is a problem for you, you might want to set a cathod follower output stage.

Enjoy.
Greg
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Old 26th July 2012, 10:04 PM   #6
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Have a look there :
ES9023 DAC PCB
#44 Jean-Paul gives informations...
ES9023 will provide you about 2.0V output, you've to choose your flavor SE or Balanced.

Phil
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Old 27th July 2012, 01:49 AM   #7
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Quote:
Originally Posted by andyjevans View Post
I doubt if I could build a whole DAC from parts, but maybe not impossible. I'd really be looking for an assembled board. I can do the PSU, case etc.

So if there were anything on the market that was a step up from my PCM1794A RAKK DAC (v.1) and gave me 2v out I'd be interested in buying and trying it.
Seeing as you're a tube kinda guy I'm surprised you haven't yet been initiated into the world of NOS. You could do worse than get a Muse Mini 4 * TDA1543 - the output resistors can be changed to give any swing you like up to the compliance limit of the DAC (which I think is 4V p-p).
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