Help me decide which ebay dac please?

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Laurence
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Post by Laurence » Sun Oct 09, 2011 8:13 pm

Yes but what is the specified transformer impedance?

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Post by Laurence » Sun Oct 09, 2011 8:20 pm

I have just realised from what you are saying that you have the transformer after the valves. My questions were with the assumption that you loaded the DAC with the transformer. My mistake.

But in any case is there an answer to the question that should you load the DAC with a transformer what would be the impedance?

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Nick
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Post by Nick » Sun Oct 09, 2011 9:57 pm

Ok. yes, DAC direct to grid, 9+9:1 transformer in anodes of 6n6p.

As for the impedance. No idea. the spec says a minimum of 1k5, but I guess the more the better.
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Paul Barker
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Post by Paul Barker » Mon Oct 10, 2011 7:22 am

I suspect the 1k5 refers to the minimum dc resistance across the DAC so as not to load too much current down. I am not sure the AC side. would be a matter for experience, surely someone has done it? I don't have a suitable transformer and mine sounds too good to purchase such costly parts.

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Post by Nick » Mon Oct 10, 2011 10:08 am

The sheets state a minimum of 1k5 AC load, but I would hope that any TX actually used would present far more load. I don;t know what the source resistance of the DAc is, but I doubt they expect it to be presented with a load that varies with frequency (and is reactive). So I guess thats what Paul means by a AC load.
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Mike H
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Post by Mike H » Mon Oct 10, 2011 1:23 pm

Nick wrote:Ok. yes, DAC direct to grid, 9+9:1 transformer in anodes of 6n6p.
Ah so each needs to have identical anode current? So you made the DC biased (no signal) values the same?

Are the cathodes commoned together?
 
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Post by Nick » Mon Oct 10, 2011 5:03 pm

Yes, its a LTP, with the balancing arrangement posted earlier. Possibly one way to force balance would be a CCS in each cathode, one trimmed to give equal current, then a bipolar electrolytic to AC couple the cathodes. That has the major advantage in that it should not drift.
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Post by Paul Barker » Mon Oct 10, 2011 5:46 pm

sorry i hadn't rechecked the data sheet. 1k5 AC load should be enough then. I wouldn't mind betting the source impedance is very low. Would not be too hard to determine it by using the laptop as a sig generator and scope for the null point with variable resistance. there again mucking about like that might kill the chip. Forget that.


I don't suppose anyone found out how Allen Wright did his special balancing act?

What you said Nick probably works.

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Post by Mike H » Mon Oct 10, 2011 6:39 pm

Nick wrote:Possibly one way to force balance would be a CCS in each cathode, one trimmed to give equal current, then a bipolar electrolytic to AC couple the cathodes. That has the major advantage in that it should not drift.
Very spookily, I was thinking along the same lines. Just not mentioned it yet.

No really I was Image
 
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Post by Andrew » Mon Oct 10, 2011 7:27 pm

I had something along those lines planned, tho' it wasn't nearly as well thought through yet.

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Laurence
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Post by Laurence » Mon Oct 10, 2011 8:38 pm

Nick wrote:Yes, its a LTP, with the balancing arrangement posted earlier. Possibly one way to force balance would be a CCS in each cathode, one trimmed to give equal current, then a bipolar electrolytic to AC couple the cathodes. That has the major advantage in that it should not drift.
Put the pair of balanced CCS's on the power supply before the transformer.

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Post by Nick » Mon Oct 10, 2011 8:51 pm

Centre tapped.
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Post by Andrew » Mon Oct 10, 2011 9:01 pm

Here's how it measures, this is just the e-bay DAC not the valve output stage.
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red_fft.jpg
FTT 1kHz
red_fr.jpg
Freq response
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Dave the bass
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Post by Dave the bass » Mon Oct 10, 2011 9:08 pm

<question from the back of the class>

"Sir! Sir!"

The 'fft' graph, is that showing the harmonics produced either side of a 1Khz signal? I've never really understood what I should be looking at here TBH.

I can see as freq rises so does the background noise level (I think).

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Post by Andrew » Mon Oct 10, 2011 10:25 pm

Well, you need to see the baseline without the DAC, every measurement system has its own signature so there is some irreducible error involved, basically the sonic signature of my 'audio analyser'.

If you look the baseline you see some of the artefacts that are present in the first instance so you need to subtract the two to get the true DAC measurement - nothing is perfect, after all.

There is indeed harmonics,you are spot on, interestingly, since the DAC is balanced, 3rd seems to dominate, as it should....see the peak at 3kH; that's 3rd, there's no 2nd which should be expected in a good balanced system; it should cancel out.

There's some noise around 50Hz and again at 150Hz that isn't on the baseline, my guess this is due to the unshielded PCB picking up the sea of hum in my gaffe and amplifying it or possibly (but unlikely, I hope) PSU noise and getting amplified. I'm not sure what the 750Hz is caused by, it could be the lower odd harmonic of the 1kHz.

The good thing is the worst is around over -90 db down, which is not bad at all in the grand scheme of things, a fart in thunder, really.

I would say the designer has done a good job on this one, measurements don't tell us everything, but they are a good place to start to understand what going on.

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It looks something like this.
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