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Old 22nd May 2012, 10:50 AM   #841
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It is a bit hard to explain. A "perfect" AD converter, say run on 44kHz would have such a steep anti alias filter so that no tone over slightly less then half the sampling rate would land on the medium ( say CD ). Reproduced over a "perfect" linear phase DA converter with the same sampling rate no tones will be added and no tones will be taken away, although it "looks" like there will be tones before and after the event because say a 1kHz square wave has pre and post ringing. That is simply the result of the brick wall filter, nothing over ca.20kHz. So what we see is not tones added but tones subtracted, namely over 20kHz. It is called the Gibbs phenomenon as far as i know.
WHEN the AD converter passes tones over half the sampling rate, so it aliases, the situation is a bit different.
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Old 22nd May 2012, 11:02 AM   #842
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Tks.
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Old 22nd May 2012, 11:09 AM   #843
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Gibbs phenomenon - Wikipedia, the free encyclopedia

it is scary, wikipedia knows even this.
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Old 22nd May 2012, 11:13 AM   #844
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You see, the more overtones there are, "the squarer the square".
In analog this is a bit different because there is not such a steep "here is something, there is nothing" situation so instead of pre- and post-ringing we get rounded edges.
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Old 22nd May 2012, 11:29 AM   #845
regal is offline regal  United States
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I am hoping with time off over memorial day that I can hook this up to my AD1862 dac. Whats a realistic THD -90dB ? Mine is the parallel 2SK369BLs Sen, have them matched and the sweet heatsink

Thinking of powering it with one of those 18V Nicad drill batteries, good idea or best to go lithium (either li-ion or LiFePo4)? I don't need long listening job general 1-3hrs per week.

thanks

Last edited by regal; 22nd May 2012 at 11:47 AM.
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Old 22nd May 2012, 03:15 PM   #846
EUVL is offline EUVL  Europe
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-90dB for the IV stage is realistic, yes. You need to add the DAC THD on top.
I have no measurement to prove, so would be interested if you can measure.

NiCad is fine. I started with alkaline dry batteries.
There is no end to what you can try with fine tuning.


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Old 23rd May 2012, 04:19 PM   #847
heb1001 is offline heb1001  China
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Quote:
Originally Posted by EUVL View Post
-90dB for the IV stage is realistic, yes.
My current numbers are:
2nd -136dB,
3rd -87.7dB,
4th and above all < -110dB
peaks of noise all < -98dB

So I'm currently limited by 3rd harmonic but I still have 12 JFETs in parallel which is definitely too many. When I measured 5 in parallel, the 3rd harmonic was -95dB ish.

I think my second set of servos will reduce all distortion. They are not finished yet. When they are, I can also snip off some JFETs to get 3rd down some more.

I can increase measured SNR by 10dB by increasing Riv from 256R to 800R at the cost of some more distortion.

I'm pretty confident of achieving -90dB.

I found this python script intended for calculating THD+N from a .wav file:

https://gist.github.com/246092

I can give that a go when I'm done fine-tuning.
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Old 24th May 2012, 05:05 PM   #848
heb1001 is offline heb1001  China
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Help - what's the best way to connect the DAC chip to the Sen box?

I had previously been connecting the Sen box to my DAC chip by plugging some twin core cable with shield into the dip sockets on my sound card after unsoldering some surface mount components to disconnect the DAC outputs from the rest of the sound card. I was using the two cores for the DAC output and the shield for the return current to the DAC ground. One cable each for L+R balanced channels. I had XLR plugs on the other end with the three pins used for the +ve, -ve and DAC gnd with the shield connected to pin 1 but not the plug. The cables went through the side of the computer with the side cover off.

This solution sounded good but was picking up some noise which I assumed was because the shield was being used for DAC gnd rather than as an actual shield connected to chassis gnd at both ends so I spent time creating some short cables to connect the DAC L and R channels to D-sub sockets on a PCI bracket and longer cables to connect the D-sub sockets to the Sen box.

This time I used USB 2.0 cable with a foil shield and four cores. For the internal cables, I pulled out one core and used three cores for the DAC +ve, -ve and gnd. I connected the shield to the D-sub socket shield (connected to the PC chassis) and left the end of the shield near the DAC unconnected. For the external cables I used 3 of the 4 cores, +ve -ve, gnd connected to the XLR plug pins and D-sub pins, one core left floating and the foil shield connected to the XLR plug and D-sub plug shields.

This allowed me to close the computer case and connect the Sen box with cables with a foil shield connected to chassis gnd at both ends and three cores to carry the DAC +ve, -ve and gnd.

Measurements showed 10dB less noise using the new cables but the listening test is disappointing: the liquid smoothness that was present before has gone.

The measurements look very clean but there might be high frequency noise I can't measure.

Anyone got any ideas what the problem is likely to be or what the best connection strategy is likely to be?

Thanks in advance.
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Old 24th May 2012, 06:35 PM   #849
EUVL is offline EUVL  Europe
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I just use 2 separate pairs of twisted wires for L & R (i_in/Gnd).
Never had any noise issues.


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Old 25th May 2012, 01:06 AM   #850
heb1001 is offline heb1001  China
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Quote:
Originally Posted by EUVL View Post
I just use 2 separate pairs of twisted wires for L & R (i_in/Gnd).
Never had any noise issues.


Patrick
So, in my first solution, i_in and Gnd were not twisted because I was using the shield for Gnd and in the second solution I'm not exactly sure. I guess that the usb cable might contain two pairs rather than all four wires twisted together in which case the second solution would have had +ve twisted with Gnd and -ve not twisted.

I'll try to find/make some three core shielded cable where all the cores are definitely twisted together and see how that goes.

It's not exactly a noise problem at -100dB but it could certainly be better.
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