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Old 2nd January 2012, 05:56 PM   #811
1audio is offline 1audio  United States
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Quote:
Originally Posted by oneoclock View Post
Hello Demian.

I think noise ground out must get to clock. Better external power to clock?

Tektronix put no ground noise. I havent problems.

I can not made Jitter measures with WaveSpectra - PC , The noise is above the harmonics of 229 Hz.

I will try with isolation transformer to the output and WaveSpectra - PC.
The isolation transformer trick does reduce some of the noise a lot. It did not reduce the jitter.

I think the next steps in improving the AB1.1 will be ground optimization. a star ground on the analog side would work great (no really HF stuff to mess it up) but its the clock to dac and clock to USB module that will be challenging to optimize. With signals clocks and power sharing grounds is possible that a bigger bypass cap could even induce jitter by creating a moving ground reference from the noise currents being bypassed.
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Old 2nd January 2012, 05:57 PM   #812
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Hm. Maybe the differences between fw revisions come from CPU activities being different.

I've been thinking about how to check this in the time domain as well. Perhaps an SPDIF transmitter (which some people have requested in its own right) would be a nice feature? Then a connected recorder could determine if the received data equal the output.

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Old 2nd January 2012, 06:04 PM   #813
UnixMan is offline UnixMan  Europe
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Quote:
Originally Posted by 1audio View Post
I think the next steps in improving the AB1.1 will be ground optimization. a star ground on the analog side would work great (no really HF stuff to mess it up) but its the clock to dac and clock to USB module that will be challenging to optimize.
it's time to implement BÝrge's idea of optoisolators on I2S and clock to uC... and of course goin' to a self-powered device.
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Old 2nd January 2012, 06:06 PM   #814
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Demian, I'm a believer in the Henry Ott ground philosophy with massive planes, good separation and return current loops minimizing themselves. Haven't really done star ground in a while. The closest I've got is to use GND "peninsulas", but that's just part of good separation.

Also, I hope to keep the boards on two layers to keep cost down. Any suggestions to improved ground routing would be much welcome!

I'll look long and hard at the layout once again. But it is difficult to keep signal and power grounds separate. Both DAC and XO (before LDOs) get their power from the same spot, so that is also where their returns meet.

BÝrge

Quote:
Originally Posted by 1audio View Post
I think the next steps in improving the AB1.1 will be ground optimization. a star ground on the analog side would work great (no really HF stuff to mess it up) but its the clock to dac and clock to USB module that will be challenging to optimize. With signals clocks and power sharing grounds is possible that a bigger bypass cap could even induce jitter by creating a moving ground reference from the noise currents being bypassed.
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Old 2nd January 2012, 06:10 PM   #815
borges is offline borges  Norway
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Originally Posted by UnixMan View Post
it's time to implement BÝrge's idea of optoisolators on I2S and clock to uC... and of course goin' to a self-powered device.
Okay, I'll see what I can do on the next generation AB!

Power supplies is interesting for a lot of reasons. Adding a single-ended wall-wart is easily done today. That will power the LDOs for the XOs and DAC. But if we want to use more advanced DACs we will need bipolar supplies. Some of us have lab supplies to hook up, but not all. So for a USB DAC to be practical with bipolar internal supplies it needs some sort of switching power supply.

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Old 2nd January 2012, 06:44 PM   #816
1audio is offline 1audio  United States
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Large ground planes are great for RF, especially when skin effect is significant. They become a problem when you are looking at 140+ dB dynamic range. If you have 1 milliOhm and 1 milliAmp the voltage drop becomes 1 uV. Invisible in a digital system (except if it moves the clock thresholds around a little) but its a lot when compared to 2V out (-106 dB). This is why ground currents can be a real challenge in design. Also 100 pS jitter is the same as -106 dB sidebands.

Multilayer I think of as a crutch on something like this. There just aren't that many connections.
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Last edited by 1audio; 2nd January 2012 at 06:47 PM. Reason: typos
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Old 2nd January 2012, 08:52 PM   #817
UnixMan is offline UnixMan  Europe
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I second 1audio about GPs and PCBs. GPs are easy and good for RF, not that much for audio. Going multilayer is probably unavoidable for a top quality DAC.

Quote:
Originally Posted by borges View Post
So for a USB DAC to be practical with bipolar internal supplies it needs some sort of switching power supply.
OMG, no wall-wart and no switchers, anywhere!

We need good, large, clean and quiet internal linear PSUs. With plenty of power to waste!

(smart use of brute-force is about the only approach that works well with audio).

You should turn to a standard 1U rack-size enclosure (perhaps even 2U if needed). With toroidal or better yet R-Core PTs, Schottky barrier rectifiers, pseudo-CCS+C smoothing (to limit current peaks and thus rectification noise), gyrator CLC filters, CCS power distribution with shunt regs on each load, plenty of reservoir capacitance everywhere, top of the line DACs in full dual-mono configuration, etc.

All of that is gonna cost quite some money. But it will pay off with really great SQ. Which is what people here is looking for in the first place.

The current AW is IMHO already at the knee of the cost-quality curve. I'm afraid that you can not get much further than that without gettin' rid of compromises... accepting the unavoidable consequent jump in costs.

But in this "market" costs are a minor problem. Audio enthusiasts are willing to pay for quality. Whatever is needed. IMHO you'd sell more top quality, no-compromise music machines at say €1000.00 each than yet another "good for the price" device at €100. Of course, for this to be true it must NOT look like a cheap PC periferal... it's just psychology, but in this field that's fundamental too. It's not just marketing: what you see (and what you feel) is what you hear too!
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Last edited by UnixMan; 2nd January 2012 at 09:19 PM.
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Old 2nd January 2012, 09:48 PM   #818
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Going multilayer is probably unavoidable for a top quality DAC.
Agreed.

Multilayer may seem like overkill, but it allows you to have equivalent power and ground planes without tripping over signals. I find that I never need more than 4 layers because there is always room for analog areas and digital areas if the power/ground planes are segregated, and this also avoids capacitive coupling between multiple planes (e.g. 6-layer).

There are plenty of inexpensive USB DAC products that do not compromise on the PCB, so it will be hard to beat their performance without moving beyond amateur 2-layer designs.
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Old 2nd January 2012, 11:27 PM   #819
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I have no great knowledge about ground, but I think this layout AVCC mass could be a possibility.Click the image to open in full size.

1audio, Noise appears after pass time and is removed by moving the connectors. I happened in the two AB-1.1 I have. Could you check this in your circuit?

I've cutting edge copper of the back plate of the audio connectors and especially miniUSB. It was ended the problem.


And this is the measure quite good of AB-1.1. Distortion at 1 KHz.
Click the image to open in full size.

Intermodulation with 19 and 20 KHz. These measures are quite good.
Click the image to open in full size.
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Old 3rd January 2012, 05:47 AM   #820
1audio is offline 1audio  United States
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Good catch. I tried removing the end plate and saw a significant improvement as well. I will redo the tests to see if they are better tomorrow.
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