ESS Sabre Reference DAC (8-channel)

Hi Dustin,

I'm using transformer output with my Buffalo II. Wired across + and -. I like it alot.

Was wondering if the ESS Sabre chip likes to see the output's + and - current returned to the DAC's AC signal ground @ AVCC/2 (by using the transformer's primary center tap), or is simply wiring the transformer from + to - (and ignoring the center tap as I have done) complete the shortest current path ?

Great job on the Sabre chip, BTW.

-Steve




Hi Steve,

To get the best out of the DAC, what you want to do is cause the current going into the power supply pins to be constant, that way you wont get power supply modulator causing harmonic distortion. This can be accomplished by returning the current back to some DC potential. The value of the DC the current gets returned to doesn't seem to matter a lot. Some have tied it to ground, some to DVcc/2 and some DVcc. Puting the DAC into a mode where its pins are at a virtual ground also acomplished this. The extra benefit of have the virtual ground is that the internal voltage coefficients of the DAC circuits are no longer able to cause THD. I have been wanting to try building up a discrete I/V for some time, but im just too busy with other things at the moment. :(


Dustin
 
Hi Dustin, Calvin

I too am extremely interested in using the ESS9018 in a single ended way, with a discrete component I/V stage. I intended to use "half" of the differential output- i.e. to feed the I/V converter from either the + or the - output, and ground. The I/V circuit can be set up to provide 1/2 AVCC on its input.

But you mention grounding "DACb". What did you mean by this please?

Many thanks

Paul


I Paul,

So you plan to use only DAC and then tie the DACb off to Ground? This is OK, but you will get better performance using both sides. I am interested to see what people come up with. :)

Dustin
 
Hi,

my aim would be to have as low a parts number count as possible in the signal path. At the moment I favour Owen´s idea as in here: http://www.diyaudio.com/forums/digi...ssic-pass-labs-d1-ess-dac-20.html#post2224186
The Q remaining is how to generate a singleended signal from this I/V-stage. Possibilities are either with a dedicated differential-to-singleended converter, be it a OPamp or a discrete stage, or to simply take just one I/V-output and to ground the second DAC-output. One could ground via a relais, or an opto-switch.
THD would probabely be higher in the second case, but I´m happy to see numbers and to hear that the THD-increase is not due to the DAC-outputs but due to the capabilities of the analog stages.
Thanks for this info Dustin.

jauu
Calvin
 
Hi Steve,

To get the best out of the DAC, what you want to do is cause the current going into the power supply pins to be constant, that way you wont get power supply modulator causing harmonic distortion. This can be accomplished by returning the current back to some DC potential. The value of the DC the current gets returned to doesn't seem to matter a lot. Some have tied it to ground, some to DVcc/2 and some DVcc. Puting the DAC into a mode where its pins are at a virtual ground also acomplished this. The extra benefit of have the virtual ground is that the internal voltage coefficients of the DAC circuits are no longer able to cause THD. I have been wanting to try building up a discrete I/V for some time, but im just too busy with other things at the moment. :(


Dustin

Hi Dustin,

I'm using a 3.3v regulated constant current source to feed the Sabre's output supply.

I guess what info I am after is are the Sabre's outputs (+, -) exclusively current sourcing, current sinking, or mixed. If they are either both sourcing or both sinking, I'll need to use the center tap to a ground reference for current return. If they are mixed (can either source or sink), no center tap connection is required as each (+) output will sink what the other (-) output sources.

Thanks,
-Steve
 
Hi Dustin,

Nice to have you around these parts. A couple of Q's while you're here .... first, what circuit design was used for the 108dB measurement for voltage mode output? I'm guessing an opamp as balanced to SE converter?
Second question, I understand there are ways to optimise the Sabre's modulators for best DNR. Russ White mentioned randomzing the modulators ... do you have information on this? I'm currently putting together my own 'C' firmware for the Sabre.

Thanks in advance for any help you can give,
Dan
 
Hi, all,

Here's another of my famously long posts, but I've tried all sorts of things to fix this final issue I'm having...

I'm STILL messing with my amp build - it has been sitting on the shelf for the past seven months but I really want to finish it now.

I bought a small prototyping adapter so I can now fit the CPU chip (from a Denon 1905) next to the Denon DSP board into my amp chassis.

For a bit of background info - my previous ramblings and a close-up photo of the DSP board + Sabre DAC are here...

http://www.diyaudio.com/forums/digi...-reference-dac-8-channel-144.html#post2007575

I've attached another photo of the current layout of the amp.

I'm now testing some THAT1200 opamps as an output stage for the Sabre (2 channel only atm). They appear to work great, and provide very good ground isolation which stops all the "ground loop" type noise I was getting before.

The THAT1200 outputs go through some 1uF M-Caps and then directly into the Sure amp module.

Firstly, the good news is that I've now confirmed that the cause of the digital noise when the Sabre first locked onto a signal was indeed caused by the default "low" DPLL bandwidth value of the Sabre. When I set it to medium bandwidth (via I2C using a buspirate), the noise disappears completely!

The problem I'm having now is that the Sabre locks onto the I2S audio from the DSP perfectly fine when using an optical input from a DVD player, but...

...If I use the optical output from the PC (tried two different sound cards), the Sabre locks on and plays audio for three seconds, then it looses lock for one second and the process repeats?

The sound cards work perfectly through optical to a commercial amp (even through a very cheap 5 metre optical cable). I tried three different cables with the Sabre DAC and I still have this "lock" problem?

The decoded I2S signals from the DSP board look identical on the scope whether I'm using the DVD player optical output (audio fine), or PC optical output (audio intermittent)?

Of course, on my amp build, the Denon DSP is responsible for decoding SPDIF and converting to I2S for the Sabre, so it's probably the way I have it all connected (as usual :eek: )...

The I2S signals from the DSP board are at 3.3v levels, but I'm getting a huge amount of overshoot? Is this likely due to incorrect impedance matching, or the use of a "1X" scope probe?

I've attached o'scope shots of the I2S bitclock and wordclock going into the Sabre board (please note the different time / division).

You can see that the overshoot is pretty huge. The flat peaks are at 0v and 3.3v (0V at same position when scope input set to "GND"), so the overshoot actually goes BELOW 0v and ABOVE 3.3v?

Apart from the overshoot, the I2S signals stay solid at ALL times even when the Sabre looses lock? Assuming the overshoot is not an artifact of the scope impedance, could it be the cause the Sabre loosing lock?

The CPU / DSP / Sabre ground is now isolated from the Sure amp / Tracopower SMPS / mains ground. I do appear to have around 130mV of noise on the CPU / DSP / Sabre ground though (around 295KHz, so very likely to be SMPS switching noise).

It's important to know that the I2S signals pass through some buffer chips and series resistors on the DSP board before they reach the Sabre.

The Sabre PCB also has a 47K pull-down resistor on each digital input line, and then has a 22 ohm series resistor between each input and the ESS chip.

The buffer chips on the DSP board are all powered from 3.3V. So, the signals follow this path...

(Note: Bit clock and Word clock generated by Sanyo LC89057 SPDIF receiver. So, clocks recovered from SPDIF PLL.)
LC89057 -> 47ohm -> 74LCX244 -> 47ohm -> (47K pull-down) -> 22ohm -> Sabre chip.


(I2S audio data generated by SHARC DSP. SHARC is also clocked by the Sanyo SPDIF receiver.)
SHARC -> 74LVX157 mux -> Sabre chip.


Note: The CPU uses the mux chip to "mute" the I2S audio signal (tie output to ground) when it detects a decoding error. The mux is not activated while the Sabre looses lock, so it's basically just a buffer.

Should I bypass some of the series resistors on the Bit clock and Word clock to try to reduce the apparent overshoot?

EDIT: The overshoot on the actual I2S audio signal is only around 0.48v above / below the flat peaks (ie. around 4.26v between lowest and highest overshoot).

Thanks in advance,
OzOnE (Ash). :cool:
 

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Hi,
The bitclock scope trace isnt very good. You'd better measure it with your probe in 10x, measure it the closest to the DAC pins, not the other side, this is where it matters. Sounds like an impedance mismatch, it oscillates quite a lot. Repost the new shots.

The wordclock, though not the best, doesn't seem to oscillate, so ok there is little overshoot so there is some impedance mismatch (hey... wires are soldered/plugged in it after all).

Also, try simply unplugging the bitclock, afair the Sabre can lock without it (not sure about it).

The problem you describe is strange, would frequencies not match it would not lock at all, sounds like a jitter problem, but why? Ok some jitter probably can pass the spdif decoder, but to this point? Tried connecting directly the optical receivers' output to the sabre in spdif mode (crappy, but worked)?
 
Hi, Nicolas,

Sorry for the late reply.

I've been reading up on impedance matching and I understand it much better now. I will try to add some resistors at the DAC end to see if I can tame the over / undershoot.

I'm not sure why the DAC doesn't lock onto the signal properly when the DSP board is using the optical input. I've found that it does work properly via coax though? This is very strange because the digital audio is permanently at the DAC in either case? Must be a strange jitter or clock skew problem?

It's likely to be a mismatch between the Denon DSP board and Sabre DAC then, as coax is working fine. The optical inputs also loose lock with other sound cards?

I tried disconnecting the bitclock, but it lost lock so appears to need it (doesn't appear to use JUST the arrival time of the audio data in I2S mode maybe?).

My USB sound card doesn't have a coax output, so I'll probably buy another one and modify it to connect it's SPDIF pin directly to the DSP board. I'm using the coax output of my on-board (Realtek) sound card atm, but it uses really poor sounding 48KHz resampling on everything. :(

btw, does anyone know if the Wolfson type USB-to-I2S boards (like the TP Opus) can actually pass Dolby Digital / DTS via I2S as well as PCM?

Hi, Adrculda - I bought the multi-channel DAC board from Nicolas as it happens ("NeoY2k" before your post). :)

It's a nicely designed board and is sounding great so far (now that I'm testing a proper I/V stage). I'm not sure if Nicolas has any boards left though?

The DSP board was removed from a Denon AVR-1905 (which had a blown output stage). I bought the amp for the purpose of "stealing" the DSP board and I used the rest of the amp for repairing other amps.

Using the DSP board was the only fairly straightforward way of getting hold of a Dolby / DTS 7.1 decoder. It was a bit of a pain re-soldering the Denon CPU though as I couldn't find an easy way of controlling the DSP board without it.

I'm still looking for a cost effective circuit for the eight I/V stages. Does anyone know if I could use just an LME49720 for both filtering AND balanced to single-ended conversion on it's own? (like the IVY III ?).

I don't need balanced outputs as I'm only feeding the Sure amp module, and the costs start to add up fast if I wanted IVY type stages for all 8 channels.

The THAT1200 is sounding fairly good as an output stage atm, but that's with no filtering for the Sabre outputs. I'm wondering if I could add some simple R/C filters to this chip, but I don't know where to start.

I also need to get a proper PCB made for the DC protection circuits / relays. I'm fairly happy with the design I'm using atm and I think it will offer fairly good protection. It currently requires a separate (ie. floating) PSU though.

The Sure amp has been tested quite loudly (just before the onset of distortion) without the DC protect circuit triggering, but I know it triggers with only around 2V DC after about 300ms (good enough for me).

The eventual idea is to write a guide for others who want to build a multichannel amp with a built-in decoder. I'm not sure I'll ever get around to finishing my amp though? :(

I suppose you could just grab the 5.1 or 7.1 audio directly from a USB sound card or modify a DVD player for multichannel I2S, but it's always nice to have a real DSP decoder for movie use.

OzOnE.
 
@Adrculda - btw, I was just checking out your ThermalTrack build thread and noticed that you'd bought a similar case to mine (although mine is single-unit sized)...

I thought that the attached photo might be somewhat amusing.

I haven't decided where to put my "Sepaker" binding posts yet. :D

Also note my highly advanced method for connecting the speaker wires during test. :eek: Don't laugh, those black strips (which look like electrical tape) are in fact carbon nanotube wraps. They also contain a mixture of Ritz Sidecar and snake oil.

OzOnE.
 

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Hi, Nicolas,

Yes, it's strange why lock is lost only on the optical input? There are quite a few things I had to bypass to get the CPU / DSP working by itself, so I've probably missed something out.

I won't waste any more of anyone's time on this. I'll just use the coax input for now and sort out an improved SPDIF receiver eventually.

For anyone who's interested in using one of these Denon DSP boards, you will also need to use the original control CPU (since I couldn't figure out the DSP board's control protocol). I had to put pull-up and pull-down resistors on all the relevant signals and connect all necessary control signals between the CPU and DSP board.

It's not as bad as it sounds, but I had to use a prototyping adapter for the CPU so I could get rid of the huge "motherboard" thingy where all the daughterboards originally plugged into. It currently has no display, but you could hook up the OSD chip to change all the settings (I think the OSD is necessary for changing things like speaker delays anyway).

So, I just have the Denon IR sensor connected now. Of course, the volume doesn't work since I've bypassed the volume chip and have connected the multichannel I2S audio directly to the Sabre board. I'm in the process of building a PIC board to convert between the Denon volume protocol to control the Sabre chip.

Here's the service manual for the AVR-1905 which I took the DSP board from (DSP on page 2, CPU on page 4)...

http://www.filedropper.com/avr1905allsdv02

OK, enough thread-jacking from me. If I ever get this project finished, I'll start a new thread.

OzOnE.
 
@Adrculda - btw, I was just checking out your ThermalTrack build thread and noticed that you'd bought a similar case to mine (although mine is single-unit sized)...

I thought that the attached photo might be somewhat amusing.

I haven't decided where to put my "Sepaker" binding posts yet. :D

Also note my highly advanced method for connecting the speaker wires during test. :eek: Don't laugh, those black strips (which look like electrical tape) are in fact carbon nanotube wraps. They also contain a mixture of Ritz Sidecar and snake oil.

OzOnE.

I ended up buying 2 :)
One will house the amp and the other will house preamp which is multiple modules
- Source Selector - Input 2 Board
- DAC
- DSP - Maybe ??
- Volume control
AND...
A selectable output section - Tube Buffer or HDAM

As you can see most of the "pieces" are decided upon, just a few left to complete the puzzle :)
 
Hi,

Sounds like a nice project - That amp / preamp combo will be HUGE though. :) (In keeping with many other projects I suppose).

I was trying to cram too much into this case at first (7 Sympatico modules plus toroidal AND DSP!), but after the positive comments about "Class-D" type amps like the Sure amp, I moved over to class-D and I now have a lot more room in the case.

(I'm planning to stack another Sure amp on top of this one if I can ever get the rest of the details sorted).

btw, does anyone know of a cheap I/V stage suitable for the Sabre chip? Would something like a modified IVY III type circuit work OK (if only single-ended outputs were needed)?...

This would save a fair bit in the cost as I would then only need eight OPA1632's and four LME49720's.

OzOnE.
P.S. I hope it's OK to post this circuit on here? Please let me know if not, and I'll remove it.
Note: it is NOT the original IVY III schematic!! It probably won't even work properly without some resistor changes etc.
 

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Hi,

Yep, that's where I got it from, but the schematic I posted was modified by me (doesn't include the second OPA1632 gain stage / differential outputs).

I didn't want anyone to use this schematic and get it mixed up with the proper IVY III schematic (especially since it's unlikely the schematic I posted will even work properly as-is).

If anyone would be kind enough to make and sell some PCBs / kits for a similar circuit which does I/V + filtering + single-ended conversion for multi-channel Sabre setups, I'm sure there would be a few people who would buy a kit.

I'm also still after a speaker protection PCB which works properly with the Sure amp (bridged outputs). I think the circuit I have will work OK, but it's not ideal as my knowledge only goes so far.

OzOnE.
EDIT: btw, a Sabre DAC using the 8 separate outputs would have different output impedances to a Buffalo type DAC (quad-stereo mode), so the I/V stage would need some changes anyway.
 
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