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Old 12th March 2010, 04:39 PM   #1471
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Hi,

The noise is not subtle in any way - when the Sabre is connected directly to the amp, there is a huge buzzing sound even when not playing music (sounds like SMPS noise).

When connected this way, I also get strange digital crackling sounds when the DAC comes out of auto-mute. This is likely to be due to lack of any I/V stages though.

When using the trafo between the Sabre and amp, the buzzing is completely absent and the sound quality is great.

Unfortunately, I haven't sorted the I2C programming for the Sabre yet - I don't have a Buffalo any more, I'm using Nicolas' (Neoy2k) 8-channel Sabre DAC which doesn't have an on-board PIC for the I2C stuff.

The DAC is running on defaults, although I can hook up the buspirate to control the Sabre from the PC if need be.

Does the pseudo differential / 7-bit mode create VERY noticable digital noise and DSP type noises in similar setups to mine, or is it a much more subtle thing?

OzOnE.

btw, I'm using an ES9008 - not sure if this makes much difference to the 7-bit / I2C stuff?
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Old 12th March 2010, 04:50 PM   #1472
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Without programming you will only use the first two channels (unless you have connected the three remaining I2S data inputs to the first), but in full differential mode with 6bit quantizer and you should not have any #noise.

Then the noise might be caused by the DSP, but due to the transformer cancels the noise you have a common mode type of noise,
and if you connect two amplifier channels as I described you will know for sure if the problem are common mode noise or anything else.

#noise sounds like the DAC or amplifier are destroyed...
More noise and distortion than music..
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Old 12th March 2010, 06:35 PM   #1473
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I have the other three I2S lines connected to the DSP board (for 7.1).

The sound quality is fine when connecting via the trafo to the amp, so it's not directly due to the DSP or DAC. The noise disappears completely whether I connect the Sabre +/- via the trafo, or even just Sabre +/GND via the trafo, so I don't think it's common mode noise.

I'll try hooking up across two amp channels as you said though. I still haven't tried a traditional +5V supply to the DSP board, so I'll build a quickie now.

I know the noise is not due to a ground loop to the PC (used as a source), as it still happens when using optical only, or even when it's disconnected from the PC completely? (The coax input on the DSP board is isolated anyway.)

The problem only occurs when connecting the Sabre directly to the Sure amp (no I/V at all). I know that not having a proper I/V or incorrect loading on the DAC outputs can cause "scratchy" horrible audio, but the buzzing sound is quite loud, and always present with a direct connection.

It worked fine when connecting the Sabre to the Sympatico modules - I know they have a differential input opamp, but both differential and single-ended connection to the Sympatico's also worked fine?

("minus" input on Sympatico's were connected to GND when trying single-ended. ie. Sabre "plus" to Symp "plus" / Sabre GND to Symp GND.)

I would be more than happy to use audio trafo's which would solve the problem, but it would be very expensive to buy 8 or 9 decent quality trafos.

Is this type of problem common when using SMPS / Tripath amps, or does it sound like a general ground-loop / no I/V type issue?

OzOnE.

Last edited by OzOnE_2k3; 12th March 2010 at 06:39 PM.
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Old 12th March 2010, 09:00 PM   #1474
NeoY2k is offline NeoY2k  France
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Sounds like a ground loop problem/no IV problem (not enough current? Hmm... 4mA in stereo mode aren't small!).

If you were to go for transformers and would need some good cheap ones, go buy EDCOR trafos. They have a new one, supposedly a Lundahl copy. May not be as good as the originals, but they know their job so they won't be bad anyway. You could try a That 1200, they are cheap, and easy to mount on protobard. If they were to provide enough performance, go for it!

Please note though that both trafo and That1200 will heavily limit the specs of the Sabre DAC. Despite that, this DAC has some sonic qualities not found in others, what this is down I don't know and it can shine through limited output circuit quality.

Please also note that no IV and driving directly the amplifier through cables a few meters long may result in strange behavior: you will get some filtering effects (loose mediums, while bass and treble don't move). At least, from what I tested.
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Old 13th March 2010, 12:14 PM   #1475
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Hi, Nicolas,

I'll definitely get some THAT 1200 chips. I still have two of the PCB's which you supplied to try them out in 2-channel stereo. I just wish that more of these electronic companies had a lower minimum order amount.

I've just found the cause of the noise problem though. For reasons I still can't explain, all I needed to do was to connect the V- output of the TracoPower 24V PSU to the PE (Power Earth) point... This also connects the PSU's V-output to the amp's chassis earth / mains earth point!

This has cured the major buzzing problem when using a direct connection (single-ended) between the Sabre and amp. The reason I'm so confused is that I tried the exact same thing two days ago with no effect.

I think I also just found the route cause of most of the ground loop noise - the "standby" PCB I'm using was from a Marantz receiver. It's only a simple small trafo for the +5V supply to the CPU, and a relay for switching the mains to the TracoPower PSU / LCDPS...

The problem was, I mounted this board to the back panel of the amp and the screw holes are connected to the GND connection which goes to the CPU / DSP / DAC! The back panel also has the 9-0-9 toroid for the LCDPS bolted to it. The metal back panel was also not making proper contact with the rest of the chassis, so it was almost floating! This explains a lot.

The 9-0-9 toriod is probably inducing a small amount of hum into the back panel too. I need to seriously rethink the layout and ensure all amp panels are properly earthed (I thought I'd already sorted this problem ages ago, but obviously the earthing points aren't good enough).

What I'm getting now with a direct single-ended connection is the usual sort of "crackling" audio effect I would have expected when not using I/V stages. This can be sorted out now, so I'll probably go with standard opamp I/V stages.

Those EDCOR trafos look super cheap, but I can see they've had some very good reviews. I will definitely buy a couple of those in the future. For now though, I'll see what the opamp I/V is like.

Can anyone suggest an opamp I/V stage for the Sabre which has had good results in the past? Please keep in mind that I'm using each DAC channel separately (not quad-stereo), so the output currents will be different.

Thanks everyone for your help, and sorry for being such a pain with the noise issue. I've obviously made a fair few basic mistakes as this is the most complex amp build I've done.

OzOnE. (finally, some success)

Oh, quick question - what's the lowest value resistor which can be safely used on each Sabre output for passive I/V ? I'm assuming that shorting the DAC outputs to ground is a VERY bad idea?
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Old 14th March 2010, 01:08 AM   #1476
NeoY2k is offline NeoY2k  France
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Aha ground current return paths are a big problem to everyone I think...
But yep, in theory, only things tied to a chassis should be the interconnect shields (if any). This is not where I would plug a trafo... I'd probably connect a +5V trafo's "ground" (with no CT) directly to the CT of the dual trafo following...But I may be wrong as well...

For best I/V, the one in the ESS datasheet... To my opinion. I wasn't pleased with the THS4131, though it still measured correct (but not up to the DAC's specs... For that you'll need a single board, and AD797s everywhere... well, you have the Datasheet too I suppose).

Shorting to ground isn't a problem, but what resistor value is another question. You have current (specified in the datasheet), you want a voltage (but which?), you need to find the resistor. Just don't forget the internal resistor in the DAC.

But why do you want to connect one of the dac outputs to the ground? Sound will be greatly degraded! Seperate dacs are used for positive and negative, so the things they have in common (distortion, glitches etc) will be attenuated by the next stage's CMRR. If you want SE output, you will need a bal to se converter circuit, like the That, or an opamp circuit. Or a trafo.

Cheers,
Nicolas
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Old 14th March 2010, 09:19 AM   #1477
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Default What EDCOR tranny type?

Hi NeoY2k,

What type of edcor tranny do you recommend?

I did like the products on their website!

Thank you

Peter
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Old 14th March 2010, 03:23 PM   #1478
anbello is offline anbello  Italy
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I would like to do buffalo24 volume control with a PIC and the code posted by Russ on posts #531 and #534.
I don't have the datasheet for the es9008 and i don't know the meaning of all the parameters so i can't know if the parameter settings on that code are optimal (it was written near two years ago an maybe need some modifications).
I would be grate if someone with more knowledge (and the datasheet) could read that code and see if the parameters are correctly set.

Thanks

Ciao
Andrea
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Old 14th March 2010, 08:35 PM   #1479
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Hi, Nicolas,

I didn't intend to short any of the DAC outputs to ground as such, but I was just wondering if it would cause damage if I did? Purely because I'm still not sure how to calculate the resistor I would need, and I wondered if a very low value resisotr would cause any damage to the DAC?

I'm assuming there's a big difference between the datasheet linked below and the full NDA one, but the basic datasheet is the only one I have.

So, in the datasheet (below), it says that the full-scale differential output current (per DAC channel?) is 4.224mA (p-p)...

For the simplest passive I/V to the Sure amp, I would only use the positive output of each DAC channel - The Sure amp says to only input a maximum of 2V p-p, so I would use a resistor of around 470 ohms to get just under 2 volts with full-scale DAC output??

ie. (ohm's law) 0.004224 Amps X 470 ohms = 1.98528 Volts?

But, datasheet also says that the DAC current is offset at half the peak range (2.122mA), so I really don't know how to calc this. I don't think the basic datasheet mentions internal resistance?

I don't know what the "virtual ground" refers to, but I'm guessing that the current offset would mean that 0.99264 Volts is produced when the DAC is idle (with a 470 ohm resistor)?

The resulting voltage offset wouldn't be a problem if I used a caps on the Sure amp inputs. I guess that an opamp I/V or trafo would be the only simple way to zero the offset though?

I've probably made all the wrong assumptions here, but how to calc?

All I can say about the AD797's is.. DAMN, those things are expensive! Not too bad for stereo, but I'm going to need at least 8 of them (for voltage out mode). This would probably cost more in total than the rest of the amp.

I'm not overly fussy about absolute quality at this stage because I'm certain that my amp will sound way better than anything else I've owned in the past (even when using cheaper opamps on the Sabre outputs).

I could put together a quick opamp board pretty easily if it would sound any good at all with cheaper opamps like OPA2134's? (Basically, I'm just itching to hear it in multichannel surround, so I'd rather get it running for now then upgrade it later on.)

@anbello, below is the datasheet that I have. This one was disclosed publicly many months ago (as Russ once said). It might not be the "full" datasheet, but it shows most of the common internal registers etc...

ES9008_DataSheet.pdf

OzOnE.
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Old 14th March 2010, 09:44 PM   #1480
anbello is offline anbello  Italy
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Quote:
Originally Posted by OzOnE_2k3 View Post

...

@anbello, below is the datasheet that I have. This one was disclosed publicly many months ago (as Russ once said). It might not be the "full" datasheet, but it shows most of the common internal registers etc...

ES9008_DataSheet.pdf

OzOnE.
Thanks a lot

Ciao
Andrea
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