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Old 2nd November 2008, 05:38 PM   #1061
Telstar is offline Telstar  Italy
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bump from updates from the ongoing projects.
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Old 5th November 2008, 04:08 AM   #1062
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Hi all,

Ok, so I finally got my amp chassis (after many months of waiting for it to arrive from Hong Kong), I also have a Sabre IC, and seven Sympatico PCBs !!....

What I'm looking to do next is to design a PCB for the Sabre chip. But, as I want to do full 7.1 surround, I was wondering if it would be possible to use multiplexors on the Sabre DAC outputs so that you could switch between "8 channel" mode or "2 channel" mode.

The mux chips would simply combine the four DAC outputs (per side) for "Stereo Quad" output in 2-channel mode.

This would give the best DNR etc. for stereo music, while leaving the option for full surround output (giving the best of both Worlds). This also saves the cost of three or four Sabre chips for people who want to do full surround on a tight budget (like me )

The question is, would the use of mux chips on the outputs do any harm to the DNR etc? (which would null the benefits of combining the outputs for 2-channel mode in the first place)?

I'm basically looking for any endorsement of this idea so I can go ahead and design a PCB. A candidate for the mux chip would be something like TI's MPC507A....

http://focus.ti.com/lit/ds/symlink/mpc507.pdf

You would need two chips (to combine the eight differential channels into two channels) - the MPC507 chips cost around 7.90 ($12) each in the UK.

The muxes should also allow for other combinations of the eight outputs into two channels (ie. 6 into 2, or 4 into 2), but this might require further routing of the "spare" outputs (depending on your application.)

As long as there are no major issues with noise or crosstalk with this method, I'm sure the Sabre will sound amazing in 8-channel mode. I'm just itching to start building this DAC now.

I've designed and built a fair few PCBs before, but mostly digital stuff (FPGA, PIC etc.), so I'm open to any and all suggestions / critique on this as I lack some of the more in-depth analog theory on this sort of stuff.

I've attached a photo of my amp (with the boards laid out) to give some "inspiration". Just so you know I'm serious!
My RevC amps are also sitting in there just for reference.

Cheers,
OzOnE.

P.S. I Just wanted to give a quick thanks to Russ White for his feedback on the Sympatico many months back on the LM3886 thread (sorry, I forgot to reply!)

Oh, and congrats to the guys and gals in the US on your new president !!
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Old 5th November 2008, 11:22 AM   #1063
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Quote:
Originally posted by OzOnE_2k3
Hi all,

Ok, so I finally got my amp chassis (after many months of waiting for it to arrive from Hong Kong), I also have a Sabre IC, and seven Sympatico PCBs !!....

What I'm looking to do next is to design a PCB for the Sabre chip. But, as I want to do full 7.1 surround, I was wondering if it would be possible to use multiplexors on the Sabre DAC outputs so that you could switch between "8 channel" mode or "2 channel" mode.

I've attached a photo of my amp (with the boards laid out) to give some "inspiration". Just so you know I'm serious!

Cheers,
OzOnE.

P.S. I Just wanted to give a quick thanks to Russ White for his feedback on the Sympatico many months back on the LM3886 thread (sorry, I forgot to reply!)

Hi OzOnE,

Nice work!

I would probably not use an analog MUX myself as they tend to impart more distortion and noise than I would like. What I would probably do is use a good quality relay. Something like a G6K.

Even better in my mind would just be to use 4 sabre chips and run them all quad stereo, and then simply rout your digital signals appropriately.

Cheers!
Russ
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Old 5th November 2008, 01:20 PM   #1064
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I would like to know for sure if ESS has no plans to do (or other company using IP core? ) a solid ASRC chip anymore, before i jump into TI's offering.
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Old 5th November 2008, 03:31 PM   #1065
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Quote:
Originally posted by OzOnE_2k3

The question is, would the use of mux chips on the outputs do any harm to the DNR etc? (which would null the benefits of combining the outputs for 2-channel mode in the first place)?

I'm basically looking for any endorsement of this idea so I can go ahead and design a PCB. A candidate for the mux chip would be something like TI's MPC507A....

http://focus.ti.com/lit/ds/symlink/mpc507.pdf
The mux chips have too much on resistance to directly combine the outputs of the Sabre.

If you used seperate op-amp I/V conversion on all 8 outputs that would give you the 7.1 you want.

Then another pair of op-amps to sum each of 4 to combine them to stereo outputs and giving you 10 total outputs from your PCB. That may be workable. Without using 4 Sabre chips

I wouldn't use the mux chip for the summing of I/V conversion circuits either, the on resistance of the mux inputs is probably not matched well enough.
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Old 5th November 2008, 04:34 PM   #1066
Telstar is offline Telstar  Italy
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Quote:
Originally posted by tritosine
I would like to know for sure if ESS has no plans to do (or other company using IP core? ) a solid ASRC chip anymore, before i jump into TI's offering.
if you want the best ASRC, get Anagram sonic2.
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Old 5th November 2008, 06:13 PM   #1067
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Thanks for your tips. I know I discussed this mux thing before on the other thread, and I thought about using relays, but I've heard that many audiophiles don't like to have too many relays in the signal path if possible - is this mainly due to microphony effects etc?

I also thought about the resistance differences when using mux chips, and the possible problems with slight phase shifts as I'm not sure how the Sabre would react to this. Does the combination of the outputs need to be as in-phase as possible for the best DNR (I'm guessing it's an obvious answer!)

If using the extra opamps to sum the outputs would work OK then I'd much prefer that method. Would it create any noticable channel crosstalk if the "combinational" opamps were kept in circuit while the other channels were working for surround duties?

I could just use four Sabre chips, but it does start to work out fairly pricey at 40 a piece. The thing is, although I'm a perfectionist at heart, I'm more than confident that the Sabre chip in multichannel mode should sound far better than my Denon amp (the RevC amps alone already do just that), so I'd be willing to sacrifice a bit of DNR on surround playback.

I suppose I could start with two channels, then add more DACs over time. (Also, I'm sure Dustin would be happier if we all bought 3 or 4 Sabre's each! - damn you Credit Crunch )

Does anyone have any circuit suggestions for differential opamp mixing / summing? Could I just use them as buffers (close the loop on each opamp, then combine the outputs via resistors)?

Again, the basics of AC theory aren't my strong point and I've never used fully differential opamps in a design before.

I know that one of the main criteria for designing the Sabre was that it was one of the best-measuring DACs available as well as the best sounding, but the question is - how significant is the audible difference between the quad and non-quad output modes (in stereo?)

(I hope this won't fan the flames, it's only meant as in enquiry!)

Regards,
OzOnE.
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Old 5th November 2008, 06:27 PM   #1068
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You could just use two chips, one for multichannel mode, one for stereo mode.
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Old 5th November 2008, 07:07 PM   #1069
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Actually, that's a good point. That would solve the other problem of the fact that in 2-channel mode, I would actually need 2.1, so combining the eight outputs of one chip would cause a problem for the sub output.

Also, you could keep all of the more "intricate" mods for the 2-channel chip, like clock upgrades etc.

I personally believe that the requirements for surround material is less than for pure stereo music listening. The multichannel stuff will still sound amazing no doubt.

btw, does anyone have any suggestions for the PSU caps on the Sympatico modules (in the UK, preferrably 10 each or less.) I've looked on Farnell + RS etc. but I'm not sure what to look for.

Also, @Brian / Russ - my front speakers are 4 ohm (M&K LCR-150's), but always with a 80Hz high pass - would this be a OK with the Sympatico modules, and what would the recommended trafo voltage be for this?

I'm assuming 4 ohms would still effectively show 2 ohms across each channel of the LM4780?

OzOnE.
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Old 5th November 2008, 07:24 PM   #1070
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Quote:
Originally posted by OzOnE_2k3
Also, @Brian / Russ - my front speakers are 4 ohm (M&K LCR-150's), but always with a 80Hz high pass - would this be a OK with the Sympatico modules, and what would the recommended trafo voltage be for this?

I'm assuming 4 ohms would still effectively show 2 ohms across each channel of the LM4780?

OzOnE.
Might want to move this part of the discussion to our support forum.

I would say 20-24V secondaries should be just fine.

4ohm speakers will not be a problem unless you try to push more than the amp is rated for. The effective impedance is not really that important. It more important to you simply keep the chip running within its specified current rating.

Cheers!
Russ
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