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Old 8th March 2010, 03:00 PM   #111
qusp is offline qusp  Australia
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also though, I did wonder about the problem of having a ground point that is too far from what 0 really is, especially if grounding the case for shielding a metal case. perhaps I could pull the 0 ref point from an anal probe
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Old 8th March 2010, 03:01 PM   #112
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2W into what? this amp will put much more into a higher impedance. but yeah like I said, nothing is set in stone, I only started considering the OPA1632 a couple of weeks ago, initially I was planning on using 4 x AD797 or a fully discrete stage thus the buffers and I have the PCBs left over from another project anyway. and I will be trying just the OPA1632, if I can get away with a lesser part count I will. I need to work on or find a pot with more friction too so the volumite doesnt move in my bag or pocket, probably will just use a washer or something I guess. the problem though is not knowing the output impedance of the sabre, I have written to them about an NDA since this will be the 3rd sabre chip I own, without it its hard to go about tuning an I/V stage except for doing what i'm doing and trying many options.

Last edited by qusp; 8th March 2010 at 03:10 PM.
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Old 8th March 2010, 03:12 PM   #113
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are you able to tell what the output impedance of the dac PCB is without breaching you're agreement or is the fact there is little in the way of the output too close to home?

sounds like the OPA1632 may well do OK though, since if it will put out 2w into ?? on +/-15v and i'm quite happy with the way the amp performs putting out 1.4W with +/-9v rails. and my battery supply will be somewhere in the middle

thing is though i'm interested in trying something a bit different as I do tend to prefer the JH13 driven by the FiQuest out of the buffalo32 than straight out of the B32. but that could just be an impedance mismatch/termination issue.

anyway i've taken too much of your time already, looking forward to the release, not too far now?? you on schedule for the revised release date since the system crash?

Last edited by qusp; 8th March 2010 at 03:29 PM.
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Old 8th March 2010, 04:25 PM   #114
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The 0V point is the reference in terms of VOCM. It is relative in the sense that it only makes sense in the context of a positive and negative voltage. It is the rails that would be moving relative to that mid point. They can move anywhere as long as they stay with the ranges set forth in the data sheet. Give it a good read all your questions are answered there. It does not significantly harm the signal for the rails to be a little asymmetrical in regard to VOCM. In fact with battery operation and using it to drive headphones in a balanced configuration I would probably let VOCM float (it has a voltage divider to internally set VOCM). You could then just use a single battery and not worry about the mid point voltage.

As for the battery operation part that really won't make any difference, It just a matter of knowing what might occur.

As for driving headphone, what is required for complex load is damping factor, not boat loads of current. The OPA1632 has superb damping factor as balanced headphone amp. I have been using it this way for years. I have not found anything better yet.

I hope you find the information your looking for.

Cheers!
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Last edited by Russ White; 8th March 2010 at 04:32 PM.
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Old 8th March 2010, 04:38 PM   #115
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The output impedance of both the Bufallo I and II is 195ohms at each output.

Cheers!
Russ
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Old 8th March 2010, 05:18 PM   #116
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ok I was actually wondering about that, IE whether it would be better to let it float and let the common mode take care of everything. i do understand the reference (I think), but reading the datasheet the other night for the OPA1632 got me thinking about the penalty it mentioned for letting it just use its own 'judgement' of what 0 is but then faced with the possible penalty of feeding it a 0 point that wasnt in the middle got me a bit overwhelmed with all the variables and which would be the better way to go. can I feed Vosc to the OPA1632? because wouldnt letting the 2 chips float result in some kind of mismatch? if the rails move in relation to 0, I dont get that as how is 0 generated other than in relation to the known quantity ie the rails? kind of a chicken and the egg thing. but yes I will give the 1632 datasheet a good read before embarking on any actual design. hoping ESS will reply sometime soon so I can get some more sabre info.

I suppose as long as they DONT communicate the 0 between each other then it just becomes a different arbitrary number, I guess robots really do dream of electric sheep VOSC just gets fed 3v3 as a chunk of whatever voltage is present and it takes the midpoint of this waveform as 0 right? so the rest of the voltage outside of this really is kinda meaningless? with regard to 0 anyway

thanks for the helping hand, as noted this is the first time i've really had to take all of this stuff into account with regard to actually designing my own PCB rather than just assembling or modifying an existing one. just got sick of trying to fit a square peg in a round hole, even though my avatar seems to think its OK I got the COD running portable BTW, sounds really nice and i'm surprised it doesnt get more attention. only running it single ended though as dac only because I started the research for a decent volume control then you announced the b32v2 and this made the volumite a possibility.

also thanks for the info, that will come in quite handy

Last edited by qusp; 8th March 2010 at 05:34 PM.
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Old 8th March 2010, 05:30 PM   #117
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In the meantime, I have spent some time reading the Sabre White Paper posted in the ESS website.

The case for current output mode is (according to footnote 16) to "cancel a slight on-chip resistor voltage coefficient"

The difference between voltage mode and current mode output stage is that in voltage mode the output connects to input of the opamp and in current mode output, the output connects to a resistor.

It seems then that the off chip resistor is the element canceling the voltage coefficient of the on-chip resistor.

Is there is a case then for a simple low value resistor as output stage? (We may not achieve -120 db THD, but perhaps a bit higher than -108) or is this line of thinking totally offbase?
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Old 8th March 2010, 08:21 PM   #118
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Quote:
Originally Posted by glt View Post
The difference between voltage mode and current mode output stage is that in voltage mode the output connects to input of the opamp and in current mode output, the output connects to a resistor.
Sorry, but this is not correct.

Using it in current mode means using it into a low (as close to zero as possible) impedance. An opamp is typically used precisely to do this.

Using it in voltage mode means running it into a higher impedance (it does not have to be very high either).

The benefit of current mode (ultra low impedance) is that the voltage the DAC outputs see does not change with current, it does not modulate, so the outputs of the DAC stay where they always are. They always see the same bias voltage. This is why performance is better. It also makes it so that thermal effects are spread evenly.

Cheers!
Russ
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Last edited by Russ White; 8th March 2010 at 08:29 PM.
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Old 8th March 2010, 08:33 PM   #119
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FWIW I did try some low value impedances, (10, 21, 100, 221R). THD was significantly impacted even with 10 ohms.
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Old 8th March 2010, 08:48 PM   #120
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Hi Russ, your experiment with resistors, was that above or below the theoretical -108 db THD in voltage mode?

I have no doubt that IVY III will result in best performance, but I want to get there in stages. (Been using OPUS 8741 since the first time it was released and incrementally added improvements along the way). Thanks...
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