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Gamma-2 (γ2) DAC Thread - Page 6

post #76 of 2085
6/29/09 at 1:32pm
Quote:
Originally Posted by amb View Post
UglyJoe, sorry, the answer is no. The opamp actually uses the VADD/2 DC offset from the DAC as a bias, because it's powered by a single supply. We don't want to block the DC there.
Ahh, I figured there was something I missed for what is happening there. Cool. Thanks, amb.
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post #77 of 2085
6/30/09 at 7:15am

I haven't met my question mark quota for the month, yet!

For getting balanced outputs (preamp with low-pass filter to be attached internally), would there be a nicer place to tap the signal grounds than the capacitor leads (C11/C12, C13/C14)?

Would there little enough risk of interactions between two sets of preamps that the SE output section could also still be used (if not, how unpopulated should the SE section be?)? If so, that would make this quite the bargain (translation: "I could afford to make a nice balanced source!") for a source+amp (HP amp, line buffer--what's the difference? ).

P.S. I can't believe the sun is coming up and I've been sitting here reading a DAC's datasheet, gamma-2 schematics, and checking out prices of things at Mouser...there's just no hope, is there? At least there's coffee.
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post #78 of 2085
6/30/09 at 7:27am
Quote:
Originally Posted by cerbie View Post
For getting balanced outputs (preamp with low-pass filter to be attached internally), would there be a nicer place to tap the signal grounds than the capacitor leads (C11/C12, C13/C14)?
For the next revision of the PCB layout I have added extra ground pads next to the L+/L- and R+/R- pads for this purpose.

Quote:
Would there little enough risk of interactions between two sets of preamps that the SE output section could also still be used
You can drive balanced and unbalanced preamps or headphone amps at the same time, there shouldn't be any interaction.

Quote:
P.S. I can't believe the sun is coming up and I've been sitting here reading a DAC's datasheet, gamma-2 schematics, and checking out prices of things at Mouser...there's just no hope, is there? At least there's coffee.
That's an almost daily occurrence for MisterX and myself...
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post #79 of 2085
6/30/09 at 9:04am
I spent the whole afternoon yesterday just reading about the upsampling/downsampling capabilities. A lot of reading to do yet. This is gonna be great, really look forward to learning more about these chips.
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post #80 of 2085
6/30/09 at 11:39am
If balanced will really be that simple, then I'll just plan to go all balanced well before I get a nice SE-only setup going . Even adding that, and assuming I'll ruin the most expensive parts as learning experiences (it's never the $.10 diode that fries), it still appears affordable to the point of wondering if it's real .

I already have other projects to do (I can finally afford to build the amp I wanted to back in September, as a Head-fi related one), so may not be in on the first boards to come out of prototyping. But, as soon as I get them taken care of, a gamma-1 plus gamma-2 it shall be.
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post #81 of 2085
6/30/09 at 12:23pm
If the board is populated as it is shown in the pics on the first page, it will have SE outs from the opamp.
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post #82 of 2085
6/30/09 at 12:25pm
might throw this in my chassis with my B22 if making it balanced is that easy
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post #83 of 2085
6/30/09 at 1:05pm
Well, AMB said earlier that if you want to run balanced connections, you are bypassing the 100K low pass filter that is on the SE output. You might have to come up with something like that for the balanced ouptuts, or risk having some high frequency stuff get through.
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post #84 of 2085
6/30/09 at 4:39pm
Quote:
Originally Posted by MrSlim View Post
Well, AMB said earlier that if you want to run balanced connections, you are bypassing the 100K low pass filter that is on the SE output. You might have to come up with something like that for the balanced ouptuts, or risk having some high frequency stuff get through
With a PSU for a HP amp in the case (+12-18V), the opamp options for the job grow by a few orders of magnitude over doing it on +5V (I'm not sure I'd trust it fully passive before the buffer section).
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post #85 of 2085
6/30/09 at 8:30pm
AMB, how bad is it using the balanced outs without the filter?

For instance, TPA's opus dac outputs come directly from the dac's outputs and there are no reports of HF noise.

Could you elaborate on this when you have the time?
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post #86 of 2085
6/30/09 at 9:03pm
I think I read a post by Beefy the other day that said the I/V is performed on the chip.
It may have been in the OPUS thread, like two days ago or something.
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post #87 of 2085
6/30/09 at 9:17pm

surely you're joking

Quote:
Originally Posted by MASantos View Post
AMB, how bad is it using the balanced outs without the filter?
You are going to have to rely on the information provided in the data sheets until we have a chance to measure "it".
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post #88 of 2085
6/30/09 at 9:35pm
As you could infer from the analog filter's fc of 100KHz, the "noise" is out-of-band and will not be directly audible. Hence you don't hear TPA Opus builders complaining about it. Nevertheless, that noise is undesirable and could (depending on nature and severity, which we do not yet know) cause the amp that the DAC is driving to perform worse.

FWIW, Wolfson recommends such a filter at the DAC's output. Our design is an enhanced version of one of Wolfson's examples.
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post #89 of 2085
6/30/09 at 10:15pm
So we could use the passive filter, or offboard differential filter, with our choice of single or dual rail power supply.
Or use what's already on the pcb for the single supply SE filter.
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post #90 of 2085
6/30/09 at 11:15pm
Quote:
Originally Posted by digger945 View Post
I think I read a post by Beefy the other day that said the I/V is performed on the chip.
It may have been in the OPUS thread, like two days ago or something.
Yep, but that is independent of any filtering.

With the OPUS, you can still use an additional linestage/filter/buffer, either IVY or Ballsie.
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