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Opamps for PCM1794 I/V conversion?
Opamps for PCM1794 I/V conversion?
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Old 10th December 2018, 08:00 PM   #1
Lingwendil is offline Lingwendil  United States
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Default Opamps for PCM1794 I/V conversion?

Hey there guys, I almost posted this in the Parts section, but figured it would get more relevant views and opinions here. I have a dual mono PCM1794 PCB I picked up a while back, and I'm getting ready to actually finish it. The only opamps that I have on hand are the OPA445, OPA547T, and a handful of OPA2132P. I have used the OPA2132P in the past for single-chip I/V with the PCM1794 and it worked well, but I wanted to order some other chips to try this time around, seeing as how I'm ordering parts anyway.

Please do not recommend off-board solutions, or alternative I/V schemes, that's not the intent at this time.



This is the style of board in question. The one I have is barebones, no DAC chips, I/V stage caps, or opamps. Perfect starting point

Click the image to open in full size.

It uses six single, and four dual opamps total, and I was wanting to spend 3~5$USD or so per chip.



I was thinking for duals, since they are used as the I/V stage chips here, I would try the OPA2132P that I have on hand, and maybe some LME49720/LM4562. I may grab some NE5532 just for comparison. I'm strongly leaning to simply using the LME49720 here. It has very good specs, and has a strong pedigree f great measurements, sound, and performance.

I also might have a set of four Burson V6 "Vivid" duals on the way to test out (samples for review) in this position as well



For singles, maybe the OPA132, OPA227, the usual I see used on these boards/builds is the NE5534, but it feels pedestrian to use these here for much other than comparison purposes. Maybe the OPA1611 on a browndog adapter? I was originally thinking LME49710, but it's unobtainium after going EOL.

Seeing as how the board uses the singles merely for differential-to-single-ended conversion, I'm not so sure a fancy-pantsy chip makes much sense here, based on prior experience, after a certain point.


Any comments would be appreciated.
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Old 10th December 2018, 08:55 PM   #2
sgrossklass is offline sgrossklass  Germany
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You wouldn't have an idea what the schematic looks like? Just for an idea what sort of impedances we are looking at. I mean, there's not going to be much variation in the I/V (good output driving abilities and decently high slew rate required), but after that, not sure.
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Old 10th December 2018, 09:20 PM   #3
Lingwendil is offline Lingwendil  United States
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As delivered, it's basically four complete sets of Fig.24 circuitry from the datasheet-

Opamps for PCM1794 I/V conversion?-pcm1794-png


It only differs really in the fact that there is an additional single opamp per channel to further sum the differential outputs from each PCM1794 to single-ended outputs. The capacitors are not yet populated, but the included resistors match the datasheet values. The I/V opamps (U1/U2) are a dual opamp in this PCB implementation, all others are singles.
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Old 11th December 2018, 02:56 AM   #4
abraxalito is online now abraxalito  United Kingdom
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AD811 would be my choice for I/V. However here you have duals and there's no dual AD811, the nearest dual is AD8017 but that's limited to 12V total supply and is only SOIC8, needing an adapter.
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Old 11th December 2018, 03:11 AM   #5
Markw4 is offline Markw4  United States
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There are Brown Dog adapters for two singles to DIP, to plug into a DIP socket. Singles can be DIP or SMD.

When I see a lot of film caps around opamps, I usually check to see if the film caps are for power bypassing. If so, they may not be very useful at higher frequencies where opamps can have instabilities, or maybe no problems. Something to be aware of is all.

Last edited by Markw4; 11th December 2018 at 03:13 AM.
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Old 11th December 2018, 03:48 AM   #6
Lingwendil is offline Lingwendil  United States
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AD811 huh? That one and the AD797 were recommended to me as well. Hmmmm.

Quote:
Originally Posted by Markw4 View Post
There are Brown Dog adapters for two singles to DIP, to plug into a DIP socket. Singles can be DIP or SMD.

When I see a lot of film caps around opamps, I usually check to see if the film caps are for power bypassing. If so, they may not be very useful at higher frequencies where opamps can have instabilities, or maybe no problems. Something to be aware of is all.
I'm not against the idea of adapters to run two singles on a dual socket, just trying to think of the best way. I'm not afraid of SMD either, and that would make going for a dual with single chips nice and compact too.

Also-
As shown, the red caps on the PCB picture are the film caps (2200/2700pf) on the schematic in the I/V and differential input summing stages, the PCB has ceramics to ground at each opamp supply pin.
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Old 11th December 2018, 03:58 AM   #7
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by Lingwendil View Post
I'm not against the idea of adapters to run two singles on a dual socket, just trying to think of the best way. I'm not afraid of SMD either, and that would make going for a dual with single chips nice and compact too.
AD811JR would be a good choice on a single-to-dual-SMD adapter but note its limited to +/- 12V operation due to running very, very toasty. On Aliexpress its about $1 a chip. I'd suggest no higher than +/- 10V rails for it. It is far and away better than AD8017 in subjective sound quality as I/V even though both chips have very similar paper specs.
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Old 11th December 2018, 04:05 AM   #8
Markw4 is offline Markw4  United States
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When using stacked dual SMD to DIP adapters, I sometimes use some stick-on heat sinks that can help a lot with heat dissipation.
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Old 11th December 2018, 04:23 AM   #9
BesPav is offline BesPav  Russian Federation
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No, mans.
PCM1794 output is really up to 7 mA.
DAC output are better to keep not more than +4:+6dbU.

This will demand for I/V resistance as low as 470 Ohms.

Provided schematic show us 750 Ohm I/V + -1/2 differential amplifier behind I/V stage.

Now check could single opamp provide THD levels comparative with PCM1794 being loaded to such resistance.

I.e. first stage opamps must be buffered for obtaining good performance. Even simple follower with tens mA bias current will greatly rise performance.

Best chips from “musical” reproduction view are LM6171, AD8065, AD797 and LT1364. Of course, properly cooked.
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Old 11th December 2018, 11:25 AM   #10
Lingwendil is offline Lingwendil  United States
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Have you got a link to some testing and distortion spectra to go with that? I'm curious, as the datasheet suggested circuit does give fantastic linearity and subjective quality when implemented correctly.
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