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AliExpress AD1865 R2R DAC
AliExpress AD1865 R2R DAC
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Old 10th October 2018, 01:27 AM   #51
Hugh Jazz is offline Hugh Jazz  Taiwan
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I suggest you continue to discuss the TDA - 2 reasons

-the first post makes it clear that this is what the thread is about. maybe ask admin to change the thread title too.
-the seller still hasn't shipped - 10 days now - so I cancelled my order. I won't be posting about the AD1865 on diyaudio.

For bandwidth - look at the mclk for an idea of the MHz noise in the DAC.

Last edited by Hugh Jazz; 10th October 2018 at 01:56 AM.
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Old 10th October 2018, 02:01 AM   #52
Hugh Jazz is offline Hugh Jazz  Taiwan
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and it timed out... was also gonna say google gain bandwidth product. with a lot of I/V "gain", the bandwidth of the op amp decreases so it is common to add an R-C or just C in parallel to the I/V resistor to limit gain at HF and mitigate the loss of bandwidth.
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Old 10th October 2018, 02:08 AM   #53
abraxalito is offline abraxalito  United Kingdom
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Thanks, I'll stay on the topic of filtering the DAC's output then as Matt asked....

Here are a couple of plots made in LTspice of the I/V circuit I posted up earlier. The first one is what happens with a 2n2 cap to GND from the DAC's output. I chose 2n2 as that's about what I recall Scott Wurcer using on the AD797. As you can see there's a very large peak in the FR. We can quite easily get rid of that though by putting a feedback cap across the 3k (opamp feedback resistor). With 150pF we get a very nicely damped response which is shown in the second plot.

Note I'm using LT1122 for this - chosen as the LT opamp most similar to OP42 (14MHz GBW, 60V/uS slew, JFET input). Changing to the AD8065 would increase the freq of the peak as its a faster part.
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File Type: png CaptoGND150pF.png (7.4 KB, 269 views)
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Old 10th October 2018, 02:23 AM   #54
abraxalito is offline abraxalito  United Kingdom
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Eyeballing the second plot the resulting circuit has roughly a 500kHz bandwidth, -3dB. That's 25X the audio bandwidth. This observation gives us a starting point for getting an answer to Matt's earlier question - how to choose the GBW for an I/V opamp?

One way to go about it would be to take the venerable NE5534 as a starting point as its used in so much audio circuitry - it has 10MHz GBW. If that's the benchmark then for 25X the bandwidth we should be looking at 250MHz GBW for this I/V application. Even the AD8065 doesn't make the grade there (155MHz) but it does have a sister part the AD8067 which has even higher GBW (540MHz). The trouble with that part is its not unity gain stable so can't work in a normal transimpedance stage as we have here.

I do have an idea for how to use it though, I'll not write about it until I actually have some of these chips in my hands to try out. Do stay tuned
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Old 11th October 2018, 12:03 PM   #55
erin is offline erin  Australia
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Originally Posted by abraxalito View Post
I Lynn Olson put a spectrum analyser on the output of his PCM63 and found a spectrum that went into the 10s of MHz.
I've read that article by Lynne Olson. After years of reading people say that op-amps are no good for IV, and trying valve IV stages and discreet transistor IV stages, and op-amp IV stages Ive come to the conclusion that op-amps are perfectly fine, in fact best for IV.

I don't like valves because each valve sounds different and consistent performance cannot be obtained. Discrete sounds fine, but no better than op-amps. I recently found a Yamaha CD-500 CD player with PCM54 DAC and slow old NJM op-amps and it sounds a little dark, but very smooth. Hard to say if the sound is due to the DAC or output stage, but I like it none the less.

I'm not convinced that op-amp bandwidth matters as much as some people say. My best sounding DAC uses AD711. I haven't tried the 844 or the Sen/zen IV stage. Perhaps they are markedly better?
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Old 12th October 2018, 05:02 AM   #56
abraxalito is offline abraxalito  United Kingdom
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I just came across this article in my LI feed, its worthy of wider dissemination as I've gotten a whole lot of benefit from KCP's writings : Push Me, Pull Me—When Supply Currents Go Rogue | Electronic Design

Your journey through the realms of I/V seems to not quite match up with my own @erin. I originally (wearing my EE's hat) considered opamps the best solution and didn't hear any problems with them. Then when I got into modding active speakers (the opamp-based XOs and chipamps) I discovered stuff which impacted the SQ out of opamps (and that's related to KCP's article). Which took me in the direction of discrete designs so that I could make everything classA. Now I'm toying with opamps again to see if they can offer some benefits compared to discretes - at this early stage in my exploration I've not gotten to the level of satisfaction obtainable with discretes but there is a certain impressiveness about the sound which initially is appealing but likely won't stay the course.
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Old 12th October 2018, 08:57 AM   #57
erin is offline erin  Australia
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@abraxalito, thanks for the interesting link to the opamp article.

I suppose for anyone to speak with authority on the subject of IV stages, a person would have had to have tried every single variety.
If a person has tried every type, what's to say that their favourite isn't just a personal preference, rather than being definitively better in all areas?

The thing I find curious is how some CDs or digital files can sound awful, and I used to think there was a problem with my DAC, or IV stage, or source, and then I played a different recording and the sound would be beautiful and faultless. This lead me to conclude that opamps are good because the opamp delivers more detail than either the discreet stage or the valve stage.

I think majority of the problem is that digital audio is so accurate and revealing of problems with the recording that if there is any issue whatsoever, it is revealed by a good DAC warts and all. The fact that some recordings sound wonderful with smooth, clear highs without a hint of fatigue demonstrates to me that it isn't the opamp causing the issue.
I personally attribute bad sound to the recording or mastering.

I listen to classical music and I find that so many analogue recordings used very shrill microphones and mic preamps, so they translate very poorly to digital.

The main point I want to make is that if opamps sound bad, then all music should sound bad, fortunately good recordings show how good opamps can sound. When I had my valve IV stage, the bad recordings still sounded bad but no music sounded better through the valve.
But for a while I talked myself into thinking it was better. Glowing bottle fever I suppose!
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Old 12th October 2018, 11:14 AM   #58
Hugh Jazz is offline Hugh Jazz  Taiwan
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Originally Posted by erin View Post
The main point I want to make is that if opamps sound bad, then....
If I may... if a modern op amp (such as the opa1611) sounds in any way deficient, it's not the op amp's fault.

Last edited by Hugh Jazz; 12th October 2018 at 11:21 AM.
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Old 12th October 2018, 01:01 PM   #59
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I'd not wish to speak with any kind of 'authority' on I/V stages, merely share experiences with them. If a person has listened to every type there would be a myriad of other circuit details which may have been uncontrolled (decoupling is one major one, grounding another) so any pronouncements would have to be nuanced.

As regards personal preference vs 'better' I view an audio system as representing a piece of glass. Certainly there are people who prefer tinted glass - but more transparent glass is going to be more revealing of recording details than anything tinted.

True enough that recordings differ a lot - a more transparent system though I've found renders poor recordings more enjoyably than a lesser one. Yep opamps deliver more 'detail' (at least in my experience) but I don't find that detail is really on the recording, rather its a slightly unnatural emphasis on certain upper mid-range frequencies.

Ultimately its about enjoyment and long-term listening satisfaction, which is why I hesitate to come to any conclusions without living with a sound for a length of time (weeks rather than days). My diet is 90% classical music and I tend to agree with Lynn Olson that its more challenging for a system to reproduce accurately than the more mainstream audiophile genres like 'girl with a guitar'.
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Old 12th October 2018, 03:47 PM   #60
Markw4 is offline Markw4  United States
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Originally Posted by abraxalito View Post
Yep opamps deliver more 'detail' (at least in my experience) but I don't find that detail is really on the recording, rather its a slightly unnatural emphasis on certain upper mid-range frequencies.
How could you know it isn't on a recording? Maybe not on the tape or other media the master was mixed down to, but how can you know the mastering ADC was perfectly transparent?
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