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tda1387 dac pcb "front end"
tda1387 dac pcb "front end"
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Old 13th August 2018, 04:42 PM   #221
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Quote:
Originally Posted by stellarelephant View Post
Thankfully I won't need to cut the fat GPIO power trace at top left since I already have made provision to send dedicated power to the board by NOT stacking the board atop my Pi stack and instead using GPIO jumpers for I2S and with power jumpers going to separate PSU.
Ahh, yes, an even simpler solution that doesn't involve physically altering the board!


Quote:
Originally Posted by stellarelephant View Post
I WILL try my hand at cutting the middle trace to get a dedicated 5V feed for the TL431 and I/V transistors (option #1). I will be using another one of Greg Stewart's modded Jamecos. Greg actually has the DAC for audition at present, so it'll be a week or so before I am able to try this.
Actually, perhaps a better approach is to simply remove R1, replace it with a through-hole resistor inline with your discrete supply's positive lead. Then you don't have to cut the trace on the board (you're "implicitly" cutting it by removing R1). It's easier and cleaner to re-solder an SMD resistor than to repair a cut trace IMO. That's a 1k resistor, which is a super-common value.


Quote:
Originally Posted by stellarelephant View Post
I have an LT3042 module that could be used to drop a 5V Jameco to 2.8V, for option #2 down the road. It isn't the best sounding regulator module in the world, but if it is superior to the TL431, perhaps it would give another SQ bump.
I'd be interested in learning about your results of that experiment. The TL431 is nice because it's cheap and easy to use; those are the main reasons it was chosen for this application.

Note there are different grades of TL431, that vary in their tolerance. I can't remember what grade I put on that batch of single-ended RPI DAC HATs (such as the one you have). I can't say for sure, but I suspect I may have gone with a cheaper/lower-grade TL431... if so, it might be worthwhile to get the best grade ("B"/0.5% IIRC) and upgrade the one on your board.


Quote:
Originally Posted by stellarelephant View Post
Your new balanced Pi Hat looks cool, and the added power and I/V flexibility make it quite attractive. Impressive 3D model too! I have not heard passive I/V yet. Are you intending to use it like this yourself or to follow with a separate I/V board? At any rate I appreciate the modular approach. I have been itching to build the new Pass B1 NuTube buffer...I wonder how that would pair up with passive I/V. All my gear is single ended. What will signal output voltage be for SE and balanced?
The design tool, KiCad, auto-generates those nice raytraced 3d models.

I will initially use it with passive I/V. I've only flirted with tda1387 in passive I/V form, never done any listening beyond checking that something works. So I want to see what passive sounds like. Maybe it's good enough for me? But right now I'm thinking of maybe creating a super-simple op-amp based I/V board, and also maybe trying to create a fancier I/V board with Abraxalito's filtering and discrete buffers. But for now, all that's just thinking out loud.

In general, the signal output voltage will depend on the I/V resistor used. Hopefully someone will correct me if I get this wrong, but I believe the math goes something like this: at 5V supply voltage, the tda1387 can output up to 1mA current. The output compliance is 3.5v (again with 5v supply), so our I/V needs to be chosen such that the output voltage never exceeds 3.5v. Using Ohm's law, V=IR, we get 3.5 = 0.001*R. Solve for R, the biggest IV resistor is 3.5k Ohm. A peak-to-peak voltage of 3.5v, results in an RMS output voltage of about 1.25V (divide 3.5V by 2.8, see here).

I'm personally OK with lower voltage coming out of my DAC, so I use a 1k resistor. So: V = 0.001 * 1k = 1V (peak-to-peak). 1V/2.8 = 357mV RMS.

The above exercise is for a single tda1387 (for a single-ended output signal).

My original front-end board had jumpers to configure for single-ended or balanced output. I didn't have room for jumpers on this new design, but you could go strictly single-ended with a little bit of board hacking (cut a trace, jumper a wire on HC86). Or you could keep the "cold" part of the circuit active, and run it to ground; or give it a "dummy" load like Abraxalito is doing on his lingDAC.

I think that 1.25V RMS is the most you'll get from the tda1387. Even if you parallel the chips, the output voltage still won't exceed 3.5V (voltage in parallel is the same), so that limits the RMS number.

You can go a bit higher than 3.5V by using more than 5V supply, as Abraxalito does. I believe he said it scales proportionally, so if you feed the tda1387 6V, the output compliance will be 4.5V (about 1.6V RMS).

Lastly, I believe since your goal is single-ended output, you could run the DAC in balanced mode, then convert to single-ended by use of transformers. Again, need real experts here to check this, but I think since you have a differential signal (i.e. signal plus its inverse), you could feed them to the transformer's primaries (wound in opposite directions), and have double the voltage output on the secondary side (assuming 1:1 transformer windings).

In fact, I've always wanted to experiment more with transformers, but the really nice ones are expensive.
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Old 14th August 2018, 10:03 AM   #222
Yosyos is offline Yosyos
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Hi matt,

Just want to thank you for very good and very musical dac!
The SQ is Pleasant to the ear, warm and soft.
I can listen to it for hours and really enjoy it.
My setup is pi>IsolatorPi by ian>kali>balanced tda1387 RPI HAT DAC
Feed by linear power supply based on lt3045

P. S i didn't hear hiss sound from the dac, that you mentioned before.

Thank's again,

Yos. IMG_20180710_165000.jpgIMG_20180710_164801.jpgIMG_20180710_165405.jpg
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Old 14th August 2018, 01:52 PM   #223
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Yosyos - that looks great, thank you for sharing! To my knowledge, you are the first person to do your own fabrication and assembly (at least, I think you are the first to talk about it). I'm also delighted to hear you are enjoying it.

Thanks again, and hope you continue to enjoy it!
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Old 16th August 2018, 09:11 PM   #224
Alexandre is offline Alexandre
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Quote:
Originally Posted by matt_garman View Post
In fact, I've always wanted to experiment more with transformers, but the really nice ones are expensive.
Matt, people seem to be happy with the Edcor PC600-15K, performance and soundwise. I believe it is inexpensive (under 10 dollars?) if you are in the US.

Quote:
Originally Posted by Mark Johnson View Post
For those who haven't seen it yet, diyAudio's own Scott Wurcer has measured an Edcor PC600-15K as used in the First Watt M2, and built a SPICE simulation model that fits his measurements. So, if anybody was planning to put M2 into LTSPICE, your task just got a little easier.

The model for 6X gain {as Nelson Pass used in the M2} is on the left; Scott also modeled another way of connecting the Edcor transformer (that nobody has yet built into an amplifier) with 11X gain, on the right.

Here is the post: LINK

Questions? Scott is the guy most likely to know the answers.
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Old 16th August 2018, 09:33 PM   #225
Alexandre is offline Alexandre
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I thought I should write this. I personally am not fond of making the dac differential/balanced by doubling the dac chips. While I enjoyed the dacs I made that way, I realized pure single ended is better when I went back to it.

The differential scheme changes the "harmonic signature" of the dac, for lack of a better term. Second harmonic is cancelled to a large extent, while third harmonic remains the same. This in my opinion (and that of many experts) is the opposite of what works well for sound. What seems to be desirable is dominant second harmonic, at a fair level (think tubes) and low odd harmonics.

I offer one short video and one article for consideration. Itīs interesting material, altough not specifically about dacs. These mastering engineers are among the very best (Bernie Grundman and Bob Katz).

YouTube

Katz's Corner Episode 25: Adventures in Distortion | InnerFidelity

Thanks,
Alex
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Old 17th August 2018, 03:20 PM   #226
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
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Originally Posted by Alexandre View Post
Matt, people seem to be happy with the Edcor PC600-15K, performance and soundwise. I believe it is inexpensive (under 10 dollars?) if you are in the US.
I actually have the Edcor TTPC15K/15K transformers. Doesn't look like they sell them any more, but I think they are practically the same as the PC-series that is currently available. I also have the Jensen JT-11P-1. (I wish I had the PCB mount version of those Jensens; their long skinny leads upset my neatness neurosis!) At some point I may have to pull these out of storage, and maybe get some lower-impedance Edcor PC series. Actually, to be honest, it's less about money-cost and more about time-cost. I have an ever-growing list of things I want to play with, but very limited time to actually do those things!


Quote:
Originally Posted by Alexandre View Post
The differential scheme changes the "harmonic signature" of the dac, for lack of a better term. Second harmonic is cancelled to a large extent, while third harmonic remains the same. This in my opinion (and that of many experts) is the opposite of what works well for sound. What seems to be desirable is dominant second harmonic, at a fair level (think tubes) and low odd harmonics.
Do you know why this is? I.e., why does the differential scheme "selectively" cancel out 2nd-order harmonics? Do you recall what the rest of your system looked like when you made this observation? In my case, I generally have to switch amps when I switch between balanced and single-ended DACs. In other words, two variables change.

I wonder if there's a way to counter-act this, to get the best of both worlds?

At any rate, I don't think I can hear it. Although now that you mention it, I might start hearing it.

I still like having differential output as an option on my designs. Even if single-ended is the goal, it's easy enough to only populate half the circuit, or run the cold output into ground (e.g. with those pseudo-differential cables), or even do like Abraxalito and present a "dummy" load for the cold DAC(s).

So many ideas to explore, so little time!
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Old 17th August 2018, 05:32 PM   #227
Alexandre is offline Alexandre
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Quote:
Originally Posted by matt_garman View Post
Do you know why this is? I.e., why does the differential scheme "selectively" cancel out 2nd-order harmonics?
Matt, good question, i donīt recall having seen a mathematical proof of this. Should be interesting. It is important to make the distinction: the differential circuit doesnīt erase harmonics of the original signal, it cancels its own even harmonics. So if a circuit is clean enough, very low distortion, I suppose itīs no issue running it balanced/differential.

OTOH, if the circuit produces some harmonics, it might be better not to cancel the most desirable second harmonic.

My amp is similar to Jumaīs, but I used LM3875 and balanced input (yes I use an amp with balanced input and a dac with single ended out and it works very well).

GC SuperSymmetry
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Old 20th August 2018, 05:29 AM   #228
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by Alexandre View Post
The differential scheme changes the "harmonic signature" of the dac, for lack of a better term. Second harmonic is cancelled to a large extent, while third harmonic remains the same. This in my opinion (and that of many experts) is the opposite of what works well for sound. What seems to be desirable is dominant second harmonic, at a fair level (think tubes) and low odd harmonics.
I'm skeptical of this argument because while it works fine for harmonics of test tones (normally single sinewaves) it doesn't to me seem to make sense when music is being played.

According to reliable research (referred to in Belcher's WW paper going back to 1978) with music there are so many tones present that intermodulation distortion totally swamps harmonic. So do those missing 2nd harmonics translate directly to missing 2nd order terms with IMD I wonder? From a quick look at images of how intermod distortion occurs the third order products are going to be closest in frequency to the original tones (2f1-f2 and 2f2-f1) so it might be that third order IMD is the most audible, but why would the absence of the second order IMD (sum and difference) tones be undesirable?
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Old 20th August 2018, 05:43 AM   #229
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by matt_garman View Post
Do you know why this is? I.e., why does the differential scheme "selectively" cancel out 2nd-order harmonics?
The way I look at it, its because the 2nd order (and all other even order terms) are in-phase on both the +ve and -ve signal wires, whereas the first, third, fifth etc remain 180o out of phase. So when the difference is taken between + and - those even order terms are the same on both sides and hence vanish.

I think the argument for differential is strongest when we consider power supply noise being cancelled in classA systems by such an arrangement. The cancellation doesn't happen if we run classAB though, contrary to what I've read elsewhere. So bridged chipamps don't benefit from supply cancellation unless heavily biassed into classA.
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Old 20th August 2018, 04:18 PM   #230
Alexandre is offline Alexandre
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Richard, I heard the differences, so I just accept it.

BTW, I heard these differences before I read about them, so it was an "independent finding"

( I even told you in an email )

I quote that article from Bob Katz:
Quote:
I've known for many years that harmonic distortion can enhance depth and ambience, but this is uncanny—it's like there's a magic reverb chamber in the tube. The ambience is enhanced in a holographic way that's totally pleasant and not at all distracting from the original mix or performance.
He is a top notch mastering engineer. Works with a lot of minimalist, acoustic recordings. He is quite accessible via email, maybe you could have a more technical discussion with him if you wish. BTW his book "Mastering Audio" is very good.

Thanks,
Alex
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