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tda1387 dac pcb "front end"
tda1387 dac pcb "front end"
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Old 12th January 2018, 05:17 AM   #51
robert1325 is offline robert1325  Netherlands
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Quote:
Originally Posted by abraxalito View Post
One of these boards, together with the up-coming filter I/V board (about to enter layout any day now) will allow construction of a lingDAC.
I also can't wait to hear this DAC with your filter& I/V board.


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Originally Posted by matt_garman View Post
I hope to, time permitting. That was the idea behind this design, to make it easy to try different I/V stages, or just go with a simple passive. The I/V I'm most interested to try is Abraxalito's.
Can the TDA1387 be soldered by hand with just patience, normal solder and a fine tip? It wasn't too bad adding caps on the chip pins.

And if going single-ended and 4 chip, would I be able to omit the logic inverter chip?
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Old 12th January 2018, 05:53 AM   #52
abraxalito is offline abraxalito  United Kingdom
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I must've soldered hundreds of TDA1387s down on 0.1" pitch perfboard. Soldering to the PCB was a doddle compared to that. Yes you'll need fine solder (I use 0.5mm) to avoid getting too much on the pads, which then leads to shorts.

You're right for single ended use there is no requirement to employ the HC86. If you omit it you'd need to wire across its pins in various places to get the I2S signal into the DAC chips.
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Old 12th January 2018, 08:40 PM   #53
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
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Originally Posted by robert1325 View Post
Can the TDA1387 be soldered by hand with just patience, normal solder and a fine tip? It wasn't too bad adding caps on the chip pins.
What I've done when manually soldering is to first put some solder on the chip pads (i.e. pre-tin the board's IC pin pads). Then I hold the IC itself in place with precision tweezers, and just "brush" my soldering iron across the pins. I've found this to generally be pretty quick and easy, and actually looks good too!

The other method is the "stovetop reflow": first I put a skillet on our cooking range, and turn it up to medium heat (I have a cheap dedicated skillet just for this purpose). I put solder paste on all surface-mount pads on the PCB, and then drop all SMD components in their place. I then place the PCB on the now-hot skillet. I crank up the heat and watch for the solder paste to melt (very obvious, goes from a dull grey to a shiny silver). Once all the solder paste has melted, I kill the heat and remove the PCB from the skillet, and let it air cool. It's fun to watch the the surface tension of the melted (fluid) solder paste perfectly align the SMD components. This results in a very clean "made by robots" look.

I've heard about people using their cooking ovens to reflow as I've described. Not sure I want to off-gas my lead-based solder paste in the same oven that we cook family food in. But some day I plan to look for a cheap toaster-oven with temperature control in a thrift store that I can dedicate to this purpose.
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Old 13th January 2018, 08:48 PM   #54
robert1325 is offline robert1325  Netherlands
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Matt and Abraxalito, thanks for your encouragement and instructions, I am going to do it! I will order up the parts and PCBs once the Lingdac I/V is finalized.

Off-topic and perhaps too soon: do you think the Lingdac I/V stage would run well straight into a 10k potentiometer to a tpa3245evm board? The 8x DAC with the AD845 I/V sounds a bit anemic on this setup. If not, I'll start working on a buffer stage (i.e. Kuartlotron or DCB1) in the mean time.
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Old 14th January 2018, 05:31 AM   #55
abraxalito is offline abraxalito  United Kingdom
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The way I have the I/V stage of lingDAC configured now it doesn't have particularly low output impedance (its about 600ohms). So I would not recommend it straight into any classD amplifier. And definitely not via a pot into a classD amp as the pot will raise the output impedance even more (to 2k5 or so with 10k pot).

The TDA1387 lends itself to a volume control based on varying the pin7 voltage which I think will be better than using a pot. I am still considering the best way to tweak pin7 on my DAC. Incidentally I strongly recommend connecting a DAC to a classD amp via a transformer for best dynamics. I guess I need to design a trafo-output DAC especially for those who want to couple to a classD (that includes myself ).
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Old 14th January 2018, 03:08 PM   #56
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Quote:
Originally Posted by abraxalito View Post
The way I have the I/V stage of lingDAC configured now it doesn't have particularly low output impedance (its about 600ohms). So I would not recommend it straight into any classD amplifier.
Can you elaborate on that?

Also, the opposite question: for the component that follows the lingDAC, what input characteristics are ideal?
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Old 14th January 2018, 03:21 PM   #57
abraxalito is offline abraxalito  United Kingdom
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There are a couple of aspects to consider here - first that many (not all) classD amps have rather a low input impedance - some as low as 3k. Putting that kind of load on the output of lingDAC is likely to make a small change to its frequency response. This is the lesser of the two issues.

The more important one is that classD amps generate a fair amount of noise (from their outputs) which can be picked up capacitively at the input - a high driving impedance here facilitates that pick-up. So to drive such amps I'd incorporate a buffer (emitter or source follower, biassed with CCS) to lower the output impedance by a factor of 10 or more. I already have a version of lingDAC with such a buffer which works fine - having a buffer means there's no longer any requirement for 4 paralleled DAC chips at the front end, even a single TDA1387 can be used.

The unbuffered lingDAC prefers to see a load of at least 20k.
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Old 31st January 2018, 04:42 AM   #58
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Default Finally...

For the single-ended RPI Hat:

OK, third time's a charm (hopefully). First batch of these I used the wrong footprint for the tda1387 chip, so they were worthless.

On the second batch, I made some mistake exporting the Gerber files, so the boards came back without any ground plane. I suppose they are not technically worthless, but I didn't want to deal with it.

This batch corrects the above two mistakes. I soldered up a couple boards tonight. Testing will have to wait for another day.

I hand-soldered everything, my last couple attempts at the "stovetop reflow" ended up not going as well as I hoped. So I thought I'd keep things simple this time.

Let's hope they work!
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Old 31st January 2018, 06:10 PM   #59
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
At least one board is working! I had a little time to give one of the completed boards a test. Worked without any issues. I didn't have any time for any kind of serious listing, but no obvious problems.

I have a pair of Overnight Sensations speakers I use as test speakers (the drivers are cheap in case I blow them up). As soon as I was confident this DAC won't kill speakers, I switched over to my Speedsters. I can say the DAC is at least resolving enough that the quality difference between the two different speakers was immediately obvious.

Once again I have a few extra boards, free to anyone who's willing to cover shipping. (I wonder if I could just drop one in an envelope and send it via first class mail, i.e. with just a stamp?)
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Old 1st February 2018, 06:24 AM   #60
luxury54 is offline luxury54  Romania
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Quote:
Originally Posted by abraxalito View Post
The way I have the I/V stage of lingDAC configured now it doesn't have particularly low output impedance (its about 600ohms). So I would not recommend it straight into any classD amplifier. And definitely not via a pot into a classD amp as the pot will raise the output impedance even more (to 2k5 or so with 10k pot).

The TDA1387 lends itself to a volume control based on varying the pin7 voltage which I think will be better than using a pot. I am still considering the best way to tweak pin7 on my DAC. Incidentally I strongly recommend connecting a DAC to a classD amp via a transformer for best dynamics. I guess I need to design a trafo-output DAC especially for those who want to couple to a classD (that includes myself ).
Hi, do you happen to have a simple schematic or sketch on how to couple an audio transformer with a single ended DAC like this one? I would also like to use it in combination with a class D amp;
also, would this type of transformer be compatible? thanks

http://ro.farnell.com/vigortronix/vt...1-1/dp/1674305
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