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My first DAC on PCM1794A - help needed.
My first DAC on PCM1794A - help needed.
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Old 6th February 2015, 02:18 PM   #1
Chulia is offline Chulia  Lithuania
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Default My first DAC on PCM1794A - help needed.

Hi fanatics! Although I have quite decent experience in electronics in general, my expertise in audiophile products is limited - I'd like to ask you to shed some light on areas that are a bit dark for me. I've red a lot and tried to find the answers myself - and most of them I've found, but some need clarification. Or maybe just the confirmation that I got them right, before I get my hands dirty :]

I'm going to use Raspberry Pi with Volumio as direct I2S source, this whole project is based on RPi-DAC project from here: RPi-DAC - Dual Mono

As I expect this DAC to be superior to my Hegel's H70 integrated one - I'm building the dual mono version, including power supply. I've marked areas with questions below:

Click the image to open in full size.

#1 - do I have to use separate LDO for generating 3.3V line, or I can take one from RPi. Without influencing the quality of course (even theoretically).

#1B - Can I use the same power supply (TOROID1) to power DACs on both channels? Or is it a big crime and a rude deviation from "dual mono" thing (to keep both channels as much separated as possible) and I should use separate transformer or its coil to the other channel?

#2 - Have I got the "merging" thing in PSU correctly (to create +15/0/-15 line from two +15 lines)?

#3 - the most important one. This last stage (is it correct to call it "output buffer"?) in original T-DAC/RPi-DAC design is mean to be used with headphones (TPA6120A used). I need it to be used with RCA outputs as a regular line input to my amp. How do I select this stage properly then? What are available designs that you would recommend for me? Will I be able to try several of them without changing I/V part too? Any info on this stage (both general and specific circuits/designs) would be a huge help.

Thanks in advance!
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Old 6th February 2015, 02:45 PM   #2
Redshift187 is offline Redshift187  Canada
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#1 I wouldn't recommend it. Everything I've heard is that the RPi PSU is being run near its maximum output, and drawing too much current from the RPi board causes instability.

#2 Without seeing the actual schematic, it looks right. Those are positive regulators, so the "ground" of the negative half will become -15V (because you'll hook the output to ground). You must use dual secondary transformers for this type of circuit, no center-taps allowed.

#3 You can use the headphone output as a line level output in most cases. As long as there isn't a lot of extra gain for the headphone output.
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Old 6th February 2015, 11:15 PM   #3
00940 is offline 00940  Belgium
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Regarding #3: you could just not use that stage and wire your rca output from the output of the filter's lme4990.
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Old 7th February 2015, 03:21 AM   #4
Chulia is offline Chulia  Lithuania
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I consider #1 and #2 answered - nothing to add here, thanks.

#3 - Seems I'm missing a lot of info about this stage :] If it's fine to skip it altogether - why anybody is even thinking about putting anything in there?

I get "why" in headphones case - there's too little gain in I/V stage I guess. But is it really completely not required when using with a regular amp setup? I think my project just become a lot more easier then :]

On the other hand - let's say we still add it. What benefits would we expect from doing that (using different designs, by increasing difficulty)?
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Old 7th February 2015, 11:46 PM   #5
Redshift187 is offline Redshift187  Canada
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I should give more detail on #2. The drawing is incorrect. That's a dual secondary transformer providing power to 4 regulators. If you want to use 4 regulators, you need 4 secondaries. As drawn, it'll short the transformer.

As for whether to have a buffer stage (headphone amp), the buffer stage will reduce unwanted effects of having capacitors on the output of the I/V stage and capacitors or volume potentiometers on the input of the power amp. Headphone amps don't always have gain, they sometimes just provide more current.
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Old 8th February 2015, 03:55 PM   #6
00940 is offline 00940  Belgium
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Quote:
Originally Posted by Chulia View Post
#3 - Seems I'm missing a lot of info about this stage :] If it's fine to skip it altogether - why anybody is even thinking about putting anything in there?

I get "why" in headphones case - there's too little gain in I/V stage I guess. But is it really completely not required when using with a regular amp setup? I think my project just become a lot more easier then :]
In the headphones cases, it's mostly needed because the opamps used in the I/V and filter stage don't have enough current output for low impedance headphones and not enough voltage output for some high impedance ones. Adding a dedicated stage for the headphones also allows to easily place a volume control in between stage 2 and 3 if required.

With a line-out, any half decent opamp used in the stage 2 filter should be able to drive the interconnect cables and the amp input impedance.
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Old 9th February 2015, 11:35 AM   #7
Chulia is offline Chulia  Lithuania
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Would it be a big crime to build all modules on separate PCBs and to run signals and power lines with short wires between them (as opposed by cramming everything on one PCB) ? As I think I'll go through the process of trying out various different modules for I/V and output (if any) stages, that would be much more convenient.

Also for PSU it seems I'll have to buy 2x32V secondaries toroid and then re-make it to produce 4x16V as I've never seen toroid with more than 2 secondaries. Maybe it's not without a reason and I should look into EI type (which, from my experience would be less ideal for PSU)?

Last edited by Chulia; 9th February 2015 at 11:41 AM.
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Old 3rd April 2015, 12:45 PM   #8
Chulia is offline Chulia  Lithuania
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I like to be through, that's the pleasure of doing things yourself, afterall :]

My DAC starts with a proper power. After doing a research on toroidal VS ei transformers, I decided to use EI in this particular project. Carefully placing and screening them of course will be done, but the question is:

Since I need so much secondaries for powering everything up, should I prefer less, but bigger trannies with more secondaries on them, or smaller ones, but more of them? My guess it's the second option, since smaller transformer (actually, a group of them) should radiate it's stray inductance in a smaller field, thus making the distance between it and the PCBs a better "isolator".
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Old 3rd April 2015, 02:06 PM   #9
gentlevoice is offline gentlevoice  Denmark
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My first DAC on PCM1794A - help needed.
Quote:
My DAC starts with a proper power. After doing a research on toroidal VS ei transformers, I decided to use EI in this particular project. Carefully placing and screening them of course will be done, but the question is:
Hi ...

Interesting project which I would say have many possible solutions depending on what you want to achieve sound-wise and/or technically.

Regarding transformers my personal consideration would be EMI since you are using regulators for each rail to the DAC. It likely is of less importance in this case if the transformer is capable of delivering high impulses (which is what I remember EIs are known for being able to do - I may be mistaken here, though). Personally, I would just get transformers with the required voltages and windings, possibly separating the digital side from the analog side. In my experience the PCM1794 is especially sensitive to VCC and VDD voltage fluctuations so a clean supply would be my choice (possibly prefiltering the TPS7A ..)

Regarding an output (your #3) directly to an RCA I would personally use the LME49990s directly. It's capable of driving 600 ohms and I suppose the input your are feeding is not lower than this ... (?)

In terms of splitting up the boards it of course can be done - but you may wish to consider any additional noise between the connecting wires, a split ground plane etc. I would say that it depends on how you do it.

Finally, having designed a couple of PCM1794 based DACs I'd like to mention this implementation as an alternative to an opamp based I/V conversion:

A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input)

Upgraded Single Board PCM1794 NOS DDDAC

I'm not saying it's better than an LME49990 based design but IMHO it's quite musical not least when given a very good power supply. Also, it is by default designed to work with a USB-to-I2S converter board which uses low jitter oscillators - a factor which in my experience is quite key to a DAC's sound.

In either way: Good luck with your project

Jesper
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Old 9th April 2015, 08:40 PM   #10
Chulia is offline Chulia  Lithuania
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I guess this could be called "not bad for an amateur". The PCB's are ready!

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