Hk Dac Pcm 1704

[mfrimu]

Hello everybody,

I found this kit in HK, and it looked interesting, so I ordered a PCB. I thought of posting it in case someone might be interrested. I like the multiple PS for each section of the circuit. It seems that the IV is a 200 ohm resistor to ground (I'm not 100% sure). It looks that there is a low pass filter using 2 op-amps as well. I'll be finished in a few days and post some results. We are making two of them, one with BG capacitors and one with Panasonic FC (to compare). I'll possibly add a tube output stage if i'm not satisfied with the sound of the op-amps.

I have a question: The digital and analog grounds are kept separate on the PCB, at are connected at one point via a unidentified component marked as a capacitor 100/16v. After a lot of research on grounding of mixed signal circuits, I found that I would be advisable to use a ferrite instead. Any comments?

Thanks,

Schematics (GIF)

 

[thadude2]

A link?

Hey mfr,

That pcb looks pretty interesting, do you happen to have a link to the site where you got it?

Thanks a bunch

 

[mfrimu]

Here it is.

http://www.diyclub.biz/catalog/default.php?cPath=23_87

 

[mfrimu]

PS regulators not working

There were a few errors on the PCB silkscreen such as some transistor polarities and a relay polarity. They were detected and components were installed properly.

Plugging the DAC, the sound is weak and missing + distorting the bass.

Checked the voltage regulators : the output is not what was expected. On the PCM1704 digital and analog regulators I get -3.74/+3.74v instead of -5/+5v. One of the regulators is giving -3.74/+4.3v. I don't know much about voltage requlators, and I wonder what might be the cause. There are Zener diodes in the requlators that match the output voltage required (5v). The 3 discrete requlators that are giving problems are exactly the same except the Zener diodes.

Help!

 

[Jocko Homo]

Analog Devices has recommended at times to use a ferrite bead. Sometimes you just have to try it...it may work better in some applications, but not all.

Yeah..........200R to ground is the load.....and then they amlipfy it. Why go to the bother of using a modern DAC, and then treat it like some ancient piece of crud. Go figure.

At least they put some circuitry between the line and the RX chip, although it is not what I would have done. That RX chips works better if driven differentially.

Jocko

 

[mfrimu]

Thanks Jocko Homo,

I'm trying the ferrite bead as an option. What do you mean by differential drive for the RX chip? Do you have any examples.

I'm going to be busy troubleshooting the DAC, since the regulators don't seem to work as specified... I have a second PCB I'll use for testing.

 

[mfrimu]

PS regulators measurements

I just measured the output voltage of a _5/+5 v regulator (no load) and I get +3.8/-3.8. The voltage across the Zener diodes is the same. I wonder if I should change the 5.6K resistor ?

 

[Jocko Homo]

You should not use the ferrite bead if the planes carry digital stuff between them, only if it is an "analog" only plane.

Planes can act as antennas........


Differential..........you could get a transformer.....if there actually are any decent ones still made......and connect each end of the secondary to one of those odd circuits. Then take one into the RX(+), the other into RX(-).

Make any sense?

Jocko

 

[mfrimu]

Thanks, I'll try to find a transformer.

Turns out that the PS regulator are pretty interresting, see the thread:thread here

I'm in the process of adjusting the regulators and see what happens.

 

[mfrimu]

Distorted output

Fixed the regulators, all at 5.2v and 14.5v for output section.

The sound is ok only with very low volume material, otherwise heavly distorted. Distortion more pronounced in the bass. I'm not familiar with standard amplification of i/v out (using resistor).

I'll try to isolate where the distortion comes from. I don't think that the filter using the OP275 is the cause (i removed the opamps, no change).

I don't know if the problem is digital or in the analog section.

I lowered the i/v resistor to 100ohms, no change in distorsion, just lower output.

Any suggestions as to what I should check first?

 

[mfrimu]

Distorted output

I've removed the opamps and checked the output of the DACs and it's the same - distorsion.

I've bypassed the TTL input circuit, no change.

The distorsion is low when digital input volume is very low, otherwise 99% distorsion.

?? I'm checking the pcb for errors

Any ideas?

 

[mfrimu]

There is a difference on PCB compared to schematics regarding the DF1704:

It is wired as if MODE=software but no connections to most control pins:

pins 3,4,5,7,9,10,11,12,13,15,16,17,18,27 are floating

pins 19,20 are HIGH

I wonder what the default software registers are set at. I've checked the datasheet and no mention of defaults....

 

[Jocko Homo]

Look on page 9......your answers are there.

Anywa, I don't think that is your problem. You could always ground pin 9 just for grins: it should operate the same way.

Jocko

 

[mfrimu]

Sound of DAC

Here is the distorted sound:

dac_distorted MP3 (465K)

Here is what it should be:


dac_source_ref MP3 (465K)

I was wrong that the bass is distorted, seems ok.

Is this digital overload or analog????

 

[mfrimu]

I can't see the default settings of the registers on page 9...

I see a table with the software registers at page 10 \, but it is not indicated what are the defaults. When the IC is left floating, what is the setup?

 

[mfrimu]

Working DAC!

I found the problem !

The CS8414 was not configured for I2S output. This was supposed to be wired on a small carrier board which mounts on the main PCB. Thanks for the help.

The DAC sounds very good. I can only compare to CI Audio VDA1 and optional VAC1 power supply. It sounds better, soundstage much wider and accurate, qualities that CI Audio had in the top range now are present in midrange and at lower freq (dynamics, etc).

 

[Jarno]

Ordering from DIYclub

Hi mfr,

I saw the DAC is now fully operational. I had a look at the DIY club site and saw they also carry pcb's for tube preamps, looks pretty interesting, good prices too. But since I haven't heard of anyone else ordering there, I was wondering what your opinion is on doing business with them (I heard shipping from Asia can be a bit "erratic")?

Thanks,

Jarno.