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mfrimu 4th May 2004 11:49 AM

Hk Dac Pcm 1704
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?


Schematics (GIF)

thadude2 7th May 2004 07:36 PM

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:cool:

mfrimu 10th May 2004 09:26 PM

Here it is.

mfrimu 17th May 2004 08:28 PM

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.


Jocko Homo 17th May 2004 09:40 PM

Analog Devices has recommended at times to use a ferrite bead. Sometimes you just have to try 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.


mfrimu 17th May 2004 09:59 PM

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 17th May 2004 10:25 PM

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 17th May 2004 10:33 PM

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........ 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?


mfrimu 18th May 2004 05:11 PM

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 20th May 2004 04:30 AM

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?

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