[mwofsi]

I have an Oritek modded Zhaolu, bought at a time when I didn't even want to open a dac let alone mod one. I wanted a solution that people were happy with and this seemed to fit the bill.

It is not producing sound other than slight background hiss. It'd been in daily use for 8 months or so. I've been in contact with Ori, but I don't want to send it back. I would like to better understand what's occuring.

I suspect perhaps the power supply, start-up circuit or failed chip.

So anyone able to let me ogle the schematic?

The power supply seems ok providing +/-15V to buffer/headamp and 5,0,0,5,1.3,1.3, to the dac board. Those two 1.3V supply lines seem strange. Do they have a purpose?

One of the yellow 8V transformer secondary wires is/was somewhat loose where it joins the board.

I've tried a manual reset, shorting pin 24 of the AD1852 to ground. No joy. High voltage on that pin is 2.4V, which seems low to me but of course AD don't seem to give a voltage for this pin they just say raise it high or hold it low.

EDIT: The spdif lock led lights when powered. I only have DMM to check with , no oscilloscope.

 

[mwofsi]

Erm....

After some desoldering and soldering the problem is found.

Turns out that Windows decided to change my default audio device for no apparent reason (patch maybe?). Found by going through the device troubleshooter from the device manager.

Nothing ventured, nothing gained.

So now that's working I have a few plans to mod the modded dac. Replace the pulse transformer with a correctly spec'ed one, add a low pass filter and create a similar output stage using better components such as the one being laid out for the CS4397 dac by fordgtlover (servoed of course!). Some Twisted Pear modules might even find their way in here at some point.

 

[mwofsi]

Quote:

Originally Posted by Asylum years internal headphone amp board

It's pretty basic stuff, single mosfet per channel amplification.

The whole thing together does sound quite good. More open and clear sounding than my perfectly good pc2707 usb dac, for instance, but there's a hashness to the highs that I've never been happy with. I hope to be able to address that.

 

[mwofsi]

Also on the list of to be improved is the ALPS pot for the headamp. Of course ALPS make some really good audio pots, but they make some very ordinary ones too.

I suspect that a litttle more care in component choice, (at increased cost to the builder) would make this modded dac the giant killer that it gets claimed to be. Of course I could be wrong and it could just be that the add-ons are just too basic and over simplified to get the results that many here strive for. Time and more modification will tell.

 

[mwofsi]

I'm not sure that it's fair to Ori to give out all the details. It's not as though I've worked out the circuit completely anyway. There seems nothing fancy about it. I believe that it is advertised as an all discreet output stage.

What it does do is convert differential dac signals to single ended without dc offset. There appears, for instance to be an opamp servo based on an LT1057 opamp. This is the same opamp supplied with the Zhaolu in the first place as an output stage isn't it? I've read Ori himself deriding this opamp. Whilst dc servos use lo-pass filters to let through 'only' dc, there is the noise of an opamp to consider. If you don't like the sound of an opamp you really shouldn't inject it back into the inputs of your output stage!

 

[Ori]

Quote:

Originally Posted by mwofsi /img/forum/go_quote.gif There appears, for instance to be an opamp servo based on an LT1057 opamp. This is the same opamp supplied with the Zhaolu in the first place as an output stage isn't it? I've read Ori himself deriding this opamp. Whilst dc servos use lo-pass filters to let through 'only' dc, there is the noise of an opamp to consider. If you don't like the sound of an opamp you really shouldn't inject it back into the inputs of your output stage!

You are of course correct the the use of opamp for the DC servo of the OMZ but that is using an opamp where it belongs. There is nothing inherently wrong with opamps if you use them for the right application.
Here are the issues:

Noise: opamp tends to have a wideband noise, just like most wideband amplifiers with current sources and lots of open loop gain. The opamp in the OMZ is in a low-pass circuit with a few Hz cutoff frequency. This limits the noise to the bandpass only - very little of it. Open up the bandwidth (like the full audio spectrum or some amps claim to go to 1 MHz even...) and you have a whole a lot more noise energy to contend with.

Distortion: One of the main issues with opamps is the concept of very high open-loop gain, which is "tamed" by means of a negative feedback. That is not an option but a basic conceptual mode of operation of an opamp. If you read about linear amplifiers and distortion you will find some literature about delay-induced distortion in a feedback circuit.
You might think that a fast opamp solves that problem, but the circuit itself is usually a bandpass, which limits the speed of signal propagation (the same issues that cause non-uniform phase in the output of an amplifier).
These distortion products may not be in the audio spectrum, but they can produce audible distortion through mixing (inter-modulation).

Effect on the audio: It's not an ideal world and there is inevitably some effect, but a servo circuit cuts off at a very low frequency. That is the range where an opamp is almost ideal!
The delay is not a factor and the noise energy is very limited. This is the only place I would use an opamp in audio and expect no major effect on sound quality.
In fact, the OMZ circuit takes advantage of the opamp in other ways, but I do not want to ellaborate on this topic.



The bottom line is simple. Electronic components have certain attributes and certain limitations. I would not use a component where it does not belong but I'm very happy to use it where it does the job right.
In the servo circuit you need a lot of gain and noise is not a major factor due to bandwidth. Opamps are ideal for that. No excuses needed and in fact I'd claim that building a discrete circuit for that application is plain stupidity and lack of understanding on the part of the designer.
I put my effort where it belongs. The audio path is a well-tuned zero-feedback circuit which is as close to ideal as they come. The "auxiliary" circuits are using the right technology as well. That's called an optimal solution.

 

[Ori]

Quote:

Originally Posted by mwofsi /img/forum/go_quote.gif Also on the list of to be improved is the ALPS pot for the headamp. Of course ALPS make some really good audio pots, but they make some very ordinary ones too.

Mike, I don't recall if you have the 22-step or the carbon-composition ALPS.
Despite is dimunitive size, the 22-step ALPS RH2702 uses internal 1% resistors. Look up the specs.
The carbon-composition is a SUPERB unit. It's not ordinary but SUPERB. You will not find many new units of similar quality, which is why I bought a NOS (!) batch of these units.
My advice to you is to not replace the pot in your OMZ. You'll find very likely that it is a mistake to do so!

Please consider what you are writing about my (or any manufacturer's) products on public forums before you hit the enter key. At least give me the courtesy of explaining it privately, given that we were engaged in a long email discussion where I "donated" many hours for free to help you with a problem that turned out to be your own setup and took you step by step through a fairly involved procedure.
Seems to me that it would have been logical and ethically correct to mention these "issues" in one of your many emails first.