Curcio CD12V DAC, info and help sought

Hey all,

Looking for documentation for the Curcio CD12V dac. Hopefully somebody has held on to it and can share. I've reached out to Joe Curcio without success.

Background: I have a working CD12V that I picked up at an Estate Sale. Didn't know what it was until I got it home. The DAC has been working and in use intermittently without significant issues since.

You can read about the score here:
DIY tube pre-amp(s) identification help needed | Audiokarma Home Audio Stereo Discussion Forums

The reason I want to locate the original documentation is because I would like sort out a couple things on the unit.

  1. There are 3 LED's on the front panel, one is obviously power, another illuminates upon "Lock" with the CD player, but the 3rd LED never illuminates. I cannot for the life of me figure out what it is for. Maybe for 48khz instead of 44.1khz? Or is it a non-functioning MUTE circuit?
  2. If it's possible to reduce the gain or output level?
  3. Maybe adding a Mute switch? It seems like I've seen that option on the few examples seen on the web. When the unit is powered up, but when there is no lock on a digital signal there is some annoying noise that will come through the system.
  4. I also want to explore adding multiple inputs. I know this won't be reflected in the documentation. I will probably look for an off the shelf DIY kit (i.e., Twisted Pear Audio) and will need to identify a place to tap power for whichever kit I locate.


Maybe y'all can help me solve the gain issue without the documentation. Curcio lists the output level as 4.4v RMS on his website. He also posts the attached schematic. I've circled R121 and R122, which look to me like they may be a voltage divider (?) and a good place to adjust the output level?

R121 and R122 appear to be 470Ω and 3.3M based on visual inspection of band codes. They are just before "LO" which I believe stands for "Line Out"?

I'm guessing the simplest solution would be to add a pot on the analog output of the DAC. But, if I can simply change out these resistors I would think that would be better than adding another component. I would really like to get the DAC output down to something like 1.5 - 2.2v output to gain match with the rest of the components in my system.

Any other suggestions? This unit is set up for 12AT7's, not 6DJ8's. Is there a 12AT7 substitute with less gain? Is there a way to adjust gain in the circuit closer to the tubes? Like in a feedback loop?

I have the most basic understanding of tube circuits and can read a schematic OK. But, figuring this out is a little over my head and need some hand-holding.

Thanks in advance!
 

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You would probably be better off just putting a pot on the output, however it looks like the circuit might sound better with maybe something closer to a 10k pot rather than a 1k Pot. You could try either or both though. However, to the extent using a larger value pot increases the output impedance of the unit, that could potentially cause some high frequency roll off of your audio depending on interconnection cable capacitance, etc. Maybe better to put the pot at the input of your preamp, say.

The 3.3meg resistor, R122, looks to me like its just to provide a discharge path for C110, its not a voltage divider to set the output voltage. R121 is probably there to assure stability (freedom from oscillation) of the output stage, although it would probably be safe if R121 value were increased (again, too much resistance and some HF roll off might occur with longer cabling, etc).
 
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You would probably be better off just putting a pot on the output, however it looks like the circuit might sound better with maybe something closer to a 10k pot rather than a 1k Pot. You could try either or both though. However, to the extent using a larger value pot increases the output impedance of the unit, that could potentially cause some high frequency roll off of your audio depending on interconnection cable capacitance, etc. Maybe better to put the pot at the input of your preamp, say.

The 3.3meg resistor, R122, looks to me like its just to provide a discharge path for C110, its not a voltage divider to set the output voltage. R121 is probably there to assure stability (freedom from oscillation) of the output stage, although it would probably be safe if R121 value were increased (again, too much resistance and some HF roll off might occur with longer cabling, etc).


Thanks for the reply. Pardon my ignorance, but can you elaborate on why a 10K pot might be better here than a 1k pot?

BTW, Curcio lists the CD12V output impedance as 300Ω
I am currently using a DIY build preamp based on the Norman Koren modded Dynaco PAS3. From what I understand the stock PAS3 has an input impedance of 250k. I don't know what the Koren circuit end up being, but Koren specifically states in his article that he designs for high input impedance so I would wager that it's still at least 250k if not higher.
Spice and the art of preamplifier design, Part 2

I also have the working Curcio Daniel 2 preamp that I got with the DAC and I may switch it in place of the Spiced PAS3. The input impedance of the Daniel II is listed as 1meg.

The interconnects between the DAC and the PRE will be no longer than 1m and I do have a bunch to choose from. I can easily measure the capacitance and select the lowest cables.
 
I dug around in my spares bin and found a 10k ALPS pot. This one was a motorized 3 deck version that I think was from a dead CD player and was the internal control for a variable output. The fore and aft decks tracked very close, but the middle deck tracked differently. So I removed the motor and installed it very close the analog output RCA's. Rather than solder to the pins of the pot I used some gold-plated female D-Sub pin connectors that I use to make my own headshell leads. This will make it easier if I want to swap out the pot.

Once I got it set up I installed it into my system and using headphones I gain matched it to the CD player feeding the DAC by switching my preamp back and forth between the two sources. I got it very, very close by ear.

While I was doing this I could not discern any loss of fidelity between the CD player and the DAC. In fact it was so close that I am actually disappointed there wasn't more benefit to having a tube stage.

Note the CD player is a highly modified Philips CD-60 with a TDA1541 dac. So essentially I think it came down to the difference of a tube (12AT7) vs opamp (lme49720ha) output stage. The mods on the Philips unit very closely follow everything in this thread...
Marantz CD-50 and CD-60, TDA1541, CDM4/19

I would say the Curcio DAC has a slightly wider soundstage and cymbals sound ever so slightly more realistic. But otherwise the difference was extremely subtle, which is kind of opposite of what I would expect I would experience due to expectation bias. Funny.

Then I took it back out and measured between input and wiper of the pot and found it was 2.6k and at about 3/4 to full output turn of the pot (225º).Not sure what that translates to in terms of -db from full output, maybe someone can tell me. But, I now have much more usable travel on my preamp volume control when using the DAC.

Here's some pics...
 

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Now I need to figure out what that unlit LED indicator is for.

Here's some pics of the leads. As you can see from the pic from previous post, the top LED lights when it locks on to the CD player. If I turn off the CD player the LED goes out. Of course the LED above the power switch if the power indicator.

After getting the Curcio Daniel 2 preamp working I learned that Curcio liked to put LED indicators in series with relay coils to light them whenever the relay is activated.

In these pics you can see the two LEDS on the front panel are wired with one hot lead (red wire) tying their Anodes together and the Cathodes are attached to the black and white wires. The working upper LED is the black wire which goes to the terminal marked "KM" on the filter(?) board. The other LED that never lights goes through the white wire to the terminal marked "KD". Both terminals appear to connect directly to the coil pins of the nearest relay to them.

So, does anyone have any insight to what "KD" might mean in Curcio nomenclature?

I think "K" is used by Curcio to denote relays. So I thought KM might be 'Mute Relay', but it definitely is not muting the output. Maybe it's muting the digital input?

Maybe KD is 'Digital Relay'? and might be if it received a certain type of digital signal. Maybe the de-emphasis idea is correct. Not sure how I could feed it a an emphasized signal to test it. Or, maybe it is for a switching between 44.1 and 48khz? Also not sure how I can test this with what I have at my disposal.

I really wish Curcio would respond to my email and send me the documentation packet.

Edit: forgot to mention that I noticed a connection between the boards of AG (audio ground?) to DG (digital ground?) as noted in the pic.
 

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I would say the Curcio DAC has a slightly wider soundstage and cymbals sound ever so slightly more realistic. But otherwise the difference was extremely subtle, which is kind of opposite of what I would expect I would experience due to expectation bias. Funny.

What could be even funnier is that expectation bias as the term is used above is actually an audio forum myth. There is really is something called expectation bias or experimenter bias, but it is something that affects professional researchers. The correct definition and supporting references can be found in the list of cognitive biases at: List of cognitive biases - Wikipedia

Regarding the experience you described of hearing some effect you did not expect, such is not uncommon.
 
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expectation bias as the term is used above is actually an audio forum myth.
Expectation bias in hearing is an actually documented (aviation safety) phenomenon. Such is the reason why the shills on audio forums try to build up expectations on potential customers by posting all sorts of praising words about how modification x, y, z turned out or high dollar component performed.
 
Radio operators sometimes believe they hear voices buried in noise. Aircraft VHF radios are notoriously noisy and distorted. Under such conditions mistakes of understanding can occur. In addition, aircraft pilots may be fatigued and or under heavy workloads when preparing for landings. In such situations errors of understanding are more likely to occur.

In short, pilots, and shipboard radio operators listening for distress calls buried in noise, make mistakes unique to their difficult listening situations.

None of it has anything to do with normal hi-fi listening. Nor is the term 'expectation bias' an accepted cognitive psychology term when defined to simply mean, 'you hear what you expect to hear.' Again, it is something that affects professional researchers in the cognitive psychology lexicon.
 
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What could be even funnier is that expectation bias as the term is used above is actually an audio forum myth. There is really is something called expectation bias or experimenter bias, but it is something that affects professional researchers. The correct definition and supporting references can be found in the list of cognitive biases at: List of cognitive biases - Wikipedia

Regarding the experience you described of hearing some effect you did not expect, such is not uncommon.

I think you and I are saying the same thing. I am fascinated by cognitive biases and do try to recognize when I am falling prey to them.

In this instance I had the expectation that a tube output stage would sound significantly better than the SS output stage of the CD player and so was on guard to be as objective as possible. Even so, I was surprised at how subtle the difference was, especially since I was using headphones.

What makes it all the more interesting is that I had been previously using the Curcio DAC with it's significantly higher output and so switching back and forth the CD12V always sounded better due to the jump in volume (can't remember the name of this phenomon). That of course would also contribute to my expectation that when level matched the DAC would sound significantly better.

So, did I successfully overcome my own expectation bias in this case? Is being aware of the potential for biases enough to sidestep them? Probably not. But, that's what I find so fascinating about them.
 
Is being aware of the potential for biases enough to sidestep them?

There is some research into that question. It turns out to depend on the particular bias. Some are essentially impossible to overcome. Some are relatively easy to avoid if one remembers to try.

With respect to hi-fi listening judgements, IME there is a learning curve involved. Vague differences such as 'one just seems to sound a little better or a little more dynamic that the other' are types that should set off warning bells. You should be looking for very specific differences easy to tell apart blind. Could be the particular distortion on a particular cymbal hit sounds different in some very distinct way. Could be some reverb tail after the end of a lyric has a certain level of detail in the decay (say, it can be distinctly heard that a reverb and a delay have been mixed to form a composite tail, and in one device the delay time is very distinct, whereas in the other device it is blurred in with the reverb so as to be much more indistinct). Whatever it is, you have to be able to memorize exactly how it is different to pass a blind test on it. Eventually, you learn what kind of things to listen for give reliable test results for you.

That's my two cents on it, anyway. I'm done talking about it for now.
 
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Radio operators sometimes believe they hear voices buried in noise. Aircraft VHF radios are notoriously noisy and distorted. Under such conditions mistakes of understanding can occur. In addition, aircraft pilots may be fatigued and or under heavy workloads when preparing for landings. In such situations errors of understanding are more likely to occur.

In short, pilots, and shipboard radio operators listening for distress calls buried in noise, make mistakes unique to their difficult listening situations.

None of it has anything to do with normal hi-fi listening.
That's odd because I'm not aware of any peer reviewed published research that confirms or even suggests that normal hi-fi listening is exempted from basic human flaws.
Nor is the term 'expectation bias' an accepted cognitive psychology term when defined to simply mean, 'you hear what you expect to hear.' Again, it is something that affects professional researchers in the cognitive psychology lexicon.
Anyone doing the typical casual subjective listening comparison of audio electronics is subjected to expectation bias especially when they have been exposed to marketing ploy which are ubiquitous online.
 
So, did I successfully overcome my own expectation bias in this case? Is being aware of the potential for biases enough to sidestep them? Probably not.
Your speculation is right. Our subconscious tendencies cannot be overcome by just thinking that it can be overcome. In order to make it happen, bias has to be controlled externally, level matched double blind listening test with quick switch (to compensate for our short aural memory span and listening position change) capability between the devices under test. Anything short of that isn't a bias controlled test.

I'm not trying to stop you from doing the listening test that you have been doing. It's important to distinguish the difference between subjective listening impression and objective listening test result.
 
Ok, new information.....

That LED that I have been trying to figure actually did something today.

I was listening to some music and the CD started skipping. When the skipping started the LED started flashing. Not with every skip, but on the big ones. It wasn't flashing with any pattern. It was coming on at the initial "tick" of every skip that seemed to set the laser back instead of moving forward if that makes sense. Also, as the song ended the mistracking continued into the fade out and as it got quieter I could hear the relay clicking in and out.

I have never seen this before. Does anyone have a clue what this means? Is it some sort of error indicator?

Fortunately the Philips is a rock steady performer and it is very rare to get any skipping, so I couldn't replicate it. In this instance I pulled the disc and there was a fingerprint on the disc that was easily cleaned.
 
It was a long time ago that I built one, from vague memory, 1st led for signal lock, 2nd absolute phase and 3rd deemphasis. If I am able to find documentation I'll let you know.

Man, if you had the documentation that would be great!

I dug through my CD collection to find my oldest CD's. Found a couple that were pre-UPC code issues from the early 80's and played them through the dac hoping at least one was a "pre-emphasis" disc. No luck there, or at least if it was a pre-E disc it didn't trip the LED.

I don't know enough about "absolute phase" to know if there would be any CD that would cause the LED to trigger if that's what it indicates. Unless, maybe absolute phase came into play if someone was using a different source such as DAT, computer, etc.

Edit: oh....it just dawned on me that the terminal the LED in question ties to is labelled "KD". So, if Curcio uses K to denote relay, then D more than likely stands for De-emphasis. Now, how do I test this?
 
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