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Curcio CD12V DAC, info and help sought
Curcio CD12V DAC, info and help sought
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Old 3rd November 2020, 06:27 PM   #1
dogwan is offline dogwan  United States
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Default 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!
Attached Images
File Type: png CD12V schem..png (189.8 KB, 114 views)

Last edited by dogwan; 3rd November 2020 at 06:34 PM. Reason: grammar
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Old 8th November 2020, 06:03 PM   #2
dogwan is offline dogwan  United States
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Bump...
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Old 9th November 2020, 01:02 PM   #3
Markw4 is offline Markw4  United States
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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).

Last edited by Markw4; 9th November 2020 at 01:04 PM.
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Old 9th November 2020, 01:20 PM   #4
tubesguy is offline tubesguy  United States
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Might the 3rd LED be for indicating deemphasis? That was a fairly standard feature back in the day.
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Old 9th November 2020, 06:02 PM   #5
dogwan is offline dogwan  United States
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Quote:
Originally Posted by Markw4 View Post
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.
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Old 9th November 2020, 06:49 PM   #6
Markw4 is offline Markw4  United States
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Distortion in the output stage may tend to be lower with a higher impedance load.
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Old 10th November 2020, 10:41 PM   #7
dogwan is offline dogwan  United States
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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 (225).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...
Attached Images
File Type: jpg 20201110_122215.jpg (605.1 KB, 74 views)
File Type: jpg 20201110_133810.jpg (408.2 KB, 71 views)
File Type: jpg 20201110_123926.jpg (605.3 KB, 71 views)
File Type: jpg 20201110_122537.jpg (333.6 KB, 71 views)
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Old 10th November 2020, 11:13 PM   #8
dogwan is offline dogwan  United States
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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.
Attached Images
File Type: jpg 20191006_115548.jpg (840.5 KB, 31 views)
File Type: jpg 20191006_115555.jpg (833.0 KB, 32 views)
File Type: jpg 20201109_233517.jpg (250.7 KB, 28 views)
File Type: jpg 20201109_233526.jpg (289.9 KB, 19 views)

Last edited by dogwan; 10th November 2020 at 11:16 PM. Reason: forgotten point
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Old 11th November 2020, 12:01 PM   #9
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by dogwan View Post
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.

Last edited by Markw4; 11th November 2020 at 12:04 PM.
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Old 11th November 2020, 03:08 PM   #10
Evenharmonics is offline Evenharmonics  United States
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Quote:
Originally Posted by Markw4 View Post
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.
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