Salas DCG3 preamp (line & headphone)

Hi Salas,
Congratulations for great design, proved in such a huge number of builds.
I was really enjoy reading this thread. Thank you for your kind and selfless sharing of knowledge.

I am trying to make something proven, following your DCG3 but, more compact as possible and fit it in some sort of smaller possible enclosure together with AK DAC I have to build.
Also I do not like THT actives as they becoming obsolete very fast. I hate obsolete components and brainstorming analyses are they fake or not, are they coming form China, TW or Korea. Also do not like to use expensive and hard to source stuff. So decide to go for standard easy to find components in SMD wherever is possible to find suitable replacement.

In that way i have one question. Can you explain me please why do you suggest use of Jfet input OPA in DC servo. There are some great OPA from TI that, i think can fulfil requirements at that position, but with Mosfet input and better regarding noise, drift, ofset, load drive and everything else comparing it to AD823?
 
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You are welcome. We have also tried some new generation best spec high impedance input op-amps in the DCG3. By the way, BJT input chips are not recommended due to their higher Ib and the 1MΩ source impedance RC filter in DCG3's servo. We need pA level input bias current for that. Yes you can use the OPA1656 as DC servo, the circuit sounds clean with it, but with AD823 keeps closer to its no servo state tonality for some reason. IMHO YMMV.

*Also available in SOIC-8 SMD (AD823ARZ-RL)
 
I will add my congratulations to Mr Salas for this really wonderful design. I have absolute respect for the part choices and why they were made. But the choices were made, what: six years ago? The sourcing of some of the parts is quite difficult today. As of this writing the DN2540 has about a one year lead time from its vendor, for example. I am wondering if a refresh of the design would be possible, with SMD devices for the JFet, current mirror, and op amp? And if the output stage current source could be made with a device that is not "unobtainium" ?
 
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Its a major rework task for me to emulate the finer subjective qualities of the design with new parts. I wouldn't just make an LT-Spice simulation and a new board layout and call it a day. When Tea-Bag's stash of DCG3's semis for mini-kits will be over and if there's still demand I might go to the trouble of a reissue.
 
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Pardon me…
Just to be sure, this is the correct way to connect the toroidy 2 x 18V 50VA?
(red — red+black — black)
Before I blow everything up I thought I‘d better ask 🫣
 

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(y)Your build's degree of +/- rails agreement is remarkable nonetheless. Especially within each channel. Indicates careful JFETs selection and good LED bars Vf consistency. The safety criterion for rails in a DCG3 is they should definitely measure below the U1 servo op-amp's +/- 18V recommended max spec.

Since introduced nobody ever reported a DCSTB overshooting that criterion. That's something for a fixed output reg without a precision voltage ref chip using an army of transistors inside it.
 
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I am slowly making progress. Just forgot to make a few pics, they'll follow.
Just mounted the board (and the mosfets) to the heatsink (yes heatsink ;) ), layout seems to become very nice, wait and see!
Just a liiittle question: On the board, on the edge next to a mosfet, is a very small position for ... something? there's a trace to a resistor, but the holes aren't specified, and they seem too small for even a LED?
 
Maybe I should have used the Switchcraft! In the video, she just easily pressed and it let go. I was bent over like a lineman pressing with all my weight before I could begrudgingly pull it out. It was NOT mounted, and it is impossible to get a hold of it not mounted. Basically I just leave it in, I only use it with this only set. I do have a roving eye for them now.

Russellc
 
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uhm... probably because I prefer it the complicated way, I'd like to hook up the I-select by AC in. DCSTB ist a center tap-configuration, can I still do it? Or must I go and use the DC in? (There, would it be "not reasonable" to steal the DC from one of the DCG3's DC in (instead of using the legs from a DCSTB's reservoir cap?)
 
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Better not blend two bridges with one AC source. Because I-Select has a bridge too. For standalone use with own Tx. Using raw DC is preferable. I-Select has a 12V chip regulator for that. You can also steal DC from the stabilized & filtered DCSTB output but why opening a possibility of noise ingress there from the cabling to the I-Select.
 
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ok I see. (I have to admit that did the "2-bridges-bungle" before, in my WLS18... Never heard anything not good so I did not realize it's not proper.)
Stealing DC from the DCSTB would be very easy as the connections are very close. I think I'll stick to my credo of keeping it complicated and get the raw DC :)
As a last question to this: I of course soldered the diodes in, can they stay or do they interfere?
Thank you so much!