ZV9X : A JFET input ZV7 - diyAudio
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Old 13th November 2011, 09:01 AM   #1
rsdio is offline rsdio  United States
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Default ZV9X : A JFET input ZV7

I have been working on a schematic for an amplifier that I would like to build, and I've decided to solicit comments here. Since I plan on making a minimum run of boards for my first build, I thought it might be prudent to ask for suggestions and perhaps even find some folks who'll want the inevitable extra boards. Please bear with me while I try to cover everything in this introductory posting, but feel free to ask questions or make suggestions.

BACKGROUND:
I have a nice DAC with balanced outputs capable of +26 dBu at about 50 Ω. This makes me believe that I do not need to design a preamp. I prefer the Super Symmetric amplifier design with balanced inputs and floating speaker outputs. It also seems like a good idea to start with the latest Zen Variation (#9), although I do not really know whether the JFET input is absolutely needed.

GENERAL GOALS:
I hope to design the PCB with multisite options for various contingencies that I might run into. Personally, I don't want to mess around with breadboarding or point-to-point wiring, and since multiple fab houses are available nearby in my city I figured I might as well start with a custom PCB. The idea is to put every extra resistor, pot, capacitor size variation, regulation, and anything else that I might anticipate on the board, so long as these extra parts can be left out and still have a functional board.

COMPATIBILITY:
I do not see many people talking about the new standard PCB sizes and mounting holes for PCB and transistors, but it seems like a good idea to follow these. I downloaded the PDF from the store and designed by PCB to fit. I do not have heat sinks or a case, yet, but hopefully it will be much easier to find a standard one.

UNIQUE CHOICES:
I put a Neutrik PCB-mount XLR balanced input, and a Neutrik PCB-mount speakON output directly on the board. This means I'll need to make a mirror image of my board if I want to build Left and Right into the same enclosure with inputs on the front and outputs on the back, with heat sinks on the left and right. I realize that this is probably a rather odd choice, but point-to-point wiring from panel mount jacks can just as easily be soldered to the PCB-mount pads if you don't like this arrangement. The Neutrik powerCON was not available in a PCB-mount package, so the board accepts soldered power inputs, regulated or unregulated.

OTHER NOTES:
I have no idea where to find the LU1014D parts, but I've designed for the surface-mount heat sink. I have a hunch the layout may need to be modified to allow more heat sinking for the JFET input transistors.

R0A-R0D are optional.

Those of you who don't like feedback can leave out the appropriate parts, but SuSy won't work without at least certain feedback.

I tried to think of anywhere that more variable resistors might prove useful, even if they're only mounted on the board long enough to experiment before replacing with precision resistors. Any suggestions here would be particularly appreciated.

I still feel like I have more reviewing and tweaking to do, but the schematic has settled for a while now so it seems ready for comment. Please let me know if I've made any obvious beginner mistakes! (but please also comment if you have anything to offer)
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Old 13th November 2011, 09:33 AM   #2
Zen Mod is offline Zen Mod  Serbia
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you'll need input caps

besides that - you'll need some source resistors for LU critters - degeneration-load line shaping purpose

look for Magura's big Donut amp(s) , or for my Donut amps thread

Magura's are in daily use , those from Donut thread (still) aren't fully tested
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Old 13th November 2011, 11:13 AM   #3
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Quote:
Originally Posted by rsdio View Post
OTHER NOTES:
I have no idea where to find the LU1014D parts, but I've designed for the surface-mount heat sink. I have a hunch the layout may need to be modified to allow more heat sinking for the JFET input transistors.
Just for an additional note , I have built a sort of ZV8 - like circuit , with the IXTH depletion Mosfet that sounds promising . Of course the curve of the LU1014D is quite different ( and the biasing ) compared to Dmosfet .
Still have some LU1014D around , but I am afraid will not be able to mach 4 of them for this design .
Thanks

Stefano .
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Old 13th November 2011, 11:43 AM   #4
rsdio is offline rsdio  United States
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Thanks for the comments.

Quote:
Originally Posted by Zen Mod View Post
you'll need input caps
Why? ZV8 & ZV9 don't have input caps.
Besides, what's wrong with DC coupling? My present DAC has output caps, but I plan on upgrading to a DC-coupled DAC when funding allows.
I must admit that I do not know how ZV8/9 expect to keep the Vgs around the ideal -1 V (besides, the JFET input is not necessarily required). Now that you have me thinking about it, I wonder how much padding I will need on the +26 dBu DAC outputs, and therefore how much extra gain I'll need in this little amp.

Quote:
besides that - you'll need some source resistors for LU critters - degeneration-load line shaping purpose
Q7 is an active current source from ZV7, and I believe it replaces the source resistors in similar designs. Again, however, I'll admit that I'm just cutting and pasting here. Are the Lovoltech JFETs different than the typical FETs in ZV7 with regard to degeneration/load-line shaping?
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Old 13th November 2011, 11:51 AM   #5
rsdio is offline rsdio  United States
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Quote:
Originally Posted by stefanobilliani View Post
I have built a sort of ZV8 - like circuit , with the IXTH depletion Mosfet that sounds promising . Of course the curve of the LU1014D is quite different ( and the biasing ) compared to Dmosfet .
Looking at ZV8, it mentions that the ideal input bias for Vgs is -1 V. This makes me hypothesize that ZV8 might be best for low-level signals around +4 dBu. What will happen when I send +/-8 V (+24 dBu)?

In other words, maybe I should alter the input transistor to handle my specific DAC output. I can pad the DAC output, but I imagine that I might turn it up to full +24 dBu with possibly even +26 dBu peaks.
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Old 13th November 2011, 12:35 PM   #6
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by rsdio View Post
Thanks for the comments.

Why? ZV8 & ZV9 don't have input caps.....
yes , but they don't have CCS below , and that critter ( CCS) needs some voltage to not be dead as Dodo

I told you - search for Donut amps ; everything ( and then some ) is drawn there


edit: here it is , just for your amusement :

Choky Susy F3 for LU1014's
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File Type: jpg dodo.jpg (33.6 KB, 385 views)
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Last edited by Zen Mod; 13th November 2011 at 12:38 PM.
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Old 13th November 2011, 01:26 PM   #7
rsdio is offline rsdio  United States
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Quote:
Originally Posted by Zen Mod View Post
I told you - search for Donut amps ; everything ( and then some ) is drawn there

edit: here it is , just for your amusement :

Choky Susy F3 for LU1014's
I remember reading that thread. Choky ain't SuSy. It needs the shorted tails like in all of the SuSy articles. Like Babowana says. It doesn't make sense to me to think of the source resistors as integral to the LUs, otherwise Papa never would have removed them to make SuSy. Wouldn't it be possible to think of the source resistors as part of the original Zen FETs? I don't think so, in either case.

Besides, Nelson Pass never commented on Choky to say that it implements his patented Super Symmetric design. We should all re-read the patent!

Admittedly, Choky may sound great, but that doesn't mean it's SuSy. Also, my Frankenstein above at the start of this thread may not work for many reasons, but that's not really related to whether Choky is SuSy.
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Old 13th November 2011, 01:30 PM   #8
rsdio is offline rsdio  United States
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Originally Posted by Zen Mod View Post
yes , but they don't have CCS below , and that critter ( CCS) needs some voltage to not be dead as Dodo
The active current source is biased by R19 & R20, and adjusted by R36. I would hope that it has some voltage! The articles mention the minimum bias.

However, are you saying that the Vgs of the Lovoltechs needs to be different because of the source biasing? I must admit that I don't understand the JFET gate biasing at all in the ZV8/9 articles...
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Old 13th November 2011, 01:52 PM   #9
Zen Mod is offline Zen Mod  Serbia
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feel free to re-read the Patent

those Donut amps are SUSY .

not splitting the hair or arguing , that's just fact .

regarding Jfets - you need to learn their biasing methodes , voltages , then you'll see what you need to change

every part need proper operating voltages
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Last edited by Zen Mod; 13th November 2011 at 01:54 PM.
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Old 13th November 2011, 02:00 PM   #10
rsdio is offline rsdio  United States
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Quote:
Originally Posted by Zen Mod View Post
feel free to re-read the Patent

those Donut amps are SUSY .

not splitting the hair or arguing , that's just fact .
Says who? The Court Jester? Papa?

Has anyone introduced a spike into one half of the Donut and watched it appear inverted on the other side? If so, it is happening via the transformer or via the tail?
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