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Simplistic NJFET RIAA
Simplistic NJFET RIAA
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Old 28th June 2020, 11:17 PM   #17971
Salas is online now Salas  Greece
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Simplistic NJFET RIAA
Quote:
Originally Posted by jphabc54 View Post
Thanks Salas -- Q1 is 11.18ma and Q2 is 11.21ma. I switched to 56dB gain and there I was able to dial in 4.00v no problem with 34v rails. I was not able to source R13 yet, so can't observe 63dB behavior.

Initial use will be with MM and HOMC carts, so it would be with the lower rails in this case, but I'm building the UFSP mainly with the aim to get a LOMC in the system in the near future.
In FSP builds it was many times the case they ended up with 32-33V rails especially when building it for HMC or MM. But there was this in the PDF instructions gain section: "Choose Q1 between channels from a 10.5-12.5mA range at 5% IDSS pair tolerance" (HMC MM). In UFSP there is more voltage space I left in case of depleted lower IDSS K369 choices in the future. To can still be trimmed for T.P in low gain modes even with say 15mA IDSS K369.

So now that I know your K369 IDSS I also know your rail is as it should for high output MC mode. It computes perfectly.
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Old 28th June 2020, 11:33 PM   #17972
jphabc54 is online now jphabc54  United States
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Thank you Salas as always! I now have two working channels up and running. Next step will be to finish up some back panel drawings and hopefully get an order off to Modushop this week.
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Old 28th June 2020, 11:40 PM   #17973
Salas is online now Salas  Greece
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Nice. Keep us posted. I know how good the Valve Itch you already got is to tube lovers, I actually have a couple of friends that stubbornly wouldn't replace it for my JFET designs, or anything else for that matter, but my assessment to them was at least have both.

I find the Valve Itch maybe more fun at medium volumes with character carts playing the blues etc. but when the going gets tough with program complexity and loudness on more neutral and dynamic audio system setups the FSP/UFSP are the more open natural players. IMHO of course.
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Old 29th June 2020, 11:57 AM   #17974
jphabc54 is online now jphabc54  United States
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I can’t imagine anything bettering the Valve Itch, but I simply couldn’t resist the opportunity to build a UFSP. I can’t wait to compare them side by side, and it will be nice that the UFSP will be ready to support low output out of the box (which will of course put me in the market for a SUT for the itch)...it..never..ends...
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Old 30th June 2020, 04:13 AM   #17975
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I finished the other channel on Saturday and spent most of Sunday listening --including a little Led Zeppelin retrospective following Salas' post...

The discussion on the choice of C2Y came just at the right time! Teabag measured C2 at 14.93nF for me, and tonight I added 220pF from the helpful little bag of C2Ys he's included in the kit -- wow, that gave just the right last little edge towards clarity and transparency -- it's pretty amazing, am very happy.

I'm surprised how little heat the MOSFETs are producing. I expected something more like the DCB1 Mez, but no, there's almost nothing. I'm using a re-purposed chassis, and basically put the MOSFETs where I could find a tapped M3 hole -- I didn't even screw down M1 which, surprising to me, even then isn't getting hot. This way, I didn't have to drill or tap any holes at all!! yes, I'm lazy -- until I'll clean everything up later.

I'll listen for a few more days and try a few things. I don't have a grounding scheme yet and also will partition off the power supply -- but it is already pretty quiet.

One thing I'm trying to understand is how the bias is wandering around, starting at 5.3V when switching on and then random walk around 4V +- 0.3V, basically. I'm trying to understand more of the design, started to read from post 1, but if you guys can point me to maybe a discussion or walkthrough or circuit description, I'll be very interested to read up!
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Old 30th June 2020, 04:19 AM   #17976
Salas is online now Salas  Greece
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Hi, nicely done up to now.

There's a circuit description link in post#1 named how it works. Q1 Q2 bias goes through warm up time of thermal stabilization when starting.

M1 is lukewarm when RawDC Vin isn't too high. VDS M1 is Vin-Vout. Dissipation Watt M1 is VDS*CCS (100mA for R1 5.6R standard). If you hot-rod CCS mA a bit (145 mA with R1 3.9R) the diss W goes quickly up. For M1 and mainly M2. M2 is shunt to the load burning excess mA at 34V level. Each channel with both Q1 Q2 engaged is about a 36mA load.

You may confirm your shunt regs correct mA settings by measuring the voltage drops across their R1 and then dividing it by R1's value in Ohm.
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Old 30th June 2020, 04:30 AM   #17977
LATB is offline LATB  United States
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Thank you, exactly what I was looking for!

Missed the pointer (to page 1698!) on first reading, and had only made it to page 916 yet

Thank you Salas for sharing so much of your knowledge and insights and time.
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Old 30th June 2020, 04:34 AM   #17978
Salas is online now Salas  Greece
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You are welcome.
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Old 30th June 2020, 07:20 PM   #17979
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Salas and Tea: Hats off to making incredible changes to the "build-ability" of this fantastic phono pre. Been having a little back and forth with Tea and it's clear to me now that the Ultra version is WAY more noob friendly than the previous build (Safe to say the FSP was pretty advanced actually). This I love since it could open up a lot more makers for this project—granted we are all somewhat rarified in this teeny world of DIY PLUS turntables and then some prerequisite OCD? Ha. As a much newer builder I most certainly needed help on the FSP build—and it required "locking" in some specifics with regards to carts etc as you know. Obviously I'll report back when Ultra is up and running...
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Old 30th June 2020, 09:11 PM   #17980
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Thanks. I was also happy to see the first builds reported working as expected without much assistance asked.
 

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