Simplistic NJFET RIAA

[Joel Wesseling]

I have The phono amp fired up after a few small issues I had to fix and now looks good.

Except:

My DC rails are Low at 31.5V with V2RX at FULL ROTATION. With VR1 at FULL ROTATION I get 3.62V at the Test points.

R3X is 9.1kohms and R6 is 27ohms - Should I increase R3X?

AC secondary is 43V
Raw DC is 57.2V
Raw DC with boards connected is 49V

Thanks Salas!

 

[Salas]

Yep, increase R3x to 10k

 

[AndrewT]

If you build this project according to the build guide, the PSU chassis is connected to safety earth, but the 0V is isolated from that earth by a network consisting of a resistor in parallel with a pair of diodes. The amplifier chassis is connected to 0V. If you connect the screen to chassis at both ends you effectively bypass that isolating network. I don't think that is a good idea. I suppose you could connect the amplifier end of the shield to the chassis through a small capacitor to have a low impedance path for high frequencies.

The screen/shield does not go to audio ground.
Screen /shield goes to chassis at both ends.

The two signal wires inside the screen/shield go to the signal terminations. They are the audio connections.

 

[nezbleu]

Sorry, I wasn't clear: again per the build guide, the amplifier chassis is "grounded" to the 0V reference (including the turntable ground post). So if you connect the shield to chassis at both ends, one end is connected to PE, the other to 0V. This is what I am trying to avoid.

 

[Salas]

Correct, if you will connect the DC umbilical cable's shield at both ends its going to bypass the anti-parallel diodes PE lift for the signal GND. Use a 100nF cap between shield and the phono chassis end if you worry about much present RFI. There is a 100nF already between (+) & (0) raw on the FSP's PCB DC input also. Since the standard grounding scheme has worked for so many builds successfully until now, better only start to think about revisions or additions in case of persistent unusual hardness in sound and/or intruding weird noises.

 

[Joel Wesseling]

Yep, increase R3x to 10k

I installed 10k in R3x. My DC rail max is now 33.5V but to obtain 3.6V at the test points I had to dial it back to 31.5V and 31.8V. Leaving DC at 33.5 I dont have the range with VR2x to obtain 3.6V.

Lower R6X ?

Thanks!

 

[Salas]

No, its correct the way you do it. VR2X is to have range allowed by R3x and to find the good TP within that range. Room temperature changes, installation breathing space changes, leave there some upper range reach just in case.

 

[Salas]

P.S. Don't worry if its not 33V-37V but near 32V for good TP. We have seen many there. Its due to tolerances between builds and my prototype had unusually strong Red Leds also.

 

[Joel Wesseling]

Okay, so I have 31.6V and 31.9V @ 3.6v TP leaving some rotational range on VR2X.

Listened all day yesterday and today. Amazing Sound!

Thanks for all of your help and for the work involved.


BTW, ground: I used 2 aluminum chassis bases(wood covers and faceplates will be built) that I constructed. I'm waiting for 4 pin connectors to arrive so I just ran 4 non shielded hook-up wires to the boards, the boards are connected to its chassis and ground terminal. TT ground wire is clipped on - No noise is present

 

[Salas]

All well then. Congratulations! What is the cartridge etc.?

 

[Joel Wesseling]

Clearaudio Maestro Wood 3.6mv, 47k - First Gen, not the new V-2
Mounted to Rb300 that I rewired and upgraded the counterweight.
Nottingham Hyperspace with PS

I installed 2.2ufd for C4 out and I'm running it into a Mezmerize. This should be good?

Input dip switch all off, so only 47k in right now.
Built to gain of 40db.

 

[Salas]

2u2 to Mez is good if using 20k volume pot

 

[Joel Wesseling]

That was my next question. I'm using a 50k pot with the 560k resistors but after some reading today I saw your statement saying, not to do this way, only as last resort etc.

so, I ordered a 20k pot and I'll install the associated resistor values.

Thanks!

 

[Salas]

BTW I have a friend who had your cartridge on a Pink Triangle with a Roksan Tabriz arm and it was a very good sound for Jazz and Rock

 

[Salas]

That was my next question. I'm using a 50k pot with the 560k resistors but after some reading today I saw your statement saying, not to do this way, only as last resort etc.

so, I ordered a 20k pot and I'll install the associated resistor values.

Thanks!

Well, its not bad using 50K pot, but with 20K it was subjectively more open sounding IMHO. Faster rise time on the scope technically due to the pot's value interaction with the highish K170 JFET input capacitance is the reason.

 

[Joel Wesseling]

I'm finishing a second Mez, 20k pot will go in.

 

[alexkosha]

Hi Nick,

I have some question for you.
I finalized my final version of Raw DC, but during my trials I might jumped little over 50V and also got nasty oscillations on KM due to bed PCB design. All that is fixed now, but I started to hear some clicks when I start FSP.
I verified and it is not from Raw DC PSU since I can't see anything on scope on it under 470R load. It is about 2mV ripple and some typical high frequency background noise, but do not see any spikes, ringing and etc..
I have 46.2-48.9V into FSP now, depend to home AC fluctuations during the day time and load.
I assume it is something got bed on shunt section.
I'm uploading recording and might you will give me some idea when to look.
Just rename it to *.m4a
Thank you.

 

[Salas]

It does not remind something PSU I think. More like something intermittent. Like a bad contact or switch or a bad gate in a JFET. Are those tap-thud sounds on both channels?

 

[alexkosha]

Yes. On both channels.


Sent from my iPhone using Tapatalk

 

[Salas]

So lets sum it up, during power up/warm up on both channels and appeared after some mishaps with the raw and the C-Mults. Right? For how long do they appear? Is there anything jumping on the scope if you probe the regulated FSP rail during that time?