All JFET open-loop RIAA pre-amp - diyAudio
Go Back   Home > Forums > Source & Line > Analogue Source

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd April 2005, 02:50 AM   #1
Mr Evil is offline Mr Evil  United Kingdom
diyAudio Member
 
Join Date: Aug 2004
Location: Behind you
Default All JFET open-loop RIAA pre-amp

My turntable has been sitting on a shelf for many years. Finally I have done something about this and designed an RIAA pre-amp. It uses JFETs since I recently bought a load and like them. It has no global feedback partly because it's an integral part of the functioning of the circuit and partly because I never go open-loop and want to do something different for once.

As per the attached schematic it consists of two differential stages each of which implements part of the filter, the first being the pole and zero at 50 and 500Hz, the second being the pole at 2122Hz (with an extra zero at 50kHz as I have seen suggested in a few places). A source-follower buffers the output.

Differential stages use 2SK389/2SJ109, output stage is 2SK170/2SJ74.

It requires some careful pot twiddling to balance the differentials (essential for correct frequency response) and zero offset. The various source degeneration resistors need to be chosen to achieve the desired gain with particular JFETs.

Since I have not listened to vinyl (at home at least) for a few years and have no other pre-amp to compare, I can't really judge how well the circuit performs, but it's certainly not bad.
Attached Images
File Type: png preamp_schematic.png (4.1 KB, 2256 views)
__________________
https://mrevil.asvachin.eu/
  Reply With Quote
Old 2nd April 2005, 06:19 AM   #2
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
You can actually get stable offset from this circuit?! Looks not bad at all. Have you considered adding current sources in the tails?
  Reply With Quote
Old 2nd April 2005, 08:29 AM   #3
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
reminds me to Hafler preamp........
I relly dunno what number,but it's on their site anyway
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
  Reply With Quote
Old 2nd April 2005, 10:40 AM   #4
Mr Evil is offline Mr Evil  United Kingdom
diyAudio Member
 
Join Date: Aug 2004
Location: Behind you
Quote:
Originally posted by analog_sa
You can actually get stable offset from this circuit?! Looks not bad at all...
Apparently yes. I wouldn't trust it with a power amp without an input cap, but the offset does seem to remain constant and low even after an hour or so of running. I suppose that using monolithic dual FETs takes care of thermal matching so offset of each pair should remain low, and since the same current flows in both differentials, any change in their offset should be roughly equal and opposite, leaving nothing at the output.


Quote:
Originally posted by analog_sa
...Have you considered adding current sources in the tails?
Indeed I have. It's just a prototype at the moment, so still room for change. I will probably change R3 for a CCS (JFET, of course ) and possibly also R5, which will give a nice boost to PSRR.
__________________
https://mrevil.asvachin.eu/
  Reply With Quote
Old 2nd April 2005, 09:13 PM   #5
ljozsef is offline ljozsef  Hungary
diyAudio Member
 
Join Date: Apr 2003
Location: Budapest
Default Balanced out

Hallo,

Did anybody think of using it with a balanced pre/amp?
I wonder if it could drive them without the actual complementary stages. Using FET or MOSFET input amps (e.g. Pass or its diy clones) it could succeed.
And with dual FETs only in the circuit the offset will be minimized.

Any thoughts?
Laci
  Reply With Quote
Old 2nd April 2005, 09:24 PM   #6
jam is offline jam  United States
diyAudio Member
 
jam's Avatar
 
Join Date: May 2001
Location: Auburn, CA, USA
You might be on to something here...........You might want to consider doubling up the fets in the first differential.

Jam
  Reply With Quote
Old 3rd April 2005, 11:08 PM   #7
jam is offline jam  United States
diyAudio Member
 
jam's Avatar
 
Join Date: May 2001
Location: Auburn, CA, USA
......for noise reasons.

Jam
  Reply With Quote
Old 4th April 2005, 12:29 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
diyAudio Member
 
Join Date: Aug 2004
Location: Behind you
Quote:
Originally posted by choky
reminds me to Hafler preamp........
I relly dunno what number,but it's on their site anyway
I searched around but couldn't seem to find that one. However it is not an unusual design: I have seen plenty of double-differential JFET amps. The only thing about it that I haven't seen before is the way filtering is accomplished.



Quote:
Originally posted by jam
You might be on to something here...........You might want to consider doubling up the fets in the first differential.

Jam
Do you think that would be worth it? Those JFETs are already low noise and not cheap. Maybe if I were going no-expense-spared, but this is something that will see only occasional use.

That makes me think of something: Is it better to have n-channel or p-channel for the input? The datasheets say noise is the same for both.



I've attached what the sims say the distortion spectrum at 1kHz should look like. Higher order harmonics are very low, which is nice.
Attached Images
File Type: png fft.png (2.3 KB, 1380 views)
__________________
https://mrevil.asvachin.eu/
  Reply With Quote
Old 4th April 2005, 07:45 PM   #9
diyAudio Member
 
Onvinyl's Avatar
 
Join Date: Aug 2002
Location: Germany
Quote:
That makes me think of something: Is it better to have n-channel or p-channel for the input? The datasheets say noise is the same for both.
I recall borbely opts for npn in his papers (fets-the new frontier in audio), and I think he did for noise reasons (not sure).

Quote:
The only thing about it that I haven't seen before is the way filtering is accomplished.
You may want to visit this thread:
Balanced MM phono stage : When Aleph Ono and Xbosoz meet...

Rüdiger
  Reply With Quote
Old 7th April 2005, 11:14 AM   #10
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Nice and Simple Circuit, but enough gain for MC ?

And how about making it fully symmetrical with differential output ? Probably not much more needed than adding another source follower on the opposite arm ??


Patrick
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
JFET SRPP RIAA Preamp wrenchone Analogue Source 249 27th March 2013 11:20 PM
Open loop bandwidth in op-amp. beppe61 Solid State 94 1st October 2007 04:56 AM
BUF04 Open Loop Bonsai Solid State 0 7th August 2006 10:09 AM
Riaa Pacific Jfet maxpou Analogue Source 23 15th April 2006 05:51 PM
Op-amp open loop bandwidth. beppe61 Solid State 18 18th February 2005 02:30 PM


New To Site? Need Help?

All times are GMT. The time now is 09:13 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2