j-FET / Triode Phono front-end

What people are working on at the moment
Post Reply
User avatar
IslandPink
Eternally single
Posts: 4569
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

j-FET / Triode Phono front-end

Post by IslandPink » Sun Jan 15, 2012 8:08 pm

Inspired by Nick, I have been messing about with my phono amplifier to try the j-FET/triode front-end, as promoted by Thorsten :

http://www.fortunecity.com/rivendell/xe ... ccata.html
(sorry about the pop-ups )
and Allen Wright :
http://www.vacuumstate.com/index.tpl?rubrik=8&lang=2
( see FVP5a )

Tomorrow I will post a circuit or two, and a startling photo of the inside of my phono amp .
"The moon was a stone's throw"

User avatar
Mike H
Amstrad Tower of Power
Posts: 16751
Joined: Sat Oct 04, 2008 5:38 pm
Location: Alternative parallel universe #57. No it's nice
Contact:

Post by Mike H » Mon Jan 16, 2012 4:22 pm

Essentially, a cascode!
 
"Nor yet is it still..."

User avatar
IslandPink
Eternally single
Posts: 4569
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

Post by IslandPink » Mon Jan 16, 2012 7:25 pm

Yes it's a cascode .
Actually , photo first . This is a general view of the inside of my phono amp currently . It illustrates what can happen if you build a circuit neatly, then use it a breadboard for a number or revisions over a 5-year period . I think the top-plate needs trading in for a newer model now. You may be able to discern how the core of the signal section was decoupled from the main top-plate by the use of a wood-router ( with 6mm straight bit ) and then some elastic 'hangers' .

Anyhow . It was... step-ups ( Tribute 1:10 ) into D3a ( triode ) into 20k RIAA , into D3a ( triode ) . Lots of gain , about 12mA in each D3a, low-impedance network , and sounded very good. The power supply is a cLCLC into a Gary Pimm current source : total current about 70mA ; 50mA to the circuit and about 20mA into a pair of VR tubes ( OD3 ) acting as a shunt regulator . Nice clean return path for the signals .

Now, it's a mess , I will initially just draw your attention to the by-passed step-ups ( masking tape ) and the little black objects around the first-stage D3a sockets .
Attachments
Phono2012_FET_flash.JPG
"The moon was a stone's throw"

User avatar
IslandPink
Eternally single
Posts: 4569
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

Post by IslandPink » Mon Jan 16, 2012 7:30 pm

Close-up of the input area, with 2-off ( parallel ) 2SK369 feeding each cathode of the first D3a . Not a great pic, needed to use flash, not really got a close-up camera ...

The 2SK369's were sourced from a secret Yorkshire-based supplier, matched for Idss .

Each 2SK369 has a 21R source resistor, formed from a 27R carbon-composition and a 100R carbon film . This was done in a rush !
Attachments
Phono2012_input.JPG
"The moon was a stone's throw"

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6789
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Mon Jan 16, 2012 10:11 pm

After a hard search I found the closest thing to the original Oz-Loesch, named so because it was an Australian enthusiasts version of Arthur Loesch's preamp, which was so favoured last century. I think the only alteration from the Oz-Loesch of this later mutation is the addition of a 26 output stage.

Image

The original was at that time the only jfet gain phono stage on the web, as such easy to find. Now it is almost as though it never existed.

I would think the 417a's are doubleunobtainium by now.I never had any. Perhaps the 6c45pi is close enough.

The OZ-Loesch predated Thorsten's copy (no relation to Arthur). In the same way that Jeremy Epstein's Free Lunch predated Thorsten's play on that.

izzy wizzy
Old Hand
Posts: 353
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone

Post by izzy wizzy » Tue Jan 17, 2012 7:41 am

[quote=The OZ-Loesch predated Thorsten's copy (no relation to Arthur).[/quote]

And I suspect Allen Wright pre-dated that lot too.

cheers,

Stephen

Andrew
Social outcast
Posts: 3970
Joined: Thu May 24, 2007 2:18 pm

Post by Andrew » Tue Jan 17, 2012 7:48 am

I would think the 417a's are doubleunobtainium by now.I never had any. Perhaps the 6c45pi is close enough.
There's some 5842 about on the bay, even one quad of what purports to be 417A, but they are going or silly money,

Andrew
Analogue, the lost world that lies between 0 and 1.

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6789
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Tue Jan 17, 2012 8:02 am

izzy wizzy wrote:
And I suspect Allen Wright pre-dated that lot too.

cheers,

Stephen
Possibly, going from just my memories the OZ-Loesch was first in the sequence of events. At the time it was the biggest breakthrough outside of "Sound Practices". There was a great buzz about it going on.

Some people are good at repackaging things and puting them on the interweb in a sustainable form, and if their pages are popular enough and they (the pages not the people) outlive the source eventually people start believing they had original thought. as long as they don't ever reference their source.

I am not saying those people don't grasp and comprehend well the original method. but in the academic world the plagiarism programs which are now automatically run on all submissions would ensure they learn to reference or they would never get into print.

izzy wizzy
Old Hand
Posts: 353
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone

Post by izzy wizzy » Tue Jan 17, 2012 8:43 am

I never heard of the Oz-Loesch until about 10 years ago so don't know when it surfaced. Sound Practices 1 is around Summer 1992. I was using fet/valve cascode front end on my phono in the late 80's but I can't remember where I got the idea so sadly can't reference my sources ;) Certainly wasn't the Oz thingy but could well have been Wright? Or something else? Oh well .... memories :)

cheers,

Stephen

izzy wizzy
Old Hand
Posts: 353
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone

Post by izzy wizzy » Tue Jan 17, 2012 8:52 am

IslandPink wrote:You may be able to discern how the core of the signal section was decoupled from the main top-plate by the use of a wood-router ( with 6mm straight bit ) and then some elastic 'hangers'.
I keep coming back to this and it makes me smile :D Brilliant!

cheers,

Stephen

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6789
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Tue Jan 17, 2012 3:11 pm

izzy wizzy wrote:I never heard of the Oz-Loesch until about 10 years ago so don't know when it surfaced. Sound Practices 1 is around Summer 1992. I was using fet/valve cascode front end on my phono in the late 80's but I can't remember where I got the idea so sadly can't reference my sources ;) Certainly wasn't the Oz thingy but could well have been Wright? Or something else? Oh well .... memories :)

cheers,

Stephen
I'll reference you then when I build one!

However a pair of good transformers satisfies me.

izzy wizzy
Old Hand
Posts: 353
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone

Post by izzy wizzy » Tue Jan 17, 2012 6:10 pm

Paul Barker wrote:However a pair of good transformers satisfies me.
And me.

I reckon this is a taste thing as the FET/valve cascode vs the stepup/valve route seems to polarise people. I just couldn't get on with the FET/valve cascode but then maybe I wasn't doing it right. It was a long time ago.

cheers,

Stephen

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6789
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Tue Jan 17, 2012 6:21 pm

Mark bought the last pair of S and B step ups AFAIK so he will be able to tell us his opinion in time.

User avatar
IslandPink
Eternally single
Posts: 4569
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

Ok hang on !

Post by IslandPink » Tue Jan 17, 2012 7:19 pm

Ok, a lot to say about this . Very very interesting ... especially this evening :!:
Now, I would like everyone to hold back from rushing out and buying any valves , because it's not as obvious what you need as you might think .

Now, apologies I still haven't scanned the circuit drawing to explain in detail . It will happen tomorrow. For now, just imagine it's like Chris Brady's , or Thorsten's , but I'm running two 2SK369's under each cathode .

I will first attempt to clear up a few of the comments and details that have sprung up . Good to see a crowd gathering .... :D
"The moon was a stone's throw"

User avatar
IslandPink
Eternally single
Posts: 4569
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

Post by IslandPink » Tue Jan 17, 2012 7:21 pm

izzy wizzy wrote:
The OZ-Loesch predated Thorsten's copy (no relation to Arthur).[/quote wrote:
And I suspect Allen Wright pre-dated that lot too.

Stephen
True, and Hewlett Packard and Tektronix pre-dated Allen !
"The moon was a stone's throw"

Post Reply

Who is online

Users browsing this forum: shane and 1 guest