Updating Leach FET preamp: New transistors????? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 8th December 2011, 03:55 PM   #1
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
Default Updating Leach FET preamp: New transistors?????

HI! I posted this same question over at A asylum (Prop Head Plaza) but I would like to get other's opinion here if possible. In about 77 or 78 I built Marshall Leach's low distortion preamp that was published in Audio Magazine. It used 2n5210/2n5087/mps8099/mps8599 transistors and 2n5457 n channel FETs. I carefully matched all transistors as to beta/hfe/d-s current. It would seem to me that transistors should have improved since that time. Am I wrong? I realize that most interest has turned away from discreet devices to large scale integrated circuits but I would also assume that the process should have improved the same process whereby discreet devices are made. My question is: Are the newer Toshiba/Hitachi discreet devices better, have fewer impurities and are easier to match than the older Motorolla, National, and Fairchild transistors I used in 1978? It is time to re-capp the preamps as their electrolytics have leaked and while I was there I was thinking about making the output stage more robust by subbing Hitachi 2sc3421s for the all Npn output stage mounted on heat sinks. I was pondering driving a 1:1 transformer (600 ohm:600 ohm balanced line) Or MJE181s?? Bad idea, Good Idea???? Thanks for your opinions. Ray
__________________
" "The fool doth think he is wise, but the wise man knows himself to be nothing but a fool." Will Shakespeare
  Reply With Quote
Old 8th December 2011, 09:06 PM   #2
diyAudio Member
 
Join Date: Feb 2004
Location: Queensland Australia
I don't know about being easier to match but there are a few low noise Toshiba Fets around that should be an improvement on the 2N5457's.
Cheers, Jonathan
__________________
"It was the Springtime of the year when aunt is calling to aunt like mastodons bellowing across primeval swamps." P.G. Wodehouse.
  Reply With Quote
Old 9th December 2011, 05:37 PM   #3
diyAudio Member
 
Join Date: Sep 2004
Location: virginia
The 2N5457 jfets have a rather modest transconductance of 1 mS. You can't get lots of voltage gain. The low gain and the 3 pF gate drain capacity results in a very small Miller effect. Dr Leach advertised the input capacity of his jfet preamp as 20 pF. That is astonishingly low and very useful if you are using a high inductance phono cartridge which needs a small terminating capacity.

The famous 2sk147 jfet had a gm of 25 mS which was achieved by a large interdigitated gate structure. The large gate geometry resulted in a gate drain capacity of 18 pF and a potentially big Miller effect. LUX had a phonepreamp where they used these jfets in a cascode circuit to reduce the Miller effect.
  Reply With Quote
Old 9th December 2011, 06:03 PM   #4
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
Thanks: I was under the impression that the first FET stage didn't have much gain and most of the gain was in the second stage 2n5087 pnp diff amp stage. I have added zener referenced current source to that tail. What about the 2sk170BL as a replacement for the 2n5457? Ray
__________________
" "The fool doth think he is wise, but the wise man knows himself to be nothing but a fool." Will Shakespeare
  Reply With Quote
Old 9th December 2011, 09:51 PM   #5
mlloyd1 is offline mlloyd1  United States
diyAudio Member
 
mlloyd1's Avatar
 
Join Date: Feb 2001
Location: illinois
i built the leach preamps years ago with the 2n5457 jfets. it sounded nice and nowadays, might be considered noisy compared to some recent diy designs. if you redesign and choose the higher gain, quieter toshiba jfets, you may want to cascode them due to the significant extra capacitance, especially if you're using mm cartridges.
i'd also tweak the riaa eq network and add an RC filter at the back to get the rolloff more correct.

however, having said all that, give it a shot, the results should be promising!

mlloyd1

Last edited by mlloyd1; 9th December 2011 at 09:53 PM.
  Reply With Quote
Old 9th December 2011, 10:18 PM   #6
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
Thanks! I have a scheme of cascoding from a book on FET written by Siliconix. I have some 2n4416A and some 2n3821 FOR THE SECOND jfet transistor in series of the cascode or should I use all 2sk170BL? Motchenbacher and Fitzen wrote a book and said the 2n3821 was the lowest noise n channel jfet they had tested. Thanks again! Ray
__________________
" "The fool doth think he is wise, but the wise man knows himself to be nothing but a fool." Will Shakespeare
  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
Need help with FET transistors SPX Parts 0 22nd February 2010 03:03 PM
Threshold FET 10/pc Phono Stage Preamp Schematics aka FET 10pc, FET-10pc, fet ten/pc krellkraver Solid State 1 8th February 2007 03:05 PM
Testing FET transistors sound_prodigy Solid State 19 11th March 2006 02:48 AM
Leach output transistors pooge Solid State 5 14th January 2005 09:26 AM
Updating old power transistors hifileslie Solid State 5 24th August 2002 04:44 PM


New To Site? Need Help?

All times are GMT. The time now is 09:36 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