Discrete Opamp Open Design - Page 173 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, 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 15th October 2012, 09:48 PM   #1721
bcarso is offline bcarso  United States
diyAudio Member
 
Join Date: May 2005
Location: Canoga Park, California
Quote:
Originally Posted by Waly View Post
Measurement was at 1KHz.

The big problem with this device is power dissipation. If you keep the JFET around Idss, Vce of the cascode device should be under 10V. ~100mW is all this SMD case can take. The Vcemax of 50V is pretty much useless.

Devices from the same tube (I got 100 pcs and fed my whole class) are matched to +/- 10%, so a cascoded LTP is feasible. I've built a discrete opamp with these devices in the input stage, 18 more transistors and 0402 RC's (everything hand soldered) in less than .25 sq. in.

Edit: 1KHz and about 12mA (that is, Idss)
Thanks! Better you and your students than me, hand soldering 0402!

If the construction is similar to other SMD JFETs, the gate lead has high thermal conductivity to the chip. Although it's not particularly convenient to provide a lot of local copper in the plane, one could go up. I'll let everyone know how my multiple 862 birds-on-a-wire construction works out. The small irony is the tempco is so low that a temperature estimate may prove difficult
  Reply With Quote
Old 15th October 2012, 10:05 PM   #1722
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by grhughes View Post
Granted I will match the BF862 jfets as carefully as I can but why can't there be a dc balance tweek up front around that diff amp. ! Ray
No penalty at all, you can make one of those 60 Ohm resistors trimmable (100 Ohm pot) and trim offset there. The easiest offset trim would be a large-ish pot across the two lower current mirror devices with the wiper to Vee. The pot is out of the signal path totally, in fact the other one would be too.

For amusement value both trims can be used together to trim offset to 0 and the Aol to essentially infinite.
__________________
Silence is so accurate.
  Reply With Quote
Old 15th October 2012, 10:19 PM   #1723
PHEONIX is offline PHEONIX  Australia
diyAudio Member
 
Join Date: Jul 2004
Location: Australia
Default Parallel JFets

Hello Brad

With the parallel jfets are you planning not to match the parts in the diff stage and just see how it works out, or have I misunderstood you.

Regards
Arthur
  Reply With Quote
Old 15th October 2012, 10:53 PM   #1724
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
"No penalty at all" Thanks a lot Scott! 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 15th October 2012, 10:55 PM   #1725
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
A quick look at jcx's idea. This is not definitive but what I see looks good. As stated before there is a long settling tail but for a line amp or preamp this should not matter. Seconds are now essentially gone, this is 600 Ohms at 20V p-p and 20kHz.
Attached Images
File Type: jpg disjcx.JPG (56.7 KB, 298 views)
File Type: jpg disjcx2.JPG (14.0 KB, 294 views)
File Type: jpg disjcx3.JPG (46.4 KB, 287 views)
__________________
Silence is so accurate.

Last edited by scott wurcer; 15th October 2012 at 11:13 PM.
  Reply With Quote
Old 15th October 2012, 11:20 PM   #1726
bcarso is offline bcarso  United States
diyAudio Member
 
Join Date: May 2005
Location: Canoga Park, California
Quote:
Originally Posted by PHEONIX View Post
Hello Brad

With the parallel jfets are you planning not to match the parts in the diff stage and just see how it works out, or have I misunderstood you.

Regards
Arthur
I'm tempted to try that, but I'll probably start with rudimentary matches. I'm especially keen to see if I can parallel these parts without oscillation. But bear in mind that I have other applications for the paralleled parts --- I'm not at this point so interested in building this particular amplifier, but happy to watch. One such app is as the primary devices in a modification of the linuxguru unity-gain buffer, and the first I will do only requires 2 862 in parallel per buffer. For some preamp apps I may use as many as 8 or 10, and for an envisioned direct ribbon mic preamp, quite a few more

I have about 200 pieces to play with, certainly beyond limits of time and patience, but I'll probably measure some more before tearing the jig down. What I really ought to do is throw things into excel and do some fancier statistics, like trying to detect if they are really Gaussian-distributed, or are things multimodal etc.

I may measure gate leakage to guess at chip temperatures. It would be really nice if the wire heatsink approach allows higher dissipations, even though these parts work well with low Vds. Cordell, in Linear Audio Vol. 4, has shown an MC preamp design with individual LSK389 pairs and individual associated current sources, and some resistance in the drains. Although not quite as fast as the 862 the LSK389s are fast enough to occasion RF oscillations too, so it is interesting that apparently no extra parts in series with the gates were required for stability.
  Reply With Quote
Old 15th October 2012, 11:24 PM   #1727
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by bcarso View Post
I'm tempted to try that, but I'll probably start with rudimentary matches. I'm especially keen to see if I can parallel these parts without oscillation. .
The BF862 in my experience can not go beyond 4 paralleled devices with even good DIY assembly. SMD ferrite beads or inductors on each group of 4 (gates) have no noise penalty.
__________________
Silence is so accurate.
  Reply With Quote
Old 15th October 2012, 11:28 PM   #1728
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by scott wurcer View Post
The BF862 in my experience can not go beyond 4 paralleled devices with even good DIY assembly. SMD ferrite beads or inductors on each group of 4 (gates) have no noise penalty.
Also, gate stopper resistors may be used. Since there is virtually no current to the gates, there shouldn't be any noise penalty.
  Reply With Quote
Old 15th October 2012, 11:33 PM   #1729
morinix is offline morinix  United States
diyAudio Member
 
Join Date: Jun 2006
Quote:
Originally Posted by scott wurcer View Post
The BF862 in my experience can not go beyond 4 paralleled devices with even good DIY assembly. SMD ferrite beads or inductors on each group of 4 (gates) have no noise penalty.
This guy seems to get away with 8 in parallel.

Hand solder 0402 parts? - That's hard! Brad Plunkett made me do it a few times
__________________
Robert
Lounge Audio
  Reply With Quote
Old 15th October 2012, 11:45 PM   #1730
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by morinix View Post
This guy seems to get away with 8 in parallel.

Hand solder 0402 parts? - That's hard! Brad Plunkett made me do it a few times

It gets touchy, the small inductors are harmless.
__________________
Silence is so accurate.
  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
Discrete OPAMP audio-gd Vendor's Bazaar 27 20th September 2012 04:02 PM
discrete opamp help blackpowderaudio Parts 0 16th December 2009 03:46 PM
THAT transistor headphone amp (250ma discrete opamp) design sanity check. Russ White Headphone Systems 19 13th December 2007 12:52 PM


New To Site? Need Help?

All times are GMT. The time now is 11:30 PM.


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