Discrete Opamp Open Design - Page 260 - 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 12th January 2013, 08:48 PM   #2591
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Here’s my final breadboard with the output on top and input on the bottom, ground planes facing each other. I used standard square Vector header pins to both connect the appropriate circuit points and feed the output pins out the bottom. I fired up the output, this time I used 2N4401/4403’s only for the diamond and Panasonic DMC/DMG series pairs for the rest, essentially the same performance. At 500 Ohm load .01% thirds only open-loop. This time I tried increasing the source impedance to see how effective the cascoding is in making the loading of the output stage negligible. 200K in series with the input did nothing visible, even at 22Meg there was no visible increase in distortion. At 4KHz the magnitude of the input impedance was ~7Meg Ohms but I didn’t separate the capacitive and resistive components. The input offset current to the output stage without matching is only 227nA.
Attached Images
File Type: jpg bbbb.JPG (21.9 KB, 517 views)
__________________
Silence is so accurate.
  Reply With Quote
Old 13th January 2013, 01:21 PM   #2592
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
Scott: To those of us who are not EEs, it is unclear to me which output transistors must be thermally coupled to which (bias) diamond transistor. What value should the bias pot be set before turn on ideally with the transistors that you used. What are the Panasonic DMC/DMG number of the transistors you used. Thanks for bearing with the un-initiated! 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 13th January 2013, 04:00 PM   #2593
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
OK here's the details. For the pot just trim it to 0 and power up for a minute or two and then increase pot till you add the desired current to the total supply current, no finicky probing of an internal node to adjust bias necessary. You can also play in situ and see if you can effect THD in your application. I noted the thermal tying.

The values vary slightly on the schematics simply because the exact values are not that critical (I know it drives some obsessives nuts). Like 39.2 Ohm snubbers rather than 49.9 Ohm, that trip to the bin they might have been gone. That being said I noticed the 357 Ohm resistors could be a tad larger the ratio to the position held by the 243 Ohm one should be closer to 5/3, this only adds 400uA in that branch which does not do much and comes out in the trim.
Attached Images
File Type: jpg ostageparts.JPG (68.4 KB, 482 views)
__________________
Silence is so accurate.

Last edited by scott wurcer; 13th January 2013 at 04:10 PM.
  Reply With Quote
Old 13th January 2013, 04:36 PM   #2594
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
The nominal supplies are 15V, 12V works fine though (grabbed the wrong symbol, sigh).
__________________
Silence is so accurate.
  Reply With Quote
Old 13th January 2013, 05:16 PM   #2595
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
Thanks so much! What gains are possible? Would this produce 40db of gain for a vinyl phono preamp (maybe two preamps with passive EQ in between). Can the design you just posted be pushed to + and - 18v? 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
Old 13th January 2013, 07:45 PM   #2596
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
Hey Scott, will you be building the BF862 version next or those little blighters still giving you the heebie jeebies? call me old fashioned but jfet inputs hold more appeal for me.

looks great though, very neat little unit
  Reply With Quote
Old 13th January 2013, 09:31 PM   #2597
diyAudio Member
 
dchisholm's Avatar
 
Join Date: Mar 2011
Location: St Louis, Mo
Quote:
Originally Posted by scott wurcer View Post
. . . I noticed the 357 Ohm resistors could be a tad larger the ratio to the position held by the 243 Ohm one should be closer to 5/3, this only adds 400uA in that branch which does not do much and comes out in the trim.
How did you determine that the ratio should be 5/3? I would have guessed that the 357 ohm resistors are almost the least-critical value in the whole design: they simply need to be small enough to ensure adequate base drive to the output devices at the full-load signal peaks (but not so small that power dissipation becomes a problem in the current source and driver devices).

Quote:
I noted the thermal tying.
What are the consequences of omitting the thermal ties (Q1 to Q4, and Q2 to Q3)? Is it primarily a bias stability problem, or are there also significant implications for signal performance? Coupling a pair of TO-92's requires a little thought when determining the layout but isn't a major problem to DIY construction. It gets more complicated to couple a TO92 driver to, say, a TO126 output device, and I think it's almost impossible to get meaningful thermal coupling between two SMT devices in this application. (To be sure, you CAN get pairs of transistors in a single SMT package but the power ratings are too low for consideration as the output devices in this circuit.)

Dale

p.s. - perhaps both these questions stem from the fact that I still don't really have a good grasp of the analytical details of the diamond buffer output stage, the advantages it provides over other output stages, etc. I'm still stuck at Fig 1 and Fig 2 of the TI (nee National Semi) App Note AN-227, where the only thing obvious is that the B-E drop of the driver stage counteracts the B-E drop of the output stage, so there is no net DC differential between the input and output terminals.
  Reply With Quote
Old 14th January 2013, 02:44 PM   #2598
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
The thermal coupling does not do that much here, but if you wanted to drive 32 Ohms it shows where it might help. The bias levels are somewhat just one best guess at how to distribute a target total supply current of 25mA. Since the input runs at 5mA a side the 5/3 ratio puts 3mA in the first rank of the diamond (a little is shunted away by the bias adjust), I just thought this was a good number enough but not too much.

40dB is fine, you might want an offset adjust or servo if DC is a problem. I forgot the trim pot on these boards.
__________________
Silence is so accurate.

Last edited by scott wurcer; 14th January 2013 at 02:47 PM.
  Reply With Quote
Old 14th January 2013, 03:42 PM   #2599
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
Very nice. So, a set of basic characteristics of the final SWOPA would be useful.... open loop BW/gain; thd; CMR; PSR and noise... basic all around performance numbers summary for the book. Any advice on max C load on output etc would be useful in future apps using the design. Thx-RNMarsh

Last edited by RNMarsh; 14th January 2013 at 03:48 PM.
  Reply With Quote
Old 15th January 2013, 04:01 PM   #2600
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Very nice proto, Scott.




Patrick
__________________
xen-audio.com
  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 04:42 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