Quad type bipolar PS for AlephJ - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 November 2014, 02:13 PM   #1
diyAudio Member
 
Join Date: Sep 2010
Default Quad type bipolar PS for AlephJ

ZM suggested that I start a new thread about powering an AlephJ from a 48VDC source using the Quad virtual ground. I have a few questions that I need to ask before I can get started to be sure I understand the circuit and source the correct parts. For reference, I have attached the pertinent part of the circuit. Questions:
1) Why are the initial two resistors not equal? I would have expected 10k/10k. Now if my math is correct, the midpoint sits at 22.7 which is 1.3 below desired and that happens to be exactly double the Vbe of a bjt. I am sure that is not coincidence, but I can't see how it works.
2) The specified bjts are, of course, obsolete. Can they be replaced with a MOSFET pair? i.e. irf240/irf9240. I assume they would need gate stoppers and that plus the difference in Vgs from Vbe would require a different value for the 9.1k R, but I don't know how to calculate it.
3) I would tend to use larger caps, but given the inherent lack of ripple of a battery supply, maybe that is not needed. How about a 0.1uF bypass?
Thanks for your input. I will be ordering all supplies while I am in US and then building everything when I get back to Panama in a few weeks so this may end up being an intermittent for me. Again, thanks for the help.
Attached Images
File Type: jpg quad virtual gnd.JPG.jpg (8.2 KB, 190 views)
  Reply With Quote
Old 2nd November 2014, 04:13 PM   #2
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
1. start with equal resistors ; check potential ; alter what's needed

2.use something in T0220 case ; decrease 2K2 value , to bleed more substantial current

3.larger caps , certainly OK ; bypass also OK
__________________
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 30th November 2014, 01:13 PM   #3
diyAudio Member
 
Join Date: Sep 2010
I thought I posted this previously, but I don't see it, so I'll try again.
Quote:
Originally Posted by Zen Mod View Post
1. start with equal resistors ; check potential ; alter what's needed
That is easy enough to do and I am sure I can end up with good results, but I would like to understand how the circuit works and WHY the two resistors are not equal. Can anyone explain it to me? Trial and error is great (and sometimes the only option), but knowledge is better!
Quote:
2.use something in T0220 case ; decrease 2K2 value , to bleed more substantial current
And I assume no heat sink as it is only passing the error current. ?A value like 1k enough? I am not sure what we are aiming for.
Quote:
3.larger caps , certainly OK ; bypass also OK
Again looking for understanding. A DC battery should have 0 ripple, so the cap is not "filtering." The only other function I know of for a PS cap is to act as a "reservoir." With AB or B this is certainly necessary, but with Class A operation, I thought it drew the same current all the time. Why, then, would we need a "reservoir?" Again, I am not opposed to using them. The caps are relatively cheap and I have several on hand. I just want to understand what they are doing.

Thanks for indulging me. To me, this hobby is part about getting great finished products, but also about understanding how they work.
  Reply With Quote
Old 30th November 2014, 02:11 PM   #4
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
resistors at bases are voltage dividers ...... and you can take transistors as emiter-followers (buffers) for that voltage level

even simpler - resistors at bases plus transistors are acting as usual bleeders , to help keeping common cap's node at half supply voltage

with 2K2 decrease , current is increased , keeping mid point voltage firmer

even battery supply needs speed ........ and caps are there for that
__________________
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 30th November 2014, 02:20 PM   #5
diyAudio Member
 
Join Date: Sep 2010
Quote:
Originally Posted by Zen Mod View Post
resistors at bases are voltage dividers ...... and you can take transistors as emiter-followers (buffers) for that voltage level
That much I understood. If they are a voltage divider, why aren't they equal? I am sure it has something to do with the Vgs of the transistors and that using mosfets instead will probably change that, but I do not understand the theory of why they should be different.

Quote:
with 2K2 decrease , current is increased , keeping mid point voltage firmer
Understood. How does one determine how much is enough? 1k. 500, 10???

Quote:
even battery supply needs speed ........ and caps are there for that
OK, speed. How does a cap provide speed? And what do we mean by speed? I think of speed as rate of change. If the amplifier will be drawing the same amperage all the time, what is changing? I thought Class A amps ran at a steady state and there was not a change in current with changing input/output. Perhaps that assumption is wrong?
Again, just trying to learn.
  Reply With Quote
Old 30th November 2014, 02:31 PM   #6
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
Originally Posted by dmtparker View Post
...... To me, this hobby is part about getting great finished products, but also about understanding how they work.
cheers to that!
__________________
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 30th November 2014, 02:38 PM   #7
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
Originally Posted by dmtparker View Post
That much I understood. If they are a voltage divider, why aren't they equal? I am sure it has something to do with the Vgs of the transistors and that using mosfets instead will probably change that, but I do not understand the theory of why they should be different.
even your two ears aren't identical , so why expecting N and P counterpart's Vbe needs to be ?

Quote:
Originally Posted by dmtparker View Post
Understood. How does one determine how much is enough? 1k. 500, 10???
more , the merrier , keeping an eye (brain) on resistor and transistor dissipation

say that 20-50mA is overkill

Quote:
Originally Posted by dmtparker View Post
......

OK, speed. How does a cap provide speed? And what do we mean by speed? I think of speed as rate of change. If the amplifier will be drawing the same amperage all the time, what is changing? I thought Class A amps ran at a steady state and there was not a change in current with changing input/output. Perhaps that assumption is wrong?
Again, just trying to learn.
if you look at Universe as big class A amp , and take it in infinitesimal small moment ..... everything is steady
whatever , if you take several of these moments , you'll see that moving to entropy is slow , but certainly one direction oriented process

so - nothing is steady

though , it helps that caps are faster to give than accu

__________________
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 30th November 2014, 03:19 PM   #8
hm4nine is offline hm4nine  Europe
diyAudio Member
 
Join Date: Mar 2005
I have applied this version to get a virtual ground in a previous power amp.
I used TO220 bipolars.
Nevertheless I also connected high wattage (5W) resistors between +/-supplies and virtual gnd - just to feel better for safty reasons....
Results were good - system never failed.
And it's a cheap way to get a quasi monaural supply in a stereo amp with a standard tranny which has 2 secondaries
  Reply With Quote
Old 30th November 2014, 03:33 PM   #9
diyAudio Member
 
Join Date: Sep 2010
Quote:
Originally Posted by Zen Mod View Post
even your two ears aren't identical , so why expecting N and P counterpart's Vbe needs to be ?
OK, so no theoretical reason for Rs to be different. In fact, theoretically they should be the same. Difference is just an observed fact that compensates for differences between 2 transistors, correct? I wonder why Quad chose the values they did, however. It would seem that it would need to be a case by case situation if you are just correcting for differences in transistor values.



Quote:
more , the merrier , keeping an eye (brain) on resistor and transistor dissipation
say that 20-50mA is overkill
So 50V/.02A=2500. two R in series (plus 2T) so ~1k each is fine. 50V/2000R=.025A so each R dissipates W=(.025)^2*1000=.625W so 2W resistors needed for safety factor of 3.


Quote:
if you look at Universe as big class A amp , and take it in infinitesimal small moment ..... everything is steady
whatever , if you take several of these moments , you'll see that moving to entropy is slow , but certainly one direction oriented process

so - nothing is steady

though , it helps that caps are faster to give than accu

Too metaphysical for me. How do caps help 'speed' other than being a reservoir for instantaneous demand? If a Class A amp draws a continuous 100W (for example), other than the ramp up (where an RC network helps avoid in rush current by making it 'slow'), even looking at each 'instant' there is 100W into cap and 100W out of cap. Does it have to do with the 'impedance' of the PS? I have seen this discussed, but don't really understand it.
  Reply With Quote
Old 30th November 2014, 04:41 PM   #10
hm4nine is offline hm4nine  Europe
diyAudio Member
 
Join Date: Mar 2005
It' s a must to connect some speed-up caps parallel to a battery. Batteries behave like big and slow caps; they can storage energy - but nothing else. It's similar if you use cheap/high ESR or expensive/low ESR caps in your design.
Personally I go a step further with connecting small caps close to drains/collectors of each power device, not only to improve ESR but also to compensate inductive components of wiring.
  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
Recommendation for premade bipolar PS board? speaker Power Supplies 0 6th June 2014 01:39 PM
Bipolar ps from single ended transformer dmtparker Power Supplies 4 13th September 2013 01:16 PM
FS: Buffalo IIISE DAC, Legato 3.1 I/V Stage, Placid HD & Bipolar PS Modules damohpi Swap Meet 2 30th July 2013 01:07 AM
Bipolar PS plus AC coupled output from opamp? leadbelly Solid State 6 28th July 2009 12:12 PM


New To Site? Need Help?

All times are GMT. The time now is 11:57 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