CCS acting strange. What's wrong? - 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 21st November 2009, 03:49 PM   #1
ventle is offline ventle  Norway
diyAudio Member
 
Join Date: Jan 2008
Default CCS acting strange. What's wrong?

The constant current source in the attached schematics is part of my quite common lin three-stage class B amplifier design. It's supposed to feed the input stage (Load 1, LTP), and the VAS stage (Load 2, darlington paired 2SD669). There's also a bootstrap capacitor (47uF) connected between the +70V rail and the junction between resistors R4 and R5, I forgot to draw this in the schematics.

The problem is that the R1 resistor, which is supposed to set the quiescent current of the VAS at approx 6.7 mA (0,67V/100R). As I've drawn into the schematics, I measured the voltage across R1 to be only 134mV.

The next problem is that the transistor Q1 is turned hard on making it drop only 6mV across its collector and emitter (Vce). This leads to the high-side ouput device (2SK1058) being turned hard on as well, while the low-side device (2SJ162) is hard off. I measure a DC voltage of 20V at the output of the amplifier...

I didn't draw the entire schematics because I believe the error can be found somewhere within the CCS drawn here. Anyone see any obvious design flaws? I attempted to remove R2 and R6 because some designs have and some don't have these resistors in place, but it didn't make any difference.
Attached Images
File Type: jpg IMG_6967.jpg (206.9 KB, 174 views)
  Reply With Quote
Old 21st November 2009, 04:15 PM   #2
Banned
 
Join Date: Apr 2005
Location: Stockholm
ventle,
you need two separate constant current sources, like this:
Attached Images
File Type: jpg 666.JPG (9.7 KB, 158 views)
  Reply With Quote
Old 21st November 2009, 04:26 PM   #3
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
I'd say your VAS is not sinking a sufficient current to keep the CCS in linear mode. Maybe no current at all. Perhaps a latch-up issue caused by the entanglement of the two sources.
  Reply With Quote
Old 21st November 2009, 04:39 PM   #4
Banned
 
Join Date: Apr 2005
Location: Stockholm
Elvee,
Quote:
I'd say your VAS is not sinking a sufficient current to keep the CCS in linear mode. Maybe no current at all. Perhaps a latch-up issue caused by the entanglement of the two sources.
No. The circuit in post #1 simply cannot work as intended.
  Reply With Quote
Old 21st November 2009, 04:45 PM   #5
ventle is offline ventle  Norway
diyAudio Member
 
Join Date: Jan 2008
Thanks Lumba.

What is the purpose of the 10k resistor (R2) in your schematics? Is it for linearizing, limiting current, or just simulating a load? In the latter case, should I omit it?
  Reply With Quote
Old 21st November 2009, 04:58 PM   #6
Banned
 
Join Date: Apr 2005
Location: Stockholm
ventle,
R2 (10k) lowers the power dissipation in T1 so you can use a better device. R2 in your circuit is not needed.
  Reply With Quote
Old 21st November 2009, 05:46 PM   #7
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by Lumba Ogir View Post
Elvee,

No. The circuit in post #1 simply cannot work as intended.
Could you elaborate?
Attached Images
File Type: gif DualCCS.gif (19.6 KB, 135 views)
  Reply With Quote
Old 21st November 2009, 06:03 PM   #8
Banned
 
Join Date: Apr 2005
Location: Stockholm
Elvee,
I personally would stick to two separate constant current sources of this type (ring of two), to provide a stable and low Vref for R7 and R1 (in post #1).
  Reply With Quote
Old 21st November 2009, 06:22 PM   #9
Banned
 
Join Date: Apr 2005
Location: Stockholm
Elvee,
...instead of this:
Attached Images
File Type: jpg 777.JPG (15.6 KB, 117 views)
  Reply With Quote
Old 21st November 2009, 06:22 PM   #10
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
This circuit is not necessarily my first choice either, but it should work more or less as intended.
When it works properly, it will be time to think of something better.
  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
My amp is acting like a transducer!? audio_moksha Tubes / Valves 9 15th March 2008 06:04 PM
LM1973 acting strange Wynand Digital Source 12 3rd November 2006 07:18 AM
What went wrong with my amp? Bass driver strange movement. chengs Tubes / Valves 10 20th October 2006 02:19 PM
PCM1804 acting up MC Digital Source 6 6th April 2005 05:54 AM
6922 acting strange........or is it ? ashok Tubes / Valves 18 14th October 2004 02:06 AM


New To Site? Need Help?

All times are GMT. The time now is 02:58 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