Current FB Input Stage Idea (power amp) - 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 27th August 2010, 12:03 PM   #1
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Default Current FB Input Stage Idea (power amp)

There are plenty of way to arrange input stage in a current feedback amp.
I have seen many. Standard is 4 transistors in a Diamond type figuration.
But also used is 2T and a bunch of resistors in a network.

The way I show in diagram uses 2T and 2 Current sources.
I have never seen this way before, so this I have figured it out myself.
But to me this method is rather obvious, so I think this must have been done before.

The heart is within the smaller rectangle.
2 current sources creates 0.6V across R3 and R13, 47 Ohm.
To get a good gain in first stage I use rather much current, 14 mA and 47 Ohms resistors.
R8 and R7, 150/10 Ohm sets the Gain of amp. Here we have x16
Current Feedback comes to R8.
I keep feedback resistance low in order to get low distortion.

VAS is only a pair of BC556/BC546 working at 5 mA

Output stage in the bigger rectangle is very standard.
Quiescent current for MJL3281/1302 is at 50 mA

Amplifier shows good performance. 25 Watt in 8 Ohm.
What is special for CFA amplifiers is the speed.
And this one shows stable -3dB at around 4 MHz
So it is very fast.
Attached Images
File Type: png current_fb_idea_1.png (12.3 KB, 267 views)
__________________
lineup
  Reply With Quote
Old 27th August 2010, 10:44 PM   #2
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Yeah!
I know.
When peole see something different from 2 transistor diffferential input
and the Blameless concept
they get nervous.


Their brain is not used to much more than blameless.
So CFB amplfiers must be suspect.


One nice thing about Current Feedback Amplifiers is
that they are symmetrical.
There was a time meself liked the usual 2T diff pair. Like blameless.
But lately I just love Symmetrical Amplifiers.
John Curl likes sym, too.


Happy Greetings
/lineup
__________________
lineup
  Reply With Quote
Old 27th August 2010, 11:09 PM   #3
wahab is offline wahab  Algeria
diyAudio Member
 
Join Date: Nov 2009
Location: algeria/france
That is similar to this one , which date from about 1977..

Designed with Otala s "ideas" in mind.
I would say, to the extreme, since open loop
gain is drastically reduced by use of degeneration and vas
loading in the aim to display a 20khz open loop BW...

Funny is that increasing the open loop gain by increasing
the impedances that load the vas allow vast reduction of
THD...
Attached Files
File Type: zip REVOX B750.zip (10.8 KB, 78 views)

Last edited by wahab; 27th August 2010 at 11:17 PM.
  Reply With Quote
Old 27th August 2010, 11:18 PM   #4
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Hi Lineup. I missed you my friend. How are you doing?
Hope everything is fine with you.
Fotios
__________________
Best Regards FOTIS ANAGNOSTOU
  Reply With Quote
Old 28th August 2010, 03:08 AM   #5
diyAudio Member
 
moschfet's Avatar
 
Join Date: Jul 2008
Location: Hannover
Hello lineup,

this is a very interesting circuit. My experience with amplifire in current-feedback-topologie ruled me to layout very tidy.

Because of the oscillations, the amps burning faster, than you can pull the plug.

Do you want to build it up in real?
  Reply With Quote
Old 28th August 2010, 03:31 AM   #6
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Smoky Mountains , Tennessee
Quote:
Originally Posted by lineup View Post
Yeah!
I know.
When peole see something different from 2 transistor diffferential input
and the Blameless concept
they get nervous.


Their brain is not used to much more than blameless.
So CFB amplfiers must be suspect.


One nice thing about Current Feedback Amplifiers is
that they are symmetrical.
There was a time meself liked the usual 2T diff pair. Like blameless.
But lately I just love Symmetrical Amplifiers.
John Curl likes sym, too.


Happy Greetings
/lineup
They are nice .... at cancelling out even order harmonics , leaving the "nasties" (odd orders) for us to hear. They have to be very low in distortion by design just so we don't hear what is "left" (the nasties).

Your amp is very similar to member SYN08's VSOP , which is a stable proven design (he made the prototype). It has very low distortion , but what distortion it does have is all odd order. Just an observation.

Electrocompianet actually uses a single ended stage to drive a symmetric stage . I guess one cancels out the evens while the other cancels out the odds.
OS
__________________
Mongrel website , always current and updated :
http://www.fidelityforce.com/ostripper
  Reply With Quote
Old 28th August 2010, 05:57 PM   #7
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
Quote:
Originally Posted by lineup View Post
There are plenty of way to arrange input stage in a current feedback amp.
I have seen many. Standard is 4 transistors in a Diamond type figuration.
But also used is 2T and a bunch of resistors in a network.

The way I show in diagram uses 2T and 2 Current sources.
I have never seen this way before, so this I have figured it out myself.
But to me this method is rather obvious, so I think this must have been done before.
Yes yes...but...current fb has poor CMRR so a separate stable PSup for the predriver is inevitable. Without that you can be sure the amp starts to oscillate at recovery from overdrive ( when supply voltage drops) . If you provide a separate stabilized supply such CFBs excel with high open loop bandwidth and little compensation problems - unlike Voltage feedback where you'll need to compensate the open loop bandwidth down and down again. Old buddy Mr. Williamson who is somewhat famous for the first HiFi tube amp, the Williamson amp, had noticed that and acted accordingly.
In replicas of that design you'll for sure find stabilized supplies for the pre-stage.

Hans Dieter
  Reply With Quote
Old 28th August 2010, 06:04 PM   #8
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
Quote:
Originally Posted by ostripper View Post

Electrocompianet actually uses a single ended stage to drive a symmetric stage . I guess one cancels out the evens while the other cancels out the odds.
OS
That is correct. It is indeed possible to have a predriver single ended that has
a particular thd and a particular distortion spectrum driving a sym power stage
that as also its thd and spectrum and these two together have less thd than
each of the stages. That depends much on open loop bandwidth and lead lag compesation but it works as such that the harmonics spectrum remains constant with rising frequency of the signal.
  Reply With Quote
Old 29th August 2010, 06:59 PM   #9
diyAudio Member
 
Join Date: Aug 2009
Location: Kaunas
What about using something like THS6012 and a discrete output stage?
  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
Current Feedback IC input in a power amp steph_tsf Solid State 16 27th April 2010 05:45 PM
Input stage idea lumanauw Solid State 29 29th July 2005 03:20 PM
current bias j-fet input 2sk170 ,2sj74 at input stage YUTK Solid State 11 2nd June 2005 04:34 AM
Input stage idea DarkOne Solid State 20 19th January 2004 08:11 AM


New To Site? Need Help?

All times are GMT. The time now is 08:09 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