Please comment for my new CFB amp

lukio · 7th February 2004, 01:57 AM

Hi everybody…..

I’m a new comer for this forum, this is very good forum, I like it v’ much, really….!

After my fisrt CFB amp has some problems that I never know,
I’ve re- design of my CFB please give me advise for this circuit…Is there any potensial problem that I might meet?? I ‘m not build this design yet, waiting for comments from you all…. As per simulation, the performance is good.. Actually, I still don’t know…

Best regards,,

Lukio

Lars Clausen · 7th February 2004, 03:17 AM

Looks pretty good to me

sss · 7th February 2004, 04:19 PM

regulators for the gain stages hhmmmmmmmmm nice

lukio · 8th February 2004, 02:38 AM

Hi Lars, Hi SSS.....

Thx for your reply...

Do you think, is there any possibility for the amp to get oscillation?? Do I need to place the miller cap. at driver stage or freq. compensation on feedback network??

Best regards,,

Lukio

subwo1 · 8th February 2004, 08:32 PM

Hi lukio. I have not come to any conclusion as to what is causing the problem in the other circuit you were having DC offset on, but maybe have a suggestion to test the stability of this circuit. Drive it with various capacitors on the output as a load and see what happens. Then add some compensation to see if you can stabilize it under all conditions. At the very least you may find the points where if needed, compensation is most effective.

Tube_Dude · 8th February 2004, 10:03 PM

Quote:

Originally posted by lukio

Do you think, is there any possibility for the amp to get oscillation?? Do I need to place the miller cap. at driver stage or freq. compensation on feedback network??

Hi.

Current Feedback amps must not have a capacitor in paralel with the feedback resistor,because is the value of the upper arm resistor of the feedback voltage divider,that define the bandwith.

If you want increase stability, increase a little the upper feedback resistor and make the some proporcional increase in the lower arm...( for the some overall closed loop gain).

Regards

lukio · 9th February 2004, 12:24 AM

Hi...

Very thx for u all .....

I think sugestion from Jorge and Subwo1 are very good, I will try to connect capacitive load and also try to change the value of my feedback resistor as you said..... I'm not try that methode yet..

Music and Amp is an art......it can express you ....

sam9 · 9th February 2004, 01:32 AM

"Current Feedback amps must not have a capacitor in paralel with the feedback resistor,"

Odd thing to say.... Since I'm listening at this very moment to a a CFB amp with just such a cap. However, mine has a damping resistor in series with; the valur is equal to that of the resistor in the lowert arm. The function of this arrangement is to create a frequency dependant voltage divider with the result that the overall gain falls back to unity at high frequencies sooner than it otherwise would. Since phase lag increases with frequency the idea is to pull the gain down before the phase lag reaches 180 deg.

All that is easy enough to say. The problem in implementation is that it seems quite hard to figure out what the value should be without experomentation. The calculations required are beyond me and Spice analysis assumes an ideal case wich isn't what you get when you build a real amp. Further, a very small change in the value can have pretty drastic results.

Tube_Dude · 9th February 2004, 01:43 AM

Quote:

Originally posted by sam9
"Current Feedback amps must not have a capacitor in paralel with the feedback resistor,"

Odd thing to say.... Since I'm listening at this very moment to a a CFB amp with just such a cap.

are you talking about the amp you descrive in the post 6 of

Today's Puzzler ??

?

sam9 · 9th February 2004, 03:15 AM

No. I'm still working with that one. The one I have in mind was completed last spring and is a 50Wer with a single output pair. It had a couple of problems as well, but the cure was simply to change the PCB to keep section as far away from the IS and VAS as possible. It was basically a Slone design but with a PCB laid out to fit the enclosure I was using.

This new one is derived from that with two main changes a nominal 100Wer with another output pair and a diode bias for the CCs. Also the layout of the PCB had some quirks due to how I wanted to mount it plus some options so I could try a couple of circuit variations. When I get it working how I want, I'll follow up that thread to tell what I did. Right now, I'm beginning to fear that I've been soldereing-desodering-resoldering some components so often that board is becomming a bit manky. I may need to etch a fresh one.

8th February 2004, 08:32 PM	#5
subwo1 diyAudio Member Join Date: Jan 2002 Location: North American Continent	Hi lukio. I have not come to any conclusion as to what is causing the problem in the other circuit you were having DC offset on, but maybe have a suggestion to test the stability of this circuit. Drive it with various capacitors on the output as a load and see what happens. Then add some compensation to see if you can stabilize it under all conditions. At the very least you may find the points where if needed, compensation is most effective. __________________ USMPS http://groups.yahoo.com/group/switchmode/

9th February 2004, 12:24 AM	#7
lukio diyAudio Member Join Date: Dec 2003 Location: Bandung, Indonesia	Good advise Hi... Very thx for u all ..... I think sugestion from Jorge and Subwo1 are very good, I will try to connect capacitive load and also try to change the value of my feedback resistor as you said..... I'm not try that methode yet.. Music and Amp is an art......it can express you ....

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7th February 2004, 03:17 AM	#2
Lars Clausen diyAudio Member Join Date: Oct 2003 Location: The Lab	Looks pretty good to me

7th February 2004, 04:19 PM	#3
sss diyAudio Member Join Date: Jul 2003 Location: Israel , haifa	regulators for the gain stages hhmmmmmmmmm nice

8th February 2004, 02:38 AM	#4
lukio diyAudio Member Join Date: Dec 2003 Location: Bandung, Indonesia	Hi Lars, Hi SSS..... Thx for your reply... Do you think, is there any possibility for the amp to get oscillation?? Do I need to place the miller cap. at driver stage or freq. compensation on feedback network?? Best regards,, Lukio

9th February 2004, 01:32 AM	#8
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	"Current Feedback amps must not have a capacitor in paralel with the feedback resistor," Odd thing to say.... Since I'm listening at this very moment to a a CFB amp with just such a cap. However, mine has a damping resistor in series with; the valur is equal to that of the resistor in the lowert arm. The function of this arrangement is to create a frequency dependant voltage divider with the result that the overall gain falls back to unity at high frequencies sooner than it otherwise would. Since phase lag increases with frequency the idea is to pull the gain down before the phase lag reaches 180 deg. All that is easy enough to say. The problem in implementation is that it seems quite hard to figure out what the value should be without experomentation. The calculations required are beyond me and Spice analysis assumes an ideal case wich isn't what you get when you build a real amp. Further, a very small change in the value can have pretty drastic results.

9th February 2004, 03:15 AM	#10
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	No. I'm still working with that one. The one I have in mind was completed last spring and is a 50Wer with a single output pair. It had a couple of problems as well, but the cure was simply to change the PCB to keep section as far away from the IS and VAS as possible. It was basically a Slone design but with a PCB laid out to fit the enclosure I was using. This new one is derived from that with two main changes a nominal 100Wer with another output pair and a diode bias for the CCs. Also the layout of the PCB had some quirks due to how I wanted to mount it plus some options so I could try a couple of circuit variations. When I get it working how I want, I'll follow up that thread to tell what I did. Right now, I'm beginning to fear that I've been soldereing-desodering-resoldering some components so often that board is becomming a bit manky. I may need to etch a fresh one.