question :post filter feedback

titanchen68 · 5th June 2006, 08:38 AM

I build a pre-filter feedback class-d amp before.
I try to change to post-filter feedback prototype.
But get some problem.
triangle wave amplitude is 4Vpp .
here is my circuit ...

titanchen68 · 5th June 2006, 08:41 AM

The pic show that when the error-amp output amplitude near 1v.
It get some distortion ....
The output of mosfet also get some distortion
But output amplitude only +/- 15v.

titanchen68 · 5th June 2006, 08:44 AM

The pic show that when the input signal become bigger.
The output of error-amp become strange like the pic shown.
And output of MOSFET become only negative side.
does anyone has any ideal ???

Tekko · 5th June 2006, 08:53 AM

Good to know im not the only one in trouble

Got a piccy of the amp ?

titanchen68 · 5th June 2006, 09:19 AM

Tekko :
do you mean you still the same problem??
have you fixed it ???
Or have any ideal for it ??

what kind of power supply you use ??

Tekko · 5th June 2006, 09:30 AM

I have not fixed it, i can´t fix it because4 im not qualified

I use a 2x12v trafo for the power section and a 12v pc supply for the logic level supply.

fredos · 5th June 2006, 12:41 PM

Hello Titanchen68

Lower or completly remove R33, or put a bigger C38, 1uF is not enought to feed the power required by 6 mosfet...Look like that the IR2110 go into undervoltage lockout when you try to much power from it. That's why error amp try to compensate.

Reduce by the way your traingle wave amplitude, you will get better linearity.

Fredos

www.d-amp.com

fredos · 5th June 2006, 12:42 PM

I just saw, if you reduce triangle wave, higger the value of R27 too...

Fredos

www.d-amp.com

titanchen68 · 5th June 2006, 12:54 PM

Fredo:
Thanks for your replying..!!
I will try all the suggestion you said .!!
I will reduce the triangle wave to 2Vpp.
Bigger the R27 to 100K ohms.
Remove the R33.

About the feedback network ....
could you teach how to design the component value ???
The capacitor parallel with the feedback resistor .
Will the cap bigger and less effect the phase ???

fredos · 5th June 2006, 01:08 PM

I did not found any good ''real life'' way to calculate accurately the feedback network. However, I will give you some real trick to ''adjust'' it. First only leave the feedback resistor in the feedback loop. Put a big capacitor (1-2.2uf) at the output with a 4 ohms load. Check if the amplifier is stable with and without 4 ohms load. If yes remove the cap and play with a small value (2.7-47pf) over your feedback resistor. If not, add capacitor from 47 to 220pf over feedback resistor to reach stability when you remove 4 ohms load. After this check stability when you remove the capacitor you add. Add a resistor-capacitor network over the existing network to get stability like you did on your schematics (C28 and R35), but always keep R35 to the same value of the feedback resistor (R31).

Wish that will help you!

Fredos

www.d-amp.com

5th June 2006, 08:53 AM	#4
Tekko diyAudio Member Join Date: Jan 2005	Good to know im not the only one in trouble Got a piccy of the amp ? __________________ The point of life is to build atleast one audio amplifier before you die.

5th June 2006, 09:30 AM	#6
Tekko diyAudio Member Join Date: Jan 2005	I have not fixed it, i can´t fix it because4 im not qualified I use a 2x12v trafo for the power section and a 12v pc supply for the logic level supply. __________________ The point of life is to build atleast one audio amplifier before you die.

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5th June 2006, 09:19 AM	#5
titanchen68 diyAudio Member Join Date: Apr 2005 Location: Taiwan Taipei	Tekko : do you mean you still the same problem?? have you fixed it ??? Or have any ideal for it ?? what kind of power supply you use ??

5th June 2006, 12:41 PM	#7
fredos diyAudio Member Join Date: Mar 2005 Location: Quebec	Hello Titanchen68 Lower or completly remove R33, or put a bigger C38, 1uF is not enought to feed the power required by 6 mosfet...Look like that the IR2110 go into undervoltage lockout when you try to much power from it. That's why error amp try to compensate. Reduce by the way your traingle wave amplitude, you will get better linearity. Fredos www.d-amp.com

5th June 2006, 12:42 PM	#8
fredos diyAudio Member Join Date: Mar 2005 Location: Quebec	I just saw, if you reduce triangle wave, higger the value of R27 too... Fredos www.d-amp.com

5th June 2006, 12:54 PM	#9
titanchen68 diyAudio Member Join Date: Apr 2005 Location: Taiwan Taipei	Fredo: Thanks for your replying..!! I will try all the suggestion you said .!! I will reduce the triangle wave to 2Vpp. Bigger the R27 to 100K ohms. Remove the R33. About the feedback network .... could you teach how to design the component value ??? The capacitor parallel with the feedback resistor . Will the cap bigger and less effect the phase ???

5th June 2006, 01:08 PM	#10
fredos diyAudio Member Join Date: Mar 2005 Location: Quebec	I did not found any good ''real life'' way to calculate accurately the feedback network. However, I will give you some real trick to ''adjust'' it. First only leave the feedback resistor in the feedback loop. Put a big capacitor (1-2.2uf) at the output with a 4 ohms load. Check if the amplifier is stable with and without 4 ohms load. If yes remove the cap and play with a small value (2.7-47pf) over your feedback resistor. If not, add capacitor from 47 to 220pf over feedback resistor to reach stability when you remove 4 ohms load. After this check stability when you remove the capacitor you add. Add a resistor-capacitor network over the existing network to get stability like you did on your schematics (C28 and R35), but always keep R35 to the same value of the feedback resistor (R31). Wish that will help you! Fredos www.d-amp.com

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