T106-2 core vs.??? core Amplifier problem

[zox2003]

Ok...I think that I might have very interesting issue here.

I have build two 200W /4ohm amplifier modules (using LM311+IR2010+IRF540Z) using core I found localy in store(unknown type but I think is ferrite) with success.
Amplifier carrier is on 300khZ,self-oscillating..tested on full power for hours (on speakers-3 way proffesional boxes) and worked great (great sound-bass,mid and highs were great).Carrier level was just 150mV.
I decided to get some samples of T106-2 core (which is a lot smaller then my core(4 inch diam.)) and try it.
I got it 2 days ago and finally tried it.
Results were not that good as I expected.
I used 2 x 20 gauge wire to make core (44 turns ) to get 40-50uH inductor....After I installed it I got ripple (carrier level) on output of 4Vp-p(old core was just 150mVp-p),offset voltage of 0.450Vmeasured using DMM (old core was 0.001V).
I found out that big offset voltage has something to do with amount of FB feeded back to comparator (I guess 3Vp-p vs. 150mVp-p) so I tried to reduce feedback but it did not work well.
Basically everything looks ok when you turn amplifier on but once you apply anything to input of LM311(even you do finger touch) it offsets Vout to 0.4V.Very strange....

Do you have any suggestion???

Regards....

[zkaiser]

You cannot just switch core (dimensions and type) and remain the same number of turns. As far as offset consurned you must be able to add some dc counter component to cancel it. When I was build iraudio2 amp opamps needed to be trimed for zero offset at out of the amplifier.

Anyway you should post your sch. also to get more accurate answer.

PS.

BTW it seems that you got 26uH not 40-50 as you thought so this is the problem.

[zox2003]

OK...here is working schematic for you guys:

[zkaiser]

Have you tried this ?

[zox2003]

No I have not.....Is that how you did yours?

Can you post how did you fix your DC offset....

regards

[zox2003]

It is interesting that on startup dc offset is 0V and then when you even just touch input pin it jumps to 0.45V and stays there....

[Eva]

One leg of the comparator is seeing 1K at DC, the other is seeing 22k. The offset of the comparator (including switching time asymmetry) is being amplified by overall gain (23). This is basic amplifier design. You can find a fix in most schematics of class AB amplifiers.

You have zero offset upon power up because the circuit is not yet oscillating, it does not start until it gets some input signal. I though this was quite obvious too.

If you were getting 150mV ripple with the old inductor, the value was way too high. If you are getting 4V now, the value is way too low. I though that RF engineers knew the values of their inductors

[zox2003]

Eva,

Thanks for reply....As far as value of inductor...I was doing this whole experiment over this past weekend so i did not have L meter at home...BTW...even at work if I need to measure inductance (BTW a lot smaller value then this inductor is-(range of up to 100nH max) I use Network Analyzer.RF HF design are a lot more sensitive and less tollerant on values then this amplifier inductor design.

But, thanks on all this informations regarding class D.You are very helpfull person in this field.I am wondering where do you live(secret location?).If you live at place I think you do then we might be originally from same location....

As far as DC offset----with other-original core I did not have a offset issue (with same part values).

So what is your recommendation to fix that problem?
What is "normal" ripple value?

Regards

[Eva]

Fix no. 1: Put 100uF or so (preferably non polar) in series with the 1K resistor from pin 2 of LM311 to ground to bring gain to unity at DC. This should divide offset by 20 or so. Next time use differential inputs to the comparator.

[zox2003]

What about inductor on T106-2 core and wire size(200W) for 300khz?