current feedback strange behaviour??

aaronboumans · 25th October 2004, 10:42 AM

I have a strange observation in my A3. I measure 0,35 V (R120)so current is not high enough. But I made a mistake, I forgot R114 in feedback-line! So I placed it back in and the voltage rises to 0,7V!!
How can this be with no signal, oscillations,groundloops? DC on base of Q105 changes when opening or closing feedbackloop.
Can someone please explain what is happening?

aaronboumans · 26th October 2004, 06:52 AM

Help? please?

wuffwaff · 27th October 2004, 09:19 AM

Hi,

the only thing I can think of at the moment is that the cap C103 is leaking. When connecting R114 more current flows through R115 and the voltage over R120 will rise.

What I don´t understand is how Vbase can change, this should stay at aprox. 0,65V.

William (who will think a bit more about it)

aaronboumans · 27th October 2004, 10:08 AM

I checked capacitor and pcb-layout is any mistakes were made. Non- what so ever... No oscillations (scope-check). I changed the current-adjust-resistor from 45K to 16.6K. Now I have 0,5V across R120 so bias is now correct. Sound is great, but I still do not understand why current-feedback does something with bias (when no signal in applied)!! And I also do not understand why I have to change 47K in 16.6K? I have never read of someone lowering this resistor to get the correct current.
Current transistor is BC550C, can this be the cause of any trouble?

wuffwaff · 27th October 2004, 10:38 AM

Hi,

the bias current you get when building the amp with the standard resistor values always differs due to tollerances in the transistors.
I got 2.4A from my A5 instead of 2.1 and also had to lower R21 (or R113 in your case).
When setting bias with pots you normally will get different values for left and right channel (I think I´ve got 47k left and 30k right channel). This also applies to the current feedback resistor.

BC550 works fine, Hfe has an influence though as the current through the base will differ (get larger for lower Hfe therefore upping the bias)

william

aaronboumans · 27th October 2004, 11:52 AM

Thanks Wuffwaff,
I think I'll just leave it as it is and won't try to understand how&why.
All 6 amps are now stable and the right current. Offset 30mV or lower. So only some tweaks and continue with active filters and preamp. When all electronics are finished building the speakers will be next.

25th October 2004, 10:42 AM	#1
aaronboumans diyAudio Member Join Date: Feb 2003 Location: deventer	current feedback strange behaviour?? I have a strange observation in my A3. I measure 0,35 V (R120)so current is not high enough. But I made a mistake, I forgot R114 in feedback-line! So I placed it back in and the voltage rises to 0,7V!! How can this be with no signal, oscillations,groundloops? DC on base of Q105 changes when opening or closing feedbackloop. Can someone please explain what is happening?

27th October 2004, 09:19 AM	#3
wuffwaff diyAudio Member Join Date: Aug 2001 Location: Ingolstadt Germany	Hi, the only thing I can think of at the moment is that the cap C103 is leaking. When connecting R114 more current flows through R115 and the voltage over R120 will rise. What I don´t understand is how Vbase can change, this should stay at aprox. 0,65V. William (who will think a bit more about it) __________________ een ooievaar is geen konijn want zijn oren zijn te klein!

27th October 2004, 10:38 AM	#5
wuffwaff diyAudio Member Join Date: Aug 2001 Location: Ingolstadt Germany	Hi, the bias current you get when building the amp with the standard resistor values always differs due to tollerances in the transistors. I got 2.4A from my A5 instead of 2.1 and also had to lower R21 (or R113 in your case). When setting bias with pots you normally will get different values for left and right channel (I think I´ve got 47k left and 30k right channel). This also applies to the current feedback resistor. BC550 works fine, Hfe has an influence though as the current through the base will differ (get larger for lower Hfe therefore upping the bias) william __________________ een ooievaar is geen konijn want zijn oren zijn te klein!

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26th October 2004, 06:52 AM	#2
aaronboumans diyAudio Member Join Date: Feb 2003 Location: deventer	Help? please?

27th October 2004, 10:08 AM	#4
aaronboumans diyAudio Member Join Date: Feb 2003 Location: deventer	I checked capacitor and pcb-layout is any mistakes were made. Non- what so ever... No oscillations (scope-check). I changed the current-adjust-resistor from 45K to 16.6K. Now I have 0,5V across R120 so bias is now correct. Sound is great, but I still do not understand why current-feedback does something with bias (when no signal in applied)!! And I also do not understand why I have to change 47K in 16.6K? I have never read of someone lowering this resistor to get the correct current. Current transistor is BC550C, can this be the cause of any trouble?

27th October 2004, 11:52 AM	#6
aaronboumans diyAudio Member Join Date: Feb 2003 Location: deventer	Thanks Wuffwaff, I think I'll just leave it as it is and won't try to understand how&why. All 6 amps are now stable and the right current. Offset 30mV or lower. So only some tweaks and continue with active filters and preamp. When all electronics are finished building the speakers will be next.

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