LM3886 driving mosfet - Page 2

pro · 26th June 2009, 08:35 AM

Hi PB
I solved the problem of the 12V.

It's the scope that is not right.

Yesterday night after many test, I have tryed to misure my gainclone that works alone with the same PS, and guess? It clipped at the same +/-12V. So, I have attached the probe to the calibration point of the scope, and for a 1 Vp/p I have read about 350mV p/p.

I have the same your simulation, but in the real life, the DC offset starts ad -100 mV with cold amp and then rises at about 750mV after warm-up. Tonight I try to put a 220u cap in series R17 as you seid and see if offset goes down.
Regards

Khron · 26th June 2009, 02:15 PM

pro , i've sent you an email yesterday (to avoid off-topic-ing here)

pro · 28th June 2009, 10:53 AM

The cap in series to R17 doesn't have any effect. There is a big difference between the simulator and real life.
The DC offset starts at 20 mV in first 10 minutes, then rises to 400 mV when the heatsink is warm( maybe it's too small), that i thing due the VBE multipler that is a bit overkill. I'm gonna try to slow down it with en emitter resistor on the BD139.
Anyway, the circuit seems to work very well. The output impedence mesured, is about 1/2 hom, for e DF of 15 at 20Hz and 1Khz on 8 hom. (remember that there is no glbal NFB, just DC.
Next step, i'll try a bootstrap for mosfet drain, bigger heatsink and some protections for mosfet (I have already burned two pairs of them)

AndrewT · 28th June 2009, 11:10 AM

where is the Vbe multiplier (bd139) mounted?

pro · 28th June 2009, 11:18 AM

The photo is of the version without VBE. Now it's between the mosfet. That one, had problems of temperature, due the 33p that was making oscillate the 3886. Without it, no oscillations.

Regards

AndrewT · 28th June 2009, 11:34 AM

what's the point in posting the wrong pic with a description?
To confuse us?
Do you want our help?

Always show/fit Vgs max Zeners.
These if reduced to match the Id vs Vds curves in the datasheet can be effective current limiters.

pro · 28th June 2009, 11:47 AM

Sorry Andrew I didn't have a new one. Just made it and upgraded the post.
Can you show a small schematic for zener or a link where to find it? Thank you.
Regards

AndrewT · 28th June 2009, 12:03 PM

read Pass & ESP and others.
The Zeners are often shown in power FET datasheets.

pro · 28th June 2009, 12:21 PM

Thank you Andrew. I'll give a look.
Regards

26th June 2009, 08:35 AM	#11
pro diyAudio Member Join Date: Feb 2004 Location: Milano	Hi PB I solved the problem of the 12V. It's the scope that is not right. Yesterday night after many test, I have tryed to misure my gainclone that works alone with the same PS, and guess? It clipped at the same +/-12V. So, I have attached the probe to the calibration point of the scope, and for a 1 Vp/p I have read about 350mV p/p. I have the same your simulation, but in the real life, the DC offset starts ad -100 mV with cold amp and then rises at about 750mV after warm-up. Tonight I try to put a 220u cap in series R17 as you seid and see if offset goes down. Regards __________________ Michele

28th June 2009, 10:53 AM	#13
pro diyAudio Member Join Date: Feb 2004 Location: Milano	The cap in series to R17 doesn't have any effect. There is a big difference between the simulator and real life. The DC offset starts at 20 mV in first 10 minutes, then rises to 400 mV when the heatsink is warm( maybe it's too small), that i thing due the VBE multipler that is a bit overkill. I'm gonna try to slow down it with en emitter resistor on the BD139. Anyway, the circuit seems to work very well. The output impedence mesured, is about 1/2 hom, for e DF of 15 at 20Hz and 1Khz on 8 hom. (remember that there is no glbal NFB, just DC. Next step, i'll try a bootstrap for mosfet drain, bigger heatsink and some protections for mosfet (I have already burned two pairs of them) __________________ Michele

28th June 2009, 11:10 AM	#14
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	where is the Vbe multiplier (bd139) mounted? __________________ regards Andrew T.

28th June 2009, 11:18 AM	#15
pro diyAudio Member Join Date: Feb 2004 Location: Milano	The photo is of the version without VBE. Now it's between the mosfet. That one, had problems of temperature, due the 33p that was making oscillate the 3886. Without it, no oscillations. Regards __________________ Michele

28th June 2009, 11:34 AM	#16
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	what's the point in posting the wrong pic with a description? To confuse us? Do you want our help? Always show/fit Vgs max Zeners. These if reduced to match the Id vs Vds curves in the datasheet can be effective current limiters. __________________ regards Andrew T.


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26th June 2009, 02:15 PM	#12
Khron diyAudio Member Join Date: Jul 2005 Location: Finland	pro , i've sent you an email yesterday (to avoid off-topic-ing here)

28th June 2009, 11:47 AM	#17
pro diyAudio Member Join Date: Feb 2004 Location: Milano	Sorry Andrew I didn't have a new one. Just made it and upgraded the post. Can you show a small schematic for zener or a link where to find it? Thank you. Regards __________________ Michele

28th June 2009, 12:03 PM	#18
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	read Pass & ESP and others. The Zeners are often shown in power FET datasheets. __________________ regards Andrew T.

28th June 2009, 12:21 PM	#19
pro diyAudio Member Join Date: Feb 2004 Location: Milano	Thank you Andrew. I'll give a look. Regards __________________ Michele

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