What do you thing about this power amp schematic?

maxpou · 19th January 2010, 08:34 PM

Hi Guys,
i would like to know your opinion on this power amp. The orignal schematic is Mosquito of Jean-Marc Plantefève with mod for vertical mosfet. Now what do you think about these modifications? Thank you! Maxpou

MarcelvdG · 19th January 2010, 08:42 PM

Is the one with the emitter followers the original and the simple one the modified circuit? Or are they both modified?

voltwide · 19th January 2010, 09:37 PM

A quite symmetrical design, not bad at all. The emitter follower is necessaryl to
charge the parallelled MOSFET gates in a reasonable time. From that point of view
I would expect the left design to exhibit far less output voltage slew rate than the rigtht one.
Anyway, a good design imho.

wahab · 19th January 2010, 10:05 PM

the design of the left doesn t work well with hexfets,
as the vas(s) have high output impedance, inadequate to
drive IRFs...the original schematic use hitachi s 2SJ162/SK1038
or the 2sj50/SK135 LATERAL FETS which can be drived in such ways..

the version of the right correct this inadequacy with
a buffer..

Lumba Ogir · 19th January 2010, 10:24 PM

Hi,
my compromise would be one emitter follower stage.

wahab · 19th January 2010, 10:35 PM

Quote:

Originally Posted by Lumba Ogir

Hi,
my compromise would be one emitter follower stage.

yes, it s enough...

maxpou · 20th January 2010, 12:42 AM

Thank you for reply Guys,
what do you think about Q18, Q19 in my input stage and my output stage without NF? Good or not?

Wahab and Lumba: Why only one EF when i read many time here that the best is triple EF? Thank you! Maxpou

wahab · 20th January 2010, 12:58 AM

Quote:

Originally Posted by maxpou

Thank you for reply Guys,
what do you think about Q18, Q19 in my input stage and my output stage without NF? Good or not?

Wahab and Lumba: Why only one EF when i read many time here that the best is triple EF? Thank you! Maxpou

hi, maxpou

triple EF is said good (although i do not exactly agree with that)
for purely bjts outputs...
despite needing some high PEAK currents to be drived correctly, the IRFs
don t need as much current drive as bipolar s final devices..

Lumba Ogir · 20th January 2010, 01:48 AM

maxpou,
everything has a price, it is advisable to keep the number of stages small. The drive current is needed to charge the MOSFET input capacitances at high frequencies (sorry no detailed explanation at this time).
Linearity will mainly depend on the output stage bias point. Q18, Q19 is OK in conjunction with R4/R5. Could you imagine a folded cascode VAS?
Very sensible design and part selection (except BC546).

fab · 20th January 2010, 02:42 AM

Hi

Ensure you have very similar IDSSmax for Q16 and Q17.
What is the purpose of Q18 and Q19?
Q20 and Q21 should be removed as indicated in previous posts.
How is done the thermal compensation? (I see a to-92 package for the VBE multiplier)
Maybe you want to protect the gates of the IRF mosfet with zener clamping diodes.
You can consider using double pole compensation instead of C8 and C9 only.
Since you do not show the value of resistors we can not comment to much on the rest.

I have made a similar design with the possibility to change the feedback point from overall feedback to non-overal feedback (like yours). Finally, I prefer the sound with overall feedback (feedback at output point).
I like the sound of my amp. However, I use 4 pairs of lateral mosfet.

Good luck

Fab

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19th January 2010, 08:42 PM	#2
MarcelvdG diyAudio Member Join Date: Mar 2003 Location: Haarlem, the Netherlands	Is the one with the emitter followers the original and the simple one the modified circuit? Or are they both modified?

19th January 2010, 09:37 PM	#3
voltwide diyAudio Member Join Date: Jan 2010	A quite symmetrical design, not bad at all. The emitter follower is necessaryl to charge the parallelled MOSFET gates in a reasonable time. From that point of view I would expect the left design to exhibit far less output voltage slew rate than the rigtht one. Anyway, a good design imho.

19th January 2010, 10:05 PM	#4
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	the design of the left doesn t work well with hexfets, as the vas(s) have high output impedance, inadequate to drive IRFs...the original schematic use hitachi s 2SJ162/SK1038 or the 2sj50/SK135 LATERAL FETS which can be drived in such ways.. the version of the right correct this inadequacy with a buffer..

19th January 2010, 10:24 PM	#5
Lumba Ogir Banned Join Date: Apr 2005 Location: Stockholm	Hi, my compromise would be one emitter follower stage.

20th January 2010, 12:42 AM	#7
maxpou diyAudio Member Join Date: Apr 2006 Location: Québec	Thank you for reply Guys, what do you think about Q18, Q19 in my input stage and my output stage without NF? Good or not? Wahab and Lumba: Why only one EF when i read many time here that the best is triple EF? Thank you! Maxpou

20th January 2010, 01:48 AM	#9
Lumba Ogir Banned Join Date: Apr 2005 Location: Stockholm	maxpou, everything has a price, it is advisable to keep the number of stages small. The drive current is needed to charge the MOSFET input capacitances at high frequencies (sorry no detailed explanation at this time). Linearity will mainly depend on the output stage bias point. Q18, Q19 is OK in conjunction with R4/R5. Could you imagine a folded cascode VAS? Very sensible design and part selection (except BC546).

20th January 2010, 02:42 AM	#10
fab diyAudio Member Join Date: Mar 2004 Location: Canada	Hi Ensure you have very similar IDSSmax for Q16 and Q17. What is the purpose of Q18 and Q19? Q20 and Q21 should be removed as indicated in previous posts. How is done the thermal compensation? (I see a to-92 package for the VBE multiplier) Maybe you want to protect the gates of the IRF mosfet with zener clamping diodes. You can consider using double pole compensation instead of C8 and C9 only. Since you do not show the value of resistors we can not comment to much on the rest. I have made a similar design with the possibility to change the feedback point from overall feedback to non-overal feedback (like yours). Finally, I prefer the sound with overall feedback (feedback at output point). I like the sound of my amp. However, I use 4 pairs of lateral mosfet. Good luck Fab

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