Hexfet power amplifier - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th January 2004, 09:59 PM   #1
diyAudio Member
 
Join Date: Jan 2004
Location: Lithuania Kaunas
Default Hexfet power amplifier

I'm planning to build a medium power(60-100W for 8ohms) amp.
I found this project at
http://users.otenet.gr/~athsam/power_amplifier_65w_hexfet_eng.htm

Now I wonder how this thing sounds. Is it worth to build it? Or it's just a waste of time and I must search for another better power amp schematics.

Any Comments would be appreciated
Attached Images
File Type: gif power_amp_65w_a.gif (16.8 KB, 2104 views)
  Reply With Quote
Old 31st January 2004, 03:14 AM   #2
subwo1 is offline subwo1  United States
diyAudio Member
 
subwo1's Avatar
 
Join Date: Jan 2002
Location: North American Continent
I tend to think that if the the mosfets are to be connected common source, then a circuit using only N-channel devices benefitting from their superior performance should do better. If an amp uses a P-channel device, it may as well use source follower configuration. The main hitch with source followers is that the gate drive circuit needs to operate about 15 volts beyond each drain power supply.
__________________
USMPS
http://groups.yahoo.com/group/switchmode/
  Reply With Quote
Old 31st January 2004, 05:14 AM   #3
ojg is offline ojg
diyAudio Member
 
Join Date: May 2003
Location: Norway
I have a feeling this amplifier will not work... The drain and source of Q12 need to be switched around. So it becomes a mirror image of Q13. See here for a similiar circuit.
  Reply With Quote
Old 31st January 2004, 05:19 AM   #4
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: GŲteborg, Sweden
Blog Entries: 4
This volt thing is true but you must also consider how amperes at 15 V. With a nornmal load (speaker) you only have to have 5-7 volts gate-source voltage. With this design you loses a coulple of watts but in decibells it's almost nothing (1 dB?). Otherwise the design is pretty normal, nothing wierd.

The amp will very much likely work since it's already built. It seems that the output stage is having local current feedback.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 31st January 2004, 09:36 AM   #5
ABO is offline ABO  Netherlands
diyAudio Member
 
Join Date: Dec 2003
Location: Amst
It is impossible to say beforehand if this amp will work.

This is an amp from Elektor magazine called hexfet amp. The problem with this amp is stability. It may, or may not be stable. The designers never figured out why it does work in one case and not in another.

I know because I e-mailed about it with the designer (t.giesberts@segment.nl).

use mica insulators for Q12,13
use ceramic insulators for Q10,11,8
very close matching of Q1-4, Q7,9, Q10,11

....and hope for the best. The sound is alright though.


I have its higher power brother with IGBT's instead of hexfets. This amp has the same stabitlity problems!

hope this helps
  Reply With Quote
Old 31st January 2004, 10:58 AM   #6
Rambi is offline Rambi  Netherlands
diyAudio Member
 
Join Date: Aug 2002
Location: Deventer, NL
Quote:
Originally posted by ABO
It is impossible to say beforehand if this amp will work.
This is an amp from Elektor magazine called hexfet amp. The problem with this amp is stability. It may, or may not be stable. The designers never figured out why it does work in one case and not in another.
Not only difficult to get stable in the sense of not oscillating, but also in DC offset. I built one (channel) of these and couldn't get the offset stable; it would 'jump' up and down and personally I don't like an amplifier that does that .
At least it didn't oscillate in my case.

If you like this 'kind' of amplifier (symmetric, mosfet output) maybe you should take a look at the cresendo ME
Problems with Crescendo ME - Need help!

A more thoughtful design, also from elektor, with a servo to keep DC offset within limits (some people don't like that either though).
Sound is supposedly OK, but I can't really comment on that since I never built/heard one.

Quote:
I have its higher power brother with IGBT's instead of hexfets. This amp has the same stabitlity problems!
Probably because it is the exact same design with IGBTs instead of mosfets

This amp was mentioned earlier on this forum as well, see
a MOSFET amp scheme

And the IGBT version too:

oscillation problem


Remco
  Reply With Quote
Old 31st January 2004, 03:55 PM   #7
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
depends, the basic topology could work ok with some changes for high frequency compensation

the pcb layout you point to sucks

so if you have a >100MHz 'scope and enough knowledge to cure the high frequency problems you may encounter with this design and you are capable of doing your own layout/prototype with good power, ground and RF practice - you wouldn't have to ask

bottom line, look elsewhere for a less problematic design
  Reply With Quote
Old 31st January 2004, 04:09 PM   #8
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Quote:
Originally posted by ojg
I have a feeling this amplifier will not work... The drain and source of Q12 need to be switched around.
I see no reason for the amplifier do not work...the output stage is a regular compound complementar feedback pair where the output devices are Mos-Fets. (nothing new under the sun!!)
__________________
Jorge
  Reply With Quote
Old 31st January 2004, 06:40 PM   #9
ojg is offline ojg
diyAudio Member
 
Join Date: May 2003
Location: Norway
Quote:
Originally posted by Tube_Dude


I see no reason for the amplifier do not work...the output stage is a regular compound complementar feedback pair where the output devices are Mos-Fets. (nothing new under the sun!!)
Hmmm, you may be right. I was confused by the drawing of Q12, usually the gate "lead" is drawn towards the source, like in Q13. As long as Vgs is 2V for both Q12 and Q13 it should be fine.
  Reply With Quote
Old 1st February 2004, 08:59 AM   #10
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Default 11.

Hi Mindaugas,

Nice to see you here! How the hell are you? Finished your new home?

I recognize this as the Elektor circuit too. I have built a couple of Elektors, notably the Crescendo and one other from Giesberts. All these amps had small, niggling problems.

First, it is over complex. Much more than it needs to be. (Per-Anders, I'm looking at you.......... grin!) Redolent with CCS and complementary input and VAS stages.

Second, instability problems are likely coming from the CFP output stage. My advice would be small mica caps, around 100pF, across the base/collector of Q10 and Q11. You might also lower gain of the driver by inserting a 0R47 source resistor in line with F1 and F2; this reduces feedback factor across the CFP.

Generally, CFPs don't work well in Class AB - they really prefer Class A.

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Need help with Giesberts HEXFET AMP Zimo Solid State 39 27th December 2011 01:23 PM
HEXFET amp - please help me fix it. raise0 Solid State 69 29th April 2010 05:06 AM
HEXFET amp silverchris Solid State 9 31st October 2006 03:38 PM
3rd generiation HEXFET vs. 5th generation HEXFET DarkOne Solid State 3 12th April 2002 07:51 PM
HexFet 60 Ychnaval Solid State 4 10th December 2001 07:16 AM


New To Site? Need Help?

All times are GMT. The time now is 05:38 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2