HEXFET amp - please help me fix it. - Page 3 - 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 14th February 2009, 08:48 PM   #21
diyAudio Member
 
EDDELARUE's Avatar
 
Join Date: Nov 2004
Location: Brasilia
Send a message via MSN to EDDELARUE Send a message via Skype™ to EDDELARUE
Quote:
Originally posted by raise0



Interesting mods EDDELARUE. Although i prefer to stick with the original design using IRF540 and IRF9540. I cannot afford to upgrade my PSU.



The mods came from own Elektor magazine ( Ton Giesberts ) , except the output transistors.
  Reply With Quote
Old 14th February 2009, 09:17 PM   #22
diyAudio Member
 
meanman1964's Avatar
 
Join Date: Oct 2006
Location: Belgium the land of chocolate
Quote:
Originally posted by EDDELARUE


The mods came from own Elektor magazine ( Ton Giesberts ) , except the output transistors.
can you send the article to me about the mods?
  Reply With Quote
Old 14th February 2009, 10:42 PM   #23
Beto is offline Beto  Brazil
diyAudio Member
 
Beto's Avatar
 
Join Date: Feb 2008
when told to use Zener diode in the gate of the fet was thinking in this example below:


Click the image to open in full size.
  Reply With Quote
Old 15th February 2009, 02:34 AM   #24
diyAudio Member
 
EDDELARUE's Avatar
 
Join Date: Nov 2004
Location: Brasilia
Send a message via MSN to EDDELARUE Send a message via Skype™ to EDDELARUE
Quote:
Originally posted by meanman1964


can you send the article to me about the mods?
Below follows the mods as Elektor - April/2001-pg.58 issue presents regarding the article Hexfet Power Amp from September/1995 issue:

- at the solder side of the board, fit a 27nF capacitor in parallel with R31;
- connect a 20k resisitor between the collector of T8 and ground;
- connect a 20k resisitor between the collector of T9 and ground;
- replace R20 by a 1.8k. ( here, the value may be other. It depends on the output power transistor that you use - IGBTs are not available and power mosfet, some has low VGSth and others has higher VGSth);
- replace R17 and R18 by 390 ohms;
- replace R3 and R4 by 33 ohms.
  Reply With Quote
Old 15th February 2009, 07:50 AM   #25
diyAudio Member
 
meanman1964's Avatar
 
Join Date: Oct 2006
Location: Belgium the land of chocolate
Default Re: I Fixed it, FINALLY.

Quote:
Originally posted by raise0
Hi! My arm is free again, after 6 months of immobilization. So last night I tried to continue my work on this hexfet amp. I started by replacing resistors and caps as suggested by elektor mag. and other members from this forum. Placed some decoupling caps, changed fuses to 5Amps, replaced trimmer P2 and T7 and R3, R4 to 33ohms. But while I was soldering the 20K resistors to the collector of T8 and T9, I FOUND IT. My PCB after a lot of part replacements lost a lot of traces. So my circuit slowly transformed, into a point to point connection. Something went wrong during that and I couldn’t find. It took several months but I saw the base of T7 connected to feedback loop (where R25,R28,R30,R31 meets) I stopped everything, fixed that trace and I powered up the amp… 220mA and 4mV to output! FINALLY …waited 20~30 minutes and connected speakers… IT WORKS! Cannot believe I was so stupid and BLIND before.
I am not sure what this connection did to the circuit and which part was “cancelled”. I mostly get it, but not quite sure. Any comment on that is welcomed. Anyway… Although it works now I will complete all the following mods to both circuits.

F1, F2 = 5A Fuse
100nF decoupling caps
27N over R31
20kohms resistors from collector T8 & T9 to ground
R3, R4 to 33Oohm
R20 to 1k8
R17, R18 to 390ohm
R25, R28 to 22ohm
R26, R29 to 120ohm

Furthermore I will place heatsinks on T8 and T9… They get extremely hot even with no signal applied. Probably they are oscillating… Also BOTH transformers make a slightly audible sound. That has no effect to output, but it wasn’t there some time ago. Any ideas on what caused that? Or is it just random. Still… both of them simultaneously? Will post again with the results, but not to soon. I plan to listen to it while I’m working on my other amp project. Here are some pics whith my mistake! The second one is a moving gif. Check it out.

Click the image to open in full size.
Click the image to open in full size.
Doesn't R3-4 need to be 33 Ohm and not 330 Ohm?
  Reply With Quote
Old 15th February 2009, 10:36 AM   #26
diyAudio Member
 
meanman1964's Avatar
 
Join Date: Oct 2006
Location: Belgium the land of chocolate
how critical is the 27n cap over R31?
I've two 33n caps could I use those as well
  Reply With Quote
Old 15th February 2009, 10:38 AM   #27
diyAudio Member
 
meanman1964's Avatar
 
Join Date: Oct 2006
Location: Belgium the land of chocolate
Quote:
Originally posted by Beto
when told to use Zener diode in the gate of the fet was thinking in this example below:


Click the image to open in full size.
Do you have a layout of this amp?
  Reply With Quote
Old 15th February 2009, 10:40 AM   #28
diyAudio Member
 
EDDELARUE's Avatar
 
Join Date: Nov 2004
Location: Brasilia
Send a message via MSN to EDDELARUE Send a message via Skype™ to EDDELARUE
Default Re: Re: I Fixed it, FINALLY.

Quote:
Originally posted by meanman1964


Doesn't R3-4 need to be 33 Ohm and not 330 Ohm?
R3 and R4 are parts of compensation network of complementary differential input stage and originally they were 100 ohms ( 100 ohms with 2.7nF in series - Elektor September/1995 ) but now the mods change both, R3 and R4, to 33 ohms each and not change the capacitors. As I said, it is compensation network and with the others mods it helps to avoid oscillations.
  Reply With Quote
Old 15th February 2009, 10:53 AM   #29
diyAudio Member
 
meanman1964's Avatar
 
Join Date: Oct 2006
Location: Belgium the land of chocolate
what about the cap over R31 does it have to be exact 27n ?
  Reply With Quote
Old 15th February 2009, 11:18 AM   #30
diyAudio Member
 
EDDELARUE's Avatar
 
Join Date: Nov 2004
Location: Brasilia
Send a message via MSN to EDDELARUE Send a message via Skype™ to EDDELARUE
Quote:
Originally posted by meanman1964
what about the cap over R31 does it have to be exact 27n ?
In this amplifier, the output stage has voltage gain ( this is not source follower - is a commom source stage ). So, it may oscillates and the 27nF capacitor will be there to avoid this. This capacitor is not critical since you keep it in safety range. There is a value below 27nF where that stage may to oscillate ( I don´t know wich ) and above that value it will narrow the bandwidth of amplifier. I believe that 27nF is a optimal value arrived in measurements made by author of the article.
  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 silverchris Solid State 9 31st October 2006 03:38 PM
HEXFET amp in Elektor Taliesin Solid State 10 21st May 2003 02:44 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 06:10 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