Blowing ouput transistors when clipping, why? - 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 11th January 2002, 07:36 AM   #1
diyAudio Member
 
Join Date: Jan 2002
Default Blowing ouput transistors when clipping, why?

I'm brand new here so I thought I would give a little background info.

Some of you may remember the company Mark V electronics that sold amplifier kits, the guy who ran the company closed up shop, but before he did he let everyone know who his supplier was. That company was Sound Master International out of Hong Kong, web address:

http://www.hkiol.org/template/0001/template.htm

A while ago I ordered some of the 300W Amp kits and built them. They worked great if I used a low power supply voltage such as +- 40V DC, if I hooked up a low impedence load such as 4 ohms and got clipping, everything was ok physically (except the produced sound, yuk). However, if I upped the voltage to the recomended +-75V, when I would get clipping I would lose half of the power transistors usually, as well as the op amp in the pre amp stage. For the life of me, I can't come up with a solution since my electronics background is limited to hobbiest activities. I can troubleshoot and repair the amps, but I would prefer to be able to drive them closer to their rated outputs to get more power from them. I tried replacing the output transistors with MJ15003's but still had the same problem.

I can post the schematic for these amps if anyone can help me troubleshoot them.

Here are the schematics and the instruction manual as well.


I'm glad that I found this forum, what a great way to get some more electronics experience.

http://www.lvcm.com/zombiess/amp/circuit.gif
http://www.lvcm.com/zombiess/amp/parts_list.gif
http://www.lvcm.com/zombiess/amp/powersupply.gif
http://www.lvcm.com/zombiess/amp/page1.gif
http://www.lvcm.com/zombiess/amp/page2.gif
http://www.lvcm.com/zombiess/amp/page4.gif
http://www.lvcm.com/zombiess/amp/page5.gif
http://www.lvcm.com/zombiess/amp/page6.gif
  Reply With Quote
Old 11th January 2002, 08:44 AM   #2
yeti is offline yeti  Germany
diyAudio Member
 
Join Date: Nov 2001
Location: Ewersbach
Default smoking amp

Did you have inductive load?
If so,inductive loads give high voltage peaks whe the amp clips!(ignition coil effect).
You can fix it by applying two diodes from the output to the supply rails.
  Reply With Quote
Old 11th January 2002, 09:24 AM   #3
subwo1 is offline subwo1  United States
diyAudio Member
 
subwo1's Avatar
 
Join Date: Jan 2002
Location: North American Continent
Sorry to hear about the trouble. Does Q12 always blow at that time? It has less resistance in its path. Unless the diagram is wrong, which I think it is, the collector of Q12 should have a collector resistor and Q11 should have only one collector resistor.
__________________
USMPS
http://groups.yahoo.com/group/switchmode/
  Reply With Quote
Old 11th January 2002, 02:02 PM   #4
blmn is offline blmn  Brazil
diyAudio Member
 
blmn's Avatar
 
Join Date: Feb 2001
Location: .
Jeremy

Did you loose Q10 and Q12 and the 2 others were ok?
  Reply With Quote
Old 11th January 2002, 04:48 PM   #5
diyAudio Member
 
Join Date: Jan 2002
BLMN, if I remember corretly, that is what would happen, but I have blown the other ones a few times, but I believe most of the time it was Q10 andQ12. I would also lose Q7 and Q8 and Q5, Q6 and Q13, but not always. It just wasn't very consistant when blowing up! I would loose IC1 as well, but not always.

If this amp would always blow the same things repeatedly, then I could probably figure it out. But it seams almost random. I checked the voltage ratings on all of the transistors and even replaced some of them with higher ratings for more headroom, but I had no luck with it. My only success has been with lowering the supply rails down. I am trying to get a new transformer to bring my rails up from +-40 to +-55- +-60 or so to see if that doesn't give me more power without blowing things up. The only other thing I can think of is to run a limiter on it. Does anyone have plans for a simple limiter to limit the max input to something like 1.2 volts?
  Reply With Quote
Old 11th January 2002, 06:06 PM   #6
subwo1 is offline subwo1  United States
diyAudio Member
 
subwo1's Avatar
 
Join Date: Jan 2002
Location: North American Continent
I have a suspicion about the way they designed the protection circuit Since the bases of Q3 and Q4 are referenced to ground instead of the output, as the power supply voltage is increased, the protection is progressively disabled. Maybe try changing the 3.9k's at the bases of Q3 and Q4 to 3 ohms while at the same time connecting the junction between the 1N4148's to the output instead of ground. I am not sure if 3 ohms is the right value for you, but if the voltage across the .5 ohm balast resistors goes up to 10 (20 amps of current), then about 50mA goes though the 180 ohm resistors on the bases, raising the voltage on the 3 ohm ones by over 100mV, added to the bias on the 1N4148, the transistor should turn on.

The diodes at the bases of Q3 and Q4 temperature compensate the protection circuit. But that may not really be needed. You may decide to just remove those diodes and use 15 ohm resistors instead of 3 ohm.
__________________
USMPS
http://groups.yahoo.com/group/switchmode/
  Reply With Quote
Old 11th January 2002, 07:06 PM   #7
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Strong possibility that the output devices are being
allowed to saturate at clipping, which results in
a slow recovery time of the saturated transistor.
This results in "cross conduction current" in which
high current flows from the positive rail through the
output stage to the negative rail, and is particularly
a problem if you clip the amp continuously at higher
frequencies. You can tell this is the case if the current
draw of the amp goes up significantly when you clip
it without a load.

If this is the case, then probably the bank of devices
which does not fail is the saturating bank.

You can make a simple limiter for about 1.3 volts with
4 signal diodes at the input, 2 pair in series for 1.3 volts
in each direction, and using an input resistor to limit the
current.
  Reply With Quote
Old 11th January 2002, 07:29 PM   #8
diyAudio Member
 
Join Date: Jan 2002
Nelson, I have tried the diode thing before, but when I was checking them with my scope and inputting a sinewave, i noticed that the output was being shaped by the diodes above about .8 volts with two of them in series. I have tried multiple types of diodes and all give a similar response. Is there a better way to do it with an op amp where it will just limit the overal signal. I guess a compressor would sort of do this, but I want the full dynamic range, just not to go above a pre-set level.
  Reply With Quote
Old 11th January 2002, 07:50 PM   #9
blmn is offline blmn  Brazil
diyAudio Member
 
blmn's Avatar
 
Join Date: Feb 2001
Location: .
Jeremy,

At a first glance, it seems to me the current sample is feeded from Q9 and Q11 only, and they are, for this voltage (75V) being working out of their safe area. heatsink size and thermal stability must be considered too. I think you have to put the voltage down and modify the circuit allowing the current sample being feeded from Q10 and Q12 too. Maybe the circuit was designed to work at +/-75V with 8ohms loads only;

subwo1,
The current in .5ohm resistor at the colector of Q11 shares the colector current of Q6 too, so, it might be right and the absence of the .5ohms resistor at Q12 seems to me not "elegant". Id prefer to put one there, but it seems to me the circuit could work without it.
  Reply With Quote
Old 11th January 2002, 07:58 PM   #10
blmn is offline blmn  Brazil
diyAudio Member
 
blmn's Avatar
 
Join Date: Feb 2001
Location: .
Jeremy

I remember Ive seen in Audioxpress magazine I kind of active limiter that fits your necessity. Maybe someone at the forum could find it since Im not with my magazines available for now.
  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
blowing transistors rtill Solid State 20 5th March 2008 02:17 PM
Using higher rated ouput transistors?? tgferg67 Solid State 11 6th January 2007 06:14 AM
LM4780 - ouput = 4.4v - PB! HELP! aceditnanar Chip Amps 6 3rd July 2006 04:10 PM
Ouput DC lawka Pass Labs 6 15th April 2005 04:31 AM


New To Site? Need Help?

All times are GMT. The time now is 08:01 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