When driving low impedance loads, what is the killer? - Prevention? - 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 3rd April 2009, 04:16 PM   #1
CBRworm is offline CBRworm  United States
diyAudio Member
 
Join Date: Jan 2007
Location: S. Florida
Default When driving low impedance loads, what is the killer? - Prevention?

Frequently there are speakers mentioned that are labeled as 'amp killers'. I own a couple pairs of these speakers, although certainly not the worst offenders.

My question is - what kills the amplifier, is it prolonged high volume music at low impedance (seems likely as heat would build quickly). What about transients - like a drum solo at loud volume that the protection never engages? How to prevent this besides the obvious replacement of speakers.

I understand that current multiplies as resistance drops, my question can not be answered simply by 'current kills amps' - if that's the answer, what's the mechanism.

Before I knew any better I used to run a pair of Infinity Quantum 5's (watkins 12" woofer) and a pair of RSIIIa's together at extremely loud volumes (live levels) during drum solos - both of these speakers operate between 2 and 4 ohms through much of their range. I did this using both a pioneer SX-1250 and an SX-1050. I have never (yet) had an amplifier failure - although I regularly push the equipment to its limits. My guideline used to be to turn the volume down a hair when the protection circuitry engaged. I have recently added an Adcom 555II (which I did kill with the same speakers, then repaired) into the mix to take the load off but I am curious which is more damaging to the amp - long term high power output which the protection circuitry will cut short, or instantaneous high current which the protect seems to ignore (is this by design?).

I know the devices derate with heat - does that imply that they can drive much higher currents for short periods of time as long as temperature is controlled (more heatsink area, better interface?)

Is it safe to assume that the output transistors are the weakest link in the chain excluding the power supply? More so with 30 year old transistors?

It seems that the common failure mode is the output transistors fry and take the drivers with them, correct? If the output transistors are replaced with a modern higher current device (MJ21193/94) that doesn't cause instability, will that add safe operating area to the entire amp, possibly protecting the drivers as well? or do you then end up overstressing the driver transistors?


My untrained brain thinks that by substituting a higher power transistor that is known to work well in this application the primary failure point should be eliminated and my fears shall be lessened. It also seems that the factory protection would protect while the outputs still had lots of headroom left adding even more protection.


I don't have any plans to ever go back to powering a wall of speakers with a reciever but I would like to be able to drive it to its limits with one set of speakers without fear of damage to the amp. And those limits would be with lower impedance speakers than the amp was perhaps designed for.

Both of the receivers mentioned above are still in daily use and both have been fully refurbed within the last few years with all new caps, new power supply transistors and zeners, new protection relays, transistors and diodes. In the case of the 1050 it was mostly preventative after a protect relay failure and the relay driver transistor failure, the 1250 lost the 24 volt power supply and as such I fixed that problem then rebuilt the rest of the board with modern components.

I prefer using the internal amplifiers 90% of the time, due at least partially to the speaker switches and protection circuitry to keep the speakers from melting in case of an amp failure.
  Reply With Quote
Old 3rd April 2009, 05:02 PM   #2
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Albany , NY (smallbany)
You are right on most of your assumptions. The term you are looking for is SOA (safe operating area) of the output devices.

You mentioned the famous mj21193/4 , they have among
the highest SOA of all the outputs.

But , that might also throw the current limiting circuitry
out of spec. What I am saying is that device substitution
is a little more involved than swapping out to a larger
device.
The 3 mechanisms are current gain , SOA , and the voltage rating
of the device.
You are right that SOA is dependant on temperature (A bigger
HS )

For example , if you change a device with a current gain
of 25 for one with 50+ , your bias will increase making for
a hot amp.
It is not like replacing a light bulb.
OS
  Reply With Quote
Old 3rd April 2009, 05:15 PM   #3
CBRworm is offline CBRworm  United States
diyAudio Member
 
Join Date: Jan 2007
Location: S. Florida
I can adjust the base bias to match the gain, I wonder if the bias thermal tracking would be different.
  Reply With Quote
Old 3rd April 2009, 05:46 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
some manufacturers undersize the PSU to increase their profit margin.
The effect of this is that the undersized PSU cannot delivery sustained power into an overloaded output stage.
The sagging voltage has very effectively protected the amplifier from catastrophic failure.
__________________
regards Andrew T.
  Reply With Quote
Old 3rd April 2009, 06:05 PM   #5
CBRworm is offline CBRworm  United States
diyAudio Member
 
Join Date: Jan 2007
Location: S. Florida
In this case the power supply seems to be able to source a good amount of current. It is a large torroidial transformer w/ 44,000uf per channel capacitance (2x(2x22,000)) and seperate rectifiers for each channel.

This receiver is (was) rated as 165wpc @ 8 ohms, 200 @ 4 ohms <.1% THD. It weighs 65 pounds. I am not looking to increase power output, just make it bulletproof. Currently I measure ~400 watt peaks with music (drums) into a 4 ohm load on both channels before clipping is visible on the scope (114v pk-pk).

It seems that it will put out well over its rated power almost indefinitely at 4 ohms - I assume that eventually the heatsinks will not be able to deal with the heat, or maybe eventually it will short circuit and die.
  Reply With Quote
Old 4th April 2009, 07:29 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
is that the Adcom you have described?
__________________
regards Andrew T.
  Reply With Quote
Old 4th April 2009, 10:11 PM   #7
CBRworm is offline CBRworm  United States
diyAudio Member
 
Join Date: Jan 2007
Location: S. Florida
No, this is the old Pioneer receiver. They measure pretty similarly at 8 ohms, the adcom has an advantage maintaining output voltage at 4 ohms - below that the Pioneer checks out (as it is designed to). I suspect the adcom would drive a dead short for a little while before failure while the pioneer is probably teetering on the brink of destruction at 3.9 ohms.

The adcom works perfectly but has no protection. 90% of the time I don't need the headroom of the adcom and would prefer to have the speakers protected against amp failure, and not have the heat of the adcom.

Although in reality the speaker pieces are still available on e-bay, but I have gone this far without damaging them . . .
  Reply With Quote
Old 5th April 2009, 10:57 PM   #8
OMNIFEX is offline OMNIFEX  Jamaica
diyAudio Member
 
OMNIFEX's Avatar
 
Join Date: Mar 2003
Location: Locked Up In The Amp Rack
Quote:
Originally posted by CBRworm
In this case the power supply seems to be able to source a good amount of current. It is a large torroidial transformer w/ 44,000uf per channel capacitance (2x(2x22,000)) and seperate rectifiers for each channel.

This receiver is (was) rated as 165wpc @ 8 ohms, 200 @ 4 ohms <.1% THD. It weighs 65 pounds. I am not looking to increase power output, just make it bulletproof.
Hi

The power supply (Transformer) is the weakest link. You only gain 0.8 dB going from 8 ohms (165 watts) to 4 ohms (200 watts) per channel.

If the power supply were robust, it would deliver 165 watts in an 8-ohm load and, 330 watts per channel @ 4 ohms. That would equate to a 3 dB gain.

Cheers!
__________________
OMNIFEX
  Reply With Quote
Old 5th April 2009, 11:17 PM   #9
Account disabled at member's request
 
Join Date: Oct 2008
Quote:
Originally posted by OMNIFEX


If the power supply were robust, it would deliver 165 watts in an 8-ohm load and, 330 watts per channel @ 4 ohms. That would equate to a 3 dB gain.

That is not possible even in theory.
  Reply With Quote
Old 5th April 2009, 11:28 PM   #10
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Hi

One thing that can kill an amp is a very reactive speaker. At some frequencies the impedance can become very low and/or exhibit a phase change. This change in phase of current wrt voltage can be very hard on outputs, BJT's especially. On a resistive load, the maximum current flows when the voltage is at maximum. Thus there is minimum voltage left across the output transistor for that period of time. But if the phase of the speaker impedance is 45 degrees (could even be greater than that) then the maximum current flows when the outputs have half (or more) of the rail voltage across it for that period of time. Since P=V*I, the instantaneous power in heat dissipated by the die of the transistor is very high and can exceed the de-rated SOA........then POOF the transistor will melt.
__________________
All the trouble I've ever been in started out as fun......
  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
Behringer DEQ ok driving multiple loads brucemck2 Subwoofers 0 18th January 2006 09:49 PM
Driving Low Impedance Loads weinstro Solid State 14 8th May 2005 09:53 PM
Gainclone driving inductive loads diyman Chip Amps 1 12th April 2004 09:53 AM
Zen driving high impedance loads supa-roo Pass Labs 2 18th April 2003 07:23 PM
Best amp choice for driving NASTY speaker loads stokessd Pass Labs 8 25th March 2003 11:32 PM


New To Site? Need Help?

All times are GMT. The time now is 12:58 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