SOA protection - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 19th January 2006, 03:45 PM   #1
diyAudio Member
Join Date: Dec 2005
Location: Kuala Lumpur
Default SOA protection

As TI now do cheap 4 channel 10 bit A/Ds which can sample at MS/s, what do you think about a SOA protection circuit that measures both voltage and current of both output devices. A 8 bit PIC or AVR cpu should be able to do the processing and control a mute and a speaker protection relay. The voltage signals would also be usable for tweeter protection and dc fault detection. I would use one A/D and one cpu per channel to avoid routing the input signal too far.

It should be possible to follow the manufacturers SOA curves much more accurately than the usual analogue circuits.
  Reply With Quote
Old 19th January 2006, 03:56 PM   #2
diyAudio Member
Workhorse's Avatar
Join Date: Dec 2004
In case of Reactive loads...Muting isn't a viable solution..because there is still finite dissipation occurs in corresponding output devices due to V-I phase Lead/Lag....which could damage the devices...
  Reply With Quote
Old 19th January 2006, 04:02 PM   #3
poobah is offline poobah  United States
diyAudio Member
poobah's Avatar
Join Date: Nov 2005
It has been my experience that unless you have forced cooling methods, excellent thermal engineering, and very well defined waveforms, you should not operate a device anywhere near its SOA.

Max rated current / 3 - 4 will make things that don't melt.

  Reply With Quote
Old 20th January 2006, 12:22 AM   #4
diyAudio Member
Join Date: Dec 2005
Location: Kuala Lumpur
My experience is that bipolar amps either:
1) have very conservative and intrusive SOA that can get into real trouble with reactive loads
2) have overkill multiple output devices in parallel with the cost issues
3) blow output devices

Judging from the postings in this forum (3) is way to common

Checking against the VI curve and talking into account the time would allow you to be certain that you are safely inside the SOA limits, instead of the "rule of thumb" limits which make bipolars usable at only about half of the rated voltage.
  Reply With Quote
Old 20th January 2006, 12:38 AM   #5
poobah is offline poobah  United States
diyAudio Member
poobah's Avatar
Join Date: Nov 2005
Guess you didn't really read my post...

It's takes a lot of engineering, and TESTING, to run devices, especially with wildly undefined waveforms at there maximum.

For a "one off" DIY project it is probably not worth the hassle. For a design to be built in the thousands it is worth the hassle.

And in the the end, when you have maximized device usage, what you have built is a modern TV set... 6-8 years and hurl it in the trash.

  Reply With Quote
Old 20th January 2006, 12:55 AM   #6
CBS240 is offline CBS240  United States
diyAudio Member
CBS240's Avatar
Join Date: Aug 2005
Location: K-town
It is best with audio to design via the DC SOA curve to be safe. The problem is that it is way to easy to get fake devices and because of the cheaper, smaller die, the SOA goes out the window usually as smoke. BJT's generally can't handle much current with a high Vce. This makes them more likely to become toast with highly reactive loads then FET's.(with the exception of genuine MJ21194/5) Personally I like BJT's, but you have to observe the limitations.
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 20th January 2006, 07:44 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
on a recent thread the designer suggested using the 10mS SOAR as the limiting factor for music based loadings.

This invites trouble.

As an alternative, I have used DC or one second SOAR and then applied a cap to allow the passage of very short term peak currents that would normally exceed the long term SOAR.
This applies to both BJT and FET output stages into reactive loads (usually a theoretical 60degree phase angle).

My main difficulty is identifying the cap value that does not permit damage to the output stage.
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 20th January 2006, 01:14 PM   #8
diyAudio Member
Join Date: Dec 2005
Location: Kuala Lumpur
Using the 10mS SOA for music is reckless, the lowest impedance of most reflex speakers is below 100Hz and the reflex tuning resonance leads to the the worst phase shifts below 100Hz too. Then play music with a lot of bass and you are going to get overload.

What I am trying to propose is using a digital method to constantly check that the devices are always inside the SOA curve by a safety margin of say 30%, not just guessing. So long as the devices are genuine, I trust reputable transistor makers to make a reliable product that meet their specs.
  Reply With Quote
Old 20th January 2006, 01:44 PM   #9
diyAudio Member
Join Date: Nov 2003
Location: Genova, Italy
In case of Reactive loads...
Hi all

Can any of you suggest a test reactive load (and the frequency
of the test signal ) to be used during simulation stage to verify SOA limits are not trespassed?

  Reply With Quote
Old 20th January 2006, 03:26 PM   #10
diyAudio Member
JensRasmussen's Avatar
Join Date: Jul 2002
Location: Denmark - Jutland
Send a message via ICQ to JensRasmussen Send a message via MSN to JensRasmussen
My two cents:

I think it is most important to define what the amp is going to be used for:

1) Active speakers (No passive X-over to worrie about)
2) 8 Ohm passive speaker
3) 4 Ohm passive speaker
4) 2 Ohm passive speaker

Case 1 and 2 are less of a problem, 3 and 4 are hard!

If you know the load it is possible to design the SOA protection to match this.

If you are unsure what the amp is going to be used for (This is often the case) you have to be conservative in your protection levle and maybe even expect case 4.

From Rods pages:
For the 3 ohm case, a reactive load (at 45 phase angle) can be simulated by using a 477uH inductor in series with the 3 ohm resistor (for a frequency of 1kHz)

  Reply With Quote


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 NAD amp! Goes in protection silversweden Car Audio 43 28th January 2014 05:44 PM
DC-protection? Skorpio Solid State 4 1st December 2007 09:09 AM
speaker protection (OR) overload protection myanmar Solid State 7 13th July 2006 08:21 AM
protection Dj BASS AMP Solid State 7 26th March 2003 09:53 AM

New To Site? Need Help?

All times are GMT. The time now is 06:28 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2