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Old 17th October 2012, 05:02 PM   #1
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Default An ultimate amp protection circuit ?

I had this idea around 1975.
....
Sorry, i leave this forum, so if you are interested in this circuit, please, consult my web site.
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Old 17th October 2012, 05:13 PM   #2
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Apart the fact that it detect all kind of problems, reason why i call-it ultimate, this idea brings a big advantage.
It fire the protection quasi instant (µs), with not the additional long delay needed by the integrator stages that usual protection circuits use, by habit, to detect DC.
Instant even in case of short circuit, and before your powe amp devices suffer. For the same reason, it can protect your fragile tweeters against very low DC levels.

This thread is opened to be a cooperative one; All of you who want to collaborate, bring ameliorations, printed board designs, experience returns etc... are highly welcomed.

Hope it will help some of you, folks
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Old 18th October 2012, 12:30 AM   #3
RNMarsh is offline RNMarsh  United States
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Such an idea (comparator) can be applied using the amps own internal error correction port, as well. amplify and rectify it and drive relay/disconnect.
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Old 18th October 2012, 01:00 AM   #4
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Separate relay drivers for left and right channels might allow the amp to finish a gig/concert with degraded sound - one channel.
Some amps, like the CS800s and PV 1.3k amps I am working on, have variable gain to allow for different inputs. The detection circuit couldn't cope with that without modification.
Relay drive circuit currents, and signal ground return currents, should be separated for less noise at end of power up reset. I prescribe an opto isolator or a coupling transformer for AC relays.
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Old 18th October 2012, 01:23 AM   #5
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Quote:
Originally Posted by RNMarsh View Post
Such an idea (comparator) can be applied using the amps own internal error correction port, as well. amplify and rectify it and drive relay/disconnect.
You are right, mr Marsh, of course. (Thanks for your interest).
Of course i had tough to that.

The major advantage would be we do not need to tune any level to compensate the gain factor of the amp.

There are several reasons why i chosed my solution.
- The virtual point where the two signals (input and feedback) are mixed together in the amp is very sensitive to any parasitic cap. (Like in the bus of a mixing console). Cabling a wire there to go to the protection board can deteriorate the amp performance or stability. Specially if your amp goes up to several Mhz.
- To find this point can be difficult for people with little electronic understanding.
- We loose the protection against DC in the input, as long as the amp can amplify-it before clipping.
-The last reason is, as i mentioned-it, i wanted this circuit to fit any existing amp, without any internal modification.

On a new amp project, it would be easy to add a single transistor, like an emitter follower directly on the board, to isolate the amp from any influence of this cable, saving the op Amp comparators. But, again, we lose the DC protection of the input. It would be OK for an input cap protected amp.
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Old 18th October 2012, 01:24 AM   #6
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I just use a simple PIC micro to monitor the output voltage.
If it goes above/below 20 volts for greater than 500ms it immediately shuts down the output relay.
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Old 18th October 2012, 01:45 AM   #7
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Quote:
Originally Posted by indianajo View Post
Separate relay drivers for left and right channels might allow the amp to finish a gig/concert with degraded sound - one channel.

Relay drive circuit currents, and signal ground return currents, should be separated for less noise at end of power up reset. I prescribe an opto isolator or a coupling transformer for AC relays.
As the circuit is unique for the two channels, it will cut the 2 channels in the same time. You are right. Better for an home amp. Easy to separate in two, if you prefer.
I used this idea when i was at the head of a big PA system rental company (Audio Analysts) and it had saved our life's amps several time with no negative effects. We used more than one amp for each way (sub bass, bass, middle, high middle, treble etc) so it was not a big problem.

I have to said that, each time the protection fired, the problem was so important that it was no way to continue the show (Orage with huge water fall in the Altec horns, cut of the HP cables before the show etc...)
Of course, for PA, you have to set the protection with enough margin that it will not fire at the littlest clipping.

About powering of the relays, this circuit is powered by a separate power supply from the amp, and has just a ground floating point in common with the amp itself. (no current).
In my home amp, there is never the slightest clic, whatever you do, from power on to power off, and even when you fire the protection at any level.
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Old 18th October 2012, 01:53 AM   #8
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Quote:
Originally Posted by nigelwright7557 View Post
If it goes above/below 20 volts for greater than 500ms it immediately shuts down the output relay.
You can adjust this protection to fire in < 0.1ms if any DC as low as 100mv.
How can your Pic detect amp HF oscillations dangerous for your tweeters ?
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Old 18th October 2012, 02:20 AM   #9
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I would like to add that this protection can detect any amp clipping and fire at the first kick drum, depending of the margin you set. Tune-it according to your needs.

May ask you, please, to do not comment endless the "so calling" inconvenience of the serial relay or/and paralleled solid state switch and it supposed sonic degradation.
(I'm unable to hear the difference with a wire in my high end system)
You are free to use this idea to any other solution (power supply rails cut, direct action inside a regulated power supply etc...) if you believe that it is better.
Just remember that, if any big cap after the relays in the rails, they will continue to discharge in your speakers if any power device short circuit, after protection had fired. Chose your poison.

This solution has been chosen to be 'universal' and able to be adapted outside an existing amp with no mod.
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Old 18th October 2012, 02:26 AM   #10
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Hi Esperado
greetings any pcb for this project to share
warm regards
andrew lebon
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