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Old 19th May 2010, 06:49 AM   #21
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by phoenix87 View Post
can u show me how to do wat u suggested ................ i will be trying it on my own a bit of suggestion would be very help full .....................


regards
aniket
If you really want to measure the current and use that as a means of tripping a relay then swap the position of the sense resistor and speaker in the circuit.
As it is you have the possibility of applying signals outside the 741's range (it's supplies). Swapping them round still gives the same volt drop to work with, so design a comparator etc to work with it.

Another idea (and these are just ideas for you... I wouldn't go down this line of protection) is to use opto isolators across the emmiter resistors in the amp. Excess current causes LED part to light. Then you use that as a control signal to a relay driver etc.

The trouble with this idea is that any DC current below the trip level is classed by your circuit as acceptable... and it's not... a few amps (much less in many cases) will destroy a speaker, but your circuit would say that was OK and a valid current.
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Old 19th May 2010, 09:43 AM   #22
dangus is offline dangus  Canada
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Quote:
Originally Posted by nigelwright7557 View Post
Wow thats a lot of components.
I can do a clip led with 5 small cheap components including the LED !
But, will it warn at -3 dB, detect clipping of either polarity and stretch brief clips to make them visible?

A version with fewer components from Rod Elliott:
Power Amplifier Clipping Indicator

There was the "Bilateral Clipping Indicator" in Audio Amateur (1975); that lacked a pulse stretcher, but it did use very few components.
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Old 19th May 2010, 10:56 AM   #23
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Wink Short circuit

Quote:
Originally Posted by phoenix87 View Post
protection in amps is a very good thing but many dont like them ...... but neways i recently designed a 500W @ 8E amp which is going for the PA use so i would like to use a shortcircuit protection for my amp ...... All u learned people can u help me with this...... Cause a PA amp does nt need to have very good sonics just average sonics with heavy protection is needed.... The protection scheme can be very simple ....

regards
phoenix

Its Clear that he needs a MAJOR thing in that protection circuit, witch is the SHORT CIRCUIT PROTECTION.

I have no idea, if any one from all of you, DEVELOPED a protection circuit, THAT Disconnects the (Load) in case of COMPLETE short circuit at the amplifiers output, while NOT blowing up the amplifier. even for 1 hour

FOX
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Old 19th May 2010, 12:57 PM   #24
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A small contribution...
redesign of leach amp pcb for integrated TO-247 output devices
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Old 19th May 2010, 03:58 PM   #25
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sakis my dear Greek friend
Concider a modified McIntosh protection circuit......
When max output power is reached, the circuit aut. lowers the input to the VAS stage. Nice and easy and no distortion
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Old 19th May 2010, 04:48 PM   #26
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Hello Jan ... how are you

fine idea ??? never seen that before ... can you point me to any schematic ???
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Old 19th May 2010, 05:22 PM   #27
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Protection circuits and pcbs you can find here:
100W Ultimate Fidelity Amplifier
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Old 19th May 2010, 05:33 PM   #28
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sakis :-)
Fine but busy like you
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Old 19th May 2010, 08:22 PM   #29
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any schematic help in modifing my circuit????????????????//



regards
aniket
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Old 20th May 2010, 04:06 AM   #30
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Default series resistor protection

I am not qualified to criticize the more complicated protection circuits, but I can contribute to Phoenix87's circuit. The measurement of DC overcurrent is interesting but much less sophisticated than the VI limit implemented with microprocessors discussed in the early part of this thread. Transistors have a "Safe Operating Area" that allows them to put out a lot of voltage for a little while, or a lot of current for a little while, but not both simultaneously. If your music source is the cannon shot of 1812 Orchestra (see music thread how to test your system recently) then you want to allow that current - which is a very short time. If your music source is solo Japanese flute, you have the transistor putting out almost no current while holding off the full rail voltage. Both conditions are okay. But if your speaker wire is shorted or crossover cap blown, then there is too much current for too much time. The mathmatics of the SOA model is so complicated that it is best implemented in software on a microprocessor, but MP development systems, programming software, and PROM burners are expensive accessories for a one off home project. Instead of the series resistor of Phoenix87's diagram, I would vote for current transformers (of which I have a couple in blown up motor drives) or as previously suggested, linear transfer opto isolators. However, once you know the output current, you have to integrate it over time and compare against a history curve of what has happened previously - mathematics that is difficult to do with discretes or op amps. Then if the math says bad, what are you going to do? Mr Sakis superrepairman says opening the output relays causes problems -I am suspicious myself of sudden changes in load impedance. Mr. Peavey in the CS800 has opted to crowbar the output with a triac and let the power supply fuse blow. That probably limits speaker damage if you use his 500 watt music power speakers, and limits damage to the amp, but is a one time event. I was hoping an early responder would point out to me a microprocessor chip and development system including software and PROM blower and integrated A/D function that is under $300 these days. But that is probably a yellow brick road type of situation.
ApexAudio, I see you are monitoring heat sink temperature to turn on the fan and then pull the speaker relays. DJoffe & I did something similar on the ST120 modification he came up with, only we cut the output tsistor bias current in half instead of pulling the speaker relays. I wonder if heat sink temperature lags the real problem, which is transistor die temperature (predicted by SOA math). I don't have the experience to know if the heat sink mostly heats up before the output transistors blow.
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Last edited by indianajo; 20th May 2010 at 04:17 AM. Reason: discuss apex
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