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Old 20th May 2010, 08:58 AM   #31
dangus is offline dangus  Canada
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I'm pretty sure I spotted a chip that had some on-chip debugging interface (like JTAG) while browsing the Digikey catalog. Probably an Atmel or PIC. JTAG interfaces are cheap to make or buy.

I thought I'd seen an ad for some chip that provided SOA protection, but I guess I was thinking of NS's SPiKe circuit that's used in the chip amps. AN-898
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Old 20th May 2010, 01:54 PM   #32
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Quote:
Originally Posted by Jan Dupont View Post
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
Crown Audio's ODEP works that way, well in a more elaborate way
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Old 20th May 2010, 02:57 PM   #33
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Yes and some (many) others
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Old 20th May 2010, 05:12 PM   #34
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Here is something I knocked together that works but can be refined. It measures the maximum voltage swing that you may want to allow across the speaker. The voltage would be more or less proportional to the current although the current may lag or lead the voltage somewhat, but you can set it to a level that is meaningful.

I used two op-amps in a threshold detection mode. The one op-amp triggers from a negative half cycle and the other from a positive. The potentiometer would set both op amps thresholds to the same but opposite value, thus they would be triggered symmetrically.

The outputs of the op-amps are OR by the two diodes D5 & D5. If the transistor Q1 is triggered by either op-amps it conducts and turns the relay on and disconnects the speakers, while Q2 latches Q1 and to reset the protection circuit the power has to be turned off.

Obviously this will not protect the amp against short circuit but will protect your speakers from a fault in the amp.

One could toy around with the previous idea of measuring the current through a series resistor and thereby disconnect the load if the amp is short circuit.

Anyway my ten cents worth.

Regards

Nico
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Old 20th May 2010, 11:55 PM   #35
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I understand what Sakis means, pro audio amplifiers should work even on a "partially" shorted load, well as long there is still a usable winding on the speaker's voice coil. You cannot afford an amplifier cutting in and out of protection/muting on a live gig, that would be embarrassing.

Some pro audio amplifier uses thermistor mounted on the main heatsink to reduce the bias level of the output stage, probably from class AB level to class B level. Some designs employs a thermistor on the bias network of the transistor that "clamps" the VAS stage to reduce the drive when the heatsink is hot.
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Old 21st May 2010, 12:00 AM   #36
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A very simple VI limiter can use a zener from the gate of the MOSFET to the output.
This limits the gate drive the harder it is driven.
The voltage across the source resistor goes and the so the zener forces less voltage to the gate.
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Old 21st May 2010, 06:10 PM   #37
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For serious pro audio where the show must go an reguardless of cost, Peavey has a PA management system that integrates multiple amps and multiple speakers, with a console where the sound man can send the sound from different amps to different speakers if there is a problem. Thousands of dollars. Uses stakon connectors, probably more transistor friendly than 1/4" phono plug that shorts almost every time it is inserted or tripped over and pulled out. Presumably the performance speakers are tested out before the amp is ever connected to them. As a home user, I'm just trying to avoid blowing a $250 set of speaker drivers if a $10 transistor goes out. Or avoid blowing up my amp if the dog yanks on the speaker wire and shorts the spade lug to the chassis.
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Old 21st May 2010, 06:21 PM   #38
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we are neither on the one nor the other side ....i haven seen amps in my place the latest 25 years that use 1/4" output plugs .... all of them uses speakon ...

then again non of us is ever going to come up with a multi thousand dollar sound management thingy ... including me that actually been looking to something more "earthy " protection enough without too much messing up with the signal ....
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Old 23rd May 2010, 11:15 PM   #39
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Uh, The Peavey CS800S amp I have that I bought but I've been afraid to use was built in 1998. The schematic of the Peavey CS800 I have been referring to in this thread, that I got by searching diyaudio, is a revision B dated 1976. The revision B has a diac-triac speaker voltage clamp, and no speaker fuses or disconnect relays. The CS800S that I bought has a "DDT" switch on the front panel that is not on the schematic. I bet one "D" stands for dynamic. Does anybody have a schematic for a Peavey CS800S? I might have a protection circuit as good as the Crown whose schematic was posted on a previous protection thread as being worth of emulation. (The crown amp had a PIC16C57 IC deciding what to do with the temperature and other inputs, that I assumed was a microprocessor because it had a clock input).
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Last edited by indianajo; 23rd May 2010 at 11:18 PM. Reason: spelling,grammer
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Old 24th May 2010, 10:09 AM   #40
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here is some thing i found out have a look everybody



Click the image to open in full size.



regards
aniket
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