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Old 8th June 2012, 07:46 AM   #1
alibear is offline alibear  United Kingdom
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Default LOUDSPEAKER PROTECTION

Hi all, can anyone advise me on output protection. Is it better to use a DC protection circuit (lots for sale on ebay) or will a suitably rated fuse be good enough. This is for use with a KSA50 Clone.
Any advice much appreciated.
Alan
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Old 8th June 2012, 07:53 AM   #2
syklab is offline syklab  Hong Kong
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Something different in speaker protection.

Protecting the speaker output using a microprocessor
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Old 8th June 2012, 01:24 PM   #3
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Default DC detection relay

Quote:
Originally Posted by alibear View Post
Hi all, can anyone advise me on output protection. Is it better to use a DC protection circuit (lots for sale on ebay) or will a suitably rated fuse be good enough. This is for use with a KSA50 Clone.
Any advice much appreciated.
Alan
This DIY design is original made for +- 45Vdc. It runs with nothing more than 100-200mA, and you can add a 220R/5w resistor or more in series with the plus and minus rails for higher voltages.:

https://picasaweb.google.com/1031761...79135858129058

For stereo layout there have been used two Erni 24V relays, but you can change the layout for any multi-connection 48V relay ( or two 24V in series):

https://picasaweb.google.com/1031761...78685275924978

Besides the DC detection on the amplifier output, the circuit also "feels" on the AC rectifier bridge. If the powertransformer "tilt" to either the plus or minus voltage side the output relays disconnects rapidly.

The circuit has saved all speakers here, and has been used since a KEF B139 burned off more than 25 years ago.

Last edited by Leakstereo20; 8th June 2012 at 01:27 PM.
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Old 8th June 2012, 01:40 PM   #4
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Leakstereo20, thanks for posting the Protection circuit. What is the DC detection threshold of the circuit you posted? Thanks.
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Old 8th June 2012, 02:18 PM   #5
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Quote:
Originally Posted by Samuel Jayaraj View Post
Leakstereo20, thanks for posting the Protection circuit. What is the DC detection threshold of the circuit you posted? Thanks.
The detection threshold is +- 8V DC, and the delay time is approx. 3 sec. . The delay time can be increased by use of a higher value of the C20 capasitor.

Check the circuit with a 100K resistor from either + or - rails to the plus speaker terminal on the output relay. The protection circuit should disconnect rapidly.

Rgds.
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Old 9th June 2012, 12:14 PM   #6
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Thanks. Isn't a DC detection threshold of + - 8V DC a little high. Voltages below that threshold would destroy delicate FR speakers and small woofers even before the protection is triggered.

I saw some Elektor design achieving + - 600 mV DC threshold but am not able to get hold of the schematic now.
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Old 9th June 2012, 01:17 PM   #7
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Yes, I also agree +/-8v DC is high too. You need less than 1v so as not damage speakers.
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Old 9th June 2012, 01:37 PM   #8
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8V is fine. The reason for DC on the output is that the output devices fail, in which case the full rail voltage is applied to the output.
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Old 9th June 2012, 04:08 PM   #9
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Quote:
Originally Posted by richie00boy View Post
8V is fine. The reason for DC on the output is that the output devices fail, in which case the full rail voltage is applied to the output.
Yes, that was the basic idea. If something goes wrong and the chip melts, which in many cases is the PNP , the full rail is applied to the output as mentioned by Richie.

Even, + - 8Vdc applied in a longer period of time, say minutes, will be in need of a lot of amperes before being able to melt the voicecoil of a modern woofer...I think. The crossover seriel capasitor blocks for DC voltage to midrange and tweeter.
If something goes wrong inside a poweramplifier there are normally a higher voltage value applied to the output than the mentioned +-8V dc.

Rgds

Last edited by Leakstereo20; 9th June 2012 at 04:28 PM.
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Old 9th June 2012, 07:28 PM   #10
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Default Change of threshold voltage

Quote:
Originally Posted by Samuel Jayaraj View Post
Thanks. Isn't a DC detection threshold of + - 8V DC a little high. Voltages below that threshold would destroy delicate FR speakers and small woofers even before the protection is triggered.

I saw some Elektor design achieving + - 600 mV DC threshold but am not able to get hold of the schematic now.
It might be possible lowering the sensor resistor values from 220K, in order to get lower threshold DC voltage values...say from the 600mV you suggested to 1V DC. This have not been on the testbench, and have to be checked.

rgds.
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