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Old 25th March 2012, 02:38 AM   #1
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Default Protecting the speaker output using a microprocessor

I have been reading a lot on this forum about how to protect the speakers in case the output transistors failed. If this will happen the speakers will be destroyed due to the rails high voltages. So I decided to design a protection system that will protect the speakers from this to happen by using a microprocessor. Another thing that brought me the attention to design it was my LM3886 built. It is running at the max rails voltage of +/- 40 volts. If one of the LM3886 will fail my speakers will be destroyed.

There are some microprocessors that are too expensive for this project so I decided to use the new NANO series from Basic Micro. Their prices start from the 3 dollars to the $12.98. They are programming using the simple basics instructions and also have the floating point. The good advantage of using a microprocessor versus hardware wise is that using the microprocessor all the changes are done in the programming and not in the hardware. You do not have to unsolder and soldered to make the changes. If you wants to change the delay just change the instruction value for the delay. The best NANO for this application is the NANO 8. It is has 8 pins microprocessor with four I/O pins. Enough I/O pins for this application.
For more information for the NANO series check it in the Basic Micro web side.

To build this project I used the NANO 28 that it is a left over from another project. It is an over killed due to too many I/O pins that will not be uses. Right now everything it is running flawless in the testing/debugging mode with no problems. The microprocessor it is programmed to do the following sequencing of event.

Power up sequencing
1– Disable the output speaker relay.
2- Enable the mute (optional).
3- Delay 4 second before enable the speaker output relay to allowed the rail voltages to stabilized.
4- Monitor the speakers output voltages to be within limit (1.0 volts).
5- if voltage is within limit then close the output relay.
6- Delay for 4 seconds to allowed the relay de-bouncing.
7- Disable the mute circuit.
8 –Continuously monitoring the speaker output voltage to be constantly changing or the rails voltage it is not reach. In case of clipping the output relay will be disable.
If the reading is constantly the same for a period of time then immediately disable the output relay, enable the mute circuit and turn on the fault led light indicating the amplifier speaker outputs has a high output voltage and must be corrected.

There are some other extra that can be added but for now just the basics to see how it is work out.

Right now I have one speaker protection system installed in my LM3886 amplifier built by myself. The LM3886 and the speaker protection printed circuit boards were fabricate by ExpressPCB using my printed board circuit layout design.
Attached are some pictures of the two boards and the amplifier. The relay board in the picture is an unfinished board waiting for a TL783 linear regulator to finish it. I will keep it as a spare.
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Old 25th March 2012, 01:33 PM   #2
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It is very important that the cpu can drive the relays to off fast enought. You might considder mosfet "relays".
One way to see if things goes very wrong is to know the max current and have a adc meassure output from an Allegro hall element. To meassure dc in the ac signal is important, but might difficult in software of a small chip.
Nevertheless, You do need several adc inputs per amp.
Another way to do the protection, is the old "analog" way, but having the outputs, from these parts trigge the cpu, and let that control the relays, fanspeed and so on.
The nano chip You are talking about might not be overkill, as You might wanna on several faults seperatly, to do reporting for a pc or equal :-).
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Old 25th March 2012, 03:05 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

The idea seems fair enough but is probably total overkill for a chip amp,
but probably with a bit more work, monitoring output current, it could
possibly replace the SOA protection of a discrete power amplifier.

I find it hard to believe a chip amp is likely to fail in the manner described.

rgds, sreten.
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Old 25th March 2012, 03:06 PM   #4
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I said is an over kill because I was looking for a minimum parts to keep cost down. Right now I am using only two analog inputs and two outputs. One for the relay, one for the mute and two to read the speakers output.This is only for a test to proved that it is possible to used a microprocessor for this application. I am using a SSR for the mute. I may consider your suggestion. using mosfets transistors instead the relay. I will try to research it. It will be easy to implement just built something that plug in the relay socket.
Thank you. I am thinking also to implement to read the heat sink temperature and disable the speaker outputs.
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Old 25th March 2012, 03:12 PM   #5
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The reason for the used of the chip amp is that it is the only amp that I can use to test the design. Normally I used Dynaco 120 that it is capacitor coupled to the speakers. This design will work or can be implemented in any direct speaker driven power amplifier.
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Old 30th March 2012, 06:48 PM   #6
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Default Updated program to include rails voltage drop

Added in the program to check for rails voltage drop to disable speaker output relay

Power up sequencing
1– Disable the output speaker relay.
2- Enable the mute (optional).
3- Delay 4 second before enable the speaker output relay to allowed the rail voltages to stabilized.
4- Monitor the speakers output voltages to be within limit (1.0 volts).
5- if voltage is within limit then close the output relay.
6- Delay for 4 seconds to allowed the relay de-bouncing.
7- Disable the mute circuit.
8 –Continuously monitoring the speaker output voltage to be constantly changing or the rails voltage it is not reach. In case of clipping the output relay will be disable.
9-Check rails voltage drop to disable speaker output relay - gathered it from Puppet threads "Amplifier pwr down "thump" issue" <<<<< Added
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Old 31st March 2012, 03:33 AM   #7
puppet is offline puppet  United States
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You do nice work tauro0221. The chassis is laid out pretty well .. nice and simple!
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Old 31st March 2012, 03:43 AM   #8
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Puppet thank you for your words.
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Old 4th April 2012, 12:14 PM   #9
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I'm developing a "micro" controlled amplifier (based on the P101 of Rod Elliott) too.
The micro (ATMega16) will drive a small 320x240 color LCD but also will control:

- +/- 56V power supply on/off
- soft-start for the (above) PSU with a protection if the bypass relay fails and the "inrush" current limiter resistors get too hot
- PSU overload protection (using 2 ADCs the rail voltages are monitored and if their value get lower than 45V, the micro turns off the PSU)
- DC speaker protection (using 2 ADCs, one for channel)
- output transistor thermal protection (using two simple thermal switches, if one of them closes the micro turns off the PSU)

The LCD will show:

- the current value of the rails voltage
- the active protection
- the output power (RMS)

The circuit (and the method) to measure the output power is still under study...I have two Allegro ACS712-30 (30 Ampere AC/DC) that I could use to measure the current absorbed by the amp, then detracting the quiescent current and multiplying it for the current rail voltage I could calculate (with approximation) the power supplied to the loudspeaker.
Any other idea to measure the output voltage without using a True-RMS to DC converter?
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Old 4th April 2012, 04:36 PM   #10
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Hi,
My design almost it is doing the same thing that your are proposing to use in your design. For the temperature you can use the LM35. The temperature output it is in millivolts proportional to the temperature. For the current I would use a low value precision resistor and read the voltage at both end. Then calculate the current using the ohms law. Since the output it is changing constantly I would use small filter to smooth it. Right now all the values I am reading are dumping to the computer. I have an output to the display no using it now. I am using a led to indicate a fault. My prototype it is working flawless in the power up / down. I simulate a high speaker reading and the relay turn off immediately.

Belief me it is so simple to do it we a microprocessor. All the changes are done software wise and no components to change. One thing that you most be sure it is to have all the inputs that you will need and add some space in the board just in case you need to add some extra components.

Right now I am working in a solid state relay to replace the speaker output relay like somebody suggested.
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