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Class D Switching Power Amplifiers and Power D/A conversion

DC speaker protection for Sure class-D amps?..
DC speaker protection for Sure class-D amps?..
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Old 4th March 2010, 09:51 AM   #21
OzOnE_2k3 is offline OzOnE_2k3  United Kingdom
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Join Date: Jul 2005
Location: Devon

As long as you check that the protection module doesn't have one input connected to circuit ground of course.

If you find a module which could work, please let me know. There are very few circuits that I've seen which will work for bridged amps (apart from the upc1237 chip ones, but it's a bit old now.)

I did think about only testing one output pin with the other pin of the DC prot circuit connected to ground. One scenario which could happen with this though is that the pin you're testing shorts to ground (output H-Bridge shorted), but the other pin stays at idle (12V), or worse (24V)!

I've just ordered a couple of diode bridges to try the original circuit. When testing it before, it worked fine for triggering on low frequencies (<3Hz or whatever it's set to), but it wasn't very sensistive to low voltage DC <4V without it false triggering at high listening volumes.

I might still use a PIC chip if it can be made very reliable. Basically, it just needs a diode bridge on the speaker outputs (after some resistors) to work as a zero-crossing detector. The PIC will then trigger the protection if the signal stays non-zero for too long. Perhaps some simple 555 chips could be used to time the non-zero signal instead?

If you have multiple amp channels, a good idea might be to allow the "protect" signal from any channel to disable all relays at once under a fault condition. This is what I hope to do at some point.

Ideally I want to put LED indicators on the protect board and have the board separately powered. This way, if one specific channel fails, the protect board will disable all relays (and switch off the amp's PSU), but also keep an LED lit which shows the channel which triggered the fault. This might give you a chance to test that amp channel for electrical or visible damage before powering up again (might save destroying other parts of the amp).

Anyway, just some thoughts.


Last edited by OzOnE_2k3; 4th March 2010 at 09:54 AM.
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Old 4th March 2010, 10:52 PM   #22
portreathbeach is offline portreathbeach  United Kingdom
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Join Date: Jan 2010
Location: Cornwall
Originally Posted by nigelwright7557 View Post
I used a small cheap PIC for DC detect.
I just dropped the voltage using two resistors into the PIC i/o one pair for +ve phase and anotehr set for the -ve phase.
If the output stays >20 volts or < -20volts for 500mS then I drop the relay.
Using the PIC also means I can have a 3 second anti thump power up delay.
I use this circuit on my two disco amps and never blown a speaker due to DC.
I accidentally shorted the output once and blew the output transistor4s but the DC protect jumped in to save the speakers.
Please could you explain a bit more about your PIC circuit. I am building a Gainclone amp and pre-amp and am using a PIC to control the pre-amp selector and volume via an IR remote. I would like to have DC speaker protection so as not to blow my speakers incase of a amp failure. I think a PIC in the amp enclosure taking care of the DC offset would be a great way to protect my speakers.

How do you get your 5V to power the PIC, and how exactly do you wire the amps output to the PIC?

One of the speaker terminals for the Gainclone is Power Ground, so would I have to use back to back diodes (in parallel) from the other terminal then through a resistor divider network and tap of that into an input in my PIC and then sense when the value is 'logic high' for a set time?

Thanks in advance

Last edited by portreathbeach; 4th March 2010 at 10:56 PM.
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Old 3rd April 2010, 06:35 PM   #23
sendler is offline sendler  United States
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Join Date: Aug 2004
Location: ny
Default Off the shelf

I finally took the time to puzzle it out. An off the shelf dc protection module should work if it has it's own supply which is independent of the power amp's. Place two resistors in series across the speaker outs and tap the center for the connection to the protection module's ground. The sense will connect to either one of the speaker terminals as is normal. The trip voltage will have to be twice normal but it should work other than that.
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Old 12th April 2010, 02:43 AM   #24
sendler is offline sendler  United States
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Location: ny
Default divider

Cristi gave me another way to set up the divider. Just set the series pair of resistors across the voltage supply of the amp and trim until the center tap creates a voltage that is equal to the offset of the output leg that you want to sense and tie this to the otherwise floating ground of the separate power supply of the speaker protection module.
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