VI/ SOA limiter for push pull amplifier.

[diy didi]

VI limiter/ SOA protection For push pull Amps.
Hi all
I have recently been getting a couple of Ashdown MAG300 bass amps with blown output stages.
All due to either shorted speaker cables or soft shorted speaker voice coils. .
I recently built a speaker protect circuit based around the uPC1237 (photo), but did not make use of pin 1 that has the overload circuitry, as I’m not sure exactly how to use it.
Anybody have a circuit or idea how to add a VI / SOA limiter to these or similar push pull amplifiers. ??
Thank you all in advance.

 

[ronsimpson]

I am very short of information.
What information do you have on a uPC1237? Can you share?

 

[alec_t]

There is a typical application circuit in the datasheet.

 

[ronsimpson]

Sorry, I meant to ask about the "VI / SOA limiter" not the amp. There are many schematics on the internet.

 

[rjenkinsgb]

I have one of those, the single 15" version - I've not managed to blow it up yet, happily!

There is a schematic here, if you don't have one:

https://schems.com/bmampscom/ashdown/Ashdown_MAG300_schem.pdf

The amp itself has no facility for current sensing. It would need either a low value resistor as a current shunt in series with the amp output (wasting power), or better a hall-effect current transducer with the output cable just threaded through it.

They typically work from DC to tens if not hundreds of KHz, so one of those feeding an op amp active precision rec would give a good current reading with no effect on the amp output.
Scale that, possibly with some short time constant filtering to avoid false trips & you should have a signal to feed the IC.


Personally, I'd probably just try a semiconductor rated fuse (FF or uR type) in line with the speaker output, rather than have the electronics & see if that worked OK. I can't remember the rated impedance offhand, but I'd estimate 16A should be suitable fr 4 Ohms, or 12A for 8 Ohms.