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JBL 27th August 2001 01:30 PM


I'm building some amplifier for myself and i want to add some sort of short circuit protection. Does anybody got an idea. (it's a bjt amp)

PassFan 28th August 2001 11:38 PM

How about a resetable circuit breaker:)

haldor 29th August 2001 12:26 AM

The idea of using a circuit breaker is good from a safety standpoint (helps prevent fires and such), but don't expect this to save the amp itself from damage.

Some designs use fuses in the output signal path, but this often won't protect your output devices from damage during a short and can cause audible distortion. You absolutely do want fuses in your +/- DC supply (to protect the transformer from a shorted output transistor). I buy power transformers that have a thermal cutout switch built into them to make sure that expensive hunk of iron doesn't melt down if I really mess up.

The most reliable way to protect against a shorted load is to use what is called foldback current limiting. What happens is that the amp senses the voltage drop across the output transistors load sharing resistors to determine what the load resistance is. The protective circuitry then reduces the output signal to stay within the safe operating range of the output devices no matter what the load is doing. When you remove the short, the amp starts output full power again (assuming the supply fuses haven't blown).

Some people don't like the effect that fold back current limiting has on the sound when it engages. Personnally I enjoy replacing output transistors a lot less. In my opinion, if the current limiting circuitry ever engages during normal use then you either have way too low impedance speakers or else you need a more powerfull amp.

Check out Randy Sloan's book on building High Power Audio Amplifiers. He goes into great detail on how to design an amp to prevent damage from a shorted output.

Phil Ouellette

PassFan 29th August 2001 12:39 AM

Hey Phil is that also known as an amplified diode . I have seen a reference to this in a number of places as being used to protect the outptu of bipolar designs. Maybe I saw it in Randys book . He has a web site also ( I think (nice guy).

haldor 29th August 2001 12:44 AM

I confess, I have no idea what an "amplified diode" means. Guess it's time to go to google and do a web search.


PassFan 29th August 2001 01:05 AM

Web site wrong
Sorry guys I got the wrong site The # for SEAL Electronics is(606)452-4135 in weeksbury KY.

Phil in Understanding Electricity and Electronics by Randy Slone on page 183 The purpose of an amplified diode circuit is to provide forward bias for output driver transistors to eliminate crossover distortion as it pertains to the amp on that page . I guess I answered my own question oh well ( I bet i'll remember it ) :)

Geoff 29th August 2001 01:15 AM

As I have said in a previous thread, if you want a good description of short circuit protection for audio amplifiers have a look at AN485 available on the On-Semi site.

ppl 29th August 2001 02:35 AM

Wow! A Blast from the Past. I rememmber that Note from years ago. Indeed it gives good Treatment on the Subject.While Your at that site also Check out AN1308 100 & 200 Watt Wideband Low Feedback Audio Power Amp. Another great read. BTW I also do not like Foldback Curent Limiting and I use rail Fuses and Bypass the Fuse With a Film cap. As far as Protecting the Transformer a Mains fuse on the Pri. Side of the transformer is the Best overall protection and is required by alot of Gov.Reg's ie UL, VDE, SA,

haldor 29th August 2001 03:39 PM

Hi ppl,

I agree that a mains fuse or circuit breaker is definitely required to prevent a fire hazard, but given the need to handle inrush currents, it is not likely to provide much protection for the transformer itself. A thermal cutout switch built into the transformer absolutely prevents overheating damage and should be standard practice for power amps (plus it's cheap, Plintron only charges a dollar or two extra for this).

Fuses in the supply rails are needed to protect the rest of the amp from a blown output transistor, but don't provide any real protection for the transistors themselves. By the time a rail fuse blows (assuming it does), the transistors themselves are usually long gone. After all, you can easily exceed the SOA for a bi-polar transistor without ever exceeding the max current capability of the amplifier (the SOA is load impedance dependent). Once secondary breakdown begins, it's by-by transistors.

I know that many DIY amp builders don't bother to include any kind of output protective circuitry (nor do they do anything to protect the speakers from DC), but this isn't any reason to recommend against taking extra steps to build reliable amps that can survive abuse.

We don't cut corners about things that often have at best a minimal effect on sound quality (like the differences between two brands of top notch caps or resistors), so why then be unwilling to go the extra mile and take some steps to prevent all that hard work from going up in smoke.


blmn 29th August 2001 10:53 PM

I had problems with distortion from protection circuits at output power stages in my last project.

It happened because the speaker impedance have expressive dips at some frequencies.

I solved the problem reducing the lower impedance limit of my protection circuit, but itīs is not the correct way, I think, since I didnīt change the power supply and the output and driver stages too.

I think the best way to deal with this problem is using some kind of soft clip limiter at the input of the amp and leave only the short circuit protection to the output circuit.

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