Hi,
I'm refering to the circuitry that protects the output devices of a power amp against overload, short circuits etc. These circuitries seem to have been introduced by RCA in their Transistor Manual, and also by other semiconductor makers, such as Siemens, in the early 1970ies and almost immediately became a quasi-standard. Protection of the speakers against DC is not the point here.
To my comprehension, every SS power amplifier needs to have this overload protection, as the melting fuses that usually are provided in the supply rails seem to protect against catastrophic failures only, such as burning down the house etc., but they do not protect the SS devices, even if they were ultra-fast acting.
As we see here in DIYAudio, not every design shows such protection. We can clearly say that overload protection is a point in amps that are attributed »PA«, but almost not in amps for home use.
So, what are the objections against those circuitries? What makes it more worthful to omit them over the value of the power devices?
Best regards!
I'm refering to the circuitry that protects the output devices of a power amp against overload, short circuits etc. These circuitries seem to have been introduced by RCA in their Transistor Manual, and also by other semiconductor makers, such as Siemens, in the early 1970ies and almost immediately became a quasi-standard. Protection of the speakers against DC is not the point here.
To my comprehension, every SS power amplifier needs to have this overload protection, as the melting fuses that usually are provided in the supply rails seem to protect against catastrophic failures only, such as burning down the house etc., but they do not protect the SS devices, even if they were ultra-fast acting.
As we see here in DIYAudio, not every design shows such protection. We can clearly say that overload protection is a point in amps that are attributed »PA«, but almost not in amps for home use.
So, what are the objections against those circuitries? What makes it more worthful to omit them over the value of the power devices?
Best regards!
Most earlier designs of overload protection involved a current foldback design that would reduce output stage current flow when overload happened. These did a wonderful job of protecting the output stage, but sounded awful on activation of protection.
With the decrease in cost of output devices and the constant improvement in handling of high current spikes, designers have changed to adding more output devices and designing the amplifier to be more immune to these brief over-current situations. It's quite simple now to just measure voltage drop across emitter resistors and open the output relay on gross over-current instead.
With the decrease in cost of output devices and the constant improvement in handling of high current spikes, designers have changed to adding more output devices and designing the amplifier to be more immune to these brief over-current situations. It's quite simple now to just measure voltage drop across emitter resistors and open the output relay on gross over-current instead.
Hi,
but there are ample designs that don't even show this relay-based cutoff, or a link to an external board for this?
Best regards!
but there are ample designs that don't even show this relay-based cutoff, or a link to an external board for this?
Best regards!
Most amplifier designs here are just that. Protection is an add on. In a pro-sound/PA design you are more likely to see protection on board as they are usually more likely to be abused. Most who build their own home HiFi amplifiers like what we've built and don't tend to abuse them as much.
VZaichenko and I have developed an excellent complete protection system that can be added to just about any amplifier and takes care of all aspects of protection. It seems to be popular with HoneyBadger builders.
http://www.diyaudio.com/forums/solid-state/264313-how-build-21st-century-protection-board.html
VZaichenko and I have developed an excellent complete protection system that can be added to just about any amplifier and takes care of all aspects of protection. It seems to be popular with HoneyBadger builders.
http://www.diyaudio.com/forums/solid-state/264313-how-build-21st-century-protection-board.html
There are circuits that protect quickly and silently against overload on the speaker line, but they are in PA amps costing $1000. Current transformers around the speaker lines, a microprocessor, that is not easy to do as an amateur. Most people don't want to pay for it.
You can also protect the output devices from speaker wire shorts if you want to. RG Keene came up with (or published) a circuit using NFET's to disconnect the rail capacitors from the OT's if something bad is detected. You use photovoltaic fet drivers like APV1122 to produce voltage above the + rail for the FET gate, without having another transformer winding. Takes 4 nfets for stereo, one for each channel rail. With 50A nfets getting down to $1.50, it is becoming viable. but the speaker current detection is the trick, then there is latching that a fault occured and lighting an indicator on the panel. I'm up to about 35 parts on my circuit, just for DC on speaker line detection. Takes an extra wall transformer stuffed in the case too, to power the latch IC & LED's since my PA amp has different speaker return voltages for the two channels.
You can also protect the output devices from speaker wire shorts if you want to. RG Keene came up with (or published) a circuit using NFET's to disconnect the rail capacitors from the OT's if something bad is detected. You use photovoltaic fet drivers like APV1122 to produce voltage above the + rail for the FET gate, without having another transformer winding. Takes 4 nfets for stereo, one for each channel rail. With 50A nfets getting down to $1.50, it is becoming viable. but the speaker current detection is the trick, then there is latching that a fault occured and lighting an indicator on the panel. I'm up to about 35 parts on my circuit, just for DC on speaker line detection. Takes an extra wall transformer stuffed in the case too, to power the latch IC & LED's since my PA amp has different speaker return voltages for the two channels.
I prefer to protect my speakers.
I use a micro to monitor the output.
If I get DC for more than 500mS I turn off the speaker relay.
Must have saved a fortune over the years in speakers when testing new amps.
I use a micro to monitor the output.
If I get DC for more than 500mS I turn off the speaker relay.
Must have saved a fortune over the years in speakers when testing new amps.
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