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Amp Protection Circuit - Here!

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Many folks here have been looking for an Amplifier Protection Schematic. Many links have been provided with many pointing to Rod's project. Here is a circuit from Elektor published in the 1980s. It is relatively cheap and the components are readily available. The circuit is very reliable.

The functions provided by the circuits are as follows:
Power on Delay, Output DC protection for speakers, Delayed speaker connection (all of these accomplished thru' 2 [or 3] relays) and Thermal protection (a fan and indicator LED can be switched on at a predetermined temperature of the heatsinks).

Not shown in the schematic are 15-0-15 Volt 15VA transformer and a 12 volt regulator (LM7812).

This is the schematic that has all functions mentioned above except the thermal part:


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The temperature sensor circuit is optional. It only switches on a Fan and indicator LED through a relay but does not disable the speakers or the input to the amp. These can be configured by the user if desired.

Schematic of the Thermal Sensor:


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The circuit needs a 12-0-12 or 15-0-15volt 300-500mA transformer. You could tap the power after the rectifier diodes and run an LM7812 to get the 12 volt DC supply for the relays.

Amp outputs are wired from NC contacts of the output relays to R1 and R4 of Figure 1. Ground connections have to be completed.

Lineup, you are right about the temp. sensing transistors. You can do a variety of things with Re3 contacts - mute the input, switch on fans, trigger a temperature variable fan circuit; or if set to sense very high temperatures of output devices/heatsink, cut off power to the output stages of the amp etc.,
Hi Samuel,

I use this DC-protection + Delayed speaker connection in all my amps with good results. It reacts pretty fast on a DC-signal.

Concerning the temperature protection, I had some problems with an "oscillating' relay, as of the hysteresis was very litttle. I managed to get a good working on, with a "big" (+-100µF)condensator over the relay-coil if I remember correctly.

Can i make only one DC protection an use two R7, (which is 100k) one conected to Left and other to Right channel? If one chanel blows up so it pass the DC voltage will it damage the other chennel? Dc will pass through 2x100k. If this can be done, can i do this allso with 10k or 33k input R on protection circuit?


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Amp Protection Circuit

Hi Samuel

Your Amp Protection Circuit is very interesting for me: it comletly protect amplifier and speakers. But I have some questions:
- if 12V AC should wired to AC2 ?
- if Thermal Sensor should be conect to IC1 ?

Many thabks for your reply.



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i've got a fairly simple one..... comes from the Apt-1
it's fairly simple, includes turn-on delay, dc protection and SOA (safe operating area) protection for both channels.

if the file sizes are right, these should upload easily.....


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and here's the soa detectors, and where they connect to the output transistors (build for each channel) the output of these detectors connect to the relay driver circuit. all that.s needed for this is a handful of transistors and a dual op amp and a relay.....

some recalculation of rc values in the soa detectors may be required for other output devices. the output devices in the apt-1 were MJ15022/23.


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