DC-Protect Circuitry

[AKN]

Another dirty solution comes to mind, just short output to blow fuse on output like this Triac solution implemented on some Crest pro amps.

Edit: I meant QSC instead Crest of course

[Fritzell]

Wow, thanks for all your answers!

Back to that PDF.
is K1 an opto isolated thyristor?

What limits the voltage on the 470uF delay capacitor?

K1 is a relay. Multisim 7 has an odd symbol for it..

I dont like the power fet protection at all

I understand this because at first I did like the Relay/contactor idea more. The good thing with relay is that if the whole amp and psu blows completely the speaker is still safe. But still, the mosfet/psu version is alot better than not no have any protection at all which i don't have now. But who knows, I might rebuilt this one in a relay version. But as said, im gonna give the mosfet version a try

I Etched the PCB this evening, It wen't quite good. The only bad thing is that the printing was a little faded on the black so I had to reinforcement some large copper areas (for the Dc currents between mosfets and connector) . But it's ok. I'll show it when it's finished. Hopefully components will show up on wednesday.

[AndrewT]

Hi,
relays cutting a DC offset from output to speaker need a very large current rating for this duty.
The contacts can arc or even worse, weld closed on high current.
Then your assumed safe protection has gone.

Close rated fuses in the supply rails before the amp output stage and let the NFB correct for modulation of the supply rails.

[parsecaudio]

Hi to all
the reasons for adding a protection circuits is not only to protect from amplifier fail but also to eliminate the ON transient when power on.

The mosfets protection are very good to protect from a DC offset in the amp output's but it does not work for bump on power on.

A problem is inside a Mos used like switch:your internal on resistance is not linear with frequency,so it is not a good solution for high end amp.

Also to use the capacitors after this Mos is not a good thing because when the protection is on and the power supply is disconnected,the charge of this cap flows on the speakers.

Another problem:
it is difficult to find a low on resistance for P channels mosfets,and the resistance are different for N and P.

For the P polarity in case of high power amp their dissipation cannot be neglected.

[fab]

Originally posted by parsecaudio
Hi chris and fab
A simple question:
you are sure that he is simpler to mount a single quad op amp(14 pins=14 holes and/or solders points ) or some transistors(minimum count of 4 + their associated components= minimum 18 holes / solders point on PCB)?

Fab,it is right that a simple discrete transistors can do the same function.Are less expensive four transistors or one single standard operational amp?

No precision needed?Why?

Hi Parsec
The intent of my comment for the use a a few discrete transistors compared to a single quad op-amp is not for cost or number of separate components (this is DIY not commercial stuff here) but to have a very high reliability due to the limited number of compoenents to get a high Mean Time Between Failure number.
A quad op-amp like the TL084 may have something like maybe 40 transistors... (I have not checked the exact number here...)

Originally posted by jacco vermeulen

The chance of an opamp failure in a circuit with 4 opamps is 4 times as high as one with 1 single opamp.
Is the chance of failure of a Quad opamp higher than that of a single opamp ?

Hi Jacco
yes 4 times (provided that all individual op-amp of the Quad are used)!


Also. I like the circuit of post #23. I do not remember what is the current high limit of the IRF540 (is it high enough for high current demands?).

[anatech]

Hi Andrew,
I've seen a few welded relays over the years, but they are in the minority. I've also seen TO-3 transistors with flame cuts in the casing and others where the arc folled the emitter lead (gone) into the socket.

I'll take relay protection any day. Mute the input at the same time and you will reduce pitting on the output relay with every turn off.

Most amplifiers use fuses before the rectifiers where they may help with lowering peak charging currents. A good thing in my book. I doubt they are audible in a properly designed amp. In a poorly designed amp, everything is audible.

Crowbars can be very effective, but they almost guarantie a service job, and not a cheap one either. Still, cheaper than some speakers.

-Chris

[AndrewT]

Hi,

Mute the input at the same time and you will reduce pitting on the output relay with every turn off

I forgot about that one, thanks for the reminder.

Crowbars can be very effective, but they almost guarantie a service job, and not a cheap one either. Still, cheaper than some speakers

if the crowbar was needed, it is time for service anyway, even if only to adjust the sensitivity of the trigger level.
If the amp had gone wrong then the only extra cost is two fuses.
If the amp had spuriously triggered, then the total cost is two fuses and some time to replace them.

With all these topological alternatives, the prime purpose is to limit damage extending beyond the first failure. That limitation includes saving speakers and transformers. Most schemes will achieve at least some of these benefits. Is there a simple (cheap) scheme that achieves all the benefits?

[matic]

I have seen some amplifiers not having any DC protection, no fuses before-after caps. Only protection was fuse on speaker. What does it do? Cat it be some soft of dc protection?

[AKN]

Originally posted by matic
I have seen some amplifiers not having any DC protection, no fuses before-after caps. Only protection was fuse on speaker. What does it do? Cat it be some soft of dc protection?

That fuse is mainly for protection of amp from to low imp load eg shorted output.

Severe amounts of DC can reach speaker and cause damage before fuse blows.

[AndrewT]

Hi,
A fuse outside the feedback loop to the speaker is a deliberate attempt to introduce distortion to the signal.
Don't.