A simple speaker protection circuit

[AndrewT]

Hi Russ,
now for a bit of tuning.

In my opinion, we want FAST detection with minimum false triggering on music and sound effects.

The DC detect will get faster as you reduce the input RC time constant.

Reducing the cap will alter both the DC time delay AND increase the minimum frequency that passes without tripping.
Your 500mS and 3Hz is maybe a good compromise, but try pushing this to become a little faster.

Finally, what about latching?
What if a semi goes short circuit and passes DC current to the speaker? The relay pulls in? What next? It must not reconnect the speaker while the fault still exists.

Will the circuit hold the speaker relay open at amp start up and delay connection until the amp output levels have settled (in those first few tens of mS)?

[d3imlay]

Originally posted by Russ White
Here is the supply circuit I am thinking of using.

Easily tuned for 110 or 220V mains.

No dependency on amp transformer at all.

The circuit will be adjust to provide a constant load even when the speaker relay is off.

Cheers!
Russ

If I understand the proposed power supply right, the negative terminal is one diode drop away from the line voltage. The detector ties the output ground of the amp to the negative DC of the supply. Assuming the chassis of the amp is tied to the output ground, that will put the chassis of the amp one diode drop away from the line voltage and pose a serious health risk. Am I missing something?

[Russ White]

Originally posted by d3imlay


If I understand the proposed power supply right, the negative terminal is one diode drop away from the line voltage. The detector ties the output ground of the amp to the negative DC of the supply. Assuming the chassis of the amp is tied to the output ground, that will put the chassis of the amp one diode drop away from the line voltage and pose a serious health risk. Am I missing something?

You are very right. It is probably not advisable at all as it is. I use this scheme for my soft start, but In that situation a relay and amp's transformer are isolating the circuit from the user. So using this circuit the same way may not work without some safety precautions, like fast blow 500ma fuse or something. The draw on the circuit is only about < 23ma.

[Russ White]

OK, I think I have an idea, I could separate the protection PS from the AMP PS via an optoisolator. That should keep every thing well isolated. I will see what I can work out. Also the case should be connected to mains earth.

[Russ White]

How about something like this.

[Russ White]

Errrr... ignore that last one. It had some major flaws. This one is better.

Cheers!
Russ

[mhennessy]

Originally posted by AndrewT
Hi Russ,
now for a bit of tuning.

In my opinion, we want FAST detection with minimum false triggering on music and sound effects.

The DC detect will get faster as you reduce the input RC time constant.

Reducing the cap will alter both the DC time delay AND increase the minimum frequency that passes without tripping.
Your 500mS and 3Hz is maybe a good compromise, but try pushing this to become a little faster.

Finally, what about latching?
What if a semi goes short circuit and passes DC current to the speaker? The relay pulls in? What next? It must not reconnect the speaker while the fault still exists.

Will the circuit hold the speaker relay open at amp start up and delay connection until the amp output levels have settled (in those first few tens of mS)?

Hi Andrew,

The input filter design is always a compromise, and the best way to improve on this is to choose a 2 pole input filter - that's what the Velleman kit does. One has to balance this against the increase in complexity - this is one area where reliability is key!

I've already spoken about latching (post #10), and I would again suggest that it is not necessary. Can you think of a situation where the DC offset will dissapear once the speaker has been disconnected? I can't (that's not to say it's impossible, of course!). But, looking at the latest schematic, it would be reasonably straightforward to reconfigure Q1/Q2 as a bistable latch...

Yes, this circuit does provide power-on delay - R4/C2 do this. Much has been said about the need to quickly discharge C2 in the event of a fault - hence the PNP EF (Q3)

Best regards,

Mark

[Russ White]

Hello Mark,

Thanks for articulating those points. I agree with you on these.

Have you looked at the optoisolated version? It has one key advantage that the relay PS is completely isolated from the audio signal(and GND). But it has one key tradoff, that is that the turn on voltage for the LEDs is around 1V as oppose to .6V for the transistors. Keep in mind, that is just the SPICE circuit, the real one will have some additional parts (like the discharging diode for the delay cap).

Cheers!
Russ

[mhennessy]

Originally posted by Russ White
OK, I think I have an idea, I could separate the protection PS from the AMP PS via an optoisolator. That should keep every thing well isolated. I will see what I can work out. Also the case should be connected to mains earth.

Hi Russ,

I've used capacitive-dropper power supplies before, and they're a mixed bag. Certainly not for the inexperienced constructor - so while this is fine for your own use, I'm not sure if I'd do it in a kit... The one published project on my site that uses one also has several warnings and disclaimers!

It would certainly be easier to use the AC from the transformer, as earlier suggested. If you're worried about accomodating a range of supply voltages, simply change the relay drive to a currect source. The rest of the ciruit can be powered from a zener and dropper resistor...

The main worry I have with the latest circuit is the detector sensitivity. The LEDs need around around 1-2mA (perhaps less because the collectors don't need to sink much). But 1-2mA through a 20K resistor suggests a voltage drop of 20-40V. So you might need to buffer the filter output, which creates more complexity.

Having said that, opto-couplers can be really usefuly building blocks, even when you don't need to isolation properties. I've seen this circuit used plenty of times before

What else? I like the connection of R1, and was going to suggest it myself earlier (if you wanted to save a 1n4148 when not using the optional DC fault LED). It makes the base look like a current source (0.6/500K - 1.2uA which is probably a bit low here for good noise immunity - earth those opto-bases!). FWIW, R10 can be omitted. R4 needs the parallel diode to discharge C2. Not sure why you have D5? Consider replacing those series diodes with a zener or LED to reduce component-count (if it matters). The two output transistors are obviously there to keep the supply voltage roughly constant, but this strikes me as wasteful. A consequence of the capacitive-dropper PSU. These could easily form Andrew's latch

Not sure why Q1/2 are different types - they both switch the same load. R12 is a bit high, probably... Never used an LT1224 before, but it looks massively over-spec'd to me. Was that selected as convenience in the simulator? Personally, a good old 741 is what I'd use here - basically anything that's cheap, as a circuit like this shouldn't make any demands of its components...

Don't know if this helps?

Mark