Speaker Protection help

[malukaluis]

Is there a simple way to protect cabs from being driven to destruction.
A friend of mine runs practice rooms where he provides guitar amphead cab combos and a few 4x12 stand alone cabs in each room, some bands to use their own amps on his speakers instead of lugging their own in.
Problem is that some of the muppets that come in with their own amps basically abuse the life out of his speakers leaving them usually open circuit. Needless to say it's costing him a small fortune in drivers.
Now short of telling him that he needs to manage the equipment use with more rigour i would like to offer him another solution.
I was thinking along the lines of maybe a soak resistor parallel the I/P and another in series with the drivers, or maybe a fuse, know that will derate the cab but i not too worried about that.

Your comments and help
Thanks

[Jazz P Bass]

A fuse is the simplest fix.
Are the 'muppets' amps rated at too much power for the cabs?
If so, there is not much to be done for that, if the band plays loud.
Draw up a list of the available equipment.
Cabs are rated at XXX.
If your (the muppets) amps are rated at more than that, bring your own cabs.

[kleuck]

For power only protection you can use some Polyswitches (you can think of them as reusable fuses) or make a relay-based protection circuit, i made one once, doesn't have to be complicated, a bunch of transistors and a relay is enough to ensure a delay (not a good idea for a tube amp, but mine was a SS amp) a current (power) protection, and a DC protection. For a guitar and tube amp, the power should be the only issue, so a Polyswitch is the simplest way, but a high impedance load can be an issue for the amp too, so a relay switching to a dummy load would be the best protection i think.

[cbarrow7625]

Eminence sells an active speaker protection system called D-fend that is powered by the audio signal. It involves a DSP and MOSFETs to smartly limit power. I have seen it demoed in action. It is amazing. It just levels off the power at the predetermined set limit. There is nothing else like it. It was developed by SLS audio for pro audio loudspeakers and is sold by Eminence under a license agreement. Check it out. The guys using it may not care for what it does to their signal but it shouldn't hurt the tube amp or the speaker.

[Enzo]

I see a different problem here. I see people with no stake in the game abusing the gear. They have no incentive to care for the gear. Give them one. Take a security deposit before renting the room. Make them responsible for blown gear. And on the other end of the scale, you have a 4x12 cab? Don't stick it full of 35 watt drivers. Fill it with 80-100 watt drivers. Most guitar amps stop at about 10 watts, sure there are exceptions, but stuff the cab so the power limit is out of reach.

Don;t let them complain that their favorite little speakers are not in the cabs, they are free to bring in their own cabs if they are disappointed.


Fusing is very difficult - you don;t want the fuse small enough that it blows on peaks all the time, but you also don;t want it so large it never blows. What do you do? And consider that a large impulse that can damage a speaker can do so way faster than a fuse can blow.

[malukaluis]

Thanks Guys. i think a belt and braces approach may be the order of the day, Enzos` suggestion of uping the power handling of the cab and Kleucks' polyswitches would offer enough protection.
I found a good article on polyswitches as I`d not heard of them before http://http://www.jaycar.co.uk/image...d/polyswit.pdf .

D-fend.... just too expensive at GBP 235.00 a non starter however if it`s the ulitmate in speaker protection here`s a link enjoy http://http://speakeressentials.com/...minence-d-fend

[olddawg]

If he has functioning cabinets right now I would just put a ceramic resistor in series with each speaker the same value as the speaker and rewire the cabinet for the impedance you want. To replace the blown speakers I would use the highest power rated Chinese knockoffs I could find and be done with it.

[cbarrow7625]

Yep, the d-fend is expensive but thought I would throw it out there as we've all seen this question before. It might be right for somebody.

A word of wisdom on poly switches; while they are technically current based devices, their actual operation is based on heat, not current draw. They do not reliably trip at a specific current draw, even with a resistive load. They trip each EXACTLY the same temperature every time. To reliably protect speakers epoxy one to the rear of each magnet so that each driver is individually protected. They are super cheap so this should not be a cost problem. Also, poly switches will not unlatch (close) until all drive current is removed and they cool. If you use one per cab, the signal will shut off ( no load and may damage the output transformer of the tube amp) and will not come back on until the player stops completely and let's the switch cool. One overpowered chord and the cab is off again. Another reason to use one per driver.

The smart money is on:1) hi wattage, cheap drivers, 2). Protect those drivers individually with poly switches so that customers are less aggravated, 3). Forget they poly switches and use a relay based protection circuit that reduces level by 3-6dB when tripped then restores everything to normal once the level is reduced. And provide an LED to let the idiots known when the protection is engaged so that they know why the level was reduced all of the sudden.

Poly switches are not ideal for driver protection without some additional work. I know from experience. I've seen lots of blown drivers from protection circuits using poly switches alone.

[malukaluis]

Yep, the d-fend is expensive but thought I would throw it out there as we've all seen this question before. It might be right for somebody.

A word of wisdom on poly switches; while they are technically current based devices, their actual operation is based on heat, not current draw. They do not roeliably trip at a specific current draw, even with a resistive load. They trip each EXACTLY the same temperature every time. To reliably protect speakers epoxy one to the rear of each magnet so that each driver is individually protected. They are super cheap so this should not be a cost problem. Also, poly switches will not unlatch (close) until all drive current is removed and they cool. If you use one per cab, the signal will shut off ( no load and may damage the output transformer of the tube amp) and will not come back on until the player stops completely and let's the switch cool. One overpowered chord and the cab is off again. Another reason to use one per driver.

The smart money is on:1) hi wattage, cheap drivers, 2). Protect those drivers individually with poly switches so that customers are less aggravated, 3). Forget they poly switches and use a relay based protection circuit that reduces level by 3-6dB when tripped then restores everything to normal once the level is reduced. And provide an LED to let the idiots known when the protection is engaged so that they know why the level was reduced all of the sudden.

Poly switches are not ideal for driver protection without some additional work. I know from experience. I've seen lots of blown drivers from protection circuits using poly switches alone.

Point taken. I like the sound of relay based protection it makes the most sense but do you have to have a separate power source for it or can it be run form the line in ? Have you a circuit ?

[cbarrow7625]

The relay can be operated just from the audio voltage. No power supply needed. Choose a 12 or 24 volt DC relay, 12 is usually fine. The voltage to the relay can be stepped down through a voltage divider if necessary.

The circuit is simple, just a high impedance resistor (1k-10k depending on how much you have to step down the voltage) followed by a single diode, then a 100-1000uF cap. In practice it is best to replace the resistor with a trim pot so that you can calibrate the trip point precisely. You are just half wave rectifying the audio then smoothing it a bit with the cap. That's it.

Alonso remember to put a reverse bias diode across the relay to catch any flyback voltage as it opens and closes.

You can have the relay open and just cut off audio or you can put resistors on there to reduce the level when tripped. The resistors will have to be able to take quite a bit of power if the idiot using it just plays it in protected mode all night. That is why you should put an LED indicator to show when it is inn protect. It is also powered by the audio. Another resistor, diode and led is all it takes to light it up.

[malukaluis]

The relay can be operated just from the audio voltage. No power supply needed. Choose a 12 or 24 volt DC relay, 12 is usually fine. The voltage to the relay can be stepped down through a voltage divider if necessary.

The circuit is simple, just a high impedance resistor (1k-10k depending on how much you have to step down the voltage) followed by a single diode, then a 100-1000uF cap. In practice it is best to replace the resistor with a trim pot so that you can calibrate the trip point precisely. You are just half wave rectifying the audio then smoothing it a bit with the cap. That's it.

Alonso remember to put a reverse bias diode across the relay to catch any flyback voltage as it opens and closes.

You can have the relay open and just cut off audio or you can put resistors on there to reduce the level when tripped. The resistors will have to be able to take quite a bit of power if the idiot using it just plays it in protected mode all night. That is why you should put an LED indicator to show when it is inn protect. It is also powered by the audio. Another resistor, diode and led is all it takes to light it up.

Thanks cbarrow7625, this is very helpful. I`m going to build one later this week. I`ll let you know how/it goes.

[R.G.]

I like Enzo's idea. People very seldom blow their own speakers. It happens, but not often.

Require a security deposit, deduct speaker replacement from the deposit. It's very likely that the speaker destruction will stop, or at least drop a lot. The people who are offended enough to refuse to do this are mostly the same customers that he doesn't want to serve.

[Ted]

You can only go so far with management of the rooms...

Security deposits are a lovely idea. But they have to be large enough to actually pay for any damage. Most rehearsal rooms I know of (and I am aware that these are shall we say fairly low-end premises) operate on cash. If you've got ten rooms, taking £100 deposit on each - when the local crackheads get wind of this you'll be robbed in about 5 seconds. Of course musicians never hang around with serious drug users so this won't be a problem.

Every bit of kit has to be tested at the beginning (to demonstrate to the hirers that it works) and end of each session. Somebody has to do that. These places usually operate on a shoestring, often staffed by part-timers who get paid by use of the rooms.

Protection devices are expensive. Home made ones may not likely be all that reliable, and good luck when the first guitarist thinks (rightly or wrongly) that your box of tricks has blown up his/her amp.

[Steve Conner]

I think you guys are trying to put a technical band-aid on a social problem. Either put badass high-powered drivers in the 4x12s so they won't blow up (Studiospares sell some Chinese 12" PA drivers that might do) or stop providing standalone cabinets.

Edit: http://www.studiospares.com/speaker-...er/invt/248290

[malukaluis]

Steve and Ted you're both right on the money. It is definitely a case where the staff on duty are handing out equipment that are not matched and i know that even with "training" they still may get it wrong. I've already started changing out blown drivers for higher rated generic ones, that is really the answer although we right back to the begging if someone brings in a power amp and mini desk.....yes apparently this has happened.
I've had a play at a relay driven protection system that cbarrow7625 had suggested, link to follow.
I have my reservations in actually building it into any of these cabs, if i did, the line in would have to be dumped into a 50 or 100w resistor when tripped, just in case a valve amp was being used, that would at least cause the user to stop playing and investigate. Any way have a look it's not very exciting but knowledge gained should be knowledge shared... well IMHO.

https://www.youtube.com/watch?v=aU6GstGROu0