Speaker Protection help

[malukaluis]

Thanks guys... i'm just finishing up my prototype and will post the circuit. On transient protection how is the value of the MOV or diode determined?

[bob p]

http://music-electronics-forum.com/t27773/

[malukaluis]

Thanks for the help, here's what i've come up with. Certainly a must for bench test speakers I think.


The LED is optional and the 8ohm 200w resistor if you're never going to use the cab on a valve amp.

[bob p]

I like to keep it simple. If I had the problem that was described earlier, I'd take the really easy way out -- I'd just put a power resistor in the cabinet.

[malukaluis]

I like to keep it simple. If I had the problem that was described earlier, I'd take the really easy way out -- I'd just put a power resistor in the cabinet.

How though? a low ohm resistor in series maybe, I can see how that would work a bit like the light bulb idea but would it protect against prolonged high input?

[Steve Conner]

Looks OK, but could I suggest using a full-wave rectifier so that a DC fault of either polarity would trigger the circuit. Also I can't see any reason for the 10k resistor in parallel with the relay coil.

[km6xz]

My own view of this is a business policy problem more than technical. How much competition is there? Are the good customers likely to leave if more control is applied to protecting the investment and viability of the business? If there are few options for other rehearsal spaces the owner can focus on culling out the bad customers who threaten his business. He can prevent users from connecting their own amps to the owner's speakers.
Here, most people do not have spaces to play except in clubs on gigs or rehearsal halls so there are dozens of them within walking distance of my apartment. All are low income businesses with a lot of risk of expensive damage where money is not as much of an issue as lack of parts to repair damaged items. I have been easing into repairing as a side business and have regular clients who own collectively about 100 room, so every day someone will bring a few amps. One owner has a SPL meter in the room and it connects to an alarm. He can get by with that because he has pretty high end gear in 15 rooms...not sure where he got the money but he had all tube amps of higher end models. The alarm goes off twice in a night and the group is restricted in renting the room again. He explained that by having top gear, they fear now having access to it due to misbehaving. I think that works for him but others struggle with the problem. There are no re-cone kits here so blowing speakers gets expensive in replacing them entirely.
To help prevent the destruction of gear I have been suggesting which models would be safest and which would be very difficult to put back together due to lack of parts. I suggest being more selective in accepting clients and refer potentially bad customers to competitors. Bad customers can do far more damage than their payments justify, good customers are worth keeping and catering to. One practice facility with 8 rooms requires a band or musician prepay a month and does not allow any rentals less than a month. If they blow the gear, he keeps the month payment and the customer had no gear to use.
I am a firm believer that playing too loud is one of the main reasons most musicians never learn anything and always suck. They never learn what the song is about or what it actually sounds like, or that the vocals never sound good. Somehow so many think that too-loud is the only way to enjoy the activity of playing without realizing it assures they will never get good.

[J M Fahey]

Agree and add: I think the problem should be met at a much more basic level:

1) the rehearsal room owner *must* have his own equipment well balanced.
If, for example, he has a 4x12" cabinet loaded with "real 20W" Greenbacks, then the head driving them *must* be 50 W RMS or less, period.
Hooking a 100W one and asking: "guys, don't play too loud, will you"? is stupid.

No SPL meter will work.
If set to stand the full band (including drummer) , it won't trigger on a single guitar amp.

And cabinet must be hardwired to head, period.
No "head swapping experiments" allowed.

2) *IF* customers want to try something else, fine , but Room policy is : "bring your own head .... *and* your own cabinet ... "
Anyway they won't be able to connect them to house cabinets.

3) Or, have only 4x12" cabinets, at least with G12T75/Jensen MOD110/Eminence Patriot Texas Heat (150W each, only $75)
No Greenbacks, Vox Blue not even Jensen C or P12N

Only safe way is to make them physically impossible to blow.

[Steve Conner]

I remember playing in one venue fitted with a SPL limiter, sheer misery. No matter how we set the amps, the drummer would still trigger it. In this case, it was a bar located in the ground floor of a tenement block, with people's homes all around, so I guess it was only fair to limit the SPL.

I like to use XLRs for speaker connectors. That would stop people plugging their own kit in.

[km6xz]

Yes, that is the situation here, SPL is the issue even though the walls are several feet thick, brick or concrete so the speakers were not the only concern with volume, he has a PA system as well to protect and limiting the drummer is a good way of lowering all the instruments. Only in the most remote industrial locations will be rehearsal halls without residences all around in the city center. One of the few things that will get a response from the police is a resident complaining about noise after midnight.
We are not that sensitive to absolute sound level, but instead, on differences in levels, like all the rest of our senses so filling a rehearsal hall with 50watt amps and a single channel pa system so only vocals sent through it, the perceived sound level would be fine and plenty loud with limited power.

[Steve Conner]

Every band I've been in has rehearsed this way, using the PA for vocals only, and relatively low-powered amps. The rehearsal rooms we use are all on industrial estates, but there's no sense trashing your ears any more than necessary.

I was once in a band that had a quiet vocalist and a bunch of acoustic instruments. To get the effect we wanted, the electric part of the band had to be very careful not to drown the acoustic part out. I used a 60 watt tube bass rig with a single 2x10" cabinet and even that was too loud at times. The drummer used brushes for some songs, and we arranged the songs carefully so everyone wasn't making a racket at once. It certainly wasn't rock and roll, but it was a nice experience in a twee kind of way. I think I learnt some sort of lesson about stage volume too.

[malukaluis]

Looks OK, but could I suggest using a full-wave rectifier so that a DC fault of either polarity would trigger the circuit. Also I can't see any reason for the 10k resistor in parallel with the relay coil.

I think you're right with a full wave rectifier, when testing with a scope there was a whole lot of noise just below the relays pull in threshold. A rectifier may suppress it....i'm now thinking a schmitt trigger for the relay .
The 10k is working in conjunction with the preset as a potential divider also current limiter of sorts for the preset as on this test with a 50w amp into an 8 ohm dummy load the preset flips the relay at about 121 ohms just below the signal clips.

[olddawg]

Well, simply fusing the speakers will achieve that, both by protecting them *and* leaving output transformers unloaded at full power.
Talk about an unexpected lightshow

Now seriously, those 4x12" should be stuffed with 100 to 150W RMS (each) PA type speakers
No 4 or 6 6L6/EL34 guitar head should be able to blow it.

Never underestimate the ability of nitwits to destroy stuff. Gee? I have always wanted to run my 6 string bass through a Big Muff and an LPB1 with my 800 watt amp into a 4x12 guitar cab just to see how it sounds. I would never do it with my own stuff but what the hell? This cab isn't mine! I remember being in a place like this in the 1980s. The bands had to play loud to get over the band in the next room. Lol. We stopped once and another band was actually modulating our PA through the AC mains. I would still do what I said before. Chinese knockoff speakers. Limiting resistor for each speaker. If you want to add extra protection, put in a red button circuit breaker next to the jack that will trip at 80% max of the cab rating at 1khz as measured without the limiting resistors. Cheap and simple. Like the customers.

[bob p]

I was once in a band that had a quiet vocalist and a bunch of acoustic instruments. To get the effect we wanted, the electric part of the band had to be very careful not to drown the acoustic part out. I used a 60 watt tube bass rig with a single 2x10" cabinet and even that was too loud at times. The drummer used brushes for some songs, and we arranged the songs carefully so everyone wasn't making a racket at once. It certainly wasn't rock and roll, but it was a nice experience in a twee kind of way. I think I learnt some sort of lesson about stage volume too.

What you're describing is the difference between professional musicians and amateurs. Professional musicians play in the way that's required by the music. Amateurs are self-indulgent and play to satisfy their own egos.

I think we're all over-thinking the speaker protection thing. If you need to limit the voltage to the cabinets, then limit the voltage to the cabinets. Just build a simple non-adjustable attenuator into the box that throws away enough voltage to protect the speakers in the worst case scenario.

[Enzo]

Why make it transparent? Calculate how large a signal should be allowed, then park some zeners across it to clip anything larger. yes it will sound horrible when they go too loud. So what? Some interstate highways have rumble strips along the edge of the road. Sounds horrible when you drift out of your lane. So you steer back where you belong. If they turn up too far and it sounds like a blown speaker, they will back off so it sounds good.