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Old 15th June 2005, 07:18 PM   #1
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Default Loudspeaker protection from overdriven amp

Ok, so here is what happened. I was A-B ing newly built speakers. Cranked the volume up a bit to much and blew the tweeters. The tweeters were are rated for 20 or 30 watts (don't remember which). The normal listening level had about 10vdc for these speakers.

I'd like to add something to protect the tweeters from more dump ****. What can you guys recommend?

thanks
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Old 15th June 2005, 08:02 PM   #2
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First thing I'd recommend is to not try and get by the auto censor like that.

The next thing is to consider increasing the slope and/or frequency of your high pass.
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Old 15th June 2005, 08:39 PM   #3
K-amps is offline K-amps  United States
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Parts express has some tweeter protectors... search those out.


If you are not too worried about SQ and want to just protect the tweets from abuse, you can insert a 10v 10w lamp in series with the tweet. This lamp would present about a 1 ohm DCR when cold and would not significantly affect the tweeter when cold. When you pass over 8 volts to the tweeter the lap will begin to sink some of that current and protect the tweeter.

I would short the lamp during critical listening....

Another option would be insert a 6v relay driven by the signal. the relay is fed by a resistor (100R) . You will have a smoothing cap (16v 2200 to 4700uf) and a 6v-8v zener paralelled across the relay's coil. The cap will stop chattering caused by the signal and the zener will protect both the coil and the cap.

The amplifier will drive the relay's coil via (in series with) a diode and a 100R resistance. If a certain threshold is reached, the cap begins to charge thereby increasing teh voltage build-up across the relay's coils untill there is enough voltage to snap the relay....click!

At this point, the relay which is configured as normally closed will open the tweeter from the amplifier.

Unless the signal is removed or reduced, it continues to keep the relay energized. You can adjust the time constants by varying the cap size and resistor values.

This method requires more components but presents nealry no loss of SQ. I am guessing a couple of resistors, a relay, a smoothing cap and a couple of diodes should do it.....
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Old 16th June 2005, 01:51 AM   #4
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I like your ideas K-amp. Could you possibly post a schematic or email one to me.

Thanks
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Old 16th June 2005, 02:29 PM   #5
K-amps is offline K-amps  United States
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I am posting a rudimentary schematic made in MS-Paint so it is not very good but you will get the idea.

Also the values of the components is up to you as at what level you want to set the trigger. Some experimentation will be needed.

R1= 50-100 ohms
R2= Use this to discharge the cap to resume listening. Leave this out initially unless needed.
D1= Any 1-2 amp generic
D2= same value as D1. This dissipates the EMF pulse the coil creates when it de-energizes. It will protect the amplifier from spikes.
ZD1= Depending on relay and C1 voltage chosen. Match this accordingly. i.e. for 6v relay get 6v zener etc. 2-3 watt will do. This will primarily protect the capacitor and the relay coil from burning up if the signal is too strong.
C1= At least 100% higher voltage than zener chosen. 2200uF to 10k uF. For 6v operation use 16v cap etc.
RL1= My guess is a 6v relay will do. 12v will not energise soon enough and tweeter will be Kaput by the time it does.

ZD1, the cap and relay are all inter-dependant voltage wise.

D1 and 2 should be minimum of 100v.

R1 will set how quickly the relay will engage once enough voltage is sensed.

C1 will charge up (larger value will make the protection forgiving, lower value will engage the relay more frequently).

R2 discharges the relay. Initially do not install this unless you feel the relay is not letting go, then experiment with a few values gradually lowering them till the relay releases in a reasonable time after the signal has gone down. As a ball park the value of R2 must be half of R1. i.e. if R1 is 100 ohm then do not use R2 below 200 ohm and so on otherwise R1 will never be able to charge the capacitor and engage the relay.

You will need to play around and see what suits your application. I cannot give you specific values of components... but these are hopefully easily available.... even Ratshack has these components.

I leave the tweeter connection to the relay switches to you.... obviously you will connect the tweeter to the normally engaged (closed) arms of the relay and as the relay engages on, the contacts are released and the tweeter gets no power.


Good luck.


K-
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Old 16th June 2005, 04:50 PM   #6
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Last night I replaced tweeters, brought volume up slowly until I reached what is nearly my normal listening level. Ran this listening level for an hour. I would want to run them a few more hours to burn them in and then take voltage output.

Your schematic is great help.
Quote:
Originally posted by K-amps
I am posting a rudimentary schematic made in MS-Paint so it is not very good but you will get the idea.
Q1- Amplifier + and grd, are those connections taken before crossover board or after tweeter highpass filter?

Quote:
Originally posted by K-amps
RL1= My guess is a 6v relay will do. 12v will not energise soon enough and tweeter will be Kaput by the time it does.
Q2- Is the relay used ac or dc?
Q3- I had already seen 10vdc before posting, I suspect voltage output will be greater than that at my listening level. Will a 6vdc relay holdup with higher voltage outputs.

Quote:
Originally posted by K-amps
As a ball park the value of R2 must be half of R1. i.e. if R1 is 100 ohm then do not use R2 below 200 ohm and so on otherwise R1 will never be able to charge the capacitor and engage the relay.
Ahhhh so that's how that works. Thanks
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Old 16th June 2005, 05:07 PM   #7
K-amps is offline K-amps  United States
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Ans 1: You can do either.... perhaps better protection of done after HF XO but then you load up the XO slightly... but not much.

Ans 2: Dc relay. The D1 will pass DC to the repay and C1.

Ans 3: Yes 6vdc will hold up because of ZD1 clamping the voltage above 6v.

FYI with my own tweeter protection I use a 12vdc relay and it engages at about 34vac from the amplifier... so consider some losses due to R1.
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