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Old 8th November 2012, 02:20 AM   #11
glennb is offline glennb  Australia
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Quote:
Originally Posted by Vrystaat View Post
Oops, just saw the original bipolars are 63v and not 35v, will get some tomorrow.
Cheers Jan
This is an old design, I think I used it about 30 years ago. Under a DC fault condition the voltage rating on the left capacitor needs to be at least half the amplifier output stage power supply. The right capacitor can be a low voltage rating, as it will only ever see about + / - 0.6V, which is the Vbe of Q1 or Q3. It is preferred to use bipolars rather than back-to-back polarized electrolytics as in normal operation the voltage across these is very low (a tiny fraction of a volt) and won't provide much polarization to keep them well formed.
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Old 8th November 2012, 08:53 AM   #12
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by Vrystaat View Post
............ just saw the original bipolars are 63v and not 35v, will get some tomorrow..............
look at the resistors.
Think about what you see.
They form a resistor ladder.
Amp output to 33k to 33k to 33k to Spkr Ground.
At the amp output you will have Vpk
After the first 33k the voltage will eventually rise to Vpk * [33k+33k] / [33k+33k+33k] = Vpk*2/3
After the second 33k the voltage will eventually rise to Vpk * 33k / [33k+33k+33k] =Vpk/3

Why would you need 35V, or 50V, or 63V capacitors?

In fault conditions where the amp output could be as high as one or other supply rail then Vpk ~ Vsupply.
The first cap must have a rating of >=2/3 of Vrail to survive worst case conditions.

BTW.
that circuit has been discussed and is not worth building. It does not work effectively. There are many that perform better.
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Last edited by AndrewT; 8th November 2012 at 08:57 AM.
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Old 8th November 2012, 10:38 AM   #13
glennb is offline glennb  Australia
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Originally Posted by AndrewT View Post
look at the resistors. Think about what you see. They form a resistor ladder. Amp output to 33k to 33k to 33k to Spkr Ground....
The third 33K is limited to + / - 0.6 V by B-E junctions of Q1 and Q3, so its not a true 1/3 voltage divider.
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Old 8th November 2012, 10:47 AM   #14
AndrewT is online now AndrewT  Scotland
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You are right.
I would not use that circuit.
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Old 9th November 2012, 10:03 AM   #15
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Default DC detect speaker protection

Thanks for the reply's and info to all,
Andrew, I have build them and they seem to work fine on + 2V/ -2V on the detect input. Can you plse recommend a better circuit that is simple to build as well. I don't want something like a crowbar , just something better that uses relay's as well.

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Jan
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Old 9th November 2012, 12:09 PM   #16
AndrewT is online now AndrewT  Scotland
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If it works and meets your targets for performance, then just carry on using it.
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Old 9th November 2012, 03:01 PM   #17
MarianB is offline MarianB  Romania
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I have used that same circuit for speaker DC protection many times, and it worked just fine every time, in stead of bipolar caps i used 47u polarised in series with negative pin as common, and i have adaped it for 12v power source, the circuit is sensitive enough and never has it failed me, and believe me when i say sometimes it would have been a disaster if it would have failed, but it did not, so i recomend it.
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Old 9th November 2012, 03:56 PM   #18
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Default DC detect speaker protection

MarianB,
Thanks for confirmation, I will be implementing them.

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Jan
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