Forward Biased

Protection for those expensive tweeters (and other speakers too!), part 5 (final)

These two drawings complete the two protection  circuits we were discussing.  The first one is  essentially just the same circuit already posted, and the second is an input-output comparator that illustrates how one can connect it to the input and output of pretty much any amplifier.

The input frequency response of the second circuit is full-range because, although the tweeters may be fed signals only down to, say, 2KHz, they should be protected from signals all the way down.

I hope it's pretty self-explanatory, but if I haven't made everything clear let me know.

Protection for those expensive tweeters, part 3

Here’s a new copy of the same circuit.  The only difference is that I have added a couple of waveforms you should see at 1) the comparator outputs, U101-a and –b, and 2) the Schmitt trigger inputs, U102-a and –c.  Nothing else has been changed. 

Protection for those expensive tweeters, part deux

Now for the fun stuff!  This’ll be easier if you open up the drawing file so you can view it as you read this.

First of all, what’s the big deal about this circuit as opposed to, say, your run-of-the-mill DC detector found included in many power amplifier circuits for protection?

It’s faster.  And it’s more sensitive.  Perfect for protecting those precious little tweeters that reproduce all those sweet high frequencies.

It will respond in much less than 100ms.  The exact time depends on an RC time constant together with the switching level of a Schmitt trigger, so, while it might work perfectly the first time, it also might need a bit of tweaking to get it just right.

First of all, you’ll notice that the circuit looks symmetrical.  That’s because it is symmetrical. ;o)  The circuit separately analyzes the positive and negative halves of the output waveform going to each speaker from its amplifier, and if it notices as little as 600mV of DC offset it will react in as little as 57 milliseconds (we’ll discuss the time constant in a minute), and to very low frequencies, as low as 4Hz.  This can easily damage your speakers, even your mids or mid-woofers, and tweeters all the more.  So let’s take a look at how all this happens.

Protection for those expensive tweeters

As I mentioned yesterday I am trying to get some content up here quickly, and as I was perusing some of my drawings for one that was pretty much ready to publish, I noticed this one.  It’s part of the protection circuitry for the 5.1 music system we’ll be discussing in more detail here as things unfold.

If your music system’s power amplifier has expensive or hard-to-find output devices, or if you want to protect your speakers from amplifier faults (and who doesn’t?), you probably have some sort of circuitry to detect any DC voltage at the amp’s speaker outputs.  As you may be aware, speakers don’t respond particularly well to DC, in part because the normal back-e.m.f. created by the generator action of moving speaker coils suddenly disappears, or nearly so, allowing a much greater current to flow.  Even a few milliseconds of this can allow considerable heat to build up.