Speaker protection needed

[Moulin]

Hi all,
I built myself a power amp a few years ago, and have now hooked it up, together with a musical fidelity P140 for the tweeters, to a pair of Tannoy monitor gold speakers which I’m bi-amping through a mini DSP eight crossover, I’m thinking it’s best to have some proper speaker protection circuitry, and came across this on eBay. This is definitely not my Field though so I’m asking the forum what they might recommend
thanks in advance.
James

https://ebay.us/m/4bhzeB

 

[Devnull]

Are you trying to protect the low frequency or high frequency drivers. For the high frequency driver just use a big cap for DC protection a couple of octaves below the crossover frequency. For current protection use a polyswitch. Not necessarily the best after it resets, it will have a little "residual" resistance after it trips.

 

[LTig]

I built myself a power amp a few years ago, and have now hooked it up, together with a musical fidelity P140 for the tweeters, to a pair of Tannoy monitor gold speakers which I’m bi-amping through a mini DSP eight crossover,

This is not bi-amping, it's called "activating" the speakers.

I’m thinking it’s best to have some proper speaker protection circuitry, and came across this on eBay. This is definitely not my Field though so I’m asking the forum what they might recommend

Difficult to say as I see nothing specified about how it works. For the tweeters a simple capacitor of sufficiently high capacitance (depends on impedance of the tweeter and the cross over frequency) may be all you need, at least as protection against DC.

 

[Moulin]

Thanks, could I ask you to be more specific? the x over is set at 1200 hz. What spec would the cap have to be?
And what is a poly switch?

 

[somebodyelse]

And what is a poly switch?

It's a trademark name for a polymeric positive temperature coefficient device. Essentially it behaves as a self-resetting fuse. I've not seen anyone test them for distortion effects in the speaker path though.

See thread Influence of output relay contacts to power amplifier distortion before picking relay-based protection products. FET based switching has lower distortion and fewer problems with age - see Neurochrome's Guardian-86 performance graphs for an example. That one is protecting against power on/off transients and DC on the output, so you would have to find or build one with sensing triggers for the sort of failures you're trying to protect against.

The idea behind the series cap is to provide enough high pass to protect the tweeter from being fed bass for a little while so you might have a chance to realise your mistake and disconnect it before it gets damaged. Its high pass frequency either needs to be far enough away from crossover not to significantly affect the response, or to be included in the crossover filter design. That's up to you as a design choice. The 'couple of octaves' @Devnull suggests is a rule of thumb for 'far enough' but if you're using a steeper crossover than usual you might be able to make it closer - you'd have to model it or test it to be sure. Presumably you already know how to do this if you've made a crossover using the miniDSP.