Horn Driver Protection

[Fitts]

I know that using a blocking capacitor is often recommended to protect HF drivers in systems using active crossovers. I personally don't like how a blocking capacitor changes the sound and use quick acting fuses instead. Fuses, however don't block low frequencies.

Does anyone have any thoughts about placing an inductor across the terminals of the horn to protect the driver as a alternative to a blocking capacitor?

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I'm going to take an ignorant shot at this (it will draw out all sorts of enthusiastic correction, I'm sure). A cap is high-pass, an inductor (coil) low-pass. I think an inductor defeats all purposes as the crossover is feeding high frequencies to the compression driver which you are then blocking with the inductor. I'm not sure why you'd want to put a series capacitor in the loop with an active crossover.

[AllanCT]

An inductor accross the speaker would be high pass as well. Dead short to the LF.

Many amplifiers don't like to drive the extra inductive load even though the speaker does count as one as well.

I prefer an active crossover and no protection at all. No caps. no fuses no passive crossovers, no light bulbs no thermal fuses.

I've tried them all and found a quality crossover and amplification system and a relatively steady hand has worked well for my aluminum diaphragms for about 25 years now.

I would think the investment in a spare pair of diaphragms $$$ would be a wiser choice than any of the protection compromises.

Just my .02, not sure what I would do if I had tube amplification however.

[marcuswilson]

A protection network for the HF driver is definitely desirable in situations where there is a possibility of mistakes.
All of my PA system horns have a 2nd order network for protection, that is a series capacitor and choke across the driver.
I calculate them as a Butterworth network at 1/2 to 1/4 of the crossover frequency for the system. In my gear I normally use an active crossover at 1KHz, so the horn networks are designed between 500Hz and 250Hz. It is enough to prevent damage in the event someone connects the horn to the wrong amp or gets the crossover wired incorrectly.

I feel the choke across the driver to be highly benificial as it affords LF damping of the diaphragm, especially from LF pressure waves coming in the front of the horn. This has been known to damage diaphragms.

Also there is protection from amplifier faults as well as the DC shift that occurs when some amps are driven hard or clip.
It also allows the horn to be hooked up without a crossover in an emergency, at low power of course.

Some manufacturers, EAW for instance, keep the passive crossover in circuit when using active crossover drive. If the crossover is 250 Hz then they set the active crossover point for the LF at 400 Hz and the HF at 100 Hz. This is as good as totally active crossovers as far as headroom etc with complete HF driver protection from the passive crossover.

If you are in a fixed installation with high quality power amps, then it would normally sound better having no passive network on the HF driver, but I have found my networks sound much better than a lone capacitor.

When using a network it is desirable to include a high power resistor of about 25 to 50 ohms across the amplifier terminals for LF stability, especially for valve amps.

One point to note is that most valve amps have a lot of subsonic output due to bias shifts and power supply voltage variations. The network on the HF driver would prevent this subsonic moving the diaphragm around and reduce distortion in the driver caused by that movement.

[Richard C.]

Marcus,
I work on power amps and always have an O-scope on the output to check for DC, distortion, and instability (oscillation). I see that on power up, most amps show a very violent initial DC swing and than a couple of lesser swings across zero before they stabilize. I have been very leery about using a power amp driving a HF driver, without protection. I have wondered about using a passive high pass filter when I started to Bi-amp. I like the idea of tuning it to an octave below the low freq. cutoff. I can use a software program to model this filter (I am curious about the phase shift introduced). I bought an electronic crossover with a phase (delay) adjustment, so I can compensate.

Are you familiar with the Electro-Voice STR tweeter protector? I always wondered if the relay would stay reliable, low contact resistance, welded contacts, etc. I like the concept, (allow full power, allow quick peaks, but open up on too much power, or DC).

Thank you as always for your concise responses to these posts. I have learned a lot!

Richard C.

[marcuswilson]

Richard,

Thanks for your kind comment.
I'm not familiar with the EV tweeter protector, but if it has a relay, it's probably too slow in the case of amplifier faults and those contacts do get poor.

I believe the two octave lower network has negligible phase shift at the frequencies of interest.

I have found the use of 24dB/oct crossovers made with unsaturable inductors and >440 VAC capacitors to be the best compression driver protection I have.