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Old 25th July 2015, 02:08 PM   #1
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Default Protecting the Altec family jewels

I am moving incrementally towards an all active DSP crossover 811B/416-8B system. The horns I picked up are driven by 806-8A's, which seem to be the red headed step-child of the Altec family tree. Nonetheless, its what I have and I don't want to fry them out of the gate.

There will be no inductors, resistors, or capacitors between the amplifier and the driver. To a cautious layperson like myself this looks like clipping high voltage cables onto the family jewels. As efficient as compression drivers are, and knowing what they sound like with milliamp's, I am anxious about this setup. Yes, the volume will be fully attenuated when the amp is turned on, but is this common to direct connect a compression driver to an amplifier with no crossover between the two?
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Old 25th July 2015, 02:55 PM   #2
Scott L is offline Scott L  United States
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I have done it, but admittedly it is living dangerous, especially if using a transistor amp. A tube amp is less cause for alarm. During your development stage, it might be wise to insert a high quality cap with a value that places it's inherent high-pass at least an octave lower than what your dsp based crossover is set at. Having said that, my personal choice would be about two octaves away. You might even consider having some fun with a by-pass switch you can use after said amplifier has been turned on, and is proven stable. See if you can hear the effect of the in-line capacitor, vs being by-passed.
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Old 25th July 2015, 06:46 PM   #3
Pano is offline Pano  United States
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Scott is right, protect those jewels! I usually made a resistor divider to go between amp and driver, 10dB is what I aimed for. It offers some protection and also gets the amp up out of the dirt. Unless you have a special low gain amp, the noise floor can be too high for compression drivers and horns without attenuation.

A big cap is not a bad idea either. You can go down to the local HVAC supply store and buy a motor run cap (not motor start). They work well in series with horns. You can bypass it with a small film cap if you like. I did that on my 811/806 combo with good success.

Good luck on the project! Keep us informed.
Take the Speaker Voltage Test!
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Old 25th July 2015, 09:40 PM   #4
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Rule-of-thumb for paralleling caps is the bypass cap is 100X to 1000X smaller than the "main" cap. It's mostly there to bypass the stray inductance of the larger cap, extending the bandwidth into the far ultrasonic.

It's entirely possible the bypass cap might add weird new colorations of its own, so you end with the worst qualities of each cap. This has happened to me several times, so I'm usually a bit cautious about these things.

Pano's suggestion of a discrete-resistor L-pad for the compression driver is a very good suggestion. It also buffers the wild impedance variations of the horn/driver combination, which makes it easier for the amplifier and any passive lowpass you might choose to employ.
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Old 27th July 2015, 08:57 PM   #5
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Originally Posted by brucegseidner View Post
IYes, the volume will be fully attenuated when the amp is turned on, but is this common to direct connect a compression driver to an amplifier with no crossover between the two?
Now that amplifiers with DC output protection are common, it is more common to not use "protection capacitors" than it is to use them.

At any rate, if you do use them, the phase and frequency response will be quite different with compared to without. The driver in the test below is a 16 ohm EV DH1AMT using an 18 MFD capacitor, note how response is affected throughout the frequency range, not just in the low range.

Although I used "protection capacitors" for around 36 years, after this test in 2011 got rid of them.

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File Type: png With and With out.png (197.5 KB, 76 views)
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Old 27th July 2015, 09:41 PM   #6
GM is offline GM  United States
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For loudspeaker protection, Littlelfuse recommends its 30R series PTC switches [pg. 71] and of course there's other PTC brands available:

Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

Last edited by GM; 27th July 2015 at 09:45 PM.
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Old 28th July 2015, 09:15 AM   #7
TBTL is offline TBTL  Germany
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You can compensate for the highpass effect of the capacitor with your DSP.
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