Unplugged speaker protection & line out

[bluemonk]

I wish to add a unplugged-speaker protection inserting a 150-270 ohm/15W resistor across speaker output.
Since I already have a line out jack taken from speaker output by a voltage divider, the question is:
- should the protection-resistor affect line-out?

Thanks in advance.

[Don Moose]

It shouldn't. The line out divider is in the k-ohm range, and your load is a tenth of that, with the real load a furhter tenth of _that_.

[bluemonk]

Actual resistors in line-out divider are 2k2/100 ohm.
That 100 ohm is in the range of the protection-load (I have at hand a 150R/15W). Is this an issue?
Should it be better a different range voltage divider (say 22k/1k)?

Thanks.

[Steve Conner]

The resistance presented by your 2k2/100 ohm divider is 2300 ohms, and it'll be in parallel with your 150 ohm protection resistor. 2300 in parallel with 150 is something like 145, so the difference is unimportant.

In any case it's a difference in the right direction: the lower the resistance, the better the protection works.

Heck, if you changed your line out divider to 220 ohm/10 ohm, it would also function as the protection resistor.

Note that the protection resistor doesn't necessarily make it safe to use the line-out jack without a speaker connected. For that, you need a proper dummy load with a resistance equal to the amp's rated impedance.

[bluemonk]

Ok, now I have the right clue.
Yes, I consider this unplugged-speaker protection just as a simple safety device, not as a dummy load.

Thanks a lot.

[jrfrond]

I do this a bit differently. I use a normally-closed switching jack, and the "knucklehead" load is connected to the switch-tab terminal, therefore the load is engaged ONLY when the plug is out, and I use an 25W resistor of appropriate value (4, 8, or 16Ω) that can double as a short-term dummy load for troubleshooting purposes.

[Steve Conner]

The shorting jack is a bad idea, because Mr. Knucklehead might just as easily forget to plug the OTHER end of the cord into a speaker.

I once suggested using Neutrik Silentjack shorting 1/4" plugs on all speaker leads, on the grounds that a tube amp is more likely to survive a short than an open.

[Diablo]

The shorting jack is a bad idea, because Mr. Knucklehead might just as easily forget to plug the OTHER end of the cord into a speaker.

I once suggested using Neutrik Silentjack shorting 1/4" plugs on all speaker leads, on the grounds that a tube amp is more likely to survive a short than an open.

Yes, but on a combo amp, someone is much more likely to unplug the cord into the chassis than to disconnect the spade terminals from the speakers......only because it's easier to do. I like this knucklehead idea and will probably do it to my combos. Ampeg did it your way Steve, on my old V4 with just a shorting jack.

[bluemonk]

Also speakers could fail

Speaker can fail (open bobbin) and player could take the amp turned on for (relatively) long time moving jack cables, turning knobs, hammering the cabinet... trying to make it sound.
IMVHO a permanent big resistor across the speaker output will cover most of possible issues.