L-Pad in Ext. Cab? or no?

Jerry Overstreet · Post by **Jerry Overstreet** » 23 Mar 2022 12:10 pm

I have a 100w. combo amp that I want to add an extension cabinet. They are 2 different designs of 8 ohms speakers. The characteristic differences in the 2 drivers and cabinets gives me the pleasing sound I want to hear, but my ext cab is overpowering the amp speaker. I'm keeping that configuration regardless, but have a question.

Is it safe to install an L-pad in the extension cab to slightly attenuate and balance out the volume level between the 2?.... and will it work like that without affecting the amp or the amp's speaker. The amp has 2 speaker jacks. They are only labeled Speakers with the minimum impedance warning. It is designed to carry no less than a 4 ohm load total.

I have found a source for a 100w rated L-pad that claims it works to control speaker volume without interfering with the 8 ohm resistance/impedance etc.

....or is there another/better way to do this in my configuration?

How about just using an audio taper pot in the speaker wiring inline instead?

Thoughts please.

Scott Swartz · Post by **Scott Swartz** » 23 Mar 2022 8:51 pm

The ones intended for hifi speakers to attenuate the midrange driver or tweeter will not stand up to the power load of a full range musical instrument speaker.

Something like a Weber MASS or a THD Hot Plate will easily do the job but those are much different cost size and complexity.

A normal audio taper pot even a 2 watt one like a Type J Allen Bradley will melt almost instantly.

There are wirewound rheostats rated for high enough power something like this:

https://www.amazon.com/Electronics-Salo ... 9661&psc=1

Note that its 4 inches in diameter.

You would be helped by the fact you are not attenuating the full power of the amp like the guys trying to record their Marshall amp turned all the way up, but things like the Weber MASS exist because relatively inexpensive L pads will not do the job.

Jerry Overstreet · Post by **Jerry Overstreet** » 24 Mar 2022 4:35 am

Thanks for those links and info Scott. That confirms my suspicions. I believe I will just leave things be. I've fried enough equipment in past years to last a lifetime.

As you allude, most of the info I've found concerns Hi Fi/Audiophile systems v. musical instrument speakers.

I'll figure some other workaround or just leave well enough alone.

Thanks again.

Donny Hinson · Post by **Donny Hinson** » 24 Mar 2022 4:38 am

A simpler solution is just to partially cover up the extension speaker. I've used a jacket or a blanket, but anything that will block and absorb some of the sound will work. You could also lay the extension speaker down, speaker facing up (if space permits), since much of perceived loudness is the "focus effect" of a speaker.

George Biner · Post by **George Biner** » 24 Mar 2022 1:19 pm

You could conceivably install a low-value (< 1 ohm), low-inductance power resistor in series with the louder cabinet -- that resistor can dissipate a little extra power before it gets to the hot speaker.

What value resistor? To estimate, you could put an identical signal thru each speaker and measure the dB sound level, that might give you some idea of how much power you'd have to burn.

You could also put the resistor in a separate box with jacks so you don't have to modify the original cabinet.

Just some ideas.

Jerry Overstreet · Post by **Jerry Overstreet** » 24 Mar 2022 1:39 pm

Thanks fellas. George, I'm not sure I have the knowledge or ability to acquire the proper and needed components to accomplish this, but I will check into the possibility.

Also, I sent you a PM on another subject.

Jerry Overstreet · Post by **Jerry Overstreet** » 25 Mar 2022 10:13 am

OK, how about reversing the polarity of the extension speaker putting it out of phase with the other one?

In general, I know that having one speaker moving out while the other is moving in will affect bottom end, but I don't know about volume. Is this even safe for the amp? I've had bad experiences with shorted out speaker wiring. I don't think that's a concern, but don't want to take chances.

I know we used to do that with 2 channel stereo systems ala John Hughey whereby there are 2 separate circuits or 2 separate opamps etc., but that's a bit different that having 2 speakers on the same mono input.

Don Downes · Post by **Don Downes** » 25 Mar 2022 11:39 am

This would be a great mid-term question on a resistance modeling exam.

Don Downes · Post by **Don Downes** » 25 Mar 2022 12:56 pm

I had a long winded reply to you question, but the simple answer IMHO is that the speakers in your extension cabinet are more efficient than the ones in your amp.

Here is a link explaining "speaker efficiency".

https://www.lifewire.com/amp-power-spea ... cy-3135077

This appears to me to be the case in your situation. Though you you asked about using a L-pad (linear), there are other factors to be considered. I would suggest that you use a T-pad. This would pretty much eliminate any issues with power ratings, and it would certainly help insofar as the "8ohm load" is an inductive load, and not purely resistive.

Here is another link that shows why a T-pad is the best choice rather than the L-pad. (forget the math and the tables. Just try and get the concept.

"An attenuator is a passive network, designed to weaken the power being supplied from a source circuit to a level suitable to the destination circuit. The theory behind attenuators comes from the voltage divider principle."

https://www.lifewire.com/amp-power-spea ... cy-3135077

Hope this helps. I'm more than happy to answer any questions.