What happens if I add an extra cab to my combo amp?

[groovehead]

I am pretty technically illiterate so please help me. I have seen several threads here discussing what happens when you add a second cab to an amp, but I am not sure if it will work with my combo amp or not. I have a 100 watt Ampeg 15" combo with a direct out to run into a house PA. I am unsure of the impedence (ohms). However, am I wrong in thinking that if I run a line out of the combo and into another passive cab with a couple 10"s, that it will split my resistance and double my wattage giving me 200 watts. Or, will I simply be splitting my 100 watts between 2 cabs at about 50 watts each? PLease give me any answers in very simple and easy to understand language. Thanks.

[randy clay]

This is what I think you are asking. If your amp has a extension speaker output, you can run speaker wire from that output to an extension cab to increase your sound coverage. This will mean your 100 watt amp is now driving two cabinets (50 watts a peice), but you have more coverage (and or different sound/tone). But your post mentions the "line out". That output will not drive a passive cabinet, but you could hook up another amp and speaker combination to increase both coverage and volume.

 

I hope this helps, and if I'm wrong, please will someone correct me.

[MattC]

well, first of all, the line out is an un-powered output. It is made only to run into the PA, but to run another cab off this jack, the cab must be powered. To run an extention cab off your combo, it would need an (extention) speaker output. I'm not entirely familiar with that Ampeg, but if I'm not mistaken, it doesn't have an ext. speaker out. bummer. (of course, I could be wrong, I haven't reaearched that amp)

but in general:

The speaker jack would be labeled something like 100 watts, 8 ohm min., 4 ohm min load, or some such. As a general rule, any amp can handle an 8 ohm load (8 ohms for your ext speaker). Many can handle a 4 ohm load (2 8-ohm speakers in parallel). Few instrument amps can handle a load smaller than 4 ohms and if it can, it will be labeled thusly.

You can calculate the impedance (# of ohms) like this (again, in a very general way): if the two cabs are in parallel and have the same impedance on their own, the total impedance is half.

(2 8-ohm cabs in parallel offers an impedance of 4 ohms).

Here is the mathematical formula:

1/(total impedance)= 1/R1 + 1/R2 + (1/R3) + ...+ (1/Rn)

R1 is the first cab, R2 is the second cab, etc.

The fewer ohms offered to the amp, the more current the amp will put out. Current is an amount of electricity. It is current that is destructive- high currents create a lot of heat and can cause shorts and fires. This is why it is important to observe impedances: too little impedance can cause your amp to go bad.

[g.]

My experience is that most bass gear manuals cover these questions specifically for their product. Usually can't go wrong with RTM (read the manual), and it can be educative as well. Some companies have printable or browser-viewable pdf maunuals of their gear on their sites.

[Edendude]

And...

 

Unless you are comfortable using your amp as an 'arc welder', you will want to check your amps minimum impedence rating, before plugging an extra cab into your 'extension speaker' jack. Most modern amps will tolerate a 4 ohm load, which is what you will end up with if your combo speaker is rated at 8 ohms, and so is the extra cab you plug in.

 

However, if the extra cab is rated at 4 ohms, as many are, you will be placing a 2.666 ohm load on your amp. This could send it up in smoke very quickly, because the current 'draw' on the amplifier will be too great for it to safely handle.

[g.]

Especially with separates, the bar is at 2 ohms. There is still gear out there though whose manufacturers' wild claims seem to be based on a world of whimsy wherein woofers are never wacked with wide-band walls of wham for any weal-wowld wegimen.

 

These poow specimens awe not what they appeaw to be.

[groovehead]

Thanks guys. Sounds like it wont work. I dont have an extnsion speaker output and the amp is rated at 4 ohms. So, since I am not comfortable using it as an arc welder, I will leave it alone. For my purposes, the amp works well and has sufficient output. I am in an acoustic style rock group and the only drums I have to play over are hand drums. However, I was just wondering if there was a way to boost my output with the current amp if I ever found myself in a situation where I would need more. I guess not.

 

Thanks again.

[Trane Francks]

Well, you really did have the solution right in your initial post. If you need more FOH sound than you have with just the amp, connect your line-out to the P.A. and let the P.A. system make up the difference. I, too, gig with a 100W combo (H&K BassForce L), and that's precisely how I handle venues where the little rig runs out of steam on its own.

[dcr]

It sounds like you're using the BA115, right? I really like that amp, & I almost bought one a little while back. What kept me from doing it was the fact that it had no capabilities for running an extension cabinet. If that's an issue for you, then either (a) rely on the PA for more coverage, as folks have wisely suggested, and/or (b) replace it with an Ampeg B2--basically a more "grown up" BA115, more wattage, and it will run an extension cab. (They come up on eBay all the time in the $400-$500 range, which is about what a new BA115 runs anyway. Resale on the BA also seems pretty decent.)