Re: Peak and Limit LED's


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Posted by Bob 'Skippy' Blechinger on December 28, 2000 at 08:18:40:

In Reply to: Peak and Limit LED's posted by Jah Rude on December 27, 2000 at 11:04:31:


: I have an Ampeg B2 stack which has the power amp built into a cabinet
: with 4 eight inch speakers and that sits on top of 1 fifteen inch speaker.
: On the back of the power amp, it has a label indicating 200watts @ 8 ohms
: (which is also labeled on the 15 cabinet), 350watts @ 4 ohms and then 4 ohms
: minimum. So, I'm not sure how many total watts I have according to
: these labels not to mention my limit and peak led's go off if I turn it
: up half way but I have to turn it up cuz it's hard for me to hear myself
: over everything (keys and drums especially). I would really appreciate it
: if someone could help me figure out how many watts I have and how much
: attention should I pay to those led's.

Jah,
You're gonna hate me for this; it gets kinda complicated...

It depends on your amp and speakers, *and* how you've got them hooked up.

Trust me. ;-)

The differing wattage measurements deal with an electrical principle called "impedance", which is related to resistance (the principle that makes lightbulbs and toasters work). One way to think of impedance is as water flowing through a pipe; higher impedance is like a small pipe, which restricts the amount of water that can flow through, and a lower impedance is like a large pipe, where a lot of water can flow through.

Speakers will have a given impedance, usually either 8 or 16 Ohms. In a cabinet, those can be wired as series (one after the other), parallel (one wire goes from positive to positive, another from negative to negative on each speaker down the line), or series-parallel (combination). We won't get into that, though, since any given cabinet will usually have an overall impedance.

What you need to do to figure out how much power you *really* have is to check the impedances of the cabinets that you have hooked up and work from there. Let's just say that both cabinets are 8 ohms impedance; if those are hooked up in series, you'll have 16 ohms total impedance, but if they're hooked up in parallel, you'll drop down to 4 ohms (there should be an indication of that on the back of the amp).

I went and looked at the Ampeg site while I was writing this, and if I'm reading you right, both of your cabinets have an impedance of 8 ohms. I'm not sure exactly how the Ampegs are wired for dual speaker cabinets (i.e., series or parallel), so I'm not absolutely sure if you've got a total of 4 ohms or not (it's unlikely that you're going to be running at a total of 16 ohms impedance; that would kinda work against the power of the amp).

I hope that gives you at least a little bit of explanation; right now, I'm half asleep, so hopefully, this didn't turn into an utterly incomprehensible mess... ;-)

With the LEDs, the "peak" should light up in little blips, but not a steady light. I'm not sure if the "limit" light means that the limiter is on, or that it's active; try defeating the limiter and see what happens to your volume and tone, ok?

Hope that helps!

-Bob



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