Get a load of this! techies welcome....

Discussion in 'Amps and Cabs [BG]' started by MarkMcCombs, May 21, 2002.

1. MarkMcCombs

Nov 26, 2000
Lee's Summit, MO
I received my new ampeg rig that I bought off ebay last night. The seller filled the entire speaker cab with packing peanuts, through the vent holes! After about an hour of trying to vacuum them out (I had to use a stocking over the vacumm cuz the peanuts clog the vacuum hose), I finally realized that there were peanuts inside the speaker drivers themselves, between the coil and frame. So, I had to take each of the 4 speakers out and remove the peanuts, as well as all the peanuts in the cab. Man, nice guy, the seller, but come on!

Anyway, in looking at each speaker, they all are stamped with "8 ohms" on them, and the cab itself is supposed to be 8 ohms; can this be right? Well, hooking up a speaker cable to the cab and using a multi-tester, I came up with an impedance of like 6.3 ohms. Now, I've heard that multi-testers testing impedance in DC circuits are off by a degree, is this true? In which way? I'm sure my cab is in fact 8-ohms, but how can I be sure?

It sounds fantastic, by the way.....I wonder what it would have sounded like full of packing peanuts!

(it's the Ampeg SVT-48HE)

Thanks,

Mark

2. FalsehoodBass

Jul 22, 2001
Denver, CO
i'm not sure abot the 6.3 or whatever, but if you have four identical speakers, wire the first two in series, wire the second two in series, then wire the pairs in parallel, the total impedance will be the same as one of the speakers.

8+8 = 16
8+8=16

1/16 +1/16 = 2/16 = 1/8

so 8 ohms is your total impedance.

and sorry about the peanuts.. atleast he was trying.

3. allan grossman

Sep 8, 2001
surreal city, usa
Here's a great definition of impedance -

http://www.geofex.com/Article_Folders/impednc.htm

Impedance is the generalization of the concept of resistance from DC to AC. That is, it's a way to represent how much current will flow with a specified (AC) voltage across the impedance. That is, if you have one volt AC across an impedance that lets one ampere of AC current flow, the impedance is defined by the AC version of Ohm's law and is one ohm.

Since AC has not only amplitude, like DC, but also frequency and phase, this introduces the possibility that an impedance will not only allow a current to flow, but will change the phase of the signal, and respond with different amplitudes and phases as frequency changes. You can have a resistor, a capacitor, and an inductor that each have an impedance of one ohm (or a Kohm or a Mohm) at any given frequency.

The resistor (ignoring the "imperfections" of parasitic capacitance, lead inductance, etc., which is usually valid at audio frequencies) will have the same impedance at every frequency. The capacitor will have an impedance that goes down with frequency (making the same assumptions as with the resistor, ignoring parasitics) and the inductor will have an impedance that goes up with frequency (ditto).

So - impedance changes with frequency.

Also, the 8 ohms stamped on the speaker is an average impedance - the speaker in use will fluctuate above and below 8 ohms - that's the reason some amps aren't 2 ohm stable - the closer impedance gets to 0 ohms the more tolerant the amplifier has to be to having its output termnals shorted out

Hope I haven't confused things too much -