# Cab(s) Impedance Relative to DB and/or Tone

Discussion in 'Amps and Cabs [BG]' started by Adlerburg, Sep 29, 2010.

Aug 18, 2010
Woodstock, NY
This is a question(s) for the amp/speaker gurus please... any insight is greatly appreciated.
Let's assume one has two 200W tube power amps with selectable impedances of 4 or 8 ohm output. One also has a pair of matched 8 ohm 410 cabs each rated to handle 400W. What would the differences in sound/db etc between driving these cabs independently by each of the power amps at 8 ohms, or driving both cabs in parallel by one of the amps at 4 ohm?
Is one more efficient? Louder? Sound better (or different)?
Thanks
-Mick

2. ### NoGraveConcern

Dec 30, 2003
Atlanta, GA
DB: +3 db when using 2 heads, assuming the cabs can take the power mechanically.
Tone: W/ 2 heads each cab will be driven with twice the power of a single head driving both. If a single head isn't enough power to cause the drivers to compress/distort then two heads may be.

Aug 18, 2010
Woodstock, NY
And the impedance? 200w into 410s at 8&#937; (times 2) vs 200w into 810's @ 4&#937; ?

4. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
Irrelevent when the 410s are driven by separate amps. When driven by one amp the total load will be 4 ohms, giving the same result as the 810.

5. ### NoGraveConcern

Dec 30, 2003
Atlanta, GA
Irrelevant because with tube power amps there is no volume (or power) relationship with impedance. They reach their rated power regardless of load.

With one head you have 200w of electrical energy being converted into mechanical energy by 8 10's.

With two heads you have 400w of electrical energy being converted into mechanical energy by 8 10's.

The only unknown in these equations is that of how the speakers will react to the doubling of power.

Aug 18, 2010
Woodstock, NY
Thanks Bill... Just to be clear running each cab separately by the two amps at 8&#937; would have double the power(output, loudness?) as running both cabs in parallel at 4&#937; out of one of the amps? Am I getting that right? And if so, 200w is still 200w whether coming from the 8&#937; or 4&#937; tap off the OTx? Pardon if these are elementary questions... but thanks for the answers!

Aug 18, 2010
Woodstock, NY
Thanks man.... I guess I was under the impression that your get double the power output as you 1/2 the resistance... Obviously incorrect.

8. ### KJung

Correct (not quite double) with most solid state amps... not with tube amps with those output transformers.

9. ### NoGraveConcern

Dec 30, 2003
Atlanta, GA
Double the power, which is 3db more "loudness," which is far less than double the loudness, more like slightly above the noticeable threshold for an increase in loudness.