I understand that but my question is what is the the output impedance without the additional cab?
This concept applies to power transfer between passive circuits and active amplifiers with high impedances.
Modern audio amps have a lower output impedance than the load. Ideally an amp has 0 output impedance.
For loads higher than 4 Ohms most amplifiers run out of the higher voltage rail needed to drive the load. Since power on the load is given by V-squared divided by R, in increase in R requires an increase in V. Ifthe amp was already maxed out at 4 Ohms, the power rating has to drop by half at 8.
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Look at the speaker specs. Itās 8 ohms because its 2x10. They should have left out the 16 ohm part and just said 8 ohms because you gonna use both drivers of courseā¦
Thank you @DaveT
That clears it up for me.
Now I can see why people would be pissed off with purchasing what they think is a 450 Watt Bass amp for $700CAN when in fact they need to spend another $400+ to get an additional 8 ohm cabinet to get the 450 watts they expected in the first place. making their total purchase $1,100.00+
Marketing an amp as 450 Watts seems almost like false advertising to me when you have to almost double the price to get the wattage you expected.
Imagine a player on a budget purchasing a Fender Rumble 200 for $250 more than a Rumble 100 only to discover he needs to spend substantially more to get the 200 Watts or else he might as well have just got the Rumble 100. Granted the Rumble 200 has a 15" speaker over the 12" in the Rumble 100 but still.
Now I wonder what marketing hype has been applied to my Rumble 100?
Is that 100 advertised wattage an RMS or Peak value because if it is Peak I really only have Rumble 70.
Based on what I have learned in this thread I would bet it is Peak.
Oh well, at least I have a line out for a house PA connection on my Rumble 100. 
I agree on combo units it shouldnāt be marketed as such, but straight heads - no one says you should only buy one cab, thatās a bad assumption people make Vs wattage claims. The amp can do it, depends what you do with it.
On combos it should state what you are getting and in smaller font what you can get with spending more on an external cab. Seems only fair.
Good grief we complicate everything on this planet.
Yeah, if someone wants to run full power on one cabinet they need to buy a 4 ohm cabinet. Thatās what I did and now Iām a little sorry about it. I like @joergkutterās plan where you get a smaller 8 ohm cabinet deliberately at half the power rating of the amp. That way you have a lightweight small rig and can stack another for full power.
The specs are even more sneaky in that they are probably rated also for 220V power. Here in the US with 120V power I get less output than the label many times.
Or thereās the part where one cabinet may put out as much at 200W as another at 400W because the sensitivities are different.
Itās a crazy topic to actually figure out. I think at the end of the day your choices are:
- Small venue - low hundreds of watts
- Loud drummer - middle hundreds of watts
- Patch to PA system
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I understand that but I was asking about the impedance at the amp output not the speaker cab impedance 
@DaveT answered this.
That is the same conclusion I have come to thanks to your insights.
Initially I was looking at getting the Ampeg Ba-210 for larger venues but now I have settled on a Rumble 500. Less weight and a 4 band Eq.
The Fender Rumble 500 specs state:
Wattage: 500 Watts @ 4 ohms (with External Speaker), 350 Watts @ 8 ohms (Internal)
Even without the external 8 ohm cab 350 Watts is a helluva lot of power and driving 2x10 speakers would probably meet most of my venue needs standalone.
Any larger venue and I would just plug into the house PA.
Do you know if the power ratings quoted are RMS or Peak values.
Again they very rarely say.
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Another tricky topic. Measurement traditions and even standards exist, but thereās no obligation to follow them. And digital amps behave differently than analogue ones. With anything audio, the question of time comes into to play. How much power for how long. The higher power numbers come from burst test signals. They are closer to peak values than RMS, but also more representative of audio waveforms.
The only manufacturer Iāve seen Audio Precision graphs of power vs. time has been for Powersoft PA amps. They could sustain their rated power for 1 second before degrading while their published competitor at the same power rating could maintain it for a tenth of a second. Looking at the waveform in the DAW itās possible to see how very short that peak is.
Tube amps were rated at RMS while digital amps rate more toward peak. This is where the idea of tube watts being louder than solid state watts comes from.
IcePower publishes extensive spec sheets on their power modules. Itās a little easier to know what they are up to.
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Thank you again for the insights.
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