Help me understand please

[Mottlefeeder]

On 07/12/2019 at 12:20, BassBunny said:

Personally I wouldn't bother with the splitter if the speakers have dual inputs. It's just introducing something you don't need and could go faulty. 

1 lead from Amp to Cab1, then a lead from Cab1 to Cab2.

I'm a pessimist/(boy scout in a previous life?), so I would go with this solution, but also buy the splitter in case one of the amp/speaker sockets failed. My mantra, if it can fail, sooner or later it will, so have a backup with you.

David

[Chienmortbb]

14 hours ago, Bill Fitzmaurice said:

If you run two identical 8 ohm cabs (or whatever impedance) voltage sensitivity goes up by 6dB compared to one. That's what's responsible for the increase in output, not power. Power goes up because the halved impedance load doubles the current draw, but that's really anecdotal. You don't run two cabs to get more power out of the amp, you run them to take advantage of the sensitivity increase.

What is voltage sensitivity?

 

 

[Bill Fitzmaurice]

Speakers aren't driven by watts, they're driven by volts. While an 8 ohm speaker sensitivity is usually quantified as dB/1 watt/1 meter that's incorrect, because the amp power isn't a constant. It varies with the actual impedance load, which varies with frequency. If amplifiers were constant power devices every frequency would be at a different level. But they're not.

An amp does provide constant voltage output at every frequency, no matter what the speaker impedance is at that frequency. Therefore the correct way to state sensitivity with an 8 ohm speaker is dB/2.83v/1 meter. If you add another identical speaker the amp output voltage remains the same, while sensitivity of the pair goes up 6dB compared to just the one.

As to why sensitivity goes up by 6dB, output is dependent on cone displacement. When you double the displacement, which is cone area multiplied by excursion, you get a 6dB increase in output. In the case of adding a second identical speaker while maintaining the same voltage the cone area doubles, excursion remains the same for both, therefore displacement doubles and output goes up by 6dB.

 

Edited December 12, 2019 by Bill Fitzmaurice

[invertigo]

What started as a simple question has evolved into an interesting discussion. Certainly a lot more knowledge here than I expected, and its very interesting to read and understand.

[Chienmortbb]

On 12/12/2019 at 12:56, Bill Fitzmaurice said:

Speakers aren't driven by watts, they're driven by volts. While an 8 ohm speaker sensitivity is usually quantified as dB/1 watt/1 meter that's incorrect, because the amp power isn't a constant. It varies with the actual impedance load, which varies with frequency. If amplifiers were constant power devices every frequency would be at a different level. But they're not.

An amp does provide constant voltage output at every frequency, no matter what the speaker impedance is at that frequency. Therefore the correct way to state sensitivity with an 8 ohm speaker is dB/2.83v/1 meter. If you add another identical speaker the amp output voltage remains the same, while sensitivity of the pair goes up 6dB compared to just the one.

As to why sensitivity goes up by 6dB, output is dependent on cone displacement. When you double the displacement, which is cone area multiplied by excursion, you get a 6dB increase in output. In the case of adding a second identical speaker while maintaining the same voltage the cone area doubles, excursion remains the same for both, therefore displacement doubles and output goes up by 6dB.

 

The problem is that valve/tube amps and some solid state amps have the same power output no matter what the impedance. Valve amps via a transformer and the TH 500 has the same output into either 4 or 8 ohms. 
 
Using a voltage to measure efficiency. falsely favours  a lower impedance speaker. 

Volts are the driving force, the PD or potential difference between two points,and without volts nothing would work. However volts only exist on their own if the voltage source is not connected to anything. Once you connect a voltage to any electrical circuit, current flows and watts are consumed. Efficiency is fundamentally power out divided by power in so using the same voltage for 4 & 8 ohm speakers makes a 4 ohm speaker seem more efficient. 

Edited December 19, 2019 by Chienmortbb

[Bill Fitzmaurice]

38 minutes ago, Chienmortbb said:

The problem is that valve/tube amps and some solid state amps have the same power output no matter what the impedance.

That's a common misconception. The load on the output valves isn't the speaker, it's the output transformer. The impedance of the output transformer primary winding is a constant, therefore the speaker load doesn't affect current delivery. With direct coupled SS amps as the load goes down current goes up, because as the name infers the speaker load is directly coupled to the output devices.

Using voltage to measure sensitivity doesn't favor a lower impedance speaker. For that matter one cannot measure sensitivity with anything other than a voltage source. One cannot hook up a meter to the amp output to read the amp power output, as no such meter exists. One may only read the voltage output. If one could read the power output it would render the sensitivity spec useless, because power output is different at every frequency. Voltage is used not only because it can be measured but also because it's constant no matter what the frequency.