Driver Efficiency: Tube vs SS

[Beach]

For two 8 ohm drivers in parallel and using a SS amp, the theoretical increase is 6 dB.

For two driver 8 ohm in parallel and using a tube amp, the user switches the output tap from 8 to 4 ohm. What is the increase when the drivers are in parallel?

What is the increase when the 8 ohm drivers are in series and the user changes the taps to 16 ohm? Thanks!

[Loren]

As long as you match the impedance correctly for the tube amp it will function the same as a solid state amp as far as speaker SPL goes.

The only difference is that solid state amps tend to deliver more power when the impedance is lower (i.e., 4Ω versus 8Ω).

[Beach]

Thanks Loren. If I understand you correctly, there will be a 3 dB gain in sensitivity for 2 drivers regardless of series or parallel (assuming the correlating change in taps)?

[Loren]

No, not really.

First, understand that every time you double the power to a speaker you increase sound pressure levels by 3 dB.

Let's look at wiring two 8Ω*speakers in series. Each speaker sees 1/2 the voltage drop across its terminals compared to a single speaker, so, only 1/4 the power (trust me) is actually dissipated to each speaker compared to a single speaker.

Each speaker now produces 6dB less SPL (Sound Pressure Level) than the single speaker configuration. That means two speakers each -6 dB down add to a -3 dB down when you add them as far as power dissipation is concerned. However, since you have two speakers you have twice the mechanical cone area compared to the single speaker and twice the air is now moved, which is another 3 dB gain. The net gain comes out to 0 dB. Confused?

So what is the advantage here? Mainly, you can take two 4 Ohm speakers and connect them to an 8 Ohm tap on your tube amp and it won't blow up.

If we look at a parallel configuration you will see that both speakers get the same power as a single speaker (voltage drop is the same across each driver), but since you have two you work the amplifier twice as hard and subsequently get a net 6 dB gain in SPL over a single speaker configuration.

If the two speakers are not collocated right next to each other the net gain will be only 3 dB due to room acoustics.

You asked a tricky question, but I hope this explanation helps.