Bob Reynolds wrote:
"This is a generalization, but the biggest difference to me between SS and tube amps is the output impedance. Tube amps typically have a much higher output impedance which will have a much larger interaction with the speaker. Thus, finding an amp/speaker pair is much more difficult.
"For high output impedance amps, we have a case of the tail wagging the dog -- the impedance curve of the speaker determines the frequency response of the amp."
This is partially correct, and I can certainly understand how the situation looks this way. It did to me too until I started designing loudspeakers intended to work well with a wide range of amplifiers, and found out things aren't as simple as they seemed. I had assumed that a solid state amp outputs the same power regardless of the impedance curve, overlooking the commonly-known characteristic of solid state amps doubling their rated power out into a 4 ohm load and halving it into a 16 ohm load. You see, most solid state amps approximate a constant voltage source - in other words, the wattage they put out goes up as the impedance of the load goes down. The output power varies inversely with a speaker's impedance swings. Let's look at an example:
Suppose we have a speaker with a 32 ohm peak in the bass region, then the impedance falls to 4 ohms at 200 Hz, then rises to 16 ohms in the 3 kHz crossover region, then remains at about 8 ohms above that. I'd probably call this a "6 ohm" speaker. Let's look at what happens when this speaker is driven by a solid state amp, putting out 2.83 volts (equal to 1 watt into an 8-ohm load):
Into that 32-ohm peak in the bass region, the amp is putting out 1/4 watt. Then it's putting out 2 watts at 200 Hz, 1/2 watt at 3 kHz, and 1 watt above that. Assuming the speaker is aimed at solid state amp owners, it's up to the designer to take all of this into account - and this is routinely done, no problem. But, my point is this: Typical solid state amps DO NOT put out the same power regardless of the speaker's impedance! It only seems that way because most speakers are designed to be used with solid state amps.
Tube amps (and transformer-coupled solid state amps) usually approximate a power source - that is, they deliver nearly constant wattage regardless of the speaker's impedance curve (as long as the speaker's impedance stays within the proper range for that amplifier). Such an amplifier would deliver approximately the same power across the roller-coaster impedance curve described above, and probably would not be a good choice with that speaker assuming it was "voiced" for a voltage-source amplifier. However, some speakers are at their best with an amplifier that acts as a power source - including electrostats and many high-efficiency speakers.
A few tube amps (and recent Nelson Pass designs) have very low damping factors (high output impedances) and approximate a current source - that is, their power output increases into a high impedance load and decreases into a low impedance load, but in practice they're usually about halfway between being a power source and a current source. In the real world, we might expect a high output impedance tube amp to deliver 2 watts into that bass impedance peak, 1/2 watt into the lower midrange dip, 1.5 watts into the midrange peak and 1 watt at high frequencies. Note that this is a 4-to-1 difference into the bass impedance peak compared with the solid state amp's output. I have measured this much difference (6 dB) on bass impedance peaks when going from a high damping factor solid state amp to a low damping factor OTL tube amp. This can be either a blessing or a curse, depending on the speaker's design.
You may have realized that the foregoing implies the smoother a speaker's impedance curve, the wider variety of amplifiers it will work well with. Such has been my experience.
So to recap, the power output of a typical constant-voltage-approximating solid state amp actually changes significantly depending on the speaker's impedance, but most modern speakers are designed with this in mind. Still, some of the most interesting loudspeakers work better with amplifiers that more closely approximate a constant-power source.
Duke
