Hey all, quick question regarding speaker impedance. For a flea watt tube amp with a nominal output impedance of roughly 6 ohms, would it be a problem driving a single driver speaker rated at 16 ohms?
Neither the amp nor the speaker will be harmed, but electrical damping will be low, reducing detail compared to a low impedance amp. Low damping is part of the tube sound. More noticable in the bass where more current is needed for cone control. Low damping means the current is delivered slowly.
Would that be an easier load than an 8 ohm speaker?
Technically, yes. It is an easier electrical load, but there are other factors, like phase at impedance dips, and speaker sensitivity that come into play in the real world.
Would the amp's output power change into 16 ohms rather than 8 ohms?
For a given output voltage, the lower impedance will draw more power, and sound louder. However if you live in the real world and adjust the volume to the same loudness then the power for each load would be the same, theoretically. But the amp must be able to make higher voltage output to achieve same loudness and power output into the 16 ohm speaker as the 8. Speakers used to be 16-32 ohms when tubes ruled, because of better damping and tubes had no problem making high voltage necessary to play loud. When SS took over, the transistors can make current easily, but voltage is limited to the PS rails, which is lower on transistors. So speaker impedances dropped to make it easier for SS amps to play loudly. They would voltage clip trying to play loudly on a 32 ohm vintage speaker.
I've read conflicting things online...one said the amp's output power would drop by half into a 16 ohm speaker. But that seems to conflict with others who say that lower speaker impedance is actually the more difficult load for a small tube amp...that you wouldn't want to drive a 4 ohm speaker with a 2-3 watt tube amp of a nominal 8 ohm output impedance.
These statements can both be true, but there are other variables that are not mentioned. The amp's power would drop in half into a double impedance load, if the volume was not turned up to match the loudness of the 8 ohm speaker.
The lower impedance is not necessarily more difficult. The amp does not think "difficult or easy." It either has enough power for all the variables at that instant or not. Enough power just relates to the level of distortion. If your amp is rated 2 watts then it will sound audibly distorted above 1 watt, and hit 10% distortion unlistenable at 2-3w. Depends on how the power rating is made. Difficult just means not enough power for the job, which means some combination of too loud SPL, too low load impedance, too capacitive phase, too high output impedance. Simply being 4 ohm load for a 6 ohm amp is not necessarily difficult if the other factors are "easy." Current source amplifiers have infinite source impedance and they can sound
wonderful when used the the right speaker.
Amp power only becomes a problem when there is not enough to do the job. If the 4 ohm speaker is 110dB sensitivity at 2.83V then the 2 watt amp will be fine for ear splitting 100dB musical peaks, although the damping is still low so you will have a tubey smooth sound rather class D etched detail, but there will be enough power. Conversely, if you try to play a 8ohm speaker with 87dB sensitivity and wild impedance plot with dips to 3 ohms and capacitive phase then you would be hearing nothing but distortion as the power limits of the amp are exceeded at 80dB SPL. The amp would soon fail from overheating.
Impedance, damping, phase, sensitivity, loudness all interact to determine a good or bad result.
Just to clarify...my 2.5 watt/channel amp has a single pair of speaker taps with a fixed output set at 4 ohms. So I'm wondering how this amp would drive a 16 ohm single driver speaker system.
That depends on how loud you want to play it, and what the impedance and phase curves of the speaker look like. Since it's a single driver speaker I think the phase and impedance will be OK. Just depends on how loud you want to go. Not too loud, I think... But hopefully the sound quality will make up for it.
