Can an amp overdamp a speaker?

Coming at this issue with my speaker designer hat on, in my opinion it is possible for a speaker to be either overdamped or underdamped by an amplifier. Most speakers today are designed with high damping factor solid state amps in mind, but then those speakers may be underdamped (too much bass) when driven by a low damping factor specialty tube amp. On the other hand, a speaker designed to work well with a low damping factor specialty tube amp may be overdamped (not enough bass) when driven by a solid state amp.

Blame for a mismatch (resulting in too much or too little bass) is often incorrectly attributed to the amp or to the speakers, when really it's a component compatibility issue.

To zero in on your second question, a higher damping factor is not always a good thing - it depends on the speakers. My speakers can be tuned for either type of amplifier, but in my opinion they perform best with a low damping factor amplifier.

Duke
dealer/manufacturer

Duke, I am confused. Can you please explain how one can tell by looking at a speaker response curve if it has a solid state or tube amp in mind? I always thought that those curves were amplifier-independent and any off-bass responses were the result of an amplifier's inability to keep up current at low impedances or room effects.

My understanding of speaker design tells me that the amplifier's output impedance affects the electrical Qe, which in turn, affects the speaker's Qt. If the damping factor is infinite (zero output source impedance) then the electrical Qe is just the voice coil and crossover resistances. Would this not be the ideal, the smallest possible value of Qe?

My view is that damping factor is way misunderstood. I think some envision the amplifier physically starting and stopping the speaker cones. It's the magnet that does that. The motion of the cone is the same no matter what the damping factor. What a high DF does is minimize the effect of any back emf, reducing distortion. Its measure also indicates the amplifier's ability to put out higher current at lower speaker impedances.

I would not even consider damping factor in choosing an amplifier, period. If an amp/speaker combo is not meant to be, you will hear it.

Gs5556, I agree with you that the primary low-frequency effect of amplifier output impedance (or damping factor) is on driver electrical Q. And the lowest possible value of Qe is not necessarily ideal.

Let me give an example of a speaker that would be suited for a low damping factor (high output impedance) amplifier. Suppose you have a very low-Q fullrange single-driver speaker whose frequency response (measured on a high damping factor amplifier) shows a gentle rolloff starting in the upper bass region, with the low bass being pretty weak. This speaker will sound thin and gutless without help in the bass region.

Okay, let's look at what happens when this speaker is driven by a tube amp with a very low damping factor, let's say a damping factor of 1 (output impedance of 8 ohms). This would be like doubling the driver's electrical Q. So in effect we would be getting a "free lunch" - about 3 dB or so of bass boost. This will probably make the speaker's tonal balance acceptable without having to add a subwoofer.

To give a real-world example, I build a speaker that is -3 dB at 35 Hz and -6 dB at 32 Hz with a solid state amp. With a low-damping-factor tube amp, this same speaker is -3 dB at 30 Hz and -6 dB at 24 Hz (this assumes appropriate re-tuning of the port.) As you can see, there is a significant "free lunch" here as well: the -6 dB point (which often predicts the actual in-room bass extension) moves about half an octave lower with the low damping factor amp. And, the response is actually smoother (less ripple) with the low damping factor amp.

I just gave away one of my secrets. Since it's over here under "Amps Preamps" instead of under "Speakers", maybe no one will notice.

Duke

Duke, I agree that using speaker equations can show that the lower cutoff frequency is a function of Qe. For example, if you assume a speaker design with a Qt of 0.707 with a driver that has a voice coil resistance of 5 ohms, a Qm of 3, 8 ohm impedance, and a Fs of 40 hz, then Qe at a DF of 1000 or greater will be 0.925 and give a lower cutoff of 40 hz. If you go down to a DF of 10, Qe is 15% higher and the lower cutoff drops 3 dB to 36 hz. But that's not a free lunch...

What you have to take into account is that the SPL ultimately depends on pressure generated by the driver. The factors affecting pressure are the driver magnet force factor, piston area, voice coil resistance and mechanical compliance. The SPL using these factors is inversely proportional to voice coil resistance. So decreasing the DF has the same effect as increasing the coil resistance so any -dB frequency drop by increasing Qt is offset by the reduction in SPL which is a wash. Otherwise, you could simply place a variable resistor in line with the driver which you can then adjust Qt to play with the frequency response in your room, independent of amplifier power.

Gs5556, the net effect of low damping factor on SPL is not the same as having a resistor in series. The modification of Qe is the same, but because there is in fact no resistor in series that is soaking up power there is no corresponding loss in SPL.

Once again, a real-world example: The low damping factor OTL amplifier I use delivers about twice the wattage into my speaker's high impedance in the bass region as it does into the 8-ohm load they present across the rest of the spectrum. That extra wattage in the bass region is indeed a free lunch.

Just for the record, note that the wattage output of a high damping factor amplifier is also modulated by the speaker's impedance curve, but in the opposite direction.

Duke