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

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

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

Dcstep, I don't know how much of a difference greater amplifier power makes. In my experience if the amplifier and speaker work well together, it doesn't really matter what the amplifier's damping factor is.

I used to think that a speaker would ring like a bell if the amp didn't have a high damping factor, but I no longer believe that. The voice coil is powered all the way through its cycle. In a properly designed speaker, the voice coil never has to depend on the amplifier's low output impedance to "put on the brakes" and stop unintended oscillation.

I have owned two amplifiers (Berning ZH-270 and Wolcott Presence P220M) that had variable damping factor controls, which imho was a useful feature for pairing up with different speakers. With neither amp did I ever encounter a speaker that sounded best with the damping factor at the highest setting.

I tend to like zero-global-feedback tube amps, which inherently tend to have a very low damping factor. This does limit speaker choice somewhat, but once I put on my speaker designer hat I see opportunity there instead of limitation.

Duke

Atmasphere's description here is EXACTLY what I heard with Wolcott amps as I turned the damping factor control up higher and higher. The loudspeakers were SoundLab fullrange electrostats:

"...[One] of the first things to go is the low frequency ambiance in the recording. Further reduction of definition [raising of damping factor] results in 'poster board' images of instruments in the soundstage."

Duke