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Power Optimized or Voltage Optimized Speakers?

After taking a needed break in March to get a few business-related things sorted out, we are back in April with this month’s Roger’s Corner. Roger again is taking some time off, but I have a feeling he might peak over my shoulder while I write this one. Our topic focuses on speakers and whether speakers are “power optimized” or “voltage optimized”. As part of the discussion, we will also touch on speaker resonance, but that can be an entire Roger’s Corner all unto itself, maybe a future topic, but for now we will try to keep that part of the discussion brief.

I often hear audiophiles claiming that certain speakers work better with solid state amps and others with tube amps, solely based on their power and/or current output. Typically, these are subjective opinions based on what they have read or “heard”. However, let us examine this a bit further starting with amplification, because the amplifier is one half of the equation that gets us to the facts of the matter. Amplifier circuits can simply be broken out into those that are designed to support the principles of the power model or the voltage model. As a rule, in the power model, power is constant while voltage is variable, while in the voltage model it is the opposite. Based on this you might say the power model represents tube amplifiers and the voltage model solid state amplifiers, and this would be accurate, but there is a gray area as well. This is where the speaker becomes the other half of the equation.

Nearly all solid state amplifiers are voltage model, but tube amplifiers may be voltage model to varying degrees. It depends on their output impedance, or output regulation as Roger would say, and its relationship to the impedance of the speaker. As a guideline, if the tube amplifier output impedance is 2 ohms or greater and the nominal speaker impedance is 8 ohms or higher, then the greater the degree to which the amplifier follows the principles of the power model. This guideline is not as applicable in situations where the output impedance falls well below 1 ohm and speaker impedance falls well below 8 ohms. As the use of feedback results in low output impedance the amplifier is now in a gray area where it may follow principles of both models. In this case a tube amplifier does not eliminate but rather reduces variations in power output. The degree of which depends on the specific impedance characteristics of the speaker at various frequencies and how the amplifier handles the load.

Another comment I hear from audiophiles I find amusing is, “This speaker is ‘hard to drive’ and current hungry so you need a solid state amplifier that can double down in power.” Well, the speaker got the reputation of being “hard to drive” for a reason and you can count on the fact that a hard to drive speaker will never sound quite right with any amplifier of any power rating. Regardless of the amount of power output, if the amplifier must work hard to drive a speaker, then it is also putting out more distortion in the process. The rise in power and the subsequent distortion produced results in higher order harmonics contributing to what the ear perceives as brightness. I doubt very much an audiophile has ever thought about why they feel the “sound” of an amplifier is bright, but there it is. It is a fact that a properly designed amplifier operates optimally when it does not have to work hard and, in the process, also produces the lowest distortion with harmonics the ear perceives as pleasing, such as 2nd and 3rd. 

So, are speakers optimized to support one model or the other? In an ideal situation it would be in the best interest of the speaker designer to build a speaker that is very flexible and can work well with both power and voltage amplifiers. However, this is not an easy task and so often speakers will clearly be mismatched when used with either a tube or solid state amplifier. In those instances where speakers can do “reasonably well” with either, the two presentations will certainly have different sonic characteristics, but the differences will be such that the resulting intrinsic distortion characteristics of the amplifier, and to a further extent room acoustics and personal taste, will determine which is preferable. Ultimately, we need to understand the intention of the speaker designer. 

I have often recommended the best amplifier to speaker matching advice will come from speaking with the speaker designer and asking them what type of amplifier(s) they use when testing and tuning their speakers. It is certainly a lot safer than relying on the information you get by reading audio reviews or perusing various audio forums. Here are a couple examples for reference. Israel Blume of Coincident Technology uses tube amplifiers, notably single-ended triode (SET) with his speakers. This pretty much tells me his speakers have higher and smoother impedance and are designed to work best with higher output impedance tube amplifiers that follow the principles of the power model. Now the Harbeth M40.1 is an example of a scenario where due to the design the impedance curve fluctuates all over the place, yet the speaker plays very nicely with both voltage model and power model amplifiers. For a good explanation click on the link for the Stereophile measurements on the speaker as performed and analyzed by John Atkinson. As Roger always mentioned to me, if you really want to know about a specific speaker check if Stereophile reviewed it and John Atkinson measured it:

https://www.stereophile.com/content/harbeth-m401-loudspeaker-measurements

Now earlier I mentioned we would talk a bit about resonance, and I will start by saying one of the funniest comments I hear from audiophiles comes from those that listen to dipole speakers. When commenting about box speakers they say something to the effect of, “I cannot listen to box speakers because I can hear the box.” My immediate response is to cheekily respond, “Oh really, tell me what the box sounds like.” To which the audiophile begins rattling off a bunch of nonsense. Well, what they do not know is that if they are really hearing the box what they are hearing is the resonance (sealed boxes have a single resonance peak, ported boxes have two), which is a peak in the impedance of the speaker. Unfortunately, the audiophiles making these comments did not understand resonance and could not tell me the resonance frequency of the box they heard. Furthermore, I tend to demoralize them a bit when I tell them that their dipole speakers, be it planar, electrostatic, or especially dynamic open baffle are not resonance free. 

As we are going to keep the discussion on resonance brief, we are just going to reference box speakers. Any box speaker with the woofer in the box is going to have resonance. Now a smart speaker designer understands the frequency of the resonance(s) and tunes the speaker accordingly. One example is my Spendor 1/2e speaker, a direct descendent of the famous Spendor BC-1 originally designed as a BBC monitor by Spencer Hughes. This speaker was tuned with the resonance in mind to be very neutral. Other speaker designers may want the coloration resulting from resonance. For me, early Harbeth speakers come to mind, as well as one of my favorite speakers to listen to, the Audio Note AN-E. When a voltage model amplifier encounters the box resonance it throttles back its power and in theory this gets flat frequency response. 

However, box resonance does not mean the speaker is voltage optimized. An example is the Audiokinesis Jazz Module which was a speaker I had the pleasure of owning for 10 years. The designer, Duke LeJeune, built this speaker, and many others, to take advantage of the amplifiers power response in a way that allows the speaker to have an extra 1/2 octave of response in the bottom octave, without coloration. Interestingly he tested and tuned the speaker using an Atma-Sphere S-30 (power model) and NuForce Reference 9 amplifier (voltage model).

So that is it for this months Rogers Corner. Hopefully I will be back next month with another edition. It might be beneficial to continue this discussion in one form or another. Perhaps touching upon something a bit controversial such as planar speakers tending to be power optimized as their impedance curve is not based on box resonance, and it is resonance that drives the impetus for the voltage model. If you take away anything from this article, I hope it is the fact that all amplifiers will benefit from higher impedance rated speakers. I personally wish we could go back to making 16- and 32-ohm speakers again. Unfortunately, the advent of solid state amplifiers in the 1970s allowed speaker designers to get a bit “lazy” and settle for 4 ohm designs with moody impedance curves and given that Class D amplifiers are now the rage and here to stay that probably will not be changing anytime soon.