Speaker choice: impedance, sensitivity, and tubes

Actusreus wrote: "I would like to hear a speaker designer's point of view regarding this common tendency in speaker design. If designers such as John DeVore and Alan Yun (Silverline Audio) understand this quite well, why don't others?"

Because most audiophiles use solid state amps, most speaker designers don't bother with making their speakers tube-friendly. Which is fine by me, because it leaves a market niche for us little guys to exploit.

Paying a lot of attention to the impedance curve reduces your driver choices, and often makes crossover design more challenging. I could probably get away with about 1/3 to maybe 1/2 fewer components in my crossovers if I didn't mind a roller-coaster impedance curve. Also as noted already, high efficiency + decent bass extension = a big box. With audio in general moving towards small-box speakers for aesthetics, big, tube-friendly speakers are going against the grain.

While impedance dips can reduce the real-world power that a tube amp can deliver, impedance peaks can be bad too because of their effect on the frequency response (assuming the speaker was designed for "voltage paradigm" solid-state amps). Let me explain:

Suppose we have an "8-ohm speaker" with twin impedance peaks in the bass region that rise to 16 ohms, and a 32-ohm peak at the crossover frequency of 2.5 kHz. At a level equal to "1 watt into 8 ohms" a solid-state amp will be delivering 1/2 watt into those bass impedance peaks, and 1/4 watt into that 32-ohm peak at the crossover frequency. If the speaker was voiced for a solid state amp, then what happens on a tube amp is this: You get 3 dB more output into the bass impedance peaks (which can make the bass sound underdamped), and you get about 6 dB more output in the crossover region (which makes the speaker sound forward and fatiguing). Your instinct would be to blame the tube amp and conclude that tube amps are "fat and forward" because the speaker sounded fine on the solid state amp, but the real culprit is the speaker's impedance curve.

The approach I take to the impedance curve is to keep it fairly high and with no significant peaks north of the bass region, so that the spectral balance doesn't change with amplifier type. To deal with the effects of the bass impedance peaks, I tune the box considerably lower than is theoretically "optimum", so that the region boosted by the port is moved down lower in frequency. The net effect is more extended bass than we normally would have gotten, without excess upper-bass thickness. This is as close to a "free lunch" as it gets, but it's only a "free lunch" when contrasted with the same speaker on a solid state amp. (Actually my port lengths are user-adjustable, for compatibility with the various amplifier types).

In my opinion the high efficiency and benign impedance curves that make for a tube-friendly speaker are generally beneficial regardless of amplifier type, so the main drawback is the larger box size imposed by the requirement for higher efficiency.

Of course mine is by no means the only legitimate approach to designing a tube-friendly speaker. The designer can let the impedance curve do whatever and simply voice the speaker for tube amps.

I've done some over-simplifying in this post, but hopefully given enough information to illustrate some of the considerations on the designer's side of the fence.

Duke
dealer/manufacturer

Duke (Audiokinesis) - thanks for that cogent explanation. However, it riases a bunch of other questions for me.

First is, when a speaker manufacturer publishes frequency response specs, how can they possibly apply to BOTH tube and SS amps at the same time. Unless the amp in someway can sense the change in impedance and adjust its power output when encountering impedance curves, peaks and valleys, how is it possible for a speaker that is designed for a SS amp to have a flat FR if a tube amp is used, and vice versa of course?

Second, if the answer is "SS is SS and tube is tube, and never the twain shall meet (pun - LOL)," isn't it a bit of a misrepresenation on the speaker manufacturer's part not to mention that its published stats only apply in the case of [blank] type amps and that results can vary if [other blank] type amps are used?

I bet that most speakers on the market today have roller coaster impedance curves. I think I read somewher that Wilson speakers have pretty wild curves here and there too, but are also considered tube friendly. I understand that ARC uses Wilsons to check the sound of it amps.

So I guess the bottom line is what is the consumer supposed to do to make an informed decision when mixing and matching components??

Thanks for your cogent response. Hopefully we'll be hearing from some the EE/tech types too.

Recently found this...

http://www.dhtrob.com/overige/tubefriendly_lsp_en.php

Thanks Ngjockey -- that's exactly what I was looking for. In fact, I think it responds to the OP.

Bifwynne poses some excellent questions.

Bifwynne: When a speaker manufacturer publishes frequency response specs, how can they possibly apply to BOTH tube and SS amps at the same time?

Duke: If the impedance curve is fairly smooth, then there will be no significant change in frequency response when going from tubes to solid state. And the added power a tube amp delivers into the (virtually inevitable) bass impedance peaks usually only changes the shape of the curve in the low end ("fattening" it up a bit) without actually extending the -3 dB point significantly (UNLESS the port tuning is lowered). So while the frequency response curve is not going to be exactly the same, we can make an educated guess that we'll probably get a little bit more bass energy with a tube amp (and not to overly complicate things, but some tube amps moreso than others).

But if the impedance curve is a roller-coaster over most of the spectrum, then the speaker's tonal balance will change significantly with amplifier type, and the best bet is to follow the speaker manufacturer's recommendations on what sort of amps to use (if that information is hard to come by, you may be able to find out what amps he used at audio shows).

Bifwynne: If the answer is "SS is SS and tube is tube, and never the twain shall meet (pun - LOL)," isn't it a bit of a misrepresenation on the speaker manufacturer's part not to mention that its published stats only apply in the case of [blank] type amps and that results can vary if [other blank] type amps are used?

Duke: Well maybe so, but there is enough... ah... "optimism"... already in the specs of many speaker manufacturers that making comparisions between speakers based on specs is somewhat "speculative" at best. Some manufacturers assume significant contribution from the room in calculating efficiency and/or bass extension, and some do not, and that will probably have a bigger effect on specs than whether a tube or solid state amp is used. But it's perfectly kosher to ask the manufacturer.

The one thing a manufacturer can't really "pad" with optimistic assumptions is the enclosure size. Assuming you're comparing the same type of enclosure (like vented boxes), if one box is significantly bigger than the other, that speaker will either go deeper or be more efficient (or maybe a little of both).

Bifwynne: I bet that most speakers on the market today have roller coaster impedance curves. I think I read somewher that Wilson speakers have pretty wild curves here and there too, but are also considered tube friendly. I understand that ARC uses Wilsons to check the sound of it amps.

Duke: If the Wilsons were "voiced" on "power paradigm" speakers (this paper should be required reading for anyone tube-curious and/or speaker geekish: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php), then they would work fine with ARC amps. Also if the speaker's impedance curve "zigs" where the frequency response curve "zags", a tube amp will tend to smooth out the frequency response.

Bifwynne: So I guess the bottom line is what is the consumer supposed to do to make an informed decision when mixing and matching components??

Duke: The ideal would be to have the anechoic frequency response of the speaker (preferably out to at least 45 degrees off-axis) along with the impedance curve, but unless the speaker has been reviewed by Stereophile or SoundStage, that information is unlikely to be available.

If a tube amp is involved, try to find out what the output impedance (or damping factor) of the amp is, if possible. Then contact the speaker manufacturer, tell him the amp's output impedance, and ask him if that amp would be a good match with his speakers. Or tell the speaker manufacturer the brand and model of amp you're looking at, and he can probably probably still give you good guidance. Or tell the amp manufacturer what speakers you're considering, and get his input.

Don't be fooled into thinking "bigger is better" when it comes to damping factor - the woofer's cone is never unpowered as long as the amp is on, so it never has to rely on "damping factor" to stop its motion; the audible effect of "damping factor" is on the shape of the frequency response curve, so good speaker/amplifier/room synergy is the key. Those three form a system, in my opinion. Anyway damping factor is simply 8 ohms divided by the output impedance of the amp in ohms, and is not an indicator of quality in and of itself. The circuit techniques that result in a very high damping factor (very low output impedance) often tend to be detrimental to sound quality, but that's another topic, probably for another forum.