We should reject hard-to-drive speakers more often

in your experience ralph, when an autoformer is used to better couple tube amps with lower impedance speakers, is there a meaningful sonic penalty?

@jjss49 IME a tube amp often won't deal with low impedances, particularly something like 1 or 2 Ohms. In those cases, there are no tradeoffs- the autoformer makes it possible to drive the speaker where it was not possible before. Its when the speaker is a bit higher impedance (4 Ohms or slightly more) where you might encounter tradeoffs.

BTW this holds true for solid state amps as well. Even though they might easily double power into a lower impedance, that's not the same as saying they sound their best doing that- and you can also measure the increased distortion. Paul Speltz, the guy that makes the ZEROs, has a letter from Steve McCormick (who makes very competent solid state amps) that says exactly that: it sound better driving 4 Ohms thru the ZEROs than doing it directly, despite being able to easily double power into 4 Ohm from 8.

Ralph, so the answer is = none. My best, MrD.

Back in the early 90s, TAS did a review of our MA-1 OTL amplifier. OTLs are not known for driving low impedances. The reviewer, Steven Stone, had a set of the old Apogee Full Range speakers, which were 1 Ohm. At the time we made an outboard autoformer called the ZMusic (similar to the ZEROs) that we installed between the amp and speaker- the ZMusic had a 1 Ohm tap. The result drove the Full Ranges quite well- despite the impedance, they otherwise were a resistive load and didn't need a lot of power. Steve commented it was the first time he had ever heard the speaker with a tube amplifier. 

What tube amps ( tubes used in the output stage ) can " adequately " drive 1 ohm Scintillas ? I am not aware of any, and would like to know, so I may sleep better tonight. My best, MrD.

If you use an outboard device like the ZEROs then its possible.

If we are talking about the original Apogee true ribbon 1 Ohm speakers, this would be news to me.

Technically Atmasphere is correct, they use the aluminum panel as the voice coil

the reasons for thermal compression in actual voice coil speakers can't possibly apply here.

Hence my use of the word 'possible'... In the case of a magnetic panel speaker like the Apogee, the 'voice coil' for lack of a better term can be heated.  It seems to me that would take some power. If it heats, then less current can be applied because the impedance is higher. That's what causes thermal compression. The thing about thermal compression is that it can happen with individual bass notes; IOW the 'voice coil' is heating and cooling dynamically with the music. If the speaker is using a magnetic principle then this can happen. How much it happens is a different matter.

I don't think ESL's are the only loudspeakers that don't suffer from thermal compression, what about full range panel speakers like apogees?

Apogees have a voice coil and so thermal compression is possible.

High power class D amps more than make up for the difference in dynamic contrast between low and high efficiency speakers. High efficiency speakers have way worse problems than thermal compression.

@jon_5912 Rather than innuendo, could you be specific about ’way worse problems’?

If any amplifier is properly designed and operating properly, it will not add dynamic contrast to the signal. The signal itself is the source of dynamic contrast. Loudspeakers only take away from that; if you value dynamic contrast, using a speaker that has the least thermal compression will bring you closer to your goal. In this light, ESLs have the least thermal compression owing to no voice coil at all; a close runner up is higher efficiency loudspeakers, in particular those that employ field coils (since the magnetic field in an electro-magnetic loudspeaker does not sag when current is applied to the voice coil).

Some argue that SETs are the most ’dynamic’ of all amplifiers, but if you use a sound level pressure meter you find out that isn’t true- its really distortion on the leading edge of transients interacting with the way the ear perceives loudness that causes this impression.

Amplifiers cannot ’make up the difference’ in terms of dynamic contrast.

My experience with ESL’s says that this low impedance (1/3rd of an Ohm) in the upper octave is quite noticeable and often pushes owners to beefier solid state amps.

@erik_squires FWIW, about 90% of our MA-2 (a 220Watt class A triode OTL) production are running on Sound Lab ESLs. Tubes work quite well with ESL57s, ESL63s and ESL98s. Most solid state amps behave as a voltage source and since ESLs in general tend to have an impedance curve that varies by about 9:1 or 10:1 from the bass region to the highs, quite often a voltage source will sound bright as the amp doubles its output again and again as frequency is increased.

For this reason, some ESL producers make their speakers low impedance in the bass and nearly a dead short in the highs, limiting the ability of the amp to drive the higher frequencies (partly due to the speaker cable impedance becoming a significant portion of the source impedance).

Even then, brightness is an ever-present danger with such amps, particularly if they have distortion rising with frequency.

I think the answer to that is the so-called Hoffman’s Iron Law

@lanx0003 Exactly!

yes low impedance in upper registers is nothing to worry about but tuning impedance curve in the lower registers to design a speakers desired characteristic. I am thinking Wilson speakers would be a good example some Wilson’s are known for having great slam in the lower registers, does this make sense or am I completely off base here?

@jeffrey75 I don’t think Wilson is doing what you’re suggesting, if that’s what you mean by ’off base’.

Take ATC speaker/driver as an example, if one desires a good low frequency extension out of a small-to-medium sealed enclosure (which I prefer) design speaker with a given size of driver, efficiency has to be sacrificed.

@lanx0003 While this is certainly true, what about large multi-driver full range speakers that are really hard to drive? They don’t get the Murphy that a smaller speaker does for low efficiency.

I might see this a bit differently because our ears hear on a logarithmic curve. I figure if 100 Watts won’t do the job, 200 or 400 Watts won’t either because 2 or 4x more power isn’t that much louder- 3 or 6dB. To get to a perceived ’twice as loud’ its generally accepted that you need 10x more power. In radio parlance this is known as ’gold plated deciBels’; 1000Watts, I think we can all agree, simply isn’t practical.

Live music needs efficient speakers to reach suitable SPL levels but for smaller spaces speaker designers can flatten the curve but the impedance does go down.

@jeffrey75 If the amplifier is behaving as a voltage source and the speaker is designed for that (and 99% are) then a flat impedance curve isn’t important, and if you look at the impedance curves of many speakers, you’ll see that most speaker designers don’t value a flat impedance curve.

Class D is going to make high sensitivity speakers obsolete.

@jon_5912 As a manufacturer of class D amplifiers I can tell you this statement isn’t correct. The advantage of higher sensitivity combined with higher impedance will always result in lower distortion from the amplifier and the speaker will have greater dynamic contrast owing to less thermal compression in the drivers, plus the speaker will be less critical of speaker cables.

Until those three problems are solved easier to drive speakers will have an advantage.

I am not sure if I need to tell you it doesn't make sense.

See above.

That a speaker is difficult to drive just means you need to get gear capable of driving it. I think the great majority of people who own such speakers have got suitable power for them. 

Of course that is very true. But like I mentioned earlier, if it were a simple thing to have the speaker be the same but otherwise higher impedance/easier to drive, it would be instantly smoother and more detailed both at the same time since the amplifier would be lower distortion.

This is audible; Steve McCormick makes a very competent solid state amplifier that can easily double power into 4 Ohms as opposed to 8 Ohms. The Anticables guy makes a device called the ZERO that is an autoformer that allows you to drive a 4 Ohm load while the amp sees an 8 Ohm load. He has a letter from Steve that states while Steve's amp can easily drive 4 Ohms, it sounds better driving the same load through the autoformer. I'm sure the autoformer has losses and the like; its pretty good bet that if it were not needed the result would be even better.

Yes, the 705s are not that hard to drive. But whether the speakers were sensitive or not was not the issue. Did I like the sound? I know two people who have Maggie's and 400 watts in each monoblock to drive them. They love them.  I see no reason to consider the fact that a speaker is hard to drive as a negative in and of itself. If you don't have or can't afford the power to drive them, then sure, it's a factor. 

@brev 

This is why it makes a difference: the harder you push the amp, the more difficult the load, the more distortion the amp will make. The other thing to consider is that not all really high powered amps are all that musical- its pretty well known that smaller amps tend to sound better.

I suspect you're using the same amp since both the speakers you mentioned are fairly easy to drive. But consider that if 100 Watts won't do it, you'd need 4 to 8x more power to do the job and as you point out, that can be expensive.

Also keep in mind that while the ZU is easy to drive, its not the only speaker that is meant to be easy to drive and there a number that IMO are also better sounding (while also being more efficient).

@brev I get that this was your experience which I don't argue. But the sensitivity of the speaker does matter. FWIW, if you have the standmount version of the 702, its a pretty easy speaker to drive despite being a little less sensitive.

@ricred1 

My room is L-shaped...front wall is 20’ wide, length is 26’, rear wall is 26’ wide, and 9’ ceiling. My room is rather "neutral", not too lively and not dark.

With those speakers in a room of that size I would expect you'll need about 200-400 Watts. I don't know of an integrated amp that makes that much power. 

Please suggest a couple of integrated amplifiers that can drive MBL 101e speakers.

My experience is limited to the mbl 101e. It was rated at about 81 dB which you would think is crazy inefficient. Its also a 4 Ohm load, climbing to 8 Ohms in the upper midrange. But it does not have any weird phase angles so there's that.

So add 6dB and you're at 87 which still isn't great, but that's a lot better than 81!

Now you have to figure the size of the room. In a moderate sized room most of the time a 100 Watt amplifier would do the job fine. So there you have a bit of info as to what to look for. I can't say more without knowing the room size and liveliness. 

Interesting...I’m considering MBL 101E speakers, but I’m hesitant due to them being hard to try.

@ricred1 MBLs are easier to drive than they might appear. That is because they are omnidirectional and when measured, the microphone is placed 1 meter away from the speaker. Most of the output of the speaker is thus not picked up. Effectively in a room you can add 6dB to their rated sensitivity. So they can be driven be amps of much less power than you might expect!

OK, thinking about this this is less about the gear and more about the awe created by hard to drive speakers. Like we somehow want to make the speakers happy because they are so demanding. Maybe we should get speakers that are happy with any amplifier instead??

@erik_squires This is a good point.

I’ve been telling people for years now that the harder the speaker is to drive, the more distortion your amplifier will make. Especially with respect to lower impedance, this is easy to see in the specs of any amplifier. Its also audible by making the amp less detailed (since distortion obscures detail) and often harsher (since more higher ordered harmonics are created).

So what I tell people is that if high quality audio reproduction is your goal, your amplifier dollar investment will be best served by a loudspeaker of higher impedance (8 Ohms or more) that is also easy to drive (no weird phase angles and higher efficiency).

Lower impedance speakers have a hidden cost of the speaker cable being far more critical to best performance. Lower efficiency speakers suffer thermal compression in their voice coils. There are some attempts to mitigate the latter problem, but whatever the technique is can also be applied to higher efficiency speakers with no downside.

If your speaker requires more than 100 Watts to really make it sing, you have an impractical speaker! This is because of the logarithmic nature of the human ear (which is why we use deciBels). If 100 Watts won’t do, how much power will you need? To make the speaker play twice as loud subjectively, you’ll need about 1000 Watts. How many amps of that power really sound like real music without being oppressive? How many can afford that? For this reason, even very expensive loudspeakers are often fairly easy to drive.

The only real advantage of 4 Ohms might be that your amp can make 3dB more power if its a solid state amp that can double power into 4 Ohm as opposed to 8. So you have a weak 3dB argument for that, if sound pressure as opposed to sound quality is your goal- good luck with it; 3dB is a slight increase in volume to the ear...

So if you want a really decent, musical system, difficult low impedance loudspeakers should be avoided. It does not matter ’how good it sounds’ to you; if that same speaker were simply higher impedance it would sound even better (speaker designers take note of this simple method of causing your speaker to sound smoother and more detailed at the same time).