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PLEASE NOTE - THIS PROJECT ARTICLE IS NOW AN "INTEREST" ARTICLE ONLY

Note that some things are not up-to-date

"The Linear Current Loudspeaker"

An Introduction

Below is the updated version of the article published by ASoN (Audiophile Society of NSW) .

The Linear Current Loudspeaker - With Near Pure Resistive Load:

I doubt that you have seen such a measurement as above.

Blue: Impedance - Red: Electrical Phase

The usual loudspeakers most of us use, it is the current that causes the speaker's Voice Coil to move. It makes inherent sense to use current. Only the edge of the Voice Coil sees the voltage, but this voltage is not in the magnetic gap. Send voltage down a line (a conductor if you will) and that voltage will be divided up along its path by various elements until it reaches zero. Send current down the same line and the current stays the same all the way through. That means that the very force that activates the speaker is constant. Voltage is never a constant.

-----------------------------------------------oooOOOooo-----------------------------------------------

The Mystery of the Current Mode Amplifier

 (Or “The Journey of the Non-Voltage Amplifier” Part 2 - Part 1 here)

Again I would like to qualify the above title; we are not so much talking about amplification but rather the delivery of power to the speakers.

The idea of using Voltage as a concept of power delivery to speakers is not difficult to understand, as the voltage signal can be displayed as it is regularly done on oscilloscopes where we can see with our eyes that the waves conform to our idea of how waves travel through both electronics and also through speakers and the air. But using “Current” as the delivery method to loudspeakers is not so intuitive and trickier to grasp.

But those familiar with Ohm’s Law, the mystery is at least not quite so great. If the load is 8 Ohm and we send down enough current, the exact same voltage will appear across the same 8 Ohm Voice Coil. But there is of course now the problem that if the 8 Ohm is not flat across the whole frequency response of the speaker, then the voltage will not be flat and neither will the frequency response.

The loudspeaker as a system needs to draw linear current at all frequencies at the same time. But this may be at odds with total current drive. There is no total solution here, but the compromises involved can still be quite acceptable.

Not clear? We shall deal with this shortly and see examples that should make it easier. But the point is clear that the speaker Voice Coil can act as V/I converter, where “V” Voltage is converted to “I” Current, and do the conversion on the fly and to avoid this as much as possible is the desired goal.

This makes compromises more acceptable even with techniques used to flatten frequency response that may seemingly compromise current drive. When the current draw is reasonably linear to start with, then there is less to go wrong as the system's summed current is at least stable. Interestingly even non-Voltage amplifiers will work better under those circumstances. Not all will agree with this approach as anything that may be perceived as V/I conversion is wrong. But 50% (or more) of something is better than 100% of nothing.

We can establish that Voltage amps have low output impedance, many times lower than the speaker’s impedance, and thus Current amps have high output impedance relative to that of the speaker. Also we have what we might call Unity Coupling amplifiers, which is somewhere in between and usually similar output impedance to that of the speaker. Single-Ended Triodes generally fit into this category, as does my JLTi amplifiers and likewise Vacuum State and other non-feedback designs. So we established the three types of amplifier power delivery to the speaker; Voltage, and non-Voltage types which are Current and the third, Unity Coupling. 

The key to using non-Voltage amplifiers is the loudspeaker, maybe even the key to Voltage amps. Not oddly, if a speaker is truly optimised for Current operation (as a linear load on the amplifier) it will work with any type of amplifier, but will work even better with amplifiers with some output impedance.

Hence the key is to have the speaker's load be as flat and as resistive as possible. So if we have an 8 Ohm speaker, it needs to flat as possible and as close as possible to that 8 Ohm target. A resistor draws the same current at any frequency, is always ruler flat and likewise for the electrical phase, which is to say that the current at all frequencies always stays neutral current, linear current. We may not forget that just as Voltage can be positive and negative, so likewise Current can be positive and negative. We prefer neither.

Current that is said to be positive can also be described as inductive. Current that is negative can also be called capacitive. Inductance is potentially non-linear, particularly the internal inductance of drivers is recognised as such. Capacitance is on the other hand is a more severe load on the amplifier and we must not combine high capacitive negative phase angle combined with very low impedance at any particular frequency. Heavy negative currents should be avoided, alas not always.

Most designers are fairly well aware of these things. Some will allow the impedance to drop way too low to my liking and also combine it with too much negative phase angle – but then they would not think for one moment to use anything but a Voltage amp capable of large current dumping ability.  They have been fed that by convention.

The ideal speaker should be resistive and as non-reactive as possible. Keep in mind that inductance (some) and capacitance (a lot) are reactive. So is anything that creates an uneven frequency response, such as cone resonances, anything that creates pressure on the cones, such as the box and a myriad of other things.

Can we combine a resistive load speaker driven by resistive (high output impedance) amplifiers? Yes, it has now finally been done and will be able to demonstrate it at ASoN’s August meeting.

Can you see the beauty of this? The amplifier now has greater immunity to non-linear reactances. Yet this is rarely explored.  Why? Because it normally would destroy the frequency response unless we draw flat linear current at all frequencies.

If you read Part 1 [was an earlier article in ASoN Newsletter ] you would have noticed that non-Voltage amps are effectively amplifiers that mimic a series resistor on the output, Unity Coupling about 4 to 8 Ohm and Current amp the effective series resistor should be a minimum of five times higher. It can be done. It will be done, and it was done!

 For this reason a new speaker was developed:

 Presenting The Linear Current Loudspeaker

This will be the first public demonstration of the The Linear Current Loudspeakers. These were nicknamed SOFF, acronym for Seriously Only For Fun speakers. But that name was provisional.

The "SOFF" speakers are based on the Visaton full-range B200 driver and the Tweeter is a modified Monacor DF300 with their matching waveguide. It is a minimum phase 1st order loudspeaker and ends up with the most amazing resistive load.

The two drivers are clearly shown in this pic:

 

 The Visaton B200 is German made and it looks all quality construction.

The Monacor DT300 with its optional waveguide:

The tweeter is modified by having the ferro-fluid removed as well as different damping material behind the dome and using a circular felt with Dacron in the hollow pole piece. Three layers of BlueTack is used to damp the insides of the waveguide.

Below shows how the ferro-fluid was removed from the gap.

The use of a waveguide is an essential part of this design. It has a significant effect on the construction of the crossover that lowers the negative phase angle in the critical upper midrange and lower treble.

Many who take their first look at this speaker will instantly think, ah horn loaded tweeter. No so quick please. Waveguides are not horns and don’t even behave like horns. It has been established that horns tend to maximize impedance transfer and causes significant High Orders Modes, called HOMs for short, and this is what characterises the honky horn sound. Waveguides avoids that and are about constant directivity and avoiding HOMs and the horn colourations. For those who want to know how Waveguides are different from horns, may I suggest you read this article:

www.gedlee.com/downloads/Horn%20Theory%20reply.pdf

The Box

People are often fascinated by what goes inside a speaker box, so here is the internals: 

Some would look at this and think “transmission line” – but I don’t call it that. Besides what is a transmission line? It seems to describe so many things. So what is this then? I call it an Aperiodic Damped Line. The reason is simple, the B200 has a high Q of 0.75 and usually a box will push that further up. But this line does the opposite. It also has a single Q peak in the way a sealed box has, but much lower peak and hence the added localised damping we shall use in the Crossover will be that much easier to first model and then construct. The damping: Approximately 450gr at the rear and about half that at the front behind the drivers.

The Crossover

The crossover is fairly complex and a total 16 elements make up the total Crossover. It was designed to have the flattest impedance and electrical phase ever done before. It is a proper two-way and was optimised to give the following result:

No longer displayed due to  commercial considerations.

Below is the actual measured electrical characteristic seen by the amplifier:

The Blue is the impedance with its scale on the left. It is very flat at 6 Ohm across 20 Hertz to 20KHz. The Red line is the electrical phase of the current required by the speaker in degrees; the scale is on the right. Zero degrees is basically a resistor, so it is apparent that the above example is as close as you will ever get to having a speaker act as a pure resistor. Note there is not the usual “saddle” below 100 Hertz that you get with a bass reflex and neither has the single peak you get with a sealed box.

Hate to sound like I am blowing my trumpet, but I do not know of anybody who has achieved such a result. Let me now show you some samples of typical impedances and phase plots:

Impedance solid line, electrical phase is dotted.

Yes Edgar, this is the Wilson Sasha speaker as measured by John Atkinson of Stereophile. Note that it drops down to 2 Ohm and also the saddle centered at near 25 Hertz that represents the tuning of the bass reflex alignment. The low 2 Ohm is at least offset as it coincides with close to zero degrees phase impedance at the same frequency. It is still a heavy load, just look at around 55 Hertz where 5 Ohm is combined with a large negative phase angle. But I gather this is rather better plot than the previous model it replaced.

BTW, the peaks above and below the saddle under 100 Hertz is very well controlled and reveals the 25 Hertz reflex tuning. I know how Dave Wilson achieved this effect as I also use it in my latest Elsinore Mark 4 speakers.

Now if you think that the above looks erratic, then take a look at a typical commercial speaker.

 

Here the impedance is above the limit of the graph, certainly not unusual to see plots reach well over 30 Ohms. But here the designer has at least managed low impedance points and large negative phase angles not to coincide at the same frequency. Yet this speaker, and the Sasha, would have difficulties with non-Voltage amplifiers. The frequency response would deviate significantly. Just how much will surprise you, so read on.

What these examples prove is that none of these speakers are resistive. Both the impedance and the electrical phase jumps all over the place. How can you even say they are 8 Ohm or 4 Ohm or whatever? That seems a rather nominal notion.

But there is a general rule to follow, determine the lowest impedance and then multiply by 1.4 to arrive at the nominal impedance. Since the Sasha’s lowest impedance is 2 Ohm, the nominal impedance is 2.8 Ohm – it is not even close to a 4 Ohm speaker. Clearly the intent is to use a solid state amplifier capable of that. The other sample above is the Acoustic Energy AE1 is 4.5 Ohm at 200 Hertz. Again multiply by 1.4 and we get 6.3 Ohm nominally; cannot be called an 8 Ohm speaker.

The Ultimate Torture Test

Now let us turn to example of a locally made commercial speaker, one that is highly regarded and designed by a friend of mine, available from Len Wallis. Won’t reveal his name except to say he is a good friend. The modeling info was acquired here when he brought a sample of this model. What I am about to do to his speaker system is a torture test that would make any designer cringe.

 Here is the very flat frequency response – but it won’t stay that way for long:

 

 Here is the impedance plot, quite normal as it goes for a Two-Way:

 

 Here is the electrical phase plot:

 

Both above plots are not unusually difficult types as demonstrated by other examples. For a Voltage amplifier should not be difficult. The lowest impedance is around 250 hertz, but the phase is near Zero at that frequency. It’s a reasonably easy load that qualifies as a nominal 4 Ohm speaker. But we are going to seriously test this and get this speaker out of its comfort zone.

Now let us see what happens when we drive this speaker from amplifiers that are of the non-Voltage types, such as Current amp, Unity Coupling amp and compare with a Voltage amp:

Oh dear! The Red line is the Voltage amp, the Green line is Unity Coupling amp and the bottom two Blue and Violet represents what Current amps would do.

So that is clearly not acceptable. But let us now look at what the different non-Voltage amplifiers do to the electrical phase. This is rather interesting:

Same colour coding here. Note as the Red plot is with the Voltage amp which in theory has near or zero output resistance. It is forced to see the full  negative phase angle whether it likes it or not. The inescapable conclusion is that the Voltage amp is far more exposed to the vagaries and sins of the speaker. As we up the virtual series resistance, the amplifier sees a much flatter negative phase and at 33 Ohm and above this speaker ends up looking near resistive load, so unlike what the Voltage amp sees.

But, it is a big BUT: The non-linear impedance, the non-resistive impedance, completely tears apart the frequency response, and the sample I have shown above is not one of the worst I could have shown.

The Linear Current Speaker Undergoes the Same Torture

OK, here goes. Can I do better or fall flat on my face? It is now the time to put the new Linear Current Loudspeaker speaker under the same microscope. Will I end up cringing as my fellow designer would cringe with the above example?

Here goes, first the frequency response:

 This is quite credible.

 Here is the modeled impedance and phase plot:

 

The bottom is the impedance and the upper is the electrical phase plot. This was simulated before the crossover was constructed and then tested to see if the result would confirm the modeling. It did.

Now for the impedance and phase plot, the actual measured result:

 Now it gets really interesting, what if we repeat the above torture test with different types of amps:

 

This is simply stunning. Right now I have a smile on my face. Again, if I may repeat, I do not know of anybody else who has achieved such a result and I have been doing speakers for more than fourty years. BTW, the electrical phase plot, there is no point showing it as it just gets flatter, flatter and flattest. At 33 Ohm it is ruler flat.

I will do the ultimate test a little later using one thousand Ohm.

Now this gives me an opportunity too good to miss. So on I go on my soap box.

DEMOLISHING A MYTH: HIGH DAMPING FACTOR

The Linear Current Loudspeaker finally puts the nail into the “damping factor” coffin. I read a review of an amplifier in the latest Hi-Fi World where it was extolled for having a damping factor of over 200. If that is so good, then what happens when we have a damping factor that is negative.

The problem I see here is that some people design amplifiers and others design speakers. Never shall the twain meet. The number of designers who does both is rather smallish. You have to see both sides of the coin. Who promotes damping factor? Mostly amplifier makers. What speaker manufacturer talks about the need for amplifiers with high damping factor?  I search my memory bank and I can’t think of any. Speaker designers think in terms of Voltage and Current required by their speakers, not so much about damping factor. They create the load and expect the use of the right amplifier to be able to match the demand of that load. Simple really. They may well recommend using Solid State Voltage amps when their speakers have non-linear impedance and electrical phase. Speaker designers seem to be occupied with speaker design and let the other mob concern themselves with designing amps. This is not ideal.

Back to damping factor: When amplifier’s output impedance is higher than the speaker’s impedance, the so-called damping factor goes negative. If we were to follow the damping factor convention, then there would be no speaker out there that would work satisfactorily with any amp with a negative damping factor. This is not true.

One designer who wants to understand both speakers and amplifiers and who has come to similar conclusions drawn here is Nelson Pass. You may want to read his article here:

www.passdiy.com/pdf/cs-amps-speakers.pdf

Pass even goes so far to state that damping factor is often too high and that over-damped bass may lead to less musically satisfying results. Now here is somebody who bothers looking at both sides of the coin. Rather than being bothered by damping factor he says we should aim at a critical damping that sounds best. He even extols the benefits of low damping factor:

“With low damping factor, the Fostex [full-range driver] became a totally different speaker. It suddenly had bottom end response and a better top end.”

I have never heard an amplifier designer of note saying that low damping factor could be desirable. But he did. What gives?

Also, why is the “top end” improved (but not just that I can attest to)? Just like real estate agents say the three most important things are location, location and location; this is a case of immunity, immunity and immunity. The removal of reactive elements improves everything across the board.

Pass also makes use of conjugal networks like I do.

What is significant is that critical damping here is arrived from parallel elements rather than any damping factor by the amplifier. This is what Pass refers to as critical damping. This is damping that is localised rather than coming from an external source supposedly controlling the speaker. Perhaps the reliance on Voltage amps has not really been helpful to speaker designers who need to think outside the box.

The primary design goal was based on the thought that a flat impedance and electrical phase would and should have both optimum and critical damping. At below 100 Hertz Bass is where the benefits of damping factor are considered a must. Not here. The flat impedance and electrical phase in the 20 Hertz to 100 Hertz should provided the ideal damping, so my antennae told me, not just a gut feeling, just a right feeling. The results are definitely proof. I get flat in room response down to 40 Hertz and a 3rd order roll-off below that. You can definitely hear bass below 40 Hertz, but the Bass quality struck me as most satisfying. It has speed and agility as well as a live factor and very out of the box. There are qualities in the Bass that makes me very happy.

So using the techniques available we can shunt neutral current in parallel with the load, the speaker. Properly attended to, this becomes the localised parallel critical damping of the speaker. Nothing more is needed.

The high damping factor of the amplifier, basically its low output impedance, may actually make it difficult and even impossible to apply critical localised damping. So with Current amps and Unity Coupling amps, it is the speaker designer who controls the result; the amplifier designer should supply, not high damping factor, but high immunity.

Will that become a trend? Time to start one.

Yet the following rules are as non-violable as laws of nature; they will stand the test of time:

Fact one: Voltage amplifiers, specifically of the Solid State kind, with significant feedback to supposedly bolster damping factor, do not have a desirable high level of immunity.

Fact Two: Classic Single-Ended Triodes so beloved by many have high level of immunity because they are Unity Coupling (got that phrase from Lynn Olson) and not Voltage amplifiers. Not just because they are Single-Ended. Similarly, a properly designed Push-Pull Triode does likewise.

Fact Three: It does not take a lot of intelligence to come to the conclusion that the speaker must be designed to suit the kind of amplifier used. But a speaker with linear load characteristics incorporating critical localised damping will have the edge when driven by non-Voltage amps.

The Linear Current Speaker goes much further than even what Pass has done and demolishes the reliance on the damping factor myth, and does so effectively. It will work with any amplifier. But it will work even better with non-Voltage amps as the localised critical damping will do its job without being short-circuited by the low output impedance of a Voltage amplifier. Speakers with localised damping will work better with non-Voltage amps. It will work with amps with more than 100 Ohm output impedance.

I want to show you one final assault on the capabilities of the Linear Current speaker, here is what it looks like with an amplifier with 1000 Ohm output impedance, that is one thousand Ohm:

 

This is as extreme as it gets. So in this instance, with this speaker the output impedance of the amplifier is irrelevant. Then the same applies to the damping factor required by this speaker.

This is a speaker that does not require any damping factor!!!

Damping factor from the amplifier is completely mute. But the amplifier’s immunity to the speaker’s reactance is enormous. And you can hear that.

For our upcoming tests at ASoN we will be guided by Nelson Pass who pointed out that 47 Ohm is adequate for nominal 8 Ohm speakers to operate in Current mode. We shall use 33 Ohm as the our speaker is a lower 6 Ohm, not nominal but real. For the Unity Coupling test we shall of course be using 6 Ohm. We will also have an actual Unity Coupling tube amplifier, the JLTi KT88.

A Fool Once, But Not Now

Now for a story that goes back to circa 1977. I went to the old Pye Australia that used to be in Chester Hill [Sydney] to buy some Foster 6.5 inch full range drivers. When I picked them up I was told I ought to get in touch with somebody at Sydney University who did testing of their speakers.

So I indeed got in touch with him and also got some computer print-outs from him that showed results from different box sizes. He asserted that the driver was over-damped for use in small sealed enclosures and recommended adding a series resistor as tabulated in his print-outs. I was truly taken aback by that. Who would recommend adding a series resistor between the amplifier and speaker? I argued the point with him and even blurted out at some point, what will happen to the amplifier’s damping factor? He would have none of it. It was irrelevant as far as he was concerned.

It took me a long time to realize that he was right. It was the alignment of the speaker, the actual final configuration of the speaker that matters. Get that right and all’s fine.

BTW, the gentleman with whom I had that argument was none other than Richard H. Small. Maybe that name ring a bell? When you look up the specs of any speaker you find the Thiele-Small parameters. Yes, it was that Small I was arguing against, the silly fool I was.

So the bottom line is this, most speakers are made in such a way they need a low source impedance to drive the non-linear impedance and electrical phase. Some are far less sensitive to this than others, just look at the impedance plot to see if that is OK. Then judge the effect it has on the alignment of the speaker system. It is a marriage, a matter of compatibility. This is a speaker that will have extreme compatibility unlike any other speaker around. But it will marry better with amplifiers that have, to use Nelson’s word, low damping factor. I would go as far as to say none!

We have but covered the essentials and this amounts to no more than an introduction.

Joe Rasmussen

At The Cross Roads

joeras@vacuumstate.com

www.customanalogue.com

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Last modified: Sunday January 05, 2014

Just had a terrible thought. If "intelligent design" is unscientific, then who will design our audio equipment?