Monday, 22 March 2010

Active versus Passive Speakers THD.

In my previous blog I explained the various different distortions that occur in passive speakers and which made active ones so much better. In this one I will present measurements made of both an active ADM9.1 and a passive version driven by a low distortion Class A test amp.

Both speakers were measured at a nominal 86 dBa at 0.5 Metres. The Mike was a Bruel & Kjaer type 4165 measuring microphone and the source was the low distortion output of an HP339.

The above is a distortion plot of the active ADM9.1 at 100 Hz and it shows the second Harmonic at -48 dB and third Harmonic at -50dB just visible above the noise level in the factory.

The above shows a passive ADM9 at 100 Hz where the second harmonic has remained at the same level as the active version but surprisingly the third Harmonic has increased by 12 dB and the fifth Harmonic, absent from the active version, is at -45 dB.

 The above shows that at 3 kHz (in the region of the crossover) second and third Hamonics are almost lost in the noise at -55 dB.

The above shows the passive version at 3 kHz, the second Harmonic is now -46 dB and the third Harmonic at - 48 dB.

Saturday, 20 March 2010

What Constitutes a Good Drive Unit?

The Picture above is of the 5" drive unit we use in our Neutron Five loudspeakers. It is excellent and typical of the best available today.

Below are measurements pinched from a DIY forum to illustrate how they compare with an extremely expensive "high end" driver from another manufacture.

Plot one shows a smooth amplitude response to way beyond where a crossover would go and plot two shows  harmonic distortion.

Plot three shows a much more expensive "hi end" driver from another manufacture, which is remarkably similar in amplitude response and plot four shows that it is in harmonic distortion as well. The reality is that either drive unit will give excellent and similar results. However there are other characteristics to consider that will also affect performance.  For example; power handling and maximum linear travel, where ours still compares well. 

What these measurements show is that the performance variations between the between modern paper diaphragm drive units from reputable manufacturers are comparatively small,  though nothing like as small as those between DACs and other electronic components.

There is however a divergence of opinion of what constitutes the best diaphragm material and many prefer  hard or metal ones. 

The above illustration is of a good quality 5" metal diaphragmed driver, which has a smoother amplitude response to 4 kHz, will probably have slightly less harmonic distortion, but which breaks up catastrophically at higher frequencies. AVI doesn't like them and wouldn't use one, but others prefer them and use an extra filter in the crossover to reduce/remove the spike. We don't like this either because it is further distorting phase. 

In our opinion the paper or soft diaphragm is the best compromise because it acts as a shock absorber to suppress and iron out the tendency that cones have to break up above a certain frequency dictated by their size. I think 1.4 kHz for a 5".

These differences aside, all high modern drivers have relatively shallow cones to aid dispersion, large diameter spiders to minimise distortion, they are spaced off the front plate by about 10-12 mm with ventilation underneath to avoid resonances and they have longer coils than the front plate to give lots of linear travel. Ours has + or - 5 mm which is very good and double that before there is risk of mechanical damage. Some may remember the "clack" "clack" of the spider hitting the front plate of an old Kef B110 in LS3/5As!

This last illustration shows a large diameter and compliant spider and ventilation through to the voice coil underneath. This is typical of almost all modern, good quality drive units and has been for some years, although small incremental improvements take place all the time.


Thursday, 18 March 2010

Nobody Understands Bass

Actually most do, especially if their music collection includes Acoustic instruments which are an excellent reference with which to judge the quality of reproduction and the amount of bass extension their system is capable of in their room.

Bass varies enormously from recording to recording (possibly by as much as + or - 10 dB), from room to room and much modern popular music is mixed harsh and bright to make it appear loud and exciting even on cheap systems. It gets more bass as well, but treble is dominant.

Active speakers have very much better control of low frequencies (all frequencies actually) and so, to the uninitiated, may appear to have less bass, but they haven't, so in this blog I'm going to attempt to show all the different factors that effect the amount that you hear.

Most music has a spectral content that covers an area a little over an octave each side of Middle C. Typically bass rolls out quite steeply from about 80 Hz and in classical music about 1 kHz.

The image above is of the opening bars of Mahler's Symphony of a 1000 or number eight. The peaks at 80 Hz are from the Organ. Note that there is a 10 dB drop at 1 kHz which is typical of acoustic music of all types. It's not as bright as pop.

This next spectral plot shows modern commercial rock music, which is not much different at the low end from the Mahler, so equally dependent on strong bass for its message, but what should be noted is the massive additional treble extending by another octave before slowly rolling out. This is typical modern music and is mixed to sound loud, exciting or good on anything. The extra treble benefits a cheap system by improving perceived clarity but can be a problem on an accurate one, especially to a younger listener who will have a massive 10 dB extra sensitivity in the 3 kHz region compared to an over 35 year old.
Our ears don't have a flat amplitude response, they peak at 3 kHz and the slowly roll out again

Now consider the amplitude response of a typical AVI 6" Two way speaker:

Measured anechoically the amplitude response is within plus or minus 1.5 dB and the minus 6 dB point is 50 Hz, so more than enough extension for all music, given that an untreated listening room tends to add bass as the frequency lowers. This is called Room Gain.

Rooms have a profound effect at low frequencies and they vary as you move or your speakers move.

The above image shows the same speaker measure in a typical listening room. Notice that the amplitude response is profoundly affected and also that this measurement would be completely different if the mike or the speakers were moved. Most enthusiasts realise this and juggle with their speakers and their listening position to achieve an acceptable result.

We're finding that about 90% of listeners do not want or need a Sub, but as you can see from the above measurement, it is quite conceivable that some might, especially if their preference is for particularly bright modern rock or large scale Classical Music, Big Bands and most important of all, with Movies where the greater scale is almost essential.

Passive speakers boom because passive crossovers prevent amplifiers from properly controlling them and cheap speakers also boom because they come with smaller magnets and less linear travel. When a cone is driven beyond the point where the magnet can control it, distortion goes up very quickly and the subjective effect is boom. ADM9.1s have expensive high performance drivers with lots of linear travel and they are active so definitely don't boom and may not have enough "bass" for some, which is not a problem because a dedicated Sub with an almost infinite variety of adjustment has been designed to provide exactly what may be required.

The option of our special Subwoofer means that ADMs are firmly in three way floor standing territory, but with the substantial advantage of being fully adjustable to suit taste, music and room acoustics, not to mention the enormous benefit it brings to movies, which just wouldn't be the same without a Subwoofer.


Saturday, 13 March 2010

Active Speakers - What are They?

There are three types of loudspeaker:
These are what most hi fi enthusiasts use, they are the cheapest to make and simple to design. Anyone with basic electronic skills can design a crossover, if they can’t there are computer programs that do it for them. Drivers are available off the shelf and The Loudspeaker Cook Book will give them a grounding.
The principle is simple, you need a separate power amp and inside the speaker box is a circuit known as a crossover that sends high frequencies to the tweeter and bass to the bass driver. In a three way it’s more complicated and you have an extra crossover to split the mid off from the lows.
Although they are simple, passive speakers are deeply flawed and introduce enormous amounts of distortion to an otherwise very pure signal from a modern amp and DAC. There are so many different types of distortion that it is difficult to describe them all, however the important ones are: Firstly the crossover must disconnect the amp from the drivers and so relinquish control of them. They are free to overhang and overshoot and secondly the crossover components add more distortion too, so it’s probably a good guess that they are a factor of a at least thousand times worse than the active equivalent. There are other problems too, but you get the picture........
In simple language they boom, tizz and congest compared to a good active speaker.
Powered speakers are simply passive speakers with the amps built in as is typical with computer speakers and the ones to be wary of because many are sold as active when they are not and will not be as good.
Active Speakers
Form very early on it has been accepted that active speakers are best and this was proven in the nineteen seventies when Op-Amps made it practical to build suitable crossovers for Public Address and Studio Monitoring. Active speakers are considerably better than passive ones, so much so that it is hard for many who’ve invested a fortune in piles of boxes to accept that a better result can be achieved with what looks like basic electronics on a plate screwed into the back of a pair of speakers, but it can.

The reasons are very simple. Firstly the amplifiers are permanently connected to the drivers and preventing them from overshooting or overhanging. Secondly the crossovers, which are before the power amps, can be much more accurate, have steeper filters and be more perfectly phase and amplitude correct. In a passive system the drivers are reacting against the crossover, but they can’t do this to an amp. Thirdly amplifiers have less distortion if the bandwidth they cover is restricted as it is in an active system. Fourthly high voltage low current amplifiers have less distortion than high current amps needed to drive passive speakers.
The difference between active and passive speakers is literally enormous and, as you can see from the illustration, this was recognised before 1954, It means that they can compare with the better headphones.
Why isn’t everyone using active speakers?
Everyone in Broadcast, Movie Making, Music and Public Address probably is, but not many hi fi enthusiasts who tend to be late adopters of new techologies, although that surely will change soon.
Hi fi enthusiasts have for years relied on subjective evaluation, they have believed that even cables sound different and they have insisted on auditioning each part of their systems separately. Even if that isn’t really possible, that’s what they think they are doing. They are also conservative and resistant to change, many are still using turntables, still will not accept the quality of MP3 or AAC and they have often spent a fortune, so aren’t keen to know that they can do better for a fraction of the cost, but they can.
There is no doubt also that conventional active speakers were more expensive than an Integrated amp and passive ones, because you still need a preamp and a CD player etc. However computers have changed all that by being the best source and cheap and we’ve (AVI)  put the DAC and preamp into remote controlled active speakers and so reduced the cost considerably. AVI has changed everything.

Wednesday, 10 March 2010

More Recent History
Since the early eighties when Hi Fi was the third most important purchase for most after a House and a car, TV technology beginning with the VCR, has taken what money people set aside for media. The market for hi end, as it is called, Hi Fi probably collapsed in 1997 as the wealthy bought expensive AV based, multi room systems. Some Hi Fi manufacturers were able to cash in on this boom, but not able to keep pace with falling prices and developing technology, so continue to contract, although the wacko tweak brigade has consolidated, collared the specialist market, grown a little and will probably be around for some years yet, albeit in the margins.
In 2001 Apple’s resurgence began with the announcement of the iPod. Heavily ridiculed at the time, it quickly became a design icon and now, nine years later, over 250 Million have sold as have 10 Billion music downloads! It’s a story of staggering success and, although Hi Fi types would howl in protest, an iPod sounds a great deal better than virtually any separates system made today. All anyone has to do to see this for themselves is either buy some good quality headphones or simply hold the standard Apple ears buds firmly in their ears, so no air escapes past them and enjoy a full, rich and detailed sound that just isn’t possible with most hi fi. In truth vast numbers of people have realised this and so it’s not surprising that the Headfi forum has about ten times the membership of a typical flat earth one.
Having recognised these developments, we stopped production of CD players in June 2006 and the rest of our separates in September the same year. Instead we concentrated on developing a system that could not only compete with the best headphones, but also one that did not require multiple boxes, not even one box, just a pair of speakers.

People today are much more discerning, more interested in style and fashion and most have immaculate homes. They don’t want more than the essentials, which usually means a computer, TV set, the option of music/games/video in one or more other rooms, but no wires and everything small and unobtrusive. The solution was easy and electronics are sufficiently small to do it. Build an entire hi fi system into a pair of speakers including the DAC and remote controlled preamplifier so that anything capable of outputting a digital signal, be it a computer, a DVD/PVR, a PS3, Xbox or TV could be the perfect source for a better and real world priced hi fi system.
It has been accepted since the early days of two and three way speaker systems that “active” (crossover before amps and one for each driver) was better, but complicated and expensive and probably not necessary for most. However all that changed in the seventies when the first audio integrated circuits (Op-Amps) appeared and compact systems were possible. Active immediately became the standard for PA systems and for studio monitoring and it might have been the same for hi fi because good models were available, but they got stymied by the then dominant subjective brigade who insisted that everything had to have it’s own box and cost a fortune. We owe them a lot because thirty odd years and little or no progress later, we were able to play catch up and introduce our ADM9.1s that benefit from the better drivers and electronic components available today.

Sunday, 7 March 2010

In The Beginning

In 1927 Compton MacKenzie founded Gramophone Magazine and although its purpose was to review Classical Music, it also introduced the idea of more faithful reproduction of recordings by describing how to sharpen thorn needles for the best reproduction of Choral music. Thus High Fidelity in the UK began and through the thirties great names like Murphy, Decca and Marconi produced high quality equipment to satisfy demand. Two of the finest were both founded before WW2, Quad and Wharefedale and when it was over, the quest for perfect sound continued and more and more companies appeared. They began by offering amplifier kits and instructions on how to build enormous brick enclosures or horns into homes to house huge old fashioned drive units! And they developed into quite a large and successful industry.
Not surprisingly the more extreme measures taken by some enthusiasts became the butt of many jokes and a record from Flanders and Swan, that is on Youtube, about High Fidelity was an excellent example of the best.

VHF radio and a bandwidth increase from 3.5 kHz to 15 kHz together long playing, vinyl LPs gave impetus to improvements in domestic equipment. This was especially true of live broadcasts of concerts by the BBC, which was arguably the best sound possible until CD replaced LPs in the eighties. The Beeb also used special heavy tracking cartridges to reduce surface noise, but these weren't suitable for home use because they quickly wore out records.

These were exciting times, Britain was a major player and our loudspeakers gained a worldwide reputation for being the best.

VHF radio had allowed listeners to hear faults in
recordings that those who made them often could not. This was because they were using Horn Monitors that were as loud as the music, but much more distorted than direct radiating speakers in better quality radios and radiograms. Wharfedale, Quad and later Kef, B & W and the companies born out of the BBC research department at Kingswood Warren all played their part in the development of less loud but far more accurate, direct radiating speakers for homes as well as studios concentrating on acoustic music, Jazz, Classical music speech and drama. Popular Music studios were the last to convert, but nearly all now use compact derivatives, usually active, of the designs pioneered by these companies. Few if any remain in British ownership, but they still maintain a presence here to reassure Far Eastern customers of their credentials.

By the early eighties, TV technology had overtaken the public's interest in hi fi and the major Japanese companies, who had by then bettered the Brits, decided to concentrate in this area and leave specialist hi fi for the locals. Sadly our industry has been in decline since. In my opinion the succeeding twenty-five years have been mired by subjective evaluation, the belief that all sorts of things that can't affect sound quality can and the rigorous denial of the value of measurement or comparison. A whole rather idiotic industry has sprung from this situation selling cables, stands, power supplies and a raft of other expensive and unnecessary add ons to keep people buying and believing there is sonic benefit, often to the tens of thousands of pound spent. At the same time the gentle humour of Flanders and Swan has been replaced by ridicule and dismissal not just by the engineering community, but also the buying public. The industry would probably have faded to obscurity sooner had it not been for Apple and the ipod for music and computers generally for giving voice to detractors who'd previously been denied the opportunity to express opinion publicly. As long as a cosy relationship between manufacturers, dealers, magazines and PR companies controlled the public domain, none of the rest of use knew just how many were as fed up with it as ourselves.

Computers have not only changed the way we access music or media generally, they have also changed how we buy and so have brought to an end the monopoly of the hi fi shop. Many still protest subjective credentials and advertise expensive multi box systems, but if you twist their arms, they will admit that 70% of their business is mail order, mass market and single box solutions.

These are exciting times as computers, TV and good quality sound converge into smaller more acceptable, easier to use packages. Customers can now buy DVD players with Hard Drives that record TV programs and store their music and photos or they can buy media computers (where Apple excel) and simply connect the digital output to and amplifier and speakers, or if they are canny and do more homework, AVIs ADM9.1s or Neutron Five 2.1 systems. These are technically superior, better sounding and more cost effective solutions that occupy far less space in any home and can be used by anyone. Old fashioned separate box hi fi systems are no longer necessary or desirable, because modern technology does so much better for a fraction of the cost. It has a been a particularly exciting time of life for me, because as I approach retirement I do so knowing that AVI has made a product for the future that most people need, want, can afford and can use and above all can enjoy more than was ever possible with the impractical angst and user unfriendliness of the dreaded separate box system of the past.