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Product: Active Yamaha NS1000M conversion
Manufacturer: Chevron Audio - UK
Prices:Electronics £1300 per set (two electronic 3-way 4th order active crossovers, six Hypex UCD 250 LPOEM amplifiers, and associated high current power supplies)
Custom machined crossover & power amplifier casework, £550.00GBP per pair
Speakon cabinet adaptor plates £90GBP per pair
Labour to fit complete conversion at Chevron, to customers own NS1000's: £250.00 pair
Matching pre-amplifier also available
YMMV due to import duties and currency volatility
Reviewer: Mark Wheeler - TNT UK
Reviewed: Autumn 2016

Yamaha NS1000M Active Version

The incredible transparency and low distortion of the Yamaha NS1000M, even 40 years on and apparently limited only by the passive crossover, invites experimenting with active crossovers to take it to the next level or even further. Martin Colloms' splendid book, High Performance Loudspeakers, includes information, including distortion, for the Yamaha NS1000M compared to a contemporary 3-way; read page 432-433 (sixth edition) for evidence. This is precisely what an owner commissioned from Chevron Audio for the Yamaha NS1000M in the pictures. Those of the Plebs Chorus with long memories recall that Chevron Audio (formerly Chevin Audio) is located in a village near to your Old Scribe. Your Old Scribe became acquainted with Chevron Audio proprietor Colin Yallop, when upgrades were reviewed in the Shanling CDT100C. Relationships in audio reviewing need to be explicit and readers should know that the work on the Shanling CDT100C was paid for. At TNT-audio we do not accept advertising, bribes or unreciprocated hospitality to procure favourable reviews. We do frequently offer and receive cups of tea.

Knowing your Old Scribe's well documented preference for active loudspeakers, Colin got in touch to invite an audition of this active crossover project, leading to these articles. This also led to meeting the Yamaha NS1000Ms' owner, engineer Gary Hargreaves, who also made the stands and tinkers with turntables. Your Old Scribe insisted on becoming familiar with the renovated passive Yamaha NS1000M that Gary had refurbished as a labour of an early audio love. Substituting the active version for the passive version requires minimising the changes of variables in order to establish what active operation has contributed to any perceived change in sound quality. Chevron supplied a balanced preamplifier for the active system and an integrated amplifier using an identical Hypex modules as those in the active amplification block. The preamplifier section of the integrated amplifier was similar to that in the class D balanced preamplifier supplied as an integrated amplifier.

The other unavoidable variables are the long pre to crossover line level cable in the active system, which contrasts with no interconnect in the integrated amplifier. In keeping with our policy of not using electrically eccentric cables as primitive system tuning devices, high quality pro sector balanced microphone cable was used. The other cable difference is that each of the low-resistance, low-capacitance loudspeaker cables from the six-pack of active amplification is less than 1m long. In the passive set up, there are similar short flying leads from the external passive crossover to the Neutrik Speakon connectors on the cabinet, plus 4m loudspeaker cables from integrated amplifier to passive crossover. This is a fair comparison because this is one of the primary set-up differences between typical domestic passive and active loudspeaker configurations. The total available power and much more wire (for the active version) are the primary variables in addition to the active crossover arrangements.

The Active Crossover

The choice of crossover slopes in the active system is Linkwitz-Riley. The original passive crossover will have been designed to:

• Match the impedance characteristics of the drivers
• Integrate the different sensitivities of the drivers at crossover points
• Integrate the dispersion characteristics of the drivers at the crossover points
• Prevent overload of the drivers with signals below their designed passband
• Correct broad frequency response anomalies of drivers

The Linkwitz-Riley (1976) crossover is impractical in passive crossovers because of high complexity with high power handling components. Siegfried Linkwitz and Russ Riley presented their filter design to the AES in 1976 (your Old Scribe was easily persuaded by their arguments in Birmingham Public Library that year). This filter topology is also known as a Butterworth squared filter, for reasons which will become obvious. The classic second order (12dB per octave) Butterworth filter (and its later relative, the constant summed amplitude CSA filter) dominated commercial designs in the 1960s and 1970s. The Butterworth filter features a maximally flat response (Q=0.707 or half root 2) and is easily achieved with a capacitor and inductor combination. Psychoacoustic research suggests that filter knees of Q=0.5 have better phase characteristics for music. Typical single component first order crossovers achieve this with obvious cost benefits but obvious driver overlap leading to uneven frequency response and off-axis response. As soon as a designer abandons the madness of trying to design and manufacture effective passive crossovers, the freedom offered by active filter designs opens the possibility of closer control of more ideal crossover slopes.

Because the Linkwitz-Riley (1976) is effectively two Butterworth filters cascaded to achieve the desired 4th order (24dB per octave) the Q (the shape of the knee between flat passband and ultimate slope) is established as the product of the two filter poles. Therefore the Linkwitz-Riley Q is 0.707 X 0.707, which is conveniently Q=0.5, felt by many to be the optimum figure for phase shift in audio filters. When converting an existing loudspeaker system to active operation, it needs to be remembered that this will change the overall character of the system in the listening room. Steeper slopes reduce the overlap between drivers around the crossover frequency. The interaction between the driver outputs creates an off-axis effect similar to a comb filter, so steeper slopes should be better if executed without detrimental side effects exceeding the advantages.

A disadvantage to simple flat passband active filters replacing well designed passive filters, is that the latter might cleverly sum with the driver response to improve the acoustic performance. The coincidental 1976 evolution of the third order Kef104aB ('aB' = 'acoustic Butterworth' designed by Malcolm Jones, who also designed the active crossovers used in the loudspeaker system described in TNT-audio.com here) achieved a more accurate acoustic response than a textbook filter design. Kef stated that previously, crossover filters had been designed primarily on the basis of the electrical response of the filter but, in reality, the acoustic roll-off is also a function of the natural roll-off of the driver response.

Set up changes in the listening room

The removal of a series inductor from the bass driver, with its finite and substantial electrical resistance, combined with the ultra-short loudspeaker cables replacing 4m of wire snaking across the listening room floor, makes for a much tighter impedance match (commonly expressed as damping factor). The passive alignment sounds like a Bessel (Qts=0.577). Looking at reliable contemporary test results (including my collected copies of HiFi Choice), the anechoic test graphs look like Bessel while some internet descriptions state Q=0.7. The NS1000 Monitors were designed for use against a wall, which would support a gentle knee, low Q alignment that would produce a response akin to Butterworth when mounted on a stand hard against the back wall. This might be where the descriptions differ as the in-room response will be substantially different from that in a large anechoic chamber.

The 5mH series inductors (chokes or coils) of the passive crossover bass have 0.4ohm dc resistance (DCR), reducing amplifier impedance control of the 4.9ohm DCR bass unit, typically by a factor of 12. Whenever a passive loudspeaker design is converted to active amplification the overall system response of amplifier-crossover-loudspeaker changing to crossover-amplifiers-loudspeaker renders a much leaner bass response in the listening room. The already low Q bass alignment of the Yamaha NS1000M becomes leaner, sounding like it is well below Q=0.5. The original Yamaha design intention for rear wall placement cannot benefit from being even closer to something it is already touching. Remembering common control room practice, an equivalent to pseudo-in-wall mounting was achieved by placing an even taller loudspeaker adjacent to the active Yamaha NS1000M. The extra pair of loudspeakers, acting as 'wings', as in those used to improve bass projection on a PA system, worked best with the minimum gap between Yamaha loudspeaker baffle and wing, with the front surfaces of each perfectly aligned. Thus, usual bass service was resumed as soon as possible.

While test disks are not the usual fare on TNT-audio.com, they are very useful when positioning loudspeakers. Rear wall placement means that the NS1000M need positioning far from the zone of neutrality, established by techniques such as the Wilson method. However, their alignment means they have their own zone of neutrality, a compound of room and alignment. This requires as much care as Allison, Naim or Isobarik/Sara generation to get the optimum in-room power response. Achieving this, as those designers intended, offers accurate musical balance, with instrumental timbres more accurately reproduced in rooms than from free-air optimised loudspeakers that are promptly plonked too close to room boundaries due to the restrictions of domestic environments. It is ironic, that in the case of the Yamaha NS1000M, the intention to achieve accuracy in tiny studio control rooms also serves us better in ordinary sitting rooms.

The efforts to achieve a reasonably flat in-room frequency response might immediately be thought to compromise perfect listening conditions in at least 2 ways. The presence of a second pair of loudspeakers in the listening room has long been an anathema to many British audiophiles. The Single Speaker Demonstration Room became a policy of certain audio dealer franchises in the 1980's after a number of commentators and dealers (Russ Andrews claims his shop was one of the first to feature such a facility). In the case of this review, the other pair of loudspeakers are a constant presence in this listening room. Another North of Hadrian's Wall company mythology claimed that even digital watches with alarms could upset a single speaker demonstration but unless there is evidence that this claim was really ever made, and tested blind, the improved frequency balance obtained using these cabinets had no drawbacks.

The downside of proximity positioning is that early reflections have detrimental effects on soundstage. Psychoacoustic research in the 1970s suggested that, in domestic rooms (which are small reverberant fields) the 'frequency response' that we think we hear, is actually a summation of on axis sound waves, direct from the loudspeaker, and what bounces around from off-axis output within 30mS (30 milliseconds) of the arrival of the on axis direct sound. Thus, we experience reflected inaccurate (look at the 30 degree and 45 degree response of most loudspeakers) energy together with the direct sound, so bass response is likely to be more balanced with midrange in a nearfield monitor designed for a close wall proximity position. However, those reflections within 30mS create potential problems at higher frequencies.

The well-known Chesky Audiophile test disc provides some of the best soundstage evaluation tests ever recorded. The drum kit and clicker depth tests do work. With many multi-way loudspeaker systems, the former demonstrate errors like tweeters leading the wavefront, such that cymbals sound way in front of the drums. No such problems with the active configured Yamaha NS1000M. However, the position of the receding clicker and drum kit is far from the recorded position up to 70feet (21M) from the microphone. The wall proximity probably hampers depth (implied by LEDR test results) and only controlled dispersion horn tweeters with low crossover points have achieved accurate depth, for your old scribe, when loudspeakers are this close to boundaries.

Money

What this costs existing owners of Yamaha NS1000M depends on how much of the work they wish to undertake themselves. The naked Chevron electronics cost £1300, for which the customer gets:

• Power supplies
• Six 250W Hypex modules (chosen as sounding the best of the range, even if somewhat overpowered for the tweeters, and configured for 180W/4ohm)
• Two 3-way Linkwitz-Riley active crossover boards
• Power amplifier flying leads with Speakon connectors

The laser cut, powder coated, two part NS1000 Speakon adaptors panels, enabling external mounting (always an improvement) of the passive crossovers or location of the active crossovers and power amplifiers are £45.00 per speaker. Again, Gary's engineering company can supply their own isolation stands to match the version supplied for review, £245.00 pair in fine texture powder black finish, with a special vibration isolated slot for the active active electronics. Labour to fit the complete conversion at Chevron, to customers own NS1000's is £250.00 pair. This adds up to an eye watering £2435GBP. For that price customers could external mount and refurbish their old passive crossovers, buy similar priced stands and still have over £1000 for a new power amplifier. We intend to establish which would be the better option.

Of course, there are also audiofools who would merrily part with this much cash for bits of wire between their existing amplifier and the tired 30 year old passive crossovers, but after reading a few pages of TNT-audio.com they too will achieve enlightenment.

Sound Quality

The active Yamaha NS1000M go further in impressing with even greater low frequency accuracy and more transparency. Rhythm improves to make them an even more likely favourite with the Flat Earth brigade. Better control of that big bass driver in its big sealed box lifts PRaT performance further. The already lean bass became slightly leaner, which is usual when changing from passive crossovers to active crossovers. It is due to better control of the loudspeaker system resonance, specifically the electrical resonance's (Qes) contribution to total system Q (Qts) because there is no longer that 0.4ohm series inductor (chokes) between amplifier and voice coil, each having a significant resistance.

The passive Yamaha NS1000M low frequency pitch accuracy, already superior to most comparable (age or price or size) reflex loaded loudspeaker systems, becomes more explicit. Without trampling the grapes of wine-speak vocabulary, your Old Scribe has few available adverbs. It is a function of better controlled, already minimal, woofer resonance. The tendency to lean balance can be offset by gently trimming the tweeter response to create more symmetry between the frequency extremes. The lack of midrange driver colouration means that this adjustment, however inaccurate, is achieved without the midrange becoming shouty. Playful use of cartridge loading can achieve similar effects for vinyl users. The box is overstuffed with fibreglass wadding and the extra damping of direct amplifier connection may invite experiments with less stuffing.

The passive Yamaha NS1000M performance had some high treble problems. The tweeter resonance was uncontrolled in the passive configuration, causing some intermodulation. This is an inevitable function of the tweeter level resistor preventing the amplifier's low output impeder control of tweeter resonant behaviour. Tweeters can resonate at their fundamental frequency (moving mass sprung by suspension) and at break-up frequencies. However, at higher listening levels a new set of intermodulation products became apparent, apparently unique to the active set up. At typical listening levels, in the low 90s deciBel range, there were no obvious high frequency problems, but active systems usually allow higher levels without distortion. As volumes headed into the high 90s, cymbal harmonics became splashy, sibilants became forward and detached from mouths, brass shifted forward and sounded overblown. Testing with spot frequency checks revealed nothing consistent. Using sine wave frequency sweeps demonstrated a descending intermodulation product with ascending frequencies above about 12kHz. This is not the behaviour of an errant electromechanical transducer, but more like an amplification PSU problem. Eventually the problem was traced to the preamplifier-cable reactive interface. A modest pair of resistors provided an inexpensive solution in the Chevron pre-amp and the intermodulation products disappeared.

Those tweeter resonances (apparent with both music and signal generator sources) are in fact better controlled when the active crossover is installed. These resonances are fewer than from soft domes anyway, and the rigidity of the domes renders them more controllable electrically by the motor circuit. Another good reason to choose this model for active drive. The much higher sensitivity of tweeters means they are usually connected via resistors (in this case a big L-pad). The beryllium tweeter has high Q resonances (narrow sharp spike), which are already less intrusive than the broad, multiple resonances of soft domes, but they become less obvious when connected directly to an amplifier output.

Once the active system was optimised it simply offered so much more of the same strengths. The transparency, the excellent timing, the wide window into the recording were respectively more transparent, more excellent and wider. Speaking of width, the soundstage extends well beyond the speakers, presumably through reduced driver overlap and more accurate filter phase. This probably explains the increased soundstage depth too. Micro dynamics improve through increased downward dynamic range.

While beryllium tweeters are noticeably superior to soft domes, the beryllium squawker (really the pukka equivalent term for a mid-range driver) is even more obviously superior to soft-dome mid-range drivers when the masking effect of passive crossovers and L-pads is removed. Vocals improve, becoming clearer and more accurate.

Macro dynamics understandably improve dramatically. At higher powers there is no hardening as the volume increases. The hardening was clearly an artefact of increasing crossover inaccuracy as components heat up and their resistance changes. Moments like an organ fill on Cobham's Snoopy's Search gain extra punch and clarity without the stifling passive crossover. Those UCD 250 LPOEM modules have been optimised for this system. With the stock usually chosen Hypex power supply they typically deliver 250W, but with the higher current supply chosen to exploiut the strengths of the Yamaha drivers, they make 180W into 4ohms. Effortless and clean are the adjectives that spriong to mid while cranking the volume as far as the Audio Research Reference 3 preamplifier will deliver.

Some Music

The Mobile Fidelity Gain 2 Original Master Recording 45rpm triple vinyl reissue of Bob Dylan's Blonde on Blonde arrived from across the Atlantic Ocean during this review.
Wha-a-a-a-a-a-a-agh? that bass guitar & percussion on 'Stuck Inside of Mobile' is so much clearer, that some excessive drum kit panning becomes more obvious at the same time as cymbal and stick types?
"The Old Scribe is so desperate for a place in Pseud's Corner that he's writing like a Melody Maker review from 1973", assert Plebs' Chorus, stage left, "And this album must be so familiar to any old fart over 50 that nothing new could ever be heard, said or be relevant in 2016"

We were so inspired that we even had to look online to see if anyone still makes brand new leopard skin pill box hats. The US West Coast pioneers The Charlatans (not to be confused with the UK West Midlands indie band) mostly mono reissue compilation indicates how accurate the active crossover system is from channel to channel. The mono image is firm throughout the frequency spectrum, neither wandering laterally nor front to back. The variable recording quality is clear but doesn't detract from musical enjoyment.

Similar comments apply to Beefheart?s much maligned Batlight Clearkid, whose eccentric microphone perspectives are explicit. It is a little more rambling even than Trout Mask Replica or Lick My Decals Off Baby, but then it was assembled from live takes, by John Peel (influential BBC radio DJ), for his label Dandelion Records.
"The Old Scribe has often stated that more than half his collection is due to hearing stuff on John Peel radio shows, so he probably heard this stuff there first, and it's inspiring a Proustian moment that undermines his judgement", comment Plebs' Chorus, stage left, with unusual attention to detail and literature even.

It takes an exceptionally poor system to screw up audiophile recordings and it takes a really musically transparent system to exploit odd or mediocre recordings, without resorting to an overlay of syrupy euphony. The rhythm of Lo Yo Yo Stuff gets laid bare, with clarity and perfect timing from bass to treble, despite being recorded from inside a wardrobe next to the drum booth. The harsh cracks in Van Vliet's self-destructive voice carry down the concrete pipe in which they were presumably recorded. The transparency and effortless dynamics actually conspire to make it difficult to describe the 'sound' of this system. It became apparent that the little balanced pre-amplifier might be a bottleneck limiting the system. The Chevron active driven, restored, Yamaha NS1000M system had become such an open window that the whole benefits from greater transparancy upstream. Substituting an Audio Research Reference 3 (a good compromise between the modern sound 5SE and its ilk, and the warmer fatter Reference Two MkI) opened the window wider, once fully warmed. Good pro sector balanced microphone cables remain in use, devoid of snake oil or pixie dust.

Ethical Dilemmas

Your Old Scribe takes a well documented position that for a given wad of cash, and active loudspeaker system does a more accurate job of converting electrical signals into air movements than a bigger amplifier and a bunch of high-power frequency dividing networks. Your Old Scribe now knows Colin & Gary and they know your Old Scribe's preferences...
"The Old Scribe is biased in favour of active systems and knows these two builders so he cannot be objective", challenge Plebs' Chorus, stage left, "Either they've been influenced by his opinion to build this system or he's been influenced by their enthusiasm. Either way this is biased in their favour"

"Equally, the Old Scribe has been accused of bias against digital amplification", contradict the usually silent Plebs' Chorus at stage right, "He didn't even like the universally praised T-amp, even after he'd added a massive linear power supply and used a pair of T-amps with active crossovers"

Conclusion

As the Plebs' Chorus would anticipate, your Old Scribe experienced the active configuration with the Yamaha NS1000M as orders of magnitude better in every way, than the passive version. But every TNT-audio.com reader would have expected that conclusion from this author.
"The real question is whether this is worth £2.5k to upgrade and convert, or would the money be better spent replacing passive crossover components and buying a big amplifier and fancy wires to hook them all up together", quoth the original Plebs' Chorus, stage left.

Now that Sterling (GBP), the pound £, is almost at parity with the €uro and the U$Dollar, how much of an amplifier, wire and replacement caps will £2,500 buy? The passive crossovers capacitors alone would take a substantial wedge for the latest low ESR types, even if eschewing audiophile types. That leaves only enough for a pre & power combo, with interconnect & loudspeaker cables, so the power amplifier could cost up to three times the amount (the cost of the replacement high power crossover components is likely to exceed the cost of an active crossover). The 200W Hypex is noted as being the best of the bunch, so the choice would be a better power supply or a completely different amplifier for about £700 plus the saved labour £250, the casework £250 and the adapter pates £90 (total £1290). Even a used power amplifier for under £1500 is going to be limited by the passive crossover compared with the active set up.

"Is the Yamaha NS1000M an ideal candidate for this treatment, or would the money be better spent on a different loudspeaker to be converted?", enquire Plebs' Chorus, stage left.
The Yamaha NS1000M is legendary for its transparency and accuracy and therefore an ideal candidate for taking those qualities further. Any other passive crossover monitor class loudspeaker (as they used to be known back in the day) would benefit equally. If an old favourite needs dragging towards modern standards of transparency, this approach will yield considerable gains and a good bang fer yer buck. The convenient reconfiguration that can be undertaken with the active crossover and power amplifiers means that the loudspeakers may be replaced in future while hanging on to the crossover and Hypex modules.

Your old scribe is biased because no one has ever successfully demonstrated that a big power amplifier and high power crossover can outperform an active crossover and multiple modest power amplifiers at similar cost. However, it is the active crossover and power amplifier package that makes the difference here; the pre-amplifier needed a modification to drive such long interconnect cables and was not as transparent as the rest of the system.

This is a viable audiophile alternative to a professional active crossover and linear amplifier 6-pack because it offers more headroom than the linear amplifiers and a more audiophile focused crossover. The active crossover and Hypex module combination assembled by Colin at Chevron Audio successfully raise the performance of these elderly loudspeakers to a level higher than would be possible with a passive upgrade of similar cost.
There is nothing to compare with the increased transparency, effortless dynamics and greater headroom of an active loudspeaker system compared to its price equivalent passive system. This electronics package, combined with sympathetic restoration of the original drivers and cabinets, brings new life to old boxes.

The extensive playlist indicates how difficult a task was presented by this system to put into words what it does and how it sounds. It has an effortless quality that is difficult to describe and a transparency that defies description because it adds little that can attract adjectives.
If the lean bass balance can be mitigated by placement and sources, enough neutrality can be fudged that this system defies being dated and simply works to reveal what music is present in the recording.

© Copyright 2016 The Old Scribe - Mark Wheeler - www.tnt-audio.com

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