"All amplifiers sound the same" (Peter Walker)

"Amplifiers DO sound different" (Martin Colloms)

The question of the ideal amplifier for driving the Quad ESL-57 speaker is not easy to answer. It is well known that the ‘57 offers a very difficult load, because it is highly reactive and changes from approximately 60 ohms in the bass to 1.8 ohms in the treble region. The popular stacked pairs present an even worse load, because when connected in parallel, the impedance falls to 0.9 ohms (at 18 kHz) which is virtually a short circuit.

In addition to the problems mentioned above, the original Quad speaker has mediocre efficiency (approximately 82 db/Watt) AND limited power handling: Original Quads will flash up immediately when the voltage on the input terminal exceeds 33 Volts (peak-peak).

From 1990 onwards a protection circuit (a diode rectifier bridge) was recommended by Quad-UK, quite similar to that used in the new ESL-63, although this only protects the treble unit and not the bass panels. Models which include the protection circuitry can be used quite safely with powerful valve and solid-state amplifiers, though a long period of hard driving has to be avoided (a different protection mode with a gas discharge tube is available from Sheldon Stokes)

Generally valve amps are not distressed by an awkward load as presented by the '57, but many solid-state amplifiers give up in a cloud of smoke. Some of these examples even damage the delicate Quad speaker before they die. The worst example of this I ever encountered was the BGW series 200, a "professional" power amplifier. The manufacturer had stated in the manual that Model 200 was stable into electrostatic speakers. Unfortunately, it was not! Within milliseconds the BGW went into oscillation and destroyed the treble units. The "Audio Critic" wrote about the BGW: "This one is a disaster…." How true!

On the other hand, a Marantz integrated amplifier (model 1060), whose manufacturer definitely excluded such a difficult load, in practice worked quite well with the Quads. Similar reports have be heard about the original Naim Nait integrated and the NAD 3020 (designed by Bjorn-Eric Edvardson). Both handle the Quads with ease.

The Acoustical Manufacturing Company Limited (better known as QUAD) only recommended their own amplifier designs, as they could not guarantee the performance of other brands on their fragile ESL-57. The valved Quad amps of yesteryear were great designs and ideal for driving the ESL, but unfortunately some of their solid-state successors were not masterpieces. Many audiophiles have tried other products with the ESL-57 and achieved better results than that obtained with Quad’s own designs. The Quad range of amps and alternative ELS-57 products are discussed here.

Stax DA 80, MA 80, DA 300
Solid State, Class A
(1972? '95 ?)

Stax has been known for their excellent headphones and earspeakers (models: Lambda, Sigma), but incidentally manufactured a range of other audiophile products including electrostatic loudspeakers, unipivot tonearms, electrostatic cartridges (!) and Class A amplifiers. The latter were developed to drive their huge electrostatic speakers ( series ESS 3 A , ELS-8X, ELS-F81 and F-83 ), which were notoriously very hard to drive (even harder then stacked Quads!). The Stax Class A amps also worked a treat with both the Quad ESL and ESL 63. Unfortunately several years ago the production line on all amps and electrostatic speakers was ceased. The STAX name is still alive but is now only active as manufacturer of electrostatic headphones.

http://www.stax.co.jp/

Pioneer M-22 Class A
Solid State, Class A
(1977 - ?)

One would not think of Pioneer as high end audiophile company, but there were several items in their history that were quite good. In particular, the M-22 Class A amp was a marvellous device, built like a Swiss watch. The sound quality was a little dark and soft, but quite nice with the Quad ESL.

Nakamichi 420/620
Solid State
(1977 - ?)

Nakamichi is well known for their famous cassette recorders, especially for their masterpiece called “The Dragon”. It was indeed Nakamichi (and not Philips) who demonstrated that the cassette medium could be used for audiophile purposes. Aside to their famous machines Nakamichi also developed several amps that were stable into electrostatics loads. The tiny 420 power amp was a cute design, driving Quads with ease (actually it blew my Quad 405 amplifier away).

http://www.nakamichiusa.com/

Additional recommended amplifiers
(Reportedly suitable but yet to be evaluated yet by the author)

Audio Note by Mr. Kondo
Shindo Laboratories
Wavac Triode Amplifiers

BRAUN CSV 13/CSV 60
Valve (EL 84, PL 500)
(1957 -)

The German Braun company, which ceased manufacturing audiophile products in 1980 and is now only active in the razor/ hair-dryer business) was the only company that was ever allowed to sell an OEM version of the legendary Quad ESL-57 speaker: the Braun LE1. The speaker was designed in 1960 by architect Prof. Dieter Rams and looked rather futuristic.

Prof. Dieter Rams remembers:

"The LE-1 was produced under licence by Quad. Fitted with large, very light membranes covering the whole front, reproduction quality was impressively clear and  transparent, according to the music that was appreciated at the time: jazz, music by Bach, Händel etc."

Braun offered a range of matching valve amps for the LE1. The CSV 60 utilized the PL500 tube, whereas the smaller CSV13 used the classic EL 84. The CSV 60 integrated amplifier was also available as power amp. The sound of the Braun amps is similar to Quad II, very euphonic and sweet, but maybe not quite “accurate”. Service may be a problem, as Braun do not offer spare parts any more, but Gehado Electronic Service or Quad-Musikwiedergabe may be able to help .

http://www.braun.com/
http://www.gehado.de/
http://www.quad-musik.de/

ELECTRO "2-Channel-Audio Power Amplifier"
Solid-State (Class A)
Norway

This legendary amplifier was developed by Mr. Matti Otala, a well known designer because of his former work for Harman Kardon (Citation series) and other companies. He caused a shock in the audio world when he revealed that feedback may produce transient-intermodulation-distortion (TIM). With the design of the "Two-Channel-Audio Power Amplifier" he had proof for his theory.

The Electro (or Electrocompaniet in some countries) was a wonderful match with the ESL-57; power output of 25 Watts was just right. The company has produced more powerful successors to the "2-Channel Audio Power Amplifier" in recent years, but sadly some of the magic sound quality of the original design was lost.

http://www.electrocompaniet.no/

Esoteric Audio Research EAR 861
Valve (Triode)
Huntingdon, England

Tim de Paravicini is one of the true masterminds associated with valve amplification audio, although he also designs solid-state amplifiers and is active in the professional audio scene (microphones, cutting lathes, tape recorders). Many recording studios and musicians (Paul McCartney, Kate Bush, Ry Cooder, Aerosmith) use EAR products. The cutting studio “The Exchange” in London cuts masters with EAR 549 amplification.

He was born in Nigeria and lived for several years in South Africa, where he started  his own business with transformers for mains and audio use. As he was very successful in the business he was invited by the Japanese LUX company to go to Japan and design tube and solid state amps. During his time at Luxman he developed  several amps:

Luxman C 1000 preamp (solid-state)
Luxman M 6000, M 4000, M 3000 power amp (solid-state)
Luxman MB 3045, MB 3600 tube amp (monoblocks)

In 1976 he went to live in England, together with his wife, where he started his own company ( Esoteric Audio Research ) and developed a series of monoblock valve amplifiers that are still regarded as some of the best ( EAR 509, 519, 529, 549 ). He also offered his custom designs to other companies, e.g. Musical Fidelity. Actually it was Mr. Paravicini who redesigned the legendary Michaelson & Austin TVA-1 so that it actually worked properly!

Tim has been a Quad ESL devotee throughout his lifetime. One of his recent designs, the EAR 861, is a triode amplifier (push-pull) with 2x35 watts, ideal for driving the original Quads. It is very popular in Japan where Tim received many prizes because of its superior sound and design. It seems that the Japanese love Tim and Tim loves the Japanese people (he is married to a Japanese woman).

Esoteric Audio Research: direct-coupled valve amp
Valve (DC coupled OTL)
Huntingdon, England

This is an extraordinary piece of equipment. The EAR output transformerless valve amp is quite different to the Futterman OTL, as it drives the electrodes of an ESL direct. Consequently it is not a stand-alone unit but it is mounted  inside the Quad speaker. The main draw back of conventional OTL is that they still need the voltage step-up via the input transformer of the ESL. Beveridge and Acoustat (USA) also had similar amplifiers in their electrostatic loudspeakers (model Beveridge II, Acoustat X). According to Mr. Tim de Paravicini the EAR direct-drive amp is available again, albeit at a price (6.800 GBP).

www.ear-yoshino.com

EINSTEIN “The Final Cut” (monoblocks)
OTL
Germany (2001-)

This could be the best of all OTL designs that were ever made, because it uses a rather advanced circuit design.  Whereas all other designer offer variations of the vintage Cyclotron theme, Mr. Ralf Weiler has developed something new and special - the “Cross Coupled Anodes Circuit". At least the specs are impressive for a valve amplifier:

Power: 55 watt  (4-8 ohm)
Bandwidth: 3 mega Hertz  (!!!)
Distortion: 0.02 %
Output Resistance: 300 milli Ohm
Damping Factor: 70

I first thought the Einstein could be a perfect match for Quads, but a recent demonstration of the Einstein OTL was not very favourable. There is a possibility that I will be able to evaluate the Einstein on my Quads soon and I am hoping for a more favourable impression. One big caveat: The Einstein OTL is ultra expensive and you will need two of them.

Goldmund SRI Mimesis
Switzerland

This is an integrated amp, comprising of fast MosFet devices in the power stage. It is a very strong amp, therefore care should be taken when connected to the fragile ’57 ( protection circuitry mandatory). It sounds very tight and fast, quite dissimilar to a vintage tube amp  like the Braun or Quad II . Maybe not everyone will love this clear and undistorted sound, but one has to agree that the Goldmund is in a special class (Sandro Boccara “Dr. Quad” drives his Quads with a Goldmund). [Editor: no, I use a Goldmund SRA basic amplifier which is stable into the Quad's capacitive load.]

Caveat: Recently the Mimesis technology was abandoned and a new power supply was incorporated called Job. According to several reports (e.g. Studiosound) the Goldmund amps utilizing the Job supply is not 100% stable into complex loads a la Quad.
www.goldmund.com

GRAAF GM-20
OTL
Italy (1998 -)

Italy is normally regarded as the land of fast cars and not of high-end audio. By recently the situation has changed and with companies like Sonus Faber, Chario, SAP and Graff there are a lot of high performance equipment available. Mr. Giovanni Mariani, the founder and chief designer of Graaf, began with normal (i.e. transformer coupled) valve amps, but recently he also designed some OTL stuff. The GM-20 uses the legendary Russian 6C33C triode that was formerly used in Russian MIG jets. Reviews indicate that the strong 20 watt GM-20 works a treat with original Quad, but unfortunately I could not get an example to evaluate with my Quads. The GM-20 can also be switched into bridging mode, in this case you will need two GM-20. There is also a larger version available, the GM 200, which obviously sounds great on Martin-Logans and Quad 989s. (caveat: the Graff OTLs are very expensive.)

www.graaf.it

Naim Audio NAP 250
Solid State
(1973 onwards)
Salisbury , England

This amp is still in production, though the contemporary production line probably does not sound as good as the original because of the odd consequences of the CE legislation (electromagnetic compatibility). Actually the NAP 250 was invented in the same time (1973) as the legendary Linn LP12 turntable, which is also still in production. Naim founder and managing director, Julian Vereker (who recently died of cancer) was an admirer of Peter Walker and his art. Julian owned a pair of Quad ESL57's until his death .

In an interview for Hifi-Review Julian Vereker said :

Hifi-Review: If you were unable for any reason to use one of your own amplifier at home, which other amplifier would you buy, and why?

Julian Vereker: "A complete QUAD valve system with their old electrostatic speakers. The bits...could be purchased through second-hand pages in the hi-fi magazines."

The NAP 250 amp offers app. 70 watts into 8 ohms, but is stable into any load, as long as the dedicated Naim speaker cable is used. The NAP 250 omits the Zobel network and needs some inductive damping from the cable (similar effects are reported about the Bedini 25/25). The original NAP 250 and other similar designs like the NAP 160 and NAP 120 all work quite well with the original Quad speaker. When a NAP 250 is driving ESL57 without protection circuitry, it may be useful to change the output voltage on the power supply boards from 41 Volts (standard) to 36 Volts. Naim Audio can supply a leaflet about this mod.

Naim Audio NAIT
Solid State
(1983-93)

Because many customers asked for an integrated amplifier, Naim Audio developed just such an item from 1983 onwards. Involved in the design of the Nait was Mr. Guy Lamotte, an ardent enthusiast of electrostatic loudspeakers. He had owned 5 pairs of Quad ESL57's, but was not completely happy with their fragile construction. So he developed a prototype electrostatic loudspeaker, code named FL-1, which was something like a battleship version of the ‘57. Press reports indicated that the FL-1 would be launched onto the market in 1987, but unfortunately Guy left the company without finalizing the speaker. Guy’s successor (Roy George) obviously had no experience with electrostatics and was not able to finish the ESL project, so Naim Audio choose to manufacture a range of mediocre dynamic loudspeakers instead.

The original NAIT (Naim Audio Integrated Amplifier) produces only 15 watts, but as it incorporates a rather stiff power supply with a massive Holden & Fisher transformer, it drives the Quads with ease (Chris Beeching also praises the Nait in his "Quadfather" article). Actually this little integrated blows the Quad 405 away, because it can supply up to 10 amperes into awkward loads, whereas the 405 fails with its mediocre 3 amperes output. The Nait was also available as single power amp, the NAP 90.

Recently the Naim production line was completely changed and the original Nait and its successors are no longer available. The current Nait 5 contains many Op Amps and is obviously not in the class of its legendary forerunners.

Radford  ST-25
1960 onwards
Valve (EL 34)
England

These are classics from Great Britain, comparable with Quad II's. There were many variants of the original amp, including mono blocks. Some enthusiasts like Ken Kessler even rate the valve designs of Arthur Radford over those of Peter J. Walker. It is difficult to source one of the original amplifiers, but in the early nineties the new owner of the Radford company offered a “Renaissance” series, which also sounded great.

These are amps without output transformers, designed by Ralph Karsten. The various Atmasphere OTL amps are an advancement on the vintage Futterman OTL theme. Some Quad owners swear by them, whilst others complain of a slight incompatibility. As Mr. Karsten has no distributor in Europe it is rather difficult to evaluate his interesting designs.
www.atma-sphere.com/

BEDINI Model 25/25  and Model 10/10
Solid-State (Class A)
1981 onwards
USA

Two pure Class A amplifiers with 25 watts and 10 watts , maybe not as stable as the ML-2, but sound wise even better. The Bedinis were a peach match with the Quads. To quote the Audio Critic : "Nothing makes the Quad electrostatic sound quite as perfect as the Model 25/25". The build quality was not in the class of Levinson or Stax, and some Bedini owners complained about break downs. Joe Bedini closed doors of his company several years ago, but recently a new investor revitalized the name Bedini and started a new production line. Old Bedini amps can be serviced by the new company.
www.bedini.com

DYNACO ST-70
Valve (EL 34)
1950 -
USA

Aside from Marantz the Dynaco Company could be regarded as the first “audiophile” company in the United States. The ST-70 tube amplifier was a design by the legendary Mr. David Hafler, who latterly started his own company. Dynaco also offered their amps as building kits for DIYers. Aside to the classic EL 34 output tubes, the ST-70 used a rare 7199 “compound tube”  which consists of a triode and a pentode. Many tuning companies (Audio Research, Van Alstine, Old Colony Sound) have been offering tuning kits for the ST-70 for decades and it is reported that with these modifications the ST-70 sounds even better. In the 90's the Pandor Company offered a new version of the classic ST-70, although this time with solid-state rectifiers and different tubes. Both versions are highly recommended, but difficult to source in Europe.

FUTTERMAN H3
Valve ( Output Transformerless)
(1956-85)
Julius Futterman, New York, USA

The H3 and successors H3a and H3aa are output transformer-less valve amps (OTL), which in  principle does not work properly into low impedances. However, Mr. Julius Futterman showed that it could be done with the help of feedback. All Futtermans (or is it Futtermen?) do indeed work a treat with the original Quad and Quad 63, despite the falling impedance. The only caveat were double Quads, where the impedance falls to 0.9 ohms in parallel drive. For such systems Mr. Futterman recommended series connection instead. He argues “For owners of double Quads I recommend wiring them in series and, if I may be allowed to boast a little, they sound fantastic.”

There are many enthusiasts who still argue that the Futterman OTL was the best for driving Quads, although build quality was rather shoddy (some amps even began to burn up!). Highly recommended, but difficult to source.

MARK LEVINSON ML-2
Solid-State (Class A)
(1980 onwards)
Mark Levinson Audio Systems, USA

The ML-2 was one of Mark Levinson's first power amp designs and was a main part in the legendary HQD system, which consisted of 4 Quads, 2 Hartley subwoofers and 2 Decca DK30 ribbon units. The ML-2  was merely the counterpart to the Quad 405 (which boosted a mythical 120 watts  - it actually had only 17 watt into 2 ohms). The ML-2 on the other hand was rated at only 20 watts into 8 ohm (Class A), but doubled its power as the impedance was halved. In reality it could drive ANY load. It was a peach of a match for the Quads, the maximum voltage output of 14 volts could hardly damage the delicate Quad. It is a pity that the ML-2 is not produced any more by the new owner (Harman). Mark had to sell the company because of financial troubles and then started the Cello company, together with his designer, Tom Colangelo. Cello made wonderful products (e.g. the Audio Suite), but this too went bankrupt recently. Mark’s new company is Red Rose Music, dedicated to valve amplification.

NEW YORK AUDIO LABORATORIES OTL-3Q  and OTL-4
Valve (Output Transformerless)
(1985-?)
NYAL, New York, USA

After Futterman's death Mr. Harvey Rosenberg  (president of the distinguished “Quad ESL Owners Club" ) took over the company and gave it a new name. Several version of the output transformer-less amplifier were available:

OTL-1: a massive monoblock monster with separate power supply
OTL-2: triode version of OTL-4
OTL-3: standard monoblock
OTL-3Q: special version for Quads
OTL-4: stereo OTL for ESL57 and ribbon tweeters.

OTL-3 was available as a special “Q” version, custom built for the old Quads and Quad 63s. One pair of the output tubes were removed and an external capacitor box was supplied to roll off the bass below 100Hz.

Unfortunately the NYAL company went bankrupt in the early 90's. There were fervent rumours that Harvey Rosenberg would manufacture again under the “Futterman” name. It was quite a shock for the audiophile community when his former site recently reported that  Mr. Rosenberg  had died in summer 2001.

VTL TT-25 Tiny Triodes
Valve ( EL 84 triode/ tetrode )
1990-2001
California, USA

Designer David Manley is one of the few masterminds associated with tube amplification. He was born in South Africa, then went to England to start Vacuum Tube Logic, but finally moved to California. In the 90’s he sold the company to his son Luke who is also a very talented designer.

After the separation from VTL, David and his young wife Eve Anna established Manley Labs, dedicated to professional audio.

The Tiny Triodes are wonderful little mono amps, using the EL 84 tube in either tetrode or triode mode (push-pull, not single-ended). The triode mode version offers about 25 watts, whilst the tetrode arrangement produces 45 watts. Both the Absolute Sound and “The Electrostatic Doctor” state that these amps are ideal for driving Quads. The contemporary “Limited Edition” of the Tiny Triodes is beautiful crafted, but offers only a 6 ohm tapping of the output transformer. According to the “Vacuum Tube Logic Booklet” a special 16 ohm tapping for the Quad speaker would bring no further advantage with this design.

http://www.vtl.com/

Audio Research Classic 30 (and similar models)
Conrad-Johnson MV-50 ( and similar models)
Dynaco Mk III monoblocks
Marantz Model 8 stereo (original or Renaissance series)
Marantz Model 9 monoblocks (original or Renaissance series)
McIntosh MC 275 (original or Renaissance series)
Quicksilver monoblocks
Classe Audio DR-2 Class A
Krell KSA 50 Class A
Pass Aleph 30 W MosFet
Spectral DMC 50 Class A

QUAD II mono amplifier
Type: Valve (KT 66)
1949-1970

These are the real classics. Maybe not the most accurate in sound terms but the mono valve amps are sweet and euphonic. In one word: MUSICAL ! The II uses the legendary KT 66 power tube, produced by GEC and Mullard. Producing only 15 watts or so output, but this is just right for the ESL57. Used II's can be found in British magazines for around £500 and modification kits are available from several sources. In 1996 a special 40th Anniversary edition was available, albeit at a price.

Both versions are collector items, like the original Quad ESL. Highly recommended !

QUAD 22
Type: Valve

The matching preamp to the II's. Funny looking, sweet sound, but a little dated. Should be overhauled with modern parts. No moving coil input.

QUAD 303
Type: Solid-state (discrete)
1968-86

The solid-state successor to the II, a rather compact 2-channel amp. The 303 was designed for driving the ’57 and  but it was also used successfully by recording studios and public address systems. As there was demand for a stronger monoblock version , Quad developed such an item for pro-audio applications (type 50E).

Avondale Audio can supply a complete overhaul of the 303, which improves its performance a lot.

QUAD 33
Type : Solid-state (discrete)
1963-82

The matching preamp to the 303 and the successor to the valve 22. Funny looking and maybe a somewhat dated sound. Incorporates a very clever filter design. No moving coil input as standard. Avondale Audio offers a complete overhaul of the 33 which improves performance significantly.

QUAD 405
Type: Solid-state (discrete + Op Amps)
1976-91

The Quad 405 has caused several hefty disputes amongst the hi-fi fraternity with some guys thinking that the 405 is one of the best amplifiers around, whereas others (Angus McKenzie, Martin Colloms, John Curl, Anthony H. Cordesman, Haden Boardman, Steve McCormack) have expressed some criticism.

The main problem with this amp is that it contains some rather inferior electronic parts and poorly designed protection circuitry. The operation amp at the input is type LM301, an early type which is unfortunately known for its bad T.I.D. and T.I.M. performance. The LM301 is powered by an unregulated DC line which degrades its performance further.

The design of the output stage is rather conservative, because the amplifier can supply its claimed output power only into a narrow band, around 8 ohms. Above and below this load the output power is drastically reduced, partly because of severe current-limiting. Actually the Quad 405 can only supply only 17/18 watts into 2-3 ohms (Reference: Hugh Ford, Hifi-Choice No 6).

The instantaneous peak current is only 3 amperes, which is very low for a supposedly 120 watt design. In comparison, the 15 watt “strong” Naim NAIT can produce a healthy 10 amperes output. Actually the ’57 would not normally need much power, but as the impedance falls to 0.9 - 1.8 ohms in the treble region, the amp can only drive this load with an abundant supply of current, otherwise the signal will clip significantly.

To play safe the 405 also included an optional limiter (switched in via a wire jumper) which lowered output power even more. With the limiter in, the 405 could only deliver 40 watts into 8 ohms, 20 into 4 ohms and almost no power into 2 ohms. The limiter was thought to be essential when driving the 57, but actually it was difficult to destroy the delicate speaker even without the limiter. Anyway, the diode rectifier bridge that was incorporated from 1990 onwards is much better in protecting the delicate treble unit.

One gets the impression that Quad wanted to manufacture a single amplifier that could satisfy BOTH professionals and audiophiles. Now the expectations of a professional power amp and for an audiophile amp are rather different: the pro-audio amp has to have huge power reserves, efficient cooling and effective protection circuitry. The domestic “audiophile” amp needs only low power and almost no protection. Quad tried to satisfy both markets in one design and failed to please anyone.

But the standard 405 as supplied by Quad proved to be a trusty workhorse and was used successful by several recording studios and broadcasters. As explained above, a real “audiophile” design it was not. Unfortunately many ESL owners still use it with the ESL-57 or 63 and never hear what their speakers are capable of.

QUAD 405-2
Solid-State (Op Amps)

As the critics could not be overheard (even in Huntingdon), Quad decided to rework the original 405 . The new version was called 405-2 and incorporated an improved Op Amp (type TL071) and  improved protection circuitry. The sound of the 405-2 was much better than the original 405, but still not outstanding. In the original 405 guise it proved to be a very stable and robust workhorse and was used successfully in several pro-audio applications.

MAC MOD 405
USA
Type: Solid-State
1980 onwards

The respected audio designer Steve McCormack, who now works for Conrad-Johnson, realized that the basic concept of the Quad 405 (“Current-Dumping”) was great, but the realisation was destroyed by poor components and drastic protection circuitry. In consequence he offered a modification kit for the 405. The ultimate Mac Mod were mono amps. The modification contained the following changes:

* Current Limiting removed

* Improved IC fitted (LM 318)

* Faster Drivers (Motorola MJE 15031)

* Faster Output Devices (Motorola 2N3773)

* Improved Resistors (metal-film 1% by Resista Mk3 or Corning RN-60)

* Improved Capacitors (Wonder Caps, Wima, dipped mica)

* Improved Wiring (Audioquest) and Plugs (Tiffany)

The Mac Mod 405 surpasses the original 405 in every respect: It was a great sounding amp which could compete with the most advanced high end amps of modern times. Harry Pearson of the “Absolute Sound” magazine wrote an enthusiastic review about it. I still regard it a great shame that Quad-UK did not take on the Mac Mod as standard, because the improved 405 is an exceptional amplifier. The only caveat about the complete Mac Mod is that the ultimate version (i.e. mono amps) has too much power for the delicate ESL-57 (protection circuitry mandatory!), though power output is quite suitable for the ESL-63.

Avondale Audio 405
England
1990 onwards

Mr. Les Wolstenholme, the owner of Avondale Audio, realized the potential of the standard Quad 405 and offered a similar modification to the Mac Mod, which also bettered the original 405 sound dramatically. In addition to Steve’s modifications, Les fitted a special external power supply to feed the Op Amps. The designer argued that in the standard 405 the Op Amp is powered by an unregulated DC supply, which is not at all good, because the Op Amp reacts quite strongly to power line fluctuations. In consequence he designed a dedicated external power supply with high-speed voltage regulators, which supply a clean and stabilized +/- 15 volt line to the Op Amp.

QUAD 44
Solid-State (Op Amps)
(1979-87)

This is the matching preamp to the 405 and 405-2. The idea behind the 44 was great, because it consisted of a motherboard and several modules, which could be exchanged as wanted (similar as Mark Levinson’s first preamp, the legendary JC-2 ). Unfortunately the sonic performance of the 44 was flawed, because the same mediocre Op-Amps (TL071) as in the 405-2 were used and they were powered by an unregulated DC line. The result was a rather shut-in sound quality. Specialists like Avondale Audio offer a complete overhaul: the TL071 Op Amp is exchanged for a modern type and separate voltage regulators are fitted. All these mods better the performance appreciably.

QUAD 306
Solid-State
(1987- 97)

A cute small amp that was designed for driving the ESL-63 and the 10L, Quad’s only dynamic speaker. The 10L was built by the Spendor Company for Quad, which was actually quite similar to the well known BBC LS3/5A monitor. Quad did not recommend the 306 for driving the ‘57,  because it had too much power. However, with the new protection circuitry there is no fear that the delicate treble panels can be damaged. Incidentally several ’57 owners praise the combination a lot ( e.g. Werner Ogiers)

QUAD 34
Solid State

The matching preamp to the 306 power amp.
 

QUAD 606/707/909
Solid-State
(1987-97)

These were real power horses intended for driving the ESL-63 and professional monitors. It was a good design utilizing an improved version of the patented “Current Dumping” circuit. Paul Messenger criticized a somewhat dated sound in "Hifi-Choice," but again build quality was high, as with all Quad amps. Many professionals and broadcasters use it, which is a testament to its performance and robust character. The succeeding 707 and 909 are quite similar to the 606.

QUAD 66
Solid State

The matching preamp to the 606 power amp, utilizing remote control.

QUAD II-40
Valve (KT 88)
(2000 onwards)

Quad-UK experienced severe financial difficulties during the 90’s. Following near bankruptcy it was taken over by the Verity group, and soon after, by the IAG group, which consists of a Chinese venture holding. IAG’s president, Mr. Stan Curtis, is known for years for excellent amplifier designs (e.g. for Cambridge) and press reviews (for Hifi for Pleasure). He quickly realized that a rejuvenated range of electrostatic speakers AND valve amplifiers would be required urgently, so they decided to bring out a new variant of the legendary II mono amplifier, the  II-40, and a completely new preamplifier called QC-24 (designed by Audio Note).

This is rather good news because with these valve amps Quad has found the way back to its legendary roots. I have not been able to evaluate the II-40s yet, but Ken Kessler has done so and speaks very enthusiastically in Hifi-News: “The best current all-tube rig below £3,500.” The new Quads are a peach match with the ESL-63 and the new 98 series, but they should also work fine with the original ESL.

In the meantime mastermind Stan Curtis was discharged from his position as president, but it is apparent that the policy of the new directors is with “keeping the legend alive,” so the original Quad philosophy is safe for the meantime. With Chinese financial backing there is the possibility that the entire production works will be transferred to National China in the not too distant future, but hopefully this will not compromise the famous build quality, servicing and Quad sound.

http://www.quad-hifi.co.uk/
--------------------------------
CREDITS

My sincere thanks to Hans Zeeuwe of the ESL Circuit, who published the article as first.

Many thanks also to Martin Colloms, Peter Aczel, Noel Keywood, Sheldon Stokes, Gary Jacobson, Sandro Boccara , Ernest Ruiz, Anders Enquist and all other devotees of the Quad ESL speaker.

My sincere thanks to Mark Leonard for correcting the article.

The author:

Christian Steingruber, Member of Audio Engineering Society - Austrian Section
Education: Tonmeister studies at the Viennese "Hochschule fuer Musik und Darstellende Kunst"

Contact :
christian.steingruber@utanet.at

PS. English is not my native language, thus apologies for any errors and failures.