Christian Steingruber

Amplification for Quads - Fifth Edition


Christian Steingruber


"All amplifiers sound the same" - Peter Walker

"Amplifiers DO sound different" - Martin Colloms


The question about 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 the load is highly reactive and changes from approximately 60 ohms in the bass to 1.8 ohms in the treble region. The popular stacked pairs of '57 present an even worse load, because when connected in parallel, the impedance at 18 kHz falls to 0.9 ohms which is almost a short wire.

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 Volt (peak-peak). From 1990 onwards a protection circuit (a diode rectifier bridge) was suggested by Quad-UK, quite similar to that used in the new ESL-63, though it protects here only the treble unit and not the bass panel. Models which include that protection circuitry can be used safely with powerful valve and solid-state amplifiers although long overdrive is not recommended.

Generally valve amps are not upset by an awkward load as presented by the '57, but many solid-state amplifiers quickly give up in a cloud of smoke. I encountered several "trannies" that immediately blew up - some of these items even damaged the delicate Quad speaker before they died. The biggest crap I ever encountered was the BGW series 200 : I connected it to my Quads as the manufacturer had stated in the manual that model 200 was stable into electrostatic speakers - actually it was not ! Within milliseconds the BGW went into oscillation and killed both treble units. It cost me a fortune to repair both the Quads and the amp. The respected Audio Critic magazine wrote about the BGW : " This one is a disaster...." How true !

On the other hand a Marantz integrated (model 1060), whose manufacturer definitely excluded such a difficult load, worked quite well with the Quads for over a decade. Similar reports can be heard about the original NAD 3020 integrated (designed by Bjorn-Eric Edvardson) which handled the Quads with ease.

The Acoustical Manufacturing Company (better known as QUAD) usually recommended only 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 their solid-state successors were not always masterpieces. Many audiophiles have tried other products with the ESL-57 and came to even better results than with Quad's own stuff. The original Quad amps and alternative products are discussed here.

Chapter One : QUAD Amps

QUAD II mono amplifier
Type : Valve ( KT 66)

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

A collectors item, like the original Quad ESL. Highly recommended !

Type : Valve

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

QUAD 303
Type : Solid-state (discrete)

The solid-state successor to the II, a rather compact 2-channel amp. The design reflects Peter Walker's conservative thinking with only few power in loads lower than 8 ohms, but it was a rather stable and robust design and was used by several recording studios and music groups (e.g. Pink Floyd). It is a fine amp for driving the ESL57. Modification kits were available from several sources ( e.g. Steve McCormack, Avondale Audio). There was also a mono version available, the 50E, which was dedicated to pro-audio applications.

Type : Solid-state (discrete)

The matching preamp to the 303 and the successor to the valve 22. Similar optics , maybe a somewhat dated sound. Very clever filter design. Several companies (Avondale Audio) offer upgrade kits. No moving coil input as standard , though several companies offered MC cards.

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

This amplifier caused several disputes, because some guys think the 405 is one of the best high end amplifiers around, whereas others ( Angus McKenzie, Martin Colloms, John Curl, Anthony H. Cordesman, Haden Boardman, Steve McCormack and the author) have expressed some criticism. The main problem with this amp is that it contains some rather bad electronic parts and a crummy protection circuitry . The operation amp at the input, which defines the performance of the whole amp, is type LM 301, a mere noise and distortion generator. In addition the Op Amp is powered by an unregulated DC line. The design of the output stage is rather conservative, because the amplifier can supply its suggested power only into a narrow band around 8 ohms. Beyond and below that 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. The instantaneous peak current is 3 amperes, which is very low for a 120 watter ( The 15 watt "strong" Naim NAIT offers a 10 amperes output) . Actually the '57 would not need much power, but as the impedance falls to 1 ohms in the treble region, the amp can only drive this load with the supply of lots of current - otherwise it gets into hefty clipping.

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 had only 40 watts into 8 ohms , 20 into 4 ohms and almost nothing 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 that limiter ( anyway, the diode rectifier bridge that was suggested 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 should please both professionals and audiophiles. Now the expectations for a professional power amp and for an audiophile amp are rather different : the pro-audio amp has to have huge power, cooling and lot of protection circuitry. The domestic "audiophile" amp needs only low power and almost no protection. Quad tried to make both in one design and failed.

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 overwork the current-limiting of the 405 and fitted an improved IC (TL071 series) and other parts. The sound is better than the original 405, but still not outstanding. As the original 405 it proved to be a very stable and robust workhorse and was used successfully in several pro-audio applications.

Type : Solid-State
1980 onwards

The respected audio designer Steve McCormack (USA) soon realized that the basic concept of the Quad 405 ("Current-Dumping") was great, but the idea was destroyed by crummy parts and a 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 in these days. Harry Pearson of the "Absolute Sound" magazine wrote an enthusiastic review about it. I still regard it a pity that Quad-UK did not overtake 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 (= mono amps) has too much power for the delicate ESL-57 (protection circuitry mandatory !), though power output is quite right for the ESL-63.

Avondale Audio 405
1990 onwards

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

Solid-State (Op Amps)

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-Amp (TL071) as in the 405 was used. In addition these sensible Op Amps were powered by an unregulated DC line. The result was a rather shut-in sound quality. Some specialists like Avondale Audio have exchanged that Op Amp against modern types and fitted separate voltage regulators, which bettered the noise performance and the sound dramatically.

QUAD 306
(1987- 97)

Quite a nice amp that was designed for driving the only dynamic Quad speaker, the 10L ( an OEM version of a small Spendor speaker, similar to the BBC LS3/5A, built by the Spendor company for Quad ). Quad did not recommend the 306 for driving the '57, though several owners did so and speak quite good about the match. Maybe it sounded a little "compressed" and "undynamic" but quite nice to the ears.

QUAD 606

This was a real power horse intended for driving the ESL-63 and professional speakers. 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 building quality was great as with all Quad amps. Used by many professionals and broadcasters. The succeeding 707 and 909 are quite similar to the 606.

Valve (KT 88)
(2000 onwards)

Quad-UK came financially into severe troubles in the 90’s. After a near death the Quad company was overtaken by the Verity group, and soon after, by the IAG group which consists of a Chinese venture holding. IAG's president, Mr. Stan Curtis, has been known for years for excellent designs ( e.g. for Cambridge) and reviews ( Hifi for Pleasure). He quickly realized that a rejuvenated range of electrostatic speakers and valve amplifiers would be in urgent need. The company decided to bring out a new variant of the legendary II mono amplifiers, the II-40s , and a completely new preamplifier called QC-24 (designed by Audio Note).

These are rather good news because with these valve amps Quad finds the way back to its legendary roots. I could not evaluate the II-40s yet, but Ken Kessler did so and speaks very enthusiastic in Hifi-News "The best current all-tube rig....below 3500 £" . 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 has been fired from his job , but obviously with the new directors the policy of "keeping the legend alive" is still valid, though the complete production run may come from National China in a not too distant future.

Quad Site

Chapter Two : BRAUN

Valve ( EL 84 , PL 500 )
(1959 - onwards)

The German Braun company (which now only manufacture razors) was the only company that was allowed to sell an OEM version of the legendary Quad ESL-57 speaker. The Braun speaker was called LE1 and offered from 1959 onwards. Only 500 pairs were manufactured. Braun also offered a range of matching valve amps for the LE1. The CSV 60 obtained the PL500 tube, whereas the smaller CSV13 used the EL 84. The CSV 60 integrated was also available as simple power amp. The sound is similar to the 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 (Auf dem Heiken 4-5, D-44351 Dortmund, Germany ) may help.



Chapter Three : Audiophile Amplifiers from USA

Valve ( EL 34 )
1950 -

I unfortunately forgot to include this classic USA amplifier in my former editions . I could not evaluate the ST-70 on my Quads yet , but Sheldon Stokes and Ernest Ruiz have tried so and speak very enthusiastic about the combination. Aside to Marantz the Dynaco company could be regarded as the first "audiophile" company in the States. Many well known engineers have been employed there , e.g. Mr. David Hafler. Actually the ST-70 was Mr. Hafler's design and it proved to be a success. What separates Dynaco from other companies is that the American company also offered their amplifiers as DIY kits and this expanded the distribution of their products even further. 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. Very recommended, but difficult to source in Europe.

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 the mere counterpart to the Quad 405 (which boosted a 120 watts but never had - 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 the impedance was halved. In reality it could drive ANY load. It was a peach 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 now runs CELLO (I have no information if there are any Cello amps available which match the ESL-57). It should be possible to source second hand ML-2s in the USA.

BEDINI 25/25
Solid-State ( Class A )
1981 onwards

A class A amplifier with 25 watts, maybe not as stable as the ML-2, but sound wise even better. The Bedini 25/25 was 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". It was difficult to get any Bedini products in former years, because Joe Bedini had closed the manufacturing. But recently he began to design and manufacture amplifiers again.

Bedini Site

Valve ( Output Transformerless )
Julius Futterman , New York, USA

The H3 and successors H3a and H3aa are output-transformer-less valve amps (OTL), a principle which is said not to work into difficult loads a la Quad. The legendary Julius Futterman showed that it could be done due to some feedback. The H3 worked a treat with the Quads. There are many enthusiasts who argue that the Futterman OTL is the best for driving original Quads. 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". Highly recommended, but difficult to source.

New York Audio Laboritories OTL-3Q
Valve ( Output Transformerless )
NYAL, New York, USA

After Mr. Futterman's death Mr. Harvey Rosenberg overtook the company and gave it a new name. As a long-standing Quad fan he was President of the "Quad ESL owners club". The OTL-3 was the successor of the H3aa. A special version of this output-transformerless amplifier was available : The OTL-3Q was a model 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 base below 100Hz. Unfortunately the company closed doors soon after and sadly not many of the OTL-3Qs were produced. There are rumours that Harvey Rosenberg wants to manufacture again OTL amplifiers under the "Futterman" name. See more in his site : Dr Gizmo

Atmasphere M 60/ MA 1/ MA 2
Valve ( Output Transformerless )

These are output transformerless amps, designed by mastermind Ralph Karsten. The various Atmaspheres are an advancement on the vintage Futterman OTL design. Some Quad owners swear on them, others complain a slight incompatibility. More about the subject as soon as I have more detailed information.

Atmasphere Site

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

These are wonderful little mono amps, using the EL 84 tube in either tetrode or triode mode (push-pull, not single-ended). Triode mode offers about 25 watts and tetrode 45 watts. Designer David Manley is one of the few masterminds as regards tube amplification. He was born in South Africa (like colleague Tim de Paravicini), then went to England to start Vacuum Tube Logic, but finally moved to California. He recently sold the company to his son Luke who is also a very talented designer. 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.

Vacuum Tube Logic

Chapter Four : Superior Amplification from Europe

Radford ST-25
1960 onwards
Valve ( EL 34 )

These are great classics from Great Britain in the class of the mono Quads . There were many variants of the original amp, including mono blocks. Some enthusiasts like Ken Kessler rate the valve designs of Arthur Radford even over those of Peter J. Walker . It is difficult to source one of these amps as original , but in the early nineties Sondex, the new owner of the Radford company, offered a "Renaissance" series which also sounded great with the Quads.

Electrocompaniet “2-Channel Audio Power Amplifier”
Solid-State (Class A)

This legendary amplifier was developed by the great Matti Otala, who discovered the important TIM distortions. Mr. Otala is also known for the designs he did for other companies ( e.g. Harman Kardon, Citation series) . The Electrocompaniet is a wonderful match with the ESL-57, power output of 25 Watts is quite right. The company offered more powerful successors to the "Two Channel Audio Power Amplifier" in recent years, sadly some of the magic of the original design got lost.

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

This amp is still in production, though the contemporary production line probably sounds not 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 too is still in production. Chairman Julian Vereker, who recently died because of cancer, was an admirer of the ESL-57 and owned a pair of Quads till his death. The NAP 250 amp offers 70 watts, is stable into any load, as long as a dedicated Naim speaker cable is used - the NAP 250 omits the Zobel network and needs some inductive damping from the cable ( similar things are reported about the Bedini 25/25). The original NAP 250 and other relative designs like the NAP 160 and NAP 120 work quite well with the original Quad speaker though none of them are in the class of a superior valve amplifier.

Naim Audio also wanted to manufacture a special electrostatic speaker , following the design of the Quad ESL-57, but using better materials and a rigid frame. Unfortunately their French designer, Mr. Guy Lamotte, left the company without finalizing the ESL, and so the managers decide to build mediocre dynamic loudspeakers instead - what a shame !

Naim Audio NAIT
Solid State

Because many customers asked for an integrated amplifier, Naim Audio manufactured such an item from 1983 onwards. The original NAIT (Naim Audio Integrated Amplifier) offers only 15 watts, but as it contains a rather stiff power supply (a 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, this was called the NAP 90. After the unexpected death of mastermind Julian Vereker due to cancer in early 2000, the production line was changed and the original NAIT amplifier ( and its successors ) are no longer available. The new line is a cheap affair, consisting merely of integrated circuits and is probably not in the class of its legendary forerunner , which was a mere discrete design and obtained a massive Holden&Fisher transformer

Esoteric Audio Research : E.A.R. 861
Valve ( Triode)
Huntingdon, England

Tim de Paravicini is one of the true masterminds in audio and has been known for decades. He was born in South Africa, but soon went to Japan to work for the LUXMAN company. He designed some of the Luxman valve amps, e.g. the MQ-3.600 amplifier which is still regarded a classic. Back in England he started his own company - Esoteric Audio Research - but also offered his work to other companies (the legendary Michaelson & Austin TVA-1 is Tim's design). He has been a Quad ESL devotee all the time.

The 861 is a triode amplifier (push-pull) with 2x35 watts, ideal for driving original Quads. It is very popular in Japan where Tim got many prizes because of its superior sound and design. It seems that the Japanese love Tim and Tim loves the Japanese people, because he is married with a Japanese woman, Mrs. Yoshino.

Esoteric Audio Research offers a lot of amplifiers, but the 861 must be the best around for driving old Quads. The EAR range includes the 534 (50 watt, pentode) and the 509 series, latter are dedicated for professionals. Many recording studios and musicians (Paul McCartney, Kate Bush, Ry Cooder, Aerosmith) use EAR products. The London situated Cutting studio "The Exchange" usually cuts records with EAR 549 amplification.

E.A.R. direct-coupled output transformer less valve amp
Valve ( DC coupled OTL)
Huntingdon, England

The EAR direct-coupled output transformerless valve amp as not a stand-alone unit like the Futterman but it had to be fitted inside the Quad speaker. The main advantage of a direct-coupled OTL valve amplifier is that it drives the electrostatic panels directly, whereas the Futterman ( and all other OTL designs ) still need the step-up via the input transformer of the Quad. Manufacturers like Beveridge and Acoustat (USA) also used this kind of direct amplification for their electrostatic loudspeakers (Beveridge II, Acoustat X). The Quad speaker had to be sent to the EAR factory where the amp was built in. Obviously not many direct-coupled EARs were sold. According to rumours the well known British recording engineer Tony Faulkner owns such a system. With the invention of the CE legislation in 1996 it became highly impossible to sell such a high-voltage device and Tim cancelled the production of the OTL ( Tim also offered a matching subwoofer for Quads ).

Yoshino Site

Italy ( 1998 - )

Italy is normally regarded as the land of fast cars , but not of high end audio. By now the situation has changed and with companies like Sonus Faber, Chario, SAP and Graff there are a lot of high enders around. Mr. Giovanni Mariani, the founder and chief designer of Graaf, began with normal (= 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 20 watt strong GM-20 works a treat with the Quad, but I could not get a item yet to evaluate it on my Quads. One caveat : The Graff is very expensive !


EINSTEIN “The Final Cut”
Germany ( 2001 -)

This is rather a beast and 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

The only problem with this amp is that probably none has heard it yet on Quads. It could be a perfect match , but only the hearing test can verify if my expectations are true. And as with the Graff one big caveat : The Einstein is ultra expensive !

Postscript :
As so much rubbish has been published on this subject, I was forced to write this article. I recently had to read in an audio magazine that “it was inadvisable to connect any old amplifier to the Quad speaker”. I do not know what “old amplification” the author of these lines had in mind, but it is exactly the old stuff (valve amplification ) which works a treat with the original Quad ESL. { He's right, you know! - Gary Jacobson }


Thanks to :

My sincere thanks to Hans Zeeuwe of the ESL circuit, who published the article at first. Many thanks also to Peter Turner (+) , Martin Colloms, Peter Aczel, Sheldon Stokes, Gary Jacobson, Ernest Ruiz ... and all other devotees of the original Quad speaker .

The Author :

Christian Steingruber , member of Audio Engineering Society - Austrian Section
Education : Tonmeister studies at the Viennese "Hochschule für Musik und Darstellende Kunst"
Contact via :
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English is not my native language , thus please apologize any errors and failures.