Quad amplifiers by Christian Steingruber
QUAD 303 (1968-86)
The solid-state successor to the II , similar optics. The 303 is 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.
QUAD 33 (1963-82)
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.
QUAD 405 (1976-91)
Solid-state (discrete + Op Amps)
This amplifier caused several disputes in the web, 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 is type LM 301, a mere noise and distortion generator. 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 (References : Angus McKenzie, Martin Colloms, Hugh Ford, Anthony H. Cordesman). To play safe the 405 also included an optional limiter (switched via a jumper) which lowered output power even more. With the limiter in, the 405 had only 50 watts into 8 ohms , 25 into 4 ohms. The limiter was thought to be essential when driving old ESL-57, but actually it was difficult to destroy the delicate speaker even without limiters.
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, an "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.
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 a better 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 successful in several pro-audio applications.
MAC MOD 405 (1980 onwards )
The respected USA designer Steve McCormick soon realized that the basic concept of the Quad 405 was great , but the idea was destroyed by crummy parts 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
- Faster output devices
- Improved Resistors (Resista metal-film 1%)
- 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" 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 a great 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.
QUAD 44 (1979-87)
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 to the first Mark Levinson preamp, the JC-1 ). Unfortunately the sonic performance of the 44 was flawed, because the same cheap Op-Amp as in the 405 were used. The result was a rather shut-in sound quality. Some specialists have exchanged that IC against modern types, which bettered the sound and the noise performance.
QUAD 306 (1987- 97)
Quite a nice amp that was designed for driving the only "dynamic" Quad speaker which was an OEM version of a small Spendor speaker ( similar to the BBC LS3/5A ) and was built by the Spendor company for Quad . It was never designed for driving the original ESL-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 ( 1987-97)
This was rather a power horse intended for driving the ESL-63 and professional speakers. It was a good design and very robust . 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.