Quad 33 and FM3

[Synchrodyne]

That improves headroom and distortion (see douglas self's site on RIAA). It also includes replacement electrolytics for all other boards, lower noise transistors and a number of resistors to adapt the circuits to 16V, and to optionally change the gain structure.

Re pickup input headroom, as I recall, the Quad 33 arrived on the scene before that became a “hot” topic in the hi-fi magazines of the era.  Then when it emerged, the 33 was sometimes criticized for its apparently modest headroom (40 mV) in the M1 disc input position.  Rarely mentioned was that the M2 position, which probably suited a wide range of cartridges, offered 120 mV.  The pundits appeared to want maximum sensitivity (2 mV or better) and maximum headroom in the same input as a measure of “goodness”, with no mention of adjusting input sensitivity to suit the actual input level and by so doing allowing sane volume control settings for normal listening.  I suppose though that also emerging at the time was the Japan Inc. approach of having relatively sensitive inputs that required very low volume control settings for normal listening, perhaps to convey the impression of a large power reserve to non-technical users.

The Quad 33 disc input stage looked, to a first approximation, to be a silicon version of the Dinsdale 1965 (1) two transistor design.  It retained the input bootstrapping that Dinsdale had used, but evidently in much “tighter” form, I’d guess providing a native input impedance of several megohms.  Thus, the disc input impedance was essentially defined by the input resistor, 68k for M1 and M2.  Quad later claimed that the input was purely resistive within ±5 degrees over the audio range.  One may ask why did not Quad adopt the three-transistor design offered by Bailey in 1966 (2), in which an emitter follower was added within the feedback loop to reduce RIAA feedback network loading on the second transistor gain stage.  One possible answer is that the Quad 33 was developed at a time when device cost was non-trivial, and so device count was a major consideration.  Thus, the extra transistor that might have been used in the disc input stage was rather required for the following multipurpose buffer stage, which provided a fixed and highish impedance load for the disc input amplifier, the same for the pair of radio inputs (bearing in mind that in 1967, it could be expected that the Quad 33 might be used with older radio tuners that did not have buffered outputs) and which also served as a tape output buffer.  That the tape output buffer was in the main signal path and not a separate side-chain was another manifestation of the need for device-count economy.

Something that the Quad 33 did have was a passive RC roll-off circuit following the disc input amplifier that compensated for the fact that a series feedback circuit has an inherent error in the HF end of the RIAA curve.  Baxandall (3) reported that this had a turnover point of 32 kHz.  I think that it could not have been precisely right for both the M1 (34 dB) and M2 (25 dB) gain settings, but presumably the error either way was negligible.  That refinement was not often found; for example, H.P. Walker did not use it in his 1973 design (4), the RIAA curve for which did show the error.  But Self (5) did use it in his 1976 version of the Bailey three-transistor circuit, to which he added a bootstrapped output load resistor for the second stage.

The ceramic cartridge input C1 was an early example of an approach later advocated by Burrows (6), and also used by H.P. Walker.  It assumed that such cartridges were self-equalized for the RIAA curve (true for all but about one by then) and so did not require such equalization in the disc input amplifier on this front.  But the 100k input impedance differentiated the cartridge output with a turnover within the audio range, and this was corrected by complementary integration done via the amplifier feedback loop.  This process was dependent upon cartridge self-capacitance, and the Quad 33 C1 input was said to be suitable for cartridges with self-capacitances in the range 450 to 900 pF.  I’d guess that it was exactly right for something like the geometric mean of that range, 636 pF.  (H.P. Walker assumed 600 pF for his 1971 design.)

That was a sea-change for Quad, in that the earlier QCII and 22 control units did differently.  With those it was assumed that higher quality ceramic (and crystal) cartridges were not self-equalized (probably reasonably true when the QCII was released).  Thus, they were subject to the same set of switched equalization curves as used for magnetic cartridges, but those curves were effectively rotated pi/2 to suit the amplitude responsive outputs of the piezo cartridges.

In the late 1970s Quad offered a modified disc input board with the M1 position matched to the requirements of the Shure V15 Mk III cartridge.  Input sensitivity was 3.8 mV (instead of 2 mV) and input resistance was 47k (instead of 68k), and there was a non-trivial added parallel capacitance, whose value I don’t recall.  This was featured at one of the late 1970s London hi-fi shows.  In a nearby room Shure was showing its V15 III, and using a Quad 33/303 combination.  I asked whether Shure was using the modified disc input board; the answer, given in something of a Gallic shrug manner, was no, although they were aware of it!  Perhaps Quad had been overzealous in this matter.  But then circa 1970, B&W had offered a passive equalizer that plugged in under the SME arm support to exactly match the Shure V15 Type II (unimproved version, I think) to the Quad 33 M1 input (7).

The incorporation of a tape output buffer was new for Quad.  The QCII and Quad 22 had used unbuffered tape outputs (as found on most, but not all valve equipment) that needed to look into an impedance of 500k or higher.  In fact the initial version of the QCII did not have tape input and output facilities; they were an early amendment.  Impedance matching was going to be an issue early in the solid-state era when new equipment was interconnected with older valve units.  This was more so for the tape recorder makers.  With valves, the provision of high input impedances, say 1M or more, was a routine matter, but it required special circuitry in the early transistor days, if it was catered for at all.  For example, the Revox A77 had an auxiliary input with 1 M impedance, achieved by bootstrapping the input of the signal input amplifier.  Ferrograph used a fet source-follower for the 2M2 line input of its Series 7, and as far as I know Uher used an input attenuator (divider) with a 1 M upper leg.

Back to the 33, Quad used an unusual approach to matching its tape output to recorder DIN inputs.  Instead of the usual series resistor to approximate constant current characteristics, it offered attenuated, very low impedance outputs (nominally 3.7 and 20 mV) that provided signal levels appropriate for tape recorder DIN inputs.  To provide one example, the Revox A77 DIN input (labelled “radio”) had a sensitivity of 2.5 mV at 33k, so the Quad 33 3.7 mV output would have been a reasonable match to it.  (Although in practice I imagine that the Quad 33 100 mV tape output would more likely have been connected to the A77 35 mV, 1 M auxiliary input.)
 
Next in the Quad 33 circuit came the tape monitor switching, with tape input attenuator network, followed by the volume control and a buffer stage that comprised an emitter follower with bootstrapped input.  The tone control circuit was basically of the Baxandall type, but using two transistors instead of the customary one.  The second transistor was an emitter follower that bootstrapped the first transistor load, thus allowing some gain, about 14 dB.  It also served as the output buffer.  That meant that the passive filter control was beyond the output buffer, but again, device count economy requirements may have precluded the addition of a second buffer stage at the very end of the chain.  (Not at the time, but later it might have been seen as desirable practice to have included an inverting buffer at the end of the chain to ensure that the control unit was not inverting overall.  But then the Quad 303 was inverting.)
 
The Quad 33 tone control may have been a very early example of a two-transistor Baxandall circuit, and one that was largely overlooked in the “history”, as it were.  Quilter published a similar two-transistor circuit in 1971 (8), essentially presenting it as an improvement over the Bailey single-transistor circuit, but he made no mention of the Quad 33 case.  H.P Walker also used a similar two-transistor circuit, and stressed the benefits obtained from the bootstrapped load.  That got him into a debate on bootstrapping in the pages of Wireless World (9), although his similar debate (stoush?) with Linsley Hood on the series- vs. shunt RIAA equalization question (10) was probably better known.  In 1973, Ellis (11) proposed a three-transistor circuit, in which gain was provided by a cascode pair with bootstrapped output load from an emitter follower.  His objective was improvement over the single-transistor circuity; he acknowledged the Quad 33 as well as Quilter’s work.

When comparing the Quad 33 with the QCII and 22, one may note a “reversal” as it were.  The QCII and 22 had a shunt feedback disc input stage but an (apparently unusual) series-feedback tone control stage.  The 33 had a series-feedback disc input stage, but a shunt-feedback tone control stage.

On the device count issue, it may be observed that contemporary amplifiers, such as the Rogers Ravensbourne, showed similar economy.  Evidently the situation changed very rapidly in the few years following 1967.  By way of example, whereas the Quad 33 (of 1967) had 12 bipolar transistors, all in the signal paths, in 1971, the first iteration of the Quad FM3, in addition to two dual-gate mosfets and three ICs, had no fewer than 14 bipolar transistors.  One was the oscillator, one was an IF impedance-matching stage, and one was the mono channel output buffer.  Four were in the power supply regulation circuit and the other seven were used for the auxiliary functions of muting and tuning indicator drive.  So, it does not look as if there were any severe constraints on the number used.  Also in 1971, the Radford SC24 control unit used 50 transistors, but it was a “luxury” model that sold for nearly twice the price of the Quad 33
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Judged by what was achieved with the Quad 33, one might say that the Quad 44 of 1979 indulged in quite a bit of “luxury”.  Each input and output was individually buffered, each was configurable by presets and/or changing modules, and there were two tape loops.  It came just before two-rail input switching arrived on the scene, so missed that useful facility.  An input module with isolated ground would also have been useful – that arrived with the Quad 66 a decade later.

Anyway, the Quad 33, whilst not above criticism, does come out as a carefully thought-out and in some ways innovative unit when analysed in the light of the norms and events of its time.  “Multum in parvo” might be an apt description.  Perhaps it’s a pity that its design and circuit features were nor covered in a detailed WW article at the time of its release. 

Cheers,

Steve

(1) Wireless World (WW) 1965 January p.03ff.
(2) WW 1966 December p.598ff.
(3) S.W. Amos, Ed; Radio, TV & Audio Technical Reference Book; Newnes; 1977; ISBN 0 408 00259 X; p.14-22.
(4) WW 1971 May p.221ff.
(5) WW 1976 November p.41ff
(6) WW 1970 February p.56ff; WW 1971 July p.321ff; WW 1971 August p.379ff.
(7) Earl, John (aka Gordon J. King); Pickups and Loudspeakers; Fountain Press; 1971; ISBN 0 852 42480 9; p.87.
(8) WW 1971 April p.199ff.
(9) WW 1972 May p.225; WW 1972 September p.423.
(10)  WW 1972 August p.389; WW 1972 November p.520; WW January 1973 pp.11,12; WW 1973 April pp.193,194.  
(11)  WW 1973 August p.378.