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Old 16th Oct 2020, 10:06 am   #1
llama
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Default QUAD 33 block diagram

It was very decent of QUAD to provide a cct diagram in their user manual and, indeed, copies of it are all over the Internet. However, it's drawn to show the signal path going in and out of the various plug-in boards and the 2 adaptors so not necessarily easy to trace a signal path through it.

I've attached a block diagram I've drawn which I hope is useful. If I've got bits of the architecture or details wrong please let me know.

I needed this to help me trace an intermittent rustling sound so here goes!
Graham
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File Type: pdf Quad33BlockDiag.pdf (136.9 KB, 304 views)
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Old 16th Oct 2020, 11:56 am   #2
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Default Re: QUAD 33 block diagram

Excellent Graham - very useful

Steve.
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Old 16th Oct 2020, 5:07 pm   #3
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Default Re: QUAD 33 block diagram

Thanks Graham, that makes it very clear.
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Old 19th Oct 2020, 7:03 am   #4
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Default Re: QUAD 33 block diagram

Excellent block schematic!

A couple of what might be called “explanatory” diagrams for the Quad 33 were included in the 1977 Newnes book “Radio, TV and Audio Technical Reference Book” edited by S.W. Amos. The Quad 33 items were in section 14, written by P.J. Baxandall. These might be useful supplements to the block schematic.

The first of these covered the switching arrangements:

Click image for larger version

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The second was a redrawing of the disc input circuit to more clearly show the arrangements for the magnetic cartridge (M1 and M2) inputs:

Click image for larger version

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I understand that Baxandall did a lot of the design work on the Quad 33 disc input stage.

Using that diagram as a starting point, one may also develop similar diagrams for the ceramic cartridge (C1) and special purpose (S1) inputs, as follows:

Click image for larger version

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Name:	Quad 33 Disc Input Amplifier Position S1.jpg
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For ceramic cartridge loading and matching, Quad chose an approach which was basically the same as that later recommended (and made “famous”) by Burrows (who used a modified 1966 Bailey disc input amplifier). That is, the cartridge was with a relatively low impedance, and the resulting bass rolloff was corrected by a complementary bass-boosting feedback circuit.

In the same vein, it may be noted that the Quad 33 tone control section also used the bootstrapped stage that was later recommended (and again made “famous) by Quilter, (who modified the 1966 Bailey tone control circuit). From c.1973, Hart offered the “Bailey-Burrows-Quilter” preamplifier kit.


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Old 19th Oct 2020, 9:37 am   #5
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Default Re: QUAD 33 block diagram

The trick used in the Quad 33 set for a ceramic cartridge was hidden by the way the diagram was drawn, with lots of options on the removable board. No-one noticed it! Also this was around the time most people in Quad's target market were moving to magnetic cartridges, so maybe nobody felt a need to unravel the diagram or pay the ceramic option much attention?

I'd noticed the bootstrap in the tone control stage and not thought further about it, it's a standard technique for reducing the loading imposed by bias components, so not surprising. The original Baxandall circuit had the nice high input impedance of a valve to work with, so seeing a bit of effort going into a transistor version is reasonable, what may be doubtful is doing one without any help and a single transistor as was commonly done.

Baxandall was a smart cookie. He knew how to do things properly, and how to engineer within cost constraints.

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Old 19th Oct 2020, 11:22 am   #6
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Default Re: QUAD 33 block diagram

There were two versions of the 33 mono/stereo switching, providing the following functionality:

Early (-> Ser# 7500):-
  • L off + R off (silent!?)
  • L->L + R->R (stereo)
  • L->L + R off (mute right)
  • L off + R->R (mute left)
Later (Ser# 7500->) adds:-
  • L->L + L->R (both left)
  • R->L + R->R (both right)
  • L->R + R->L (swapped)
(- via perhaps less than completely obvious switch combinations...)

I modified my early 33 to be closer to the later style, and in the course of doing so found it useful to draw up the two versions:-

Early:-
Click image for larger version

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Later:-
Click image for larger version

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Both can be equally confusing!

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File Type: pdf Quad 33 Early Switching.pdf (44.1 KB, 73 views)
File Type: pdf Quad 33 Later Switching.pdf (44.0 KB, 84 views)

Last edited by Trigon.; 19th Oct 2020 at 11:29 am.
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Old 20th Oct 2020, 12:47 am   #7
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Default Re: QUAD 33 block diagram

Quote:
Originally Posted by Radio Wrangler View Post
The original Baxandall circuit had the nice high input impedance of a valve to work with, so seeing a bit of effort going into a transistor version is reasonable, what may be doubtful is doing one without any help and a single transistor as was commonly done.
Indeed. Baxandall, writing in Amos, put it quite succinctly: “The cheapest versions use just a single transistor, but lower distortion and lower output impedance may be obtained by using a transistor pair, or an integrated circuit operational amplifier…”

Quote:
Originally Posted by Radio Wrangler View Post
Baxandall was a smart cookie. He knew how to do things properly, and how to engineer within cost constraints.
And an excellent technical writer as well


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Old 20th Oct 2020, 1:21 am   #8
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Default Re: QUAD 33 block diagram

Quote:
Originally Posted by Trigon. View Post
There were two versions of the 33 mono/stereo switching, …

Both can be equally confusing!

Very! Another quirk was that Radio 1 had a separate stereo and mono inputs (albeit both on the same 5-pin DIN connectors) the latter selected for any mono mode. This was to suit the FMII and FM3 tuners, which had separate mono outputs, but no manual stereo-mono switch. Rather they relied on the Quad 33 to perform this function.


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Old 20th Oct 2020, 8:27 am   #9
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Default Re: QUAD 33 block diagram

The 20Hz filter in Baxandall's block diagram is not well designed alas. Spice modelling of the circuit around TR400 does indeed produce a second order roll off at about ~20Hz, but also has a resonant peak of 2 - 3dB at 27Hz (dependent on volume control setting, and hence source resistance).

This can be entirely eliminated by changing the tapping point on R404/R405 while keeping the sum of them the same (to preserve the bias point of the transistor).

Changing R404 to 1k and R405 to 3.9k entirely eliminates the resonant peak, has a -3dB point at 20Hz, and it -25dB at 5Hz.

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Old 20th Oct 2020, 8:51 am   #10
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Default Re: QUAD 33 block diagram

Would not that change somewhat reduce the signal level fed onwards to the tone control network?

And would the apparent bootstrapping of R402 by C401 be less effective with a lesser signal at the bottom end of R402? My take was that the bootstrapping was done so that the Tr400 stage appeared to have a very high input impedance when looked at from the volume control.


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Old 20th Oct 2020, 9:41 am   #11
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Default Re: QUAD 33 block diagram

Yes - indeed the bootstrapping reduced the loading on the volume control. But the secondary purpose was to introduce a 20Hz second order filter, as shown in the Baxandall block diagram.

Changing the tapping on R404/R405 reduces the output level by -2.5dB. However moving the output connection to the emitter of TR400 retains the resonance peak attribute, but increases the output level by 1dB.

The solution that retains output level *and* a non-peaky response is a resistor chain from the emitter 270-750-3k9. Bootstrap cap goes from the junction of the 750/3k9 and the output from the junction of the 270/750.

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Old 20th Oct 2020, 9:53 am   #12
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Default Re: QUAD 33 block diagram

These are the comparative curves. As designed, with the peak. And incorporating the mod detailed in post 11. The volume control setting is 50%, and it is fed from a 330n capacitor (as it is from the tape board emitter follower).

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Old 22nd Oct 2020, 10:32 pm   #13
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Default Re: QUAD 33 block diagram

Thanks for that, Craig.

That I think is the most difficult part of the Quad 33 circuit to unravel. It is simultaneously a bootstrapped emitter follower buffer stage and an RC feedback type high-pass filter. I suppose one approach (a bit beyond my reach though, I think) would be to map out what it would look like for the buffer function alone, and for the filter function alone, in the latter case letting the input impedance lie where it fell, and then see how the two overlapped and combined. The disc input stage looks fearsome at first glance, but separating the M1/M2, C1 and S1 cases makes it easier.

As I understand it, Baxandall was the first to use RC feedback filters for audio low-pass and high-pass purposes, based upon prior more general work by RCA, and just before Sallen and Key developed their circuits.


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Old 23rd Oct 2020, 12:22 am   #14
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Default Re: QUAD 33 block diagram

I think there was something earlier going on at Bell Labs about active filtering methods. With the number of repeats they used of every circuit, the phone companies were after any slight saving they could get, and they had some of the people who formulated filter design methodologies. Don't have access to BSTJ going back to the year dot... I'd have gone right to where it was on the shelves.

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Old 28th Oct 2020, 1:27 am   #15
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Default Re: QUAD 33 block diagram

Baxandall published his feedback filter circuits in Wireless World (WW) 1955 January and February, as part of an article: “Gramophone and Microphone Preamplifier”. He gave the impression that he had done the circuit development work quite a bit earlier. One could infer that his feedback tone control, developed in 1950 and published in 1952, was actually part of that project. In the article he showed how a feedback filter could be equivalent to a passive L-C filter. The RCA prior work referred to was from RCA Review 1950 December, namely: “Design of High-Pass, Low-Pass and Band-Pass Filters using R-C Networks and Direct-Current Feedback Amplifiers”, by C.C. Schumard, p.356ff. It runs to 31 pages!

In his chapter in Amos, 1977, Baxandall showed Sallen & Key filters for audio low-pass and high-pass applications, suggesting that by then he had seen such, based upon unity gain solid-state circuits, as superseding his original design, which was based upon valved gain stages.

Returning to the Quad 33 ceramic cartridge input, whilst it largely went unremarked, it did get a mention in the Hi-Fi News (HFN) review of 1968 April. Therein it was said that the “optimum” source impedance (cartridge impedance) was 560 pF. Quad quoted a range of 450 to 800 pF as being suitable, the geometric mean of which was 600 pF. The last-mentioned was also the number upon which H.P. Walker based his circuit, which was of the Bailey-Burrows type although it may have been independent of Burrows’ work.

An interesting point is that the input of the Quad disc preamplifier was bootstrapped (following the Dinsdale Mk II precedent), so in its naked state, probably had a sufficiently high (say 2M or more) impedance to provide for high impedance loading of ceramic cartridges. Nonetheless Quad chose the low impedance route. It would appear that whilst Dinsdale’s two-transistor circuit provided a general precedent, his bootstrapped input was not generally followed, at least not by Bailey, H.P. Walker and others, although it was used by Revox in its A50 and A78 amplifiers, the same basic three-transistor preamplifier, with bootstrapped input, also being used for the A77 tape recorder.


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Old 28th Oct 2020, 10:01 am   #16
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Default Re: QUAD 33 block diagram

In the early 70s there was a series of articles in WW by Girling and Good covering active filter design.

Iliffe missed out, this should have been published as a book. That series is quite a good general reference and makes good reading.

David
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