Quad 909 Technical details.

Discussion in 'Hi-Fi and General Audio' started by Hodges, Mar 6, 2024.

  1. Hodges

    Hodges

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    Thanks for that - I appreciate the effort. The mechanism is still an extremely grey are - well for me anyway. But I will take the time to read some more historical articles which may make things a little less confusing. For example, Walker talks about Feed - forward and error signal, yet everything to date refers to it simply being NFB. It may well be, this simplifies matters for the authors, yet it still does not explain how the switching rate of the dumpers can be controlled, 4 times in one cycle and maintain distortion and noise figures approaching that of class A designs.

    I reiterate however, the dumpers must be switched on sufficiently far enough from their non linear region, to ensure that the complex make-up of the audio doesn't provoke non linearity by hovering between on and off. This could occur where the drive (audio) is only just above the threshold of Dumping action. That would put it closer to Class C, which I previous said. I don't see how it can be Class AB as this would but it at a point where the alternative half of the output stage are at the threshold of conduction, virtually making the principle of 'Dumping' redundant.

    Still, I am just chewing the cud, whilst I try to get a handle on it all.

    Cheers,
     
    Hodges, May 24, 2024
    #21
  2. Hodges

    Arkless Electronics

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    I think you need to spend more time with those articles. It seems you are not understanding how current dumping works. I gave the example of the Crown DC-300A as an amplifier that works in a simple and crude way (output stage operation anyway!) to use the output devices as "current dumping" in the way you appear to think the Quad works... they are VERY different though! It is unfortunately not easy to understand Quad's current dumping idea.

    Whilst it may seem counter-intuitive the 405 and later dumpers do indeed have some forward bias on the output devices. It reduces distortion, especially at HF. I have designed a few Quad style current dumpers and can confirm that a little bit of forward bias really helps. The precision of the LCR bridge has a big effect on THD also.

    The class A amp should not be thought of as driving the speakers really, and in fact there is a 47R resistor (part of the bridge) betwixt the class A amp and the load! It controls the dumpers and "fills in" the gap as they start to conduct. The magic is in the way the bridge and feedforward make this process theoretically perfect so there can be theoretically close to zero distortion.

    Don't worry, even some of the worlds leading luminaries in audio electronics at the time couldn't agree on how it works, IF it works as claimed even and whether it really uses feedforward or feedback!

    It is a very clever idea but even by the time it went into production was a bit of "a solution looking for a problem". 10-15 years earlier and it would have been much more important as germanium transistors were much more sensitive to thermal runaway and overheating issues and of much lower performance than silicon in many ways.

    Note that the patent on current dumping ran out some years ago now but there hasn't been a slew of rival current dumping amplifiers coming to market!
     
    Arkless Electronics, May 24, 2024
    #22
    Arg, Hodges and Sergeauckland like this.
  3. Hodges

    Hodges

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    Thanks Arg;

    ''If you are not going to admit that the dumpers are a Class B amplifier (they are), then we can't move on.'' - ''if one wants to be really picky, one could argue they are Class C because the conduction cycle of the output transistors is not quite 180 degrees.'' Now which do you want me to agree to? One knowledgeable individual pointed out that Walker believed he could get away without any bias, but discovered it was less than successful and added the diode - making it Class AB.

    Whatever mode is irrelevant really - I just want to get a succinct paragraph that describes how it all functions - not necessarily a mathematical analysis, although that would be most welcome. If my memory serves me correctly, then the turn - on voltage of a Silicon transistor is 0.7volts. So an AC swing of 2 volts P to P is not going to place any great burden upon the Class A section. A little more for insurance purposes would help, but either way it is well within the rating of the Class A amp.

    Now, for me, the whole question revolves around getting the dumpers, to switch on at a point where they proceed to conduct over the linear part of their characteristic. It is how that is achieved that remains unanswered. We can do an analysis of the static conditions - but dynamically, what is doing what? Add to the mix the fact that, whatever it is, it is doing it at 4 X the rate of the audio signal!

    I accept that the Dumpers may not be fast devices - indeed it was my poor explanation that led to the confusion. What I mean is, the speed at which they are switched on has to be incredibly fast. So; whatever the Feed - forward signal is, needs to be damned quick! But how is it derived, shaped, so that it is very precise? Analogous to this, it seems it acts like a ''Trigger'', holding the Dumpers out of circuit, until the threshold is crossed, then swiftly switching on the base - emitter junctions- very quickly. Up to this point, the Class A amp can handle the output signal power; and probably more, until dumper switch - on occurs.

    Thanks for continuing input.
     
    Hodges, May 25, 2024
    #23
  4. Hodges

    Hodges

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    I continue to cogitate over the issue of CD, because I find it so fascinating. I doubt I will ever get to the root of it all, but continue to marvel at how well my 909 performs.

    For the moment my Sugden Headmaster remains an ''Ornament'', just as my 33 has become, given I am driving the 909 from a Alps zero - gain/potentiometer, preamp, with a further 20db attenuation between the two. They are being driven by a Zen V2 and it seems a huge leap in quality over my 303/33 combination. I just never tire of listening to it, particularly as I don't think I have heard my Spendors perform at this level - ever. Ok, they have new ferrite drivers and clearly aren't capable of handling high power levels, but when in their comfort zone, they bring a realism to the music that never seemed to be there before.

    What continues to surprise is the Bass response. Certainly, the 909's output is sustained better than either the 303 or the 405 and this, I think, gives whatever the Spendors are capable of; at the lower registers, a fighting chance of producing some decent bass. Ok, it's not going to crack the mortar between the brickwork of our lounge, but they certainly don't leave me feeling cheated out of anything - Neil Diamond is not made to sound like Olivia Newton John!

    However, there are two other issues I am looking at, The first being the lack of tone controls, particularly when listening at very low volumes and my hearing drops off a cliff. I am reluctant to add anything by way of gain for the sake of getting a little bass and treble lift at low level, so my 33 will not be coming back into service any time soon. But equally, I am not all that confident about using some form of loudness control. I have bought a couple of loudness control circuits to experiment with, but I don't want the loudness to be continuously adjustable over a wide range. Just a pre-set amount of lift at a pre-determined volume will suffice, which can be switched in or out depending upon normal listening or back-ground level. I accept that tone controls can compensate - to a degree - for some ''Cackhanded'' recording imbalance, but rather than that, I am more inclined to ''Bin'' poor recordings than attempt to correct them.

    The second issue relates to ''Burr - Brown'' Audio Processing. I understand the company was taken over by ''Texas Instruments'' about 20 years ago. My Yamaha CD player uses it, or so I am told. However, so too does the Zen V2. I have never had the opportunity to listen seriously to any top class D to A Streamers so I cannot fairly compare my system with any. However, what did become apparent to me was the way the V2 processes the LSB - Least Significant Bits, i.e. those bits which contribute the finer detail of musical content, the region where some words of a song could be confused for others, because of lack of definition - sibilance creating confusion. Clearly, the higher the bit rate, the greater the accuracy. However, the ability to resolve high bit rates, constantly is an imperative if one wants to achieve the greatest fidelity.

    So, my question is: - What is so special about ''Burr - Brown'' that it seen as superior to other types of audio processing? And indeed, is it?
     
    Hodges, Jun 13, 2024 at 10:37 AM
    #24
  5. Hodges

    Arkless Electronics

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    The 33 and 303 are not really up to much, though very good in their day, and the 405 has excessive bass roll off. Various mods are available to soup them up somewhat from a variety of companies (when I get a delivery of round tuit's I may do some mods I worked out ages ago on my own pair of 405-2's and see what they are then capable of. I don't use them as I have designs of my own which thrash them. 303 could be modded to sound vastly better also according to CAD simulations of it I made).

    Some older B & O amps had variable loudness compensation which adjusted in step with the volume.... unfortunately couldn't be switched off!

    Burr Brown were a leading manufacturer of IC's of all varieties but specialised in analogue (low noise op amps etc) and DAC's/ADC's.
    Sadly many more companies have been sucked in by bigger ones to form "mega corps" since BB were bought out and the first thing that usually happens is "consolidation" which means loads of fantastic parts are obsoleted over night. TI also bought out National Semiconductor who had just brought out a fantastic range of super low distortion and noise op amps for audio about a year before.... yep TI obsoleted about 2/3rds of the new op amp range within months.
     
    Arkless Electronics, Jun 13, 2024 at 1:47 PM
    #25
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