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Quad HIFI Ideas?

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Re: Quad HIFI Ideas?

Postby Arpangel » Sat Jun 08, 2019 7:40 am

Hugh Robjohns wrote:
John Willett wrote:Ooh - so the 34 *does* have that wonderful exclusive Quad tilt control like the 44. :thumbup:

Yes, but it has fewer inputs, no input customisation options, and a much less flexible and adjustable set of vinyl filters.

The first question to answer is whether the lack of a channel is down to the power amp or the preamp.

Unless the OP is experienced in electronic fault finding and repair, I'd strongly recommend getting the whole system to someone who is -- otherwise there's a risk of doing more harm than good.

I can definitely recommend AmpLabs in Worcester (http://www.amp-labs.co.uk/servicing.htm), but there are countless others with the appropriate experience around the UK and elsewhere...

H

I like the filters on my 33, and those plug in cards, but my God, whoever did the soldering on those? it's worse than mine, and that's saying something!
Looking good Ian......like.....PPHHHWWOOOOORRR!!!

:D
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Re: Quad HIFI Ideas?

Postby Ian Shaw » Sun Jun 09, 2019 2:00 pm

Well, I seem to have turned my house into a HIFI graveyard this weekend. After reconnecting up my Arcam system one of my B&Ws made a farting noise & went dead, so I'm now using Dad's tiny old Missions with that. Strangely though, the display on my Arcam CD player has sprung back into life after being dead for some years. I guess that's what happens with the finely balanced & delicate nature of HIFI seperates :?

But I have a least decided to go the whole hog & get the Quad refurbed by the guy Hugh mentioned.

It's strangely comforting, staring at the insides of something that's been put together by hand & not just knocked out by the gazillion in a huge factory. My kids can't quite believe the weight of the power amp or why it's even needed to drive some old speakers.

Thanks for all the input.

BTW can someone easily explain 'current dumping'? It was obviously good enough to slap a patent on. From what I can gather it requires a high powered amp which gives whatever is needed to a lower powered amp?

Cheers

Ian
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Re: Quad HIFI Ideas?

Postby James Perrett » Sun Jun 09, 2019 2:08 pm

Ian Shaw wrote:BTW can someone easily explain 'current dumping'? It was obviously good enough to slap a patent on. From what I can gather it requires a high powered amp which gives whatever is needed to a lower powered amp?

I've seen it described as feed forward rather than feedback. It uses two amplifiers - a big class B amplifier to provide most of the power and a small class A amplifier. Quad's simple explanation was that the big amplifier provided a signal that was roughly correct while the small accurate class A amplifier provided the final adjustment so that the output was exactly right.
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Re: Quad HIFI Ideas?

Postby ef37a » Sun Jun 09, 2019 2:21 pm

"BTW can someone easily explain 'current dumping'? It was obviously good enough to slap a patent on. From what I can gather it requires a high powered amp which gives whatever is needed to a lower powered amp?"

Hah! I spent a nifty on a book and although Current Dumping was only a small part of the many things I hoped to learn, there was next to naff all about it!

I do recall at the time the pages of Wireless World (we are not worthy) were rife with claims and counter claims that CD was just a fancy way of using negative feedback.

I probably just don't have the math and will never really understand it.

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Re: Quad HIFI Ideas?

Postby James Perrett » Sun Jun 09, 2019 2:56 pm

ef37a wrote:Hah! I spent a nifty on a book and although Current Dumping was only a small part of the many things I hoped to learn, there was next to naff all about it!

That's probably down to Doug Self not rating it as a useful design. John Linsley Hood has a few paragraphs on it in his "Valve and Transistor Audio Amplifiers" book but there are also plenty of web resources covering the 405 and its successors.

PS - Amazon is listing a copy of the Linsley Hood book for under a tenner from one of their alternative suppliers at the moment.
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Re: Quad HIFI Ideas?

Postby ef37a » Sun Jun 09, 2019 3:28 pm

James Perrett wrote:
ef37a wrote:Hah! I spent a nifty on a book and although Current Dumping was only a small part of the many things I hoped to learn, there was next to naff all about it!

That's probably down to Doug Self not rating it as a useful design. John Linsley Hood has a few paragraphs on it in his "Valve and Transistor Audio Amplifiers" book but there are also plenty of web resources covering the 405 and its successors.

PS - Amazon is listing a copy of the Linsley Hood book for under a tenner from one of their alternative suppliers at the moment.

Thanks for that James, daughter can treat her old dad for that Dad's Day!

There is another amp design D S seemed a bit cold one? The "Edwin" amp. There you had the OP Transistors "just" not conducting and the speaker was driven at very low levels by the driver transistors*. As the drive current increase the voltage across the 100Rs in the power trans base circuits increase and caused them to start to conduct. All under feedback control. The design could not achieve distortion levels in 4 decimal places as do the best of Self's offerings but I am not sure we need such low figures, not for any but THE most esoteric purposes anyway?

*The Sinclcair Z50 was an Edwin op stage and was pretty good even if "50 watts" WAS rather optimistic! I actually built two "rips" using what I had available, TIP41/42 iirc and used them in bridge mode to drive a 10 foot column speaker. That system chucked it out I can tell thee! Sounded a hell of a lot better than the 15" disco "bins" you saw in the day, at least for vocal signals.

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Re: Quad HIFI Ideas?

Postby Hugh Robjohns » Sun Jun 09, 2019 4:43 pm

Ian Shaw wrote:BTW can someone easily explain 'current dumping'? It was obviously good enough to slap a patent on. From what I can gather it requires a high powered amp which gives whatever is needed to a lower powered amp?

There's lots of stuff across the web in varying levels of complexity about the current dumping concept. One of the most succinct is from Richard Brice: http://www.richardbrice.net/curdump.htm

But your description is broadly correct. A highly accurate, low-powered, class-A amplifier feeds into a passive network which ultimately feeds the speaker. And for low level signals, that's all there is...

But when a louder signal comes along, the class-A amp isnt powerful enough, so a simple class-B amplifier kicks in to deliver the appropriate oomph through the network to the speaker -- it 'dumps' the required current into the load.

However, a normal class-B amplifier suffers from gross cross-over distortion; as the signal moves from its positive side of the cycle to the negative side both of the class-B's output transistors switch off for a while and stop providing the required current to the speaker.

A class-AB amplifier cures that problem by biasing those transistors such that they never turn off completely... But that brings drawbacks with power and heat dissipation, as well as issues around controlling the bias current and maintaining stability.

The Quad design sticks with the simple class-B output stage, but cures the cross-over distortion problem by using that class-A stage to 'fill in' the gaps, feeding the corrective signal forward to the speaker through that passive network again.

It is an ingenious, very elegant, and extremely effective design, but Quad's patent prevented widespread adoption of it, so it never really received the praise it deserved.

H
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Re: Quad HIFI Ideas?

Postby Ian Shaw » Sun Jun 09, 2019 7:04 pm

Hugh Robjohns wrote:
Ian Shaw wrote:BTW can someone easily explain 'current dumping'? It was obviously good enough to slap a patent on. From what I can gather it requires a high powered amp which gives whatever is needed to a lower powered amp?

There's lots of stuff across the web in varying levels of complexity about the current dumping concept. One of the most succinct is from Richard Brice: http://www.richardbrice.net/curdump.htm

But your description is broadly correct. A highly accurate, low-powered, class-A amplifier feeds into a passive network which ultimately feeds the speaker. And for low level signals, that's all there is...

But when a louder signal comes along, the class-A amp isnt powerful enough, so a simple class-B amplifier kicks in to deliver the appropriate oomph through the network to the speaker -- it 'dumps' the required current into the load.

However, a normal class-B amplifier suffers from gross cross-over distortion; as the signal moves from its positive side of the cycle to the negative side both of the class-B's output transistors switch off for a while and stop providing the required current to the speaker.

A class-AB amplifier cures that problem by biasing those transistors such that they never turn off completely... But that brings drawbacks with power and heat dissipation, as well as issues around controlling the bias current and maintaining stability.

The Quad design sticks with the simple class-B output stage, but cures the cross-over distortion problem by using that class-A stage to 'fill in' the gaps, feeding the corrective signal forward to the speaker through that passive network again.

It is an ingenious, very elegant, and extremely effective design, but Quad's patent prevented widespread adoption of it, so it never really received the praise it deserved.

H

Thanks Hugh, that'll do nicely
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Re: Quad HIFI Ideas?

Postby ef37a » Sun Jun 09, 2019 7:44 pm

Yes thanks Hugh I do now have a better grasp.

I would venture to say that the output stage is not class "B" as it is normally considered? The transistors are at zero bias current (class C? ) whereas conventional class B, both valve and transistor, always has a small current flowing in the output stage and the complications of this were what Quad were trying to eliminate. Many would say they succeeded?

Self is not keen perhaps because the distortion figures for the 405 look decided pedestrian compared to his own designs which typically achieve an order better. 0.001% up to 1kHz and still .005% at 10kHz.

The idea of complimentary transistor at zero bias in the loop of a high gain op amp has been used in countless cheap mixers and other circuits as a headphone amplifier. Never read a complaint in a review!

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