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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

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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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Re: Quad HIFI Ideas?

Postby Forsty » Sat Jul 20, 2019 9:19 pm

I found this thread while I was wondering if I should get a Quad system. I recently bought a friends pair of B&W CM9 and Im in need of an amp. I really like the design and simplicity of the Quad amps (Elite Pre/QSP, Artera Play/Stereo). Plus it seems like the prices are dropping the last weeks, for example I found an Elite QSP for under 700€ (brand new).

But it's really hard to find reviews about these modern systems, I found only 2-3 reviews which all were quite positive for the price class. Is Quad really so unpopular nowadays? If so, why exactly?
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Re: Quad HIFI Ideas?

Postby ef37a » Sun Jul 21, 2019 8:24 am

Forsty wrote:I found this thread while I was wondering if I should get a Quad system. I recently bought a friends pair of B&W CM9 and Im in need of an amp. I really like the design and simplicity of the Quad amps (Elite Pre/QSP, Artera Play/Stereo). Plus it seems like the prices are dropping the last weeks, for example I found an Elite QSP for under 700€ (brand new).

But it's really hard to find reviews about these modern systems, I found only 2-3 reviews which all were quite positive for the price class. Is Quad really so unpopular nowadays? If so, why exactly?

If I were in the position of putting together a top grade hi fi system I would go for one of the Self amp designs.
They have reduced all form of distortion except crossover* distortion to below the noise floor of the latest (I think?) AP test rig, at least below 3kHz or so and even at 10kHz it is in the third decimal place.

That said, you now need to find a suitable passive speaker and so upon reflection, MY dream system would probably be some very expensive active monitors!

*Even that is very low indeed and should bother no one. Self does have a fix. A "displacer" that all, but eliminates Xover distortion but it does reduce efficiency considerably and seems a little inelegant to me. (but...WTF do I know!)

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Re: Quad HIFI Ideas?

Postby Arpangel » Sun Jul 21, 2019 8:47 am

ef37a wrote:
Forsty wrote:I found this thread while I was wondering if I should get a Quad system. I recently bought a friends pair of B&W CM9 and Im in need of an amp. I really like the design and simplicity of the Quad amps (Elite Pre/QSP, Artera Play/Stereo). Plus it seems like the prices are dropping the last weeks, for example I found an Elite QSP for under 700€ (brand new).

But it's really hard to find reviews about these modern systems, I found only 2-3 reviews which all were quite positive for the price class. Is Quad really so unpopular nowadays? If so, why exactly?

If I were in the position of putting together a top grade hi fi system I would go for one of the Self amp designs.
They have reduced all form of distortion except crossover* distortion to below the noise floor of the latest (I think?) AP test rig, at least below 3kHz or so and even at 10kHz it is in the third decimal place.

That said, you now need to find a suitable passive speaker and so upon reflection, MY dream system would probably be some very expensive active monitors!

*Even that is very low indeed and should bother no one. Self does have a fix. A "displacer" that all, but eliminates Xover distortion but it does reduce efficiency considerably and seems a little inelegant to me. (but...WTF do I know!)

Dave.

I've found, some people go for an ultimate uncoloured sound, only to find that it's the coloration of a system that makes it.
Naim, Linn, etc, promoted this transparency in the 80's and a lot of people including me didn't like it, it sounded cold and scratchy.
Quad isnt't as popular, simply because it's not bling, it's very old school, personally I like a bit of bling, McIntosh, JBL, Michell, all that stuff, if I was going to put together a fun system.
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Re: Quad HIFI Ideas?

Postby ef37a » Sun Jul 21, 2019 11:39 am

Arpangel wrote:
ef37a wrote:
Forsty wrote:I found this thread while I was wondering if I should get a Quad system. I recently bought a friends pair of B&W CM9 and Im in need of an amp. I really like the design and simplicity of the Quad amps (Elite Pre/QSP, Artera Play/Stereo). Plus it seems like the prices are dropping the last weeks, for example I found an Elite QSP for under 700€ (brand new).

But it's really hard to find reviews about these modern systems, I found only 2-3 reviews which all were quite positive for the price class. Is Quad really so unpopular nowadays? If so, why exactly?

If I were in the position of putting together a top grade hi fi system I would go for one of the Self amp designs.
They have reduced all form of distortion except crossover* distortion to below the noise floor of the latest (I think?) AP test rig, at least below 3kHz or so and even at 10kHz it is in the third decimal place.

That said, you now need to find a suitable passive speaker and so upon reflection, MY dream system would probably be some very expensive active monitors!

*Even that is very low indeed and should bother no one. Self does have a fix. A "displacer" that all, but eliminates Xover distortion but it does reduce efficiency considerably and seems a little inelegant to me. (but...WTF do I know!)

Dave.

I've found, some people go for an ultimate uncoloured sound, only to find that it's the coloration of a system that makes it.
Naim, Linn, etc, promoted this transparency in the 80's and a lot of people including me didn't like it, it sounded cold and scratchy.
Quad isnt't as popular, simply because it's not bling, it's very old school, personally I like a bit of bling, McIntosh, JBL, Michell, all that stuff, if I was going to put together a fun system.

Then sir, WTGR you are not talking about "High Fidelity".

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Re: Quad HIFI Ideas?

Postby Sam Spoons » Sun Jul 21, 2019 12:19 pm

Speaking of which I have always wondered why HiFi systems have 'tone controls'
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Re: Quad HIFI Ideas?

Postby John Willett » Sun Jul 21, 2019 1:37 pm

Sam Spoons wrote:Speaking of which I have always wondered why HiFi systems have 'tone controls'

I always liked the controls on the Quad 44 pre-amp., they were very subtle and "tuned" the sound without changing the character of the music. :thumbup:
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Re: Quad HIFI Ideas?

Postby blinddrew » Sun Jul 21, 2019 1:50 pm

Sam Spoons wrote:Speaking of which I have always wondered why HiFi systems have 'tone controls'
That's before we even get to the dreaded graphic equaliser! ;)

To go back to the question about why things aren't in demand anymore, it's probably got a bunch to do with the fact that HiFi separates in general aren't in demand much anymore.
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Re: Quad HIFI Ideas?

Postby Wonks » Sun Jul 21, 2019 2:04 pm

Sam Spoons wrote:Speaking of which I have always wondered why HiFi systems have 'tone controls'
And then make a big feature of their 'tone bypass' switch!
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Re: Quad HIFI Ideas?

Postby Sam Spoons » Sun Jul 21, 2019 2:06 pm

Wonks wrote:
Sam Spoons wrote:Speaking of which I have always wondered why HiFi systems have 'tone controls'
And then make a big feature of their 'tone bypass' switch!

:bouncy: :bouncy: :bouncy:
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Re: Quad HIFI Ideas?

Postby ef37a » Sun Jul 21, 2019 3:24 pm

Sam Spoons wrote:Speaking of which I have always wondered why HiFi systems have 'tone controls'

Because loudspeakers and rooms are not all the same nor perfect!

"Back in the day" neither were phono cartridges . Who here recalls the dreaded "V15 mid suck out"? But one thing you could make virtually "subjectively perfect" was the amplifier chain. RIAA EQ could be better than + or - 1dB 20 to 20 and distortion kept to inaudible levels throughout the chain. What point striving for ever better loudspeakers if the amp stamps it own character on the sound?

The fact is, if one pre/power setup sounds different from another, one of them (or both) are "wrong"!

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Re: Quad HIFI Ideas?

Postby Hugh Robjohns » Sun Jul 21, 2019 4:08 pm

Most of my hi-fi setups don't have EQ options, just because that's the way most 'quality' hi-fi has been built and sold since the 70s.

However, I acquired a Quad 44/405 rig a while ago, mostly just out of interest. I'm not very impressed, to be honest and I can see why there are so many 'mods' detailed on line...

But the Tilt EQ is actually remarkably useful -- not to correct for the speakers or the room, or even the turntable cartridge, but to compensate for the very variable tonal balance due to both musical era and different mastering engineers.

It's amazing how many mainstream commercial tracks could benefit from a little corrective EQ... IMHO :ugeek:
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