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Quad 303 Power Amp

Postby vinyl_junkie » Sat Apr 21, 2012 7:58 pm

This is a bit of a strange one but I have a real hankering to try a Quad 303 for my hi-fi needs, not really for critical listening but just to enjoy music on... Mainly vinyl records and some CD's

Will be using a 1976 Bozak mixer pre-amp and still not decided on speakers yet but basically any one still use these and how do they stack up next to more modern amps class A/B designs?

I'm pretty sure Hugh can chime in on this as I'm sure you would have used one at some point or another.
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Re: Quad 303 Power Amp

Postby ef37a » Sat Apr 21, 2012 11:48 pm

vinyl_junkie wrote:This is a bit of a strange one but I have a real hankering to try a Quad 303 for my hi-fi needs, not really for critical listening but just to enjoy music on... Mainly vinyl records and some CD's

Will be using a 1976 Bozak mixer pre-amp and still not decided on speakers yet but basically any one still use these and how do they stack up next to more modern amps class A/B designs?

I'm pretty sure Hugh can chime in on this as I'm sure you would have used one at some point or another.
If it was "to factory spec" I doubt that you could tell one from any other well designed amplifier of similar power delivery but power delivery might well be a problem since they were rated at 45W per chan' and that is pretty low these days into modern compact speakers.

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Re: Quad 303 Power Amp

Postby Guy Johnson » Sun Apr 22, 2012 1:06 pm

I really don't like Quads ... Brittle; a zizzy top, a flabby bottom with un-involving mids, and a proclivity to sound more 'different' on various models of speaker than better amps do. And very sensitive to different speaker-cables, which probably goes with the more different sounds on various models of speaker: maybe a lower tolerance to varying reactive loads on the negative feedback topology?

Many people do like them of course; it's a matter of taste. And I've heard some Quad-amped systems that sound quite nice — despite the above statement! Listen before you buy in case you're in the dislike camp..

I'll now get my coat — to shelter from the blasts form Quad lovers!
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Re: Quad 303 Power Amp

Postby Hugh Robjohns » Sun Apr 22, 2012 1:27 pm

I owned a quad 33 and 303 system for years. Used it as my office music system for a while. The 33 preamplifier was noisy compared to modern systems, but very flexible and cleverly designed, and the eq options were ideal for records.

The 303 was a solid, simple amp. A little underpowered, perhaps, but okay for ns10s and other bookshelf speakers in the near field. I always felt it had a soft and flabby sound and have no regrets in flogging it to a quad lover ;)

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Re: Quad 303 Power Amp

Postby ef37a » Sun Apr 22, 2012 3:01 pm

Hugh Robjohns wrote:I owned a quad 33 and 303 system for years. Used it as my office music system for a while. The 33 preamplifier was noisy compared to modern systems, but very flexible and cleverly designed, and the eq options were ideal for records.

The 303 was a solid, simple amp. A little underpowered, perhaps, but okay for ns10s and other bookshelf speakers in the near field. I always felt it had a soft and flabby sound and have no regrets in flogging it to a quad lover ;)

Hugh
Damned with feint praise eh Hugh! If Hugh "suffered one for years" they can't be that bad! Probably the AC coupling shows its age in the LF region, not an amp perhaps for a peaky reflex speaker?

I can't see why a 303 would have "impedance problems"? One of the few truly stable amps of its day, had to be to drive the ESL which has killed many a lesser amplifier!

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Re: Quad 303 Power Amp

Postby James Perrett » Mon Apr 23, 2012 10:10 am

ef37a wrote:
I can't see why a 303 would have "impedance problems"? One of the few truly stable amps of its day, had to be to drive the ESL which has killed many a lesser amplifier!

One of the standard test that hifi reviewers used to do when reviewing an amplifier in the '70's was to show an oscilloscope display of the amp driving an 8 ohm load with 2uF in parallel - supposedly a simple simulation of an electrostatic speaker. Lesser amps would show all kinds of nasties while Quad amps tended to be fairly close to the ideal.

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Re: Quad 303 Power Amp

Postby ef37a » Mon Apr 23, 2012 10:38 am

James Perrett wrote:
ef37a wrote:
I can't see why a 303 would have "impedance problems"? One of the few truly stable amps of its day, had to be to drive the ESL which has killed many a lesser amplifier!

One of the standard test that hifi reviewers used to do when reviewing an amplifier in the '70's was to show an oscilloscope display of the amp driving an 8 ohm load with 2uF in parallel - supposedly a simple simulation of an electrostatic speaker. Lesser amps would show all kinds of nasties while Quad amps tended to be fairly close to the ideal.

James.
Quite! And the 303 would not give a monkeys' WHAT speaker cable you used. The AV firm I was with had the place wired in twin 5A solid PVC lighting cable..WITH the proper terminations of course!

The KEFs were thusly fed from the 303 in various rooms.

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Re: Quad 303 Power Amp

Postby Guy Johnson » Mon Apr 23, 2012 11:51 am

I only report what I've heard! And the impedance/negative feedback thing is only a theory off the top of me head. Which, with the lack of hair these days means that some brain-power does tend to leak out. Wow, word-association: —> Leak!
Those transistor amps ... they were the grainy-est sounding top end ever! The big de-coupling cap on the outputs being one of the culprits there, I think. Groovy looking things at the time, though.
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Re: Quad 303 Power Amp

Postby ef37a » Mon Apr 23, 2012 12:24 pm

Guy Johnson wrote:I only report what I've heard! And the impedance/negative feedback thing is only a theory off the top of me head. Which, with the lack of hair these days means that some brain-power does tend to leak out. Wow, word-association: —> Leak!
Those transistor amps ... they were the grainy-est sounding top end ever! The big de-coupling cap on the outputs being one of the culprits there, I think. Groovy looking things at the time, though.

I doubt it. Certainly electrolytic capacitors cause distortion if there is more than a few tens of millivolts of signal across them but since that 2,200mfd (I think) coupler had a reactance of only 0.7Ohms at even 100Hz I cannot see it affecting the "top end"?

In any event signal currents flow thru' the caps in a DC coupled supply just as much as in an AC coupled one.

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Re: Quad 303 Power Amp

Postby Guy Johnson » Mon Apr 23, 2012 6:04 pm

Quite. But it's what I heard ... tried putting the feed to the Leak's power stage through a more modern nice-sounding amp and much to my surprise (as I'd blamed the preamp/tone controls) the nastiness had gone.

Basic theory is all very well, but seems not to be the whole story.

What about such things in capacitors as eddy-currents in the electrolytics' foil? Self-capacitance between layers? Minute amounts of movement between the 'plates' and insulation due to electrostatic forces generated by rapid current and voltage swings? Non-linear behaviours in all that constructional complexity of an electrolytic. And that's just taking the capacitor on its own; let alone the rest of the components ...

If there's an observable difference, and the obvious bits of theory can't explain it, then there's a reason, and it's fun to think of some possibilities. These may be right or wrong.
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Re: Quad 303 Power Amp

Postby ef37a » Mon Apr 23, 2012 6:23 pm

Guy Johnson wrote:Quite. But it's what I heard ... tried putting the feed to the Leak's power stage through a more modern nice-sounding amp and much to my surprise (as I'd blamed the preamp/tone controls) the nastiness had gone.

Basic theory is all very well, but seems not to be the whole story.

What about such things in capacitors as eddy-currents in the electrolytics' foil? Self-capacitance between layers? Minute amounts of movement between the 'plates' and insulation due to electrostatic forces generated by rapid current and voltage swings? Non-linear behaviours in all that constructional complexity of an electrolytic. And that's just taking the capacitor on its own; let alone the rest of the components ...

If there's an observable difference, and the obvious bits of theory can't explain it, then there's a reason, and it's fun to think of some possibilities. These may be right or wrong.

There has never been, as far as I am aware, a definitive test that proved that one brand of very good power amplifier sounded any different to another of the same quality and many technical people would say that if they did there is something "wrong" with one or both of them.

It seems that despite the efforts of the Baxendalls and Walkers of this world (Wireless that is!) we have not moved far from myth and snake oil.

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Re: Quad 303 Power Amp

Postby Hugh Robjohns » Mon Apr 23, 2012 7:43 pm

It's the old idea that any 'adequate' system is indistinguishable from any other 'adequate' system... Which is true.

The sad thing is that not all systems are adequate, despite the aims and claims of their makers.

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Re: Quad 303 Power Amp

Postby ef37a » Mon Apr 23, 2012 10:13 pm

Hugh Robjohns wrote:It's the old idea that any 'adequate' system is indistinguishable from any other 'adequate' system... Which is true.

The sad thing is that not all systems are adequate, despite the aims and claims of their makers.

Hugh

Even if the "maker" and the tester panel do not know which amp is which?
But I really want to know because I lost touch with the industry about the time "hi-fi" went beardy/tweaky/subjectively silly. Was the test ever done and the money paid out?

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Re: Quad 303 Power Amp

Postby Folderol » Mon Apr 23, 2012 11:04 pm

My 2d...
At one time I would have said there probably was a noticeable difference if you had keen enough ears (which I certainly don't have these days), but today any HiFi amp that is working properly within it's ratings <- important that bit < would likely be indistinguishable from any other.

After all, the idea of a HiFi amp is a wire with gain. Unlike any instrument amp or signal processing setup you absolutely don't want any colouration of the signal.
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Re: Quad 303 Power Amp

Postby ef37a » Tue Apr 24, 2012 7:27 am

Hi Will,
That is basically my position, "subjectively perfect" power amplifiers have been with us for decades.

The emergence and general acceptance of the switching power amp seems to indicate that most people take a "black box" approach to the devices. After all, the cogniscenti were very resistant to transistorized amplifiers for years, and not without good reason in many cases but now there would be few audiophiles that would say valves are better than the best solid state designs, more pleasant to listen to perhaps (soggy bass!) but not as accurate.

One would therefore expect an amplifier that works on an entirely different principle (class A,class AB debates are trifling in comparison!) to be fiercely resisted but no, many designs seem to have slipped in under the Tweaks radar very nicely!

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Re: Quad 303 Power Amp

Postby Guy Johnson » Tue Apr 24, 2012 1:35 pm

Yes, there certainly is a lot of daft (and expensive) tweakery / quackery out there all right!

There's also a lot of stuff in audio that may or may not be true, but it's worth asking questions, such some of the things picked up on, that would have been scoffed at years ago — like tranny crossover distortion, jittery digital implementations coupled with crappy analogue gear. Subtle RFI problems, low-level linearity ... the list goes on.

What about 'perfect' circuits, that in their design and manufacture the earthing's all over the place. No longer 'perfect' in my book. May or may not make a difference, but there is a difference; it's no longer in the 'theoretically correct' condition the diagramme was.§

There are loads of traps like this. Bear in mind that there are effects waiting to be found; we don't know everything — a lot, but not everything.¶





§ This statement does not equate to disbelieving theory in any way.
¶ This statement does not equate to supporting daft tweakery etc.
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Re: Quad 303 Power Amp

Postby Hugh Robjohns » Tue Apr 24, 2012 1:43 pm

ef37a wrote:That is basically my position, "subjectively perfect" power amplifiers have been with us for decades.

If that was the case, and the Quad 303 was as perfect as Mr Walker claimed, then it would still be in production, surely? All the components are still available, after all.

It was a very good amp in its day, and is still useful today (although underpowered in many situations), but the technology has moved on quite a lot in the intervening 40+ years.

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Re: Quad 303 Power Amp

Postby Folderol » Tue Apr 24, 2012 7:40 pm

Well I had to do a bit of research to remind myself, but as I thought this amp has both asymmetric supplies and quasi-complimentary output. Lots, indeed most amps, of that period were the same and the problems tend to show up gradually with extended use.

Quite a nice drawing here:
http://www.reocities.com/ResearchTriangle/Lab/6722/quad303cirb.html

I certainly would never have thought of them as perfect.

One particular issue is the output coupling capacitor. I've lost count of how many times I've been able to apparently dramatically 'improve' the bass response simply by replacing one that had virtually dried out. It's an evil thing to do to an innocent cap - hitting it with those massive ripple currents at all sorts of frequencies! That said, I don't think I've replaced one on a Quad.

Another feature of this is that in a typical setup, on negative half-cycles the speaker sees the O/P cap and the lower O/P transistor, but on the upper half-cycle it sees the O/P cap, the upper O/P transistor and the PSU reservoir cap, or in this case voltage regulator. Not nice!

With any quasi complementary O/P stage it is almost impossible to get symmetrical crossover behaviour. The usual workaround is to go well into the A region - sort of Ab rather than AB.

A modern amp with split rail and true complementary outputs is much easier to control, and is not likely to sound just perceptibly slightly different at the end of a long session, due to shifted bias point and hot capacitors.

P.S.
I accept that some of that difference may in fact be due to heated speaker coils!
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Re: Quad 303 Power Amp

Postby vinyl_junkie » Tue Apr 24, 2012 11:51 pm

Brilliant stuff guys, thanks for all the reply's!

Thanks to being more skint this month I have seen sense and will be sticking to what I already have seeing as it does the job perfectly well.... until the next craving for old gear that is hahah The 303 looks like such a nice little charming amp I couldn't help but to dig deeper into it and find out more LOL
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Re: Quad 303 Power Amp

Postby ef37a » Tue Apr 24, 2012 11:58 pm

Hugh Robjohns wrote:
ef37a wrote:That is basically my position, "subjectively perfect" power amplifiers have been with us for decades.

If that was the case, and the Quad 303 was as perfect as Mr Walker claimed, then it would still be in production, surely? All the components are still available, after all.

It was a very good amp in its day, and is still useful today (although unpowered in many situations), but the technology has moved on quite a lot in the intervening 40+ years.

hugh
Walker did not claim his amplifier was perfect. What he said was (and I am sure you know this!) was that if an audible difference cannot be detected in a signal that passes through an amplifier (suitably loaded, signal attenuated) or bypasses it, such an amplifier, any make of amplifier, can be judged to be subjectively perfect. As Will says, this assumes the amplifier is never driven into overload, a reasonable condition for a test but not perhaps one that obtains in the real world. The problem of the bias point shifting due to thermal dynamics was solved in the later current dumpers. Other people such a Sugden took a different approach with pure class A but such is the huge efficiency penalty I always thought it a design steam hammer to crack a nut!

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