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Old 13th February 2012, 09:59 AM   #2351
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Originally Posted by PB2 View Post
Hi Bob,

I've not seen much discussion around here of the Quad 405 amplifier. I do remember the articles in the AES and Wireless World years ago but I don't think any of them showed a full schematic. I recently found the service manual and I have to say that it is clever and fairly simple. Here's the service manual in case you've not seen it. The N1 and N2 modules are output current limiting and are often removed since they tend to trip too early:
http://www.keith-snook.info/Schemati...ice%20Data.pdf

I see things that I'd do differently but it is an interesting design.

Here is an AES paper that discusses the unique output stage:
http://quad405.com/jaes.pdf

And this page has reviews, mods, etc:
Quad 405 Information Page

Wondering if you studied it in detail and what you think of it.
Hi Pete,

Sorry to be late on this, and it looks like you've already gotten some good answers. I have never been enamored of the Quad current dumping arrangement. I must admit I have not studied it in detail, but it appears that when the dumper devices turn on there will be a potentially significant change in net output stage transconductance, leading to crossover distortion.

Cheers,
Bob
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Old 13th February 2012, 10:15 PM   #2352
PB2 is offline PB2  United States
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Well, I would not agree that they were good answers and you seem rather dismissive of anything not fitting certain criteria of yours. Do you really think that the final design with the output stage bridge has significant crossover distortion? I realize that the crossover distortion is not completely nulled out, but the levels are low and probably below the limits of audibility. Even Vanderkooy and Lipshitz took the time to study it in detail and publish 2 papers about it. One in 1978 in Wireless World:
http://quad405.com/lips.pdf

and the other in 1980 in the AES:
http://quad405.com/jaes.pdf

From a DIY view, or even a purists view you can hold whatever opinions involving elegance in design and complexity that float your boat, however as professional engineers we are usually expected to design to a price point keeping in mind or better knowing/testing the limits of audibility unless there are other prevailing requirements.

I'd think that you'd be interested even simply from a historical perspective.

I found the 405 schematics to be poorly drawn and if anyone is interested, the 606 is easier to read plainly showing the bridge components:
http://quad405.com/606schematic.pdf
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Last edited by PB2; 13th February 2012 at 10:37 PM.
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Old 13th February 2012, 10:37 PM   #2353
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@Pete -- in the 606, could you explain what T15 and T16 do? I really don't get it.
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Old 13th February 2012, 11:35 PM   #2354
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It was mentioned in the Quad 909 clone thread that the positive supply is a few volts higher than the negative and my theory was that this was to compensate for the high saturation voltage of the VAS along with the preceeding emitter followers. I'm really not sure if they have anything to do with the pos and neg voltage difference, or if they were trying to provide some sort of ground isolation for the front end, which really doesn't make sense to me. These are my thoughts without having looked into it too deeply - wouldn't be surprised if they are both wrong, lol!

EDIT: Just noticed, they're not using a center tap off the power transformer! That's it, and it is how they get the slightly offset supply. I suppose the load current goes through the PSU caps, but I'm not totally comfortable with that ground circuit without further analysis.
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Last edited by PB2; 13th February 2012 at 11:47 PM.
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Old 13th February 2012, 11:43 PM   #2355
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If anyone is suggesting that the Quad design is no better than a Class B amp with high feedback then please just clarify and come right out and say it. I never really liked the Quad design reading about it years ago, but I am warming up to it and rereading the two papers. I believe that once distortion levels are reasonably low it is the behavior of the amp in overload, and recovery that sets better amps apart from others. High feedback amps do not usually have the best overload recovery so perhaps this is a consideration. I've not looked at all at how the Quad design behaves in overload. I'm aware that the best thing is to have enough power to never go into overload but in real world situations this is not usually practical.

I would say that the Quad 405 has more than decent performance for a Class B amp from 1976, but they say that it is probably limited by the OP amp front end and perhaps the not fully optimized Class A amp. The 606 and later amps eliminated the OP amp (it is just a DC servo) and it would be interesting to see a test report of these later designs.

It is interesting that in part 2 of the 1978 paper the authors state that Peter Baxandall "has recently justified his approach quite clearly to us. We hope he will expand his elegant argument in his own series of articles." Does anyone know if he ever published this work, and if it is available online?
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Last edited by PB2; 14th February 2012 at 12:08 AM.
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Old 14th February 2012, 12:46 AM   #2356
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Originally Posted by PB2 View Post
Well, I would not agree that they were good answers and you seem rather dismissive of anything not fitting certain criteria of yours. Do you really think that the final design with the output stage bridge has significant crossover distortion? I realize that the crossover distortion is not completely nulled out, but the levels are low and probably below the limits of audibility. Even Vanderkooy and Lipshitz took the time to study it in detail and publish 2 papers about it. One in 1978 in Wireless World:
http://quad405.com/lips.pdf

and the other in 1980 in the AES:
http://quad405.com/jaes.pdf

From a DIY view, or even a purists view you can hold whatever opinions involving elegance in design and complexity that float your boat, however as professional engineers we are usually expected to design to a price point keeping in mind or better knowing/testing the limits of audibility unless there are other prevailing requirements.

I'd think that you'd be interested even simply from a historical perspective.

I found the 405 schematics to be poorly drawn and if anyone is interested, the 606 is easier to read plainly showing the bridge components:
http://quad405.com/606schematic.pdf
Hi Pete,

I'm sorry my answer disappointed you, and I did not intend to be dismissive of the quad. Its probably been 30 years since I studied the Quad, and I just don't remember much about the details, and I don't have the time right now to research it. It may have been a clever idea, but that does not necessarily mean it was effective in providing good measureable or audible performance.

Cheers,
Bob
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Old 14th February 2012, 01:19 AM   #2357
PB2 is offline PB2  United States
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Hi Bob,

I certainly understand time constraints, no problem.
It did measure reasonably well and many did like the sound
of the units without output protection.

Pete
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Last edited by PB2; 14th February 2012 at 01:22 AM.
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Old 14th February 2012, 12:25 PM   #2358
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Default Quad current dumping amp

Quote:
Originally Posted by PB2 View Post
If anyone is suggesting that the Quad design is no better than a Class B amp with high feedback then please just clarify and come right out and say it.
[...]
Hi Pete,

Well, one might argue that the power bandwidth is limited by the capabilities of the class A (helper) amp. So don't expect a perfect square wave response. However, I wouldn't consider this as a real issue as long as the amp is being used for the purpose it is intended: reproducing music.

Another issue might be the low damping factor at higher frequencies (due to the 3uH coil*). Given the presence of other inductances (from loudspeaker leads and voice coils, which might be even much higher), we should put the limited damping in the right perspective. Again, I wouldn't consider this as a serious flaw.

Even to 21st century standards, it's still an amazingly good amp.

Cheers,
E.

edit: * only 1.5uH in the Quad 909
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Last edited by Edmond Stuart; 14th February 2012 at 12:43 PM.
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Old 19th February 2012, 02:15 PM   #2359
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I've arrived more than 12 months after the party began but having finally got round to ordering Bob's book and having spent the last couple of weeks browsing through it I have to say I'm impressed.

Bob... you have a lot to answer for

My signature says it all, but your style of writing on LTSpice (and I groaned when I first saw "so much" devoted to it) seemed so clear and concise that it has encouraged me to have a go, starting with following your diff amp example. It was just seeing how to get started with it all that I needed... the basic operations and keystrokes. There are a couple of things I am a bit stuck on so I will post those questions elsewhere... and hope you Spice experts can help.

Really enjoying the book anyhow...
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Old 19th February 2012, 05:48 PM   #2360
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Originally Posted by Mooly View Post
I've arrived more than 12 months after the party began but having finally got round to ordering Bob's book and having spent the last couple of weeks browsing through it I have to say I'm impressed.

Bob... you have a lot to answer for

My signature says it all, but your style of writing on LTSpice (and I groaned when I first saw "so much" devoted to it) seemed so clear and concise that it has encouraged me to have a go, starting with following your diff amp example. It was just seeing how to get started with it all that I needed... the basic operations and keystrokes. There are a couple of things I am a bit stuck on so I will post those questions elsewhere... and hope you Spice experts can help.

Really enjoying the book anyhow...
Hi Mooly,

Thanks very very much for your kind words about my book, and I'm really glad you are finding it useful. That is my definition of success as a writer. I'm especially glad that the LTspice section got you going quickly. Like a lot of software, LTspice has a lot of capabilities, functions and shortcuts. I has been my own experience that just the basics are needed to really get a lot out of it and develop confidence, later picking up some of the other stuff as you need it or discover it.

Best regards,
Bob
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