Hi Edmond,
Understood, but who is talking about using 1/4 watt resistors in such a location?
Anyway, it is a good point to look at the peak power dissipation of the resistor during turn-on, and this is something that can be SPICEd.
Of course, resistors are rated for continuous power dissipation. Most can handle several times that in peak dissipation and suffer no reduction in reliability.
As pointed out earlier, it also depends a lot on how fast the power supply comes up.
Consider some back-of-the-envelope numbers.
Suppose at turn-on we have 100 Amps through the rectifier, maximum. To simplify and make things more worst-case, assume this 100A persists over the full time it takes to charge the reservoir capacitor to 60V.
Assume we have a 50,000 uF reservoir capacitor.
The voltage slew rate on the rail will be I/C = 100A/0.05F = 2000 V/s.
The rail will come up in 30ms.
Assume that the rail voltage slew rate (SR) is applied directly to a 100 uF filter capacitor (ignore the effect of the series resistor to simplify and be more worst-case).
The capacitor current will be SR * C = 2000V/s * 0.0001F = 0.2A.
If we run this through a 10-ohm resistor, the peak voltage drop will be 2V and the peak dissipation will be 2V * 0.2A = 0.4W.
This seems too small even for me to believe. What have I done wrong here? The math? The assumptions?
Cheers,
Bob
Hi Bob,
I also was puzzled. We both made a wrong assumption by thinking that the front-end PSU was directly tied to the main PSU via 10 Ohms resistors. That's not the case. After rereading syn08's description of the PSU, it appears that the front-end supply rails are switched on by means of relays. (see the final note on this page: Power Supply ). No wonder that the resistors were blown.
My apologies for the confusion.
BTW, did you try the VDMOS sub-threshold enhancements as mentioned on page 2304?
Cheers,
E.
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/Schematics/QUAD 405 Service Data.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.
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/Schematics/QUAD 405 Service Data.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.
I've read up on Black's Feedforward, Quad, Vanderkooy (…and Annison, Danyuk, Sandman, Stochino) paper's circuits, simmed a few alternatives
after thinking about it I come to the question that if you can measure the error to amplify and add in to cancel the main amp distortion - why don't you just use more feedback?
for audio power amps we have the ability to use plenty negative feedback loop gain over an extended definition of audio frequency range if allowed to use RET or MOSFET output Q to push unity loop gain intercept up and use higher order compensation, or nested feedback (including Bob's, Hawksford's EC)
feedback error disappears into noise for really high loop gain feedback amps except for the last few octaves of audio
the feedforward schemes can knock down the >20 kHz errors ~ 20-30 dB – but at some cost in parts, design complexity for arguably inaudible “improvement”
and the cancellation is very sensitive to gain/power coupling network tolerances
remember also that the audible IMD products are reduced by the feedback at the product frequency – so any high frequency difference products folding down into audio are reduced by the high loop gain feedback
feedforward may just reduce Bob's THD 20 kHz metric without adding to/giving any of the implied "goodness" at actually audible frequencies that we hope the THD20 correlates with
http://www.diyaudio.com/forums/solid-state/184725-alternative-topologies-3.html#post2506046
http://www.diyaudio.com/forums/soli...in-composite-op-amp-circuits.html#post2415805
after thinking about it I come to the question that if you can measure the error to amplify and add in to cancel the main amp distortion - why don't you just use more feedback?
for audio power amps we have the ability to use plenty negative feedback loop gain over an extended definition of audio frequency range if allowed to use RET or MOSFET output Q to push unity loop gain intercept up and use higher order compensation, or nested feedback (including Bob's, Hawksford's EC)
feedback error disappears into noise for really high loop gain feedback amps except for the last few octaves of audio
the feedforward schemes can knock down the >20 kHz errors ~ 20-30 dB – but at some cost in parts, design complexity for arguably inaudible “improvement”
and the cancellation is very sensitive to gain/power coupling network tolerances
remember also that the audible IMD products are reduced by the feedback at the product frequency – so any high frequency difference products folding down into audio are reduced by the high loop gain feedback
feedforward may just reduce Bob's THD 20 kHz metric without adding to/giving any of the implied "goodness" at actually audible frequencies that we hope the THD20 correlates with
http://www.diyaudio.com/forums/solid-state/184725-alternative-topologies-3.html#post2506046
http://www.diyaudio.com/forums/soli...in-composite-op-amp-circuits.html#post2415805
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Thanks jcx for the interesting comments and links.
You mention the use of better output devices - sure it is clear that we can do better with better devices but this design is not much more complex than others and seems to do very well with inexpensive, slow devices. You say that cancellation is very sensitive to gain/power coupling network tolerances which I'm sure is true if your goal is perfect cancellation but why not just accept a 20 dB improvement and say good enough. And really, 1% resistors are not very expensive. I also like the advantage of there being no thermal tracking issue with this design.
I have a rough idea of where the audible limits of distortion are and I believe that many designs here are well past those limits. But to have a low distortion class B design like the Quad with no thermal adjustment or tracking issues is an interesting alternative.
Interesting that this thread just started on the Quad 909:
http://www.diyaudio.com/forums/solid-state/206460-quad-909-clone.html
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I don't see the justification for /attraction of the "Class C" dumper - its not like thermal runaway in Class AB is intractable or even expensive in component count to control - unless you're Doug Self trying to nail his "Optimum Class B" bias point
with any bias at all the feedforwad has lots less work to do, the mismatch isn't so critical as when you're trying to fix a 1.2 V deadzone at zero crossing
Thorsten’s fixing up a power chip amp with Stochino’s Feedforward circuit may be interesting if you want to avoid power circuit design – the power chip amps are so slow you can’t do that much for them with added (nested) negative feedback
with any bias at all the feedforwad has lots less work to do, the mismatch isn't so critical as when you're trying to fix a 1.2 V deadzone at zero crossing
Thorsten’s fixing up a power chip amp with Stochino’s Feedforward circuit may be interesting if you want to avoid power circuit design – the power chip amps are so slow you can’t do that much for them with added (nested) negative feedback
We obviously have different perspectives based on this and our past discussions. There's no reason why I wouldn't bias it up a bit more say to a .2V dead zone, it is an obvious modification. Quad did eventually add one diode to reduce the deadzone to .6V. There's been a lot of discussion about thermal tracking, output devices with built in diodes and thus some are concerned about it - I like the fact that all that goes away. I don't have any issue with power circuit design I just like to review all the options with a level headed view of the advantages/disadvantages.
I'd like to interject here that there are many advantages to a combination of Class A and Class B operation (NOT AB) such as used by McIntosh in tube amp design, where a relatively low-power Class A driver stage drives the load through the crossover region, and where the Class B output stage then drives the rest of the signal swing.
This design was successfully transferred to solid-state by Mattes in the "Sharma" circuit many years ago, and something similar was done also by Dynaco. Negative feedback then sorts it all out for low levels of THD. I think of this as a feedback form of current dumping. I haven't done a web search for this design and don't know if it has any adherents these days, but it should....
This design was successfully transferred to solid-state by Mattes in the "Sharma" circuit many years ago, and something similar was done also by Dynaco. Negative feedback then sorts it all out for low levels of THD. I think of this as a feedback form of current dumping. I haven't done a web search for this design and don't know if it has any adherents these days, but it should....
the Vanderkooy, Lipshitz (quad/jaes.pdf) paper spells out the requirement for “true feedforward”
if you're talking "bootstrapping" a heavy bias floating supply Class A with the Class B high V stage I think the analysis shows you have to consider it as a feedback amplifier - possibly with nested loops although there is some isolation by the load's own impedance - I think it is at best equivalent to their fig 4a
"true" feedforward requires non-interaction of the 2 amps - one shouldn't affect the stability of the other - with fixed impedance transmission line loads Black proposed "biconjugate network" coupling
in SE audio amps the practical solution seems to be essentially a power XO with the correction amp controlling the load at high frequency
I certainly enjoy the intellectual exercise studying Black's Feedforward, knowing its historical relation to negative feedback
I just don't yet see it as anything but a curiosity in practical audio power amps if you aren't limited by other considerations from "maxing out" negative feedback, open loop linearity
but at this late date I think most audio power amp design can be characterized as "conceptual art" - so whatever floats your boat is fine
"con·cep·tu·al art
Noun:
Art in which the idea presented by the artist is considered more important than the finished product, if any exists
if you're talking "bootstrapping" a heavy bias floating supply Class A with the Class B high V stage I think the analysis shows you have to consider it as a feedback amplifier - possibly with nested loops although there is some isolation by the load's own impedance - I think it is at best equivalent to their fig 4a
"true" feedforward requires non-interaction of the 2 amps - one shouldn't affect the stability of the other - with fixed impedance transmission line loads Black proposed "biconjugate network" coupling
in SE audio amps the practical solution seems to be essentially a power XO with the correction amp controlling the load at high frequency
I certainly enjoy the intellectual exercise studying Black's Feedforward, knowing its historical relation to negative feedback
I just don't yet see it as anything but a curiosity in practical audio power amps if you aren't limited by other considerations from "maxing out" negative feedback, open loop linearity
but at this late date I think most audio power amp design can be characterized as "conceptual art" - so whatever floats your boat is fine
"con·cep·tu·al art
Noun:
Art in which the idea presented by the artist is considered more important than the finished product, if any exists
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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
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
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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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.
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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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?
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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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
Quad current dumping amp
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
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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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...
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...
I've arrived more than 12 months after the party began
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