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Old 17th July 2007, 10:21 AM   #11
Thomo is offline Thomo  United Kingdom
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My Quad has been singing nicely for quite a while now, and I'm fairly pleased with it.

I have a question though, is it possible to change C5 to a larger value, higher quality cap?

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Old 17th July 2007, 10:42 AM   #12
MarkMcK is offline MarkMcK  United States
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Question for anyone knowing the history of the Quad 405. When first introduced it boasted innovative circuit design. I believe they called it current dumping.

The schematic posted here, however, is an old fashioned quasi complementary design. What happened to all the innovation? When did Quad abandon it?

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Old 17th July 2007, 11:14 AM   #13
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Quad used NPN output devices for robustness (and cost I guess). The 'Current Dumping' scheme means that the lack of linearity of the upper and lower totem-pole transistors isn't a problem, so you can use a quasi-comp output without any performance issues.
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Old 18th July 2007, 10:23 PM   #14
405man is offline 405man  Scotland
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The Quad 405 is an error canceling amplifier and uses a reactive bridge to combine the output of a class A stage with an unbiased output stage. If you look at the last page here

It is easier to see the configuration than in the original Quad schematic

This is a link to an original article from Quad

and this one to further discussion about this inovative design

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Old 19th July 2007, 08:02 AM   #15
dcd is offline dcd  United Kingdom
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Alex mm

You appear to have copied one of the features of the 405 PCB accurately - you have an electrolytic capacitor along side the R30/R31

L2 should not be 3.3uH - If you are aiming for accuracy and as you now have the bridge implemented correctly you should make it the calculated value.

The original design articles show how the bridge is derived using resistors and everyone now believes that the use of an inductor L2 is simply to reduce power losses when in fact it could be a 0.3-0.5R resistor with little loss but for the fact that it is fitted to prevent switching spikes from getting to the output and as such it is best that L2 (and all the inductors) are a bit lossy to prevent ringing.


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Old 4th September 2007, 09:26 PM   #16
alex mm is offline alex mm  Romania
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Default QUAD with MOSFET output transistors

another version of QUAD and PCB .Amplifier will bee tested this weekend ,i hope
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Old 5th September 2007, 06:54 AM   #17
geminni is offline geminni  Serbia
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Mosfet for output? Can't wait for results!
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Old 5th September 2007, 09:32 AM   #18
405man is offline 405man  Scotland
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Do you have a schematic

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Old 5th September 2007, 10:37 AM   #19
tvicol is offline tvicol  Romania
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Default Quad 405 Mos-fet


This version was made at Keith suggestion. Thanks Keith !

Schematic vas posted on a Romanian forum.
Here is the link
QUAD 405 Reborn MOS-FET output

We are currently testing, so this is not final.
You are invited to comment this implementation.

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