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Old 19th April 2012, 07:43 PM   #1
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Default quad 405

Hello, I tried to use the search function but there is an endless list of threads.
I asked myself if anyone has the original scheme of quad 405 or even a version with mosfets!
should be better with mosfet right?
Thanks to all
LC
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Old 20th April 2012, 08:49 AM   #2
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Old 20th April 2012, 10:09 AM   #3
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Quote:
Originally Posted by luigicosta View Post
Hello, I tried to use the search function but there is an endless list of threads.
I asked myself if anyone has the original scheme of quad 405 or even a version with mosfets!
should be better with mosfet right?
Thanks to all
LC
I think that's what you need.
QUAD 405-2 Amplifier Modification and Information

look for an schematic evolution archive

All the versions build depending on serial number
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Old 20th April 2012, 10:27 AM   #4
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thanks to all!
LC
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Old 20th April 2012, 11:37 AM   #5
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When I repaired and revised an old QUAD-405 almost a decade ago, I found the information from Bernd Ludwig very useful, see:

Quad 405 mods

I didn't apply all his modifications, but mainly the ones that are included in the QUAD 405-2.

By the way, when you use an op-amp with sufficiently low DC input current (such as a FET op-amp) and are prepared to make the subsonic filtering a bit less effective for very low frequencies, you can easily replace the 100 uF tantalium elco with a 2.2 uF film capacitor when you modify some resistors. The following values work fine with a TL071 and an input sensitivity of 0.75 V RMS, but change the slope of the subsonic filter to first-order below 2.3557 Hz:

C1=680 nF

R3=22.1 kohm

R6=221 kohm

C4=68 nF

R4=68 kohm

R5=56 kohm

C2=2.2 uF

The attached text file contains the related mathematics. When you reduce the corner frequency, you may also need to change the DC protection time constant to prevent false triggering. I've only tried the values above, which give basically the same response as the original design but without needing 100 uF.
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File Type: txt Q405lp.txt (16.7 KB, 52 views)
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