405-2 Power on problem

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25-Jul-2019 12:06 AM

roquad

roquad

Posts: 5

I have bought a 405 -2 that has had the Ludwig mods done.

It has the annoying problem of a big thump coming through the speakers at switch/power on, switch off is ok.

Other than this problem the amp sounds excellent, is there anything I can do about this?

Thanks for any help,

Ro


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02-Feb-2020 05:24 PM

DavidA-C

DavidA-C

Posts: 5

I have a similar efect on my 405-2  and have made the same observation on my posting "Do you get a switch-on 'thump' from reducing the i/p stage gain (405-2)?" (last Oct).

From data sheets, reducing the opamp gain should produce lower distortion rather than keeping at the original value of 15.  However, this leads to higher current - 15/4.6 times higher throught the opamp.  Do we want lower (theorectically) distortion, and with harmonics that are more accpetable than from a LM301 or TL071?

Solutions? (i): A "slow start" circuit (as in the 909/Artera), i.e.: power on to "standby" and then triggered on - conect the i/p after a short period of time.  I have yet to try that one, but I will; (ii): Higher levels of reservoir capacitance, as seen in https://www.desmith.net/NMdS/Electronics/QUAD_upgrades.html ?  There's obviously scope when you consider the size of electrotlytics that were used when this amplifier was relased '79/80.

Will this "thump" cause an issue with a loudspeaker at switch-on?  For some, yes: moving coil lousdpeakers would be best off disconnected at one of the terminals at switch on.



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