QUAD 405 power off noise

[ClassA]

There is a noise from speaker combined with speaker drive unit moving towardthe the cabin when I turned off my QUAD 405. Could anyone let me know what wrong with it and which part need to be replaced? Thanks! 🙁

 

[Tajzmaj]

Like pfiuuuuu....and than thump...and woofer cone movements....
Check Bernd Ludwig's text ....google it.
It is because op amp shuts down before rest of the amp....
You can modify a lot....add a lot ....or just leave as is. It is nothing wrong with your amp it is just old Quad.
Cheers, Taj

 

[raul_77]

Quad 405-2 upgrades

http://www.desmith.net/NMdS/Electronics/Quad405/405_Qw_14.pdf

 

[CopperTop]

My experience of a Quad 405 Mk 1 clone was good, without any unpleasantness at power up or down. However, when I played about with replacing the LM318 op amp with something more modern, I got thumps at power up and, I think, even a squeal of oscillation at power down.

 

[Zen Mod]

change all electrolit caps on pcbs
if those are leaky and old , then main reservoir caps are too , so change them also


been there .......

 

[ClassA]

Thank you all for these kindly suggestions. Is it possible due to a faulty C2 or C6?

 

[Tajzmaj]

If amplifier works normally otherwise than answer is no.
Even re-caped 405 will produce some turn off noise.
Regards

 

[ClassA]

Thank you, Tajzmaj.
Do you know what will be the difference if C2 (100uF/3V, tantalum cap) is replaced by general aluminum electrolytic Cap?

 

[Tajzmaj]

Almost none.
Just replace all caps like ZM suggested but don't be surprised if noise won't disappear.
Have you exchange op amps with something not original?
Cheers

 

[ClassA]

I just replaced op amp with OPA134 combined with two 100n wima caps for decoupling. Now the power off noise is decreased to a slight "Ssssss" and followed by a shorter con movement. Is this normal for all 405 amps?

Regards,
ClassA

 

[Tajzmaj]

I'm afraid yes.
Did you check supply voltage at op amp? It should be +-15V. I don't know about 134 op amp but if it draws too much current...
Check datasheet or measure it. And be careful. Don't short something.
If noise bothers you it is possible to install some sort of dc protection using relay which act as power on delay as well. There is plenty of such ideas around. Just google it. Again ,,Bernd Ludwig,, text. It is a lot useful information there.
Cheers

 

[Tajzmaj]

Just to make it easier.
Regards
http://www.desmith.net/NMdS/Electronics/Quad405/405_Qw_14.pdf

 

[ClassA]

Thanks for all of your help,especially, Tajzmaj.
Now This 405 works fine and sounds good. I think the degraded op-amp might be the root cause.

 

[noSmoking]

Hi,
I bought a set of boards from Jim's Audio (quad 405),and have been reading what you guys have posted ,Any words of wisdom/changes before I turn on the Iron to build it?

Thanks for your help!

 

[Tajzmaj]

Hi ..
First one is Ludwig Bernd text for which I already posted link and the other is this one.
So check both and decide.
Cheers, Taj
QUAD 405 Amplifier Information and Modification

 

[noSmoking]

Hi Tajzmaj,
I will indeed,

Thanks,