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Thomo 14th July 2007 07:48 PM

Quad 405 Op-Amp Power Supply
Hi everyone. I am a relative noob, but have successfully modified a Marantz CD67 and my Quad405.

What I would like to know, is it possible to feed the opamp with its own regulated low-noise power supply?

If it's possible, is it worth it?

Any help will be greatly appreciated.

Regards, Lee.

Craig405 14th July 2007 08:05 PM


Yes it is possible with a few alterations to the PCB's, a simple series pass transistor can be driven from the centre point of the 15v zener and a resistor divider.

You can try it, it cant hurt the sound, it may help when the rails start to ripple under high current draw.
The op amp is a pretty important part of the circuit as the first stage and it provides most of the gain too.
You can upgrade the op amp to a burr brown high performance one, apparently this makes a HUGE difference over the TL071, i used an OPA134PA when i built a clone a while back.

Good luck

Thomo 14th July 2007 08:11 PM

Hi, thanks for your reply.

I have already upgraded the opamp to OPA604, and found this to be a dramatic improvement over the original. I have also altered the circuit to be non-inverting, and reduced the gain, basically all the Keith Snook mods.

I was thinking of completely disconnecting the opamps current supply and feeding it via its own tx + rectifier + LM317. I've done this extensively in my cd player to great effect.

Would this work do you think?

Regards, Lee.

Craig405 14th July 2007 08:19 PM

If youve got the parts whip it up and give it a try!
maybe just on one channel and see if the difference is apparent :)
I cant see why it wouldnt be absolutely fine...

Make sure you keep the 100nf cap soldered right next to the op-amp supply pins, or on the pin itself.
Get the centre tapped ground of your second transformer connected to the ground bus of the Quads power transformer and it should be fine.

Thomo 14th July 2007 08:21 PM

I've got plenty of parts - so I think I'll try it tomorrow!

Thanks for your advice on the grounding, I may well have forgotten that.



Craig405 14th July 2007 08:22 PM

This thread has an interesting Quad schematic where regulators are used in the supply about 1/2 way down first page.

good luck

pytyu 19th July 2007 12:49 AM

Power supply for Quad 405

Can somebody tell if there is a problem in supplying the Quad 405 with +/- 60V instead of +/-50V? I supplied a Quad module with +/-63V and it sounded OK


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