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La Mer 23rd November 2008 08:21 AM

Help: inductances in original Quad405.
I'd like to build according to sch of Quad405 by myself (DIY), because I like this great amplifier, but at same time I don't have a chance to have this original one.

During the progress in prepairing components, one problem confused me: are L1 and L2 inductances has magnetic core nor not? Which kind of magnetic core it used? Which is the corret size of the inductances? And how many circles that the inductances have? Which diameter size of the wire for the inductances?

As much as anyone here who has knowledge and experience on original Quad405, could you please kindly help me?

Thank you very much.

pintur 23rd November 2008 10:40 AM

quad405 inductances
2 Attachment(s)

here a pic with the sizes of the two inductances that must be wound without magnetic core up.

Be advised that L2 is critical, being a component of the bridge having the task to correct errors.

Enjoy with this great design.



La Mer 25th November 2008 05:35 AM

Hi, pintur.

Thank you for your help and picture which you offered, I also like Quad405 very much for its great design.

Fortunately I got a meter to measure inductance exactly or very similar to correct one 3uH.

I also had the pic you as above, by this way, it seems more easy to make the inductance. But it more or less seems different from which inductances from original amplifier itself.

So here if there's anyone who has experience or know something about original machine, to see if the original machine has inductance with or without core. Than we can understand why designer and engineer of 405 used inductance with or without core. :whazzat:

pintur 25th November 2008 10:00 AM

Hi La Mer,

inductances like in the picture are currently used by the 405' diyers and however winding without core is the best choice for audio purpose.

If the size is right, there is no reason why they might seem like the originals.



La Mer 26th November 2008 10:27 AM

Thank you, Pintur. I will anyway try it.

sreten 26th November 2008 02:28 PM


The purpose of the bridge inductor means it must be air cored.


La Mer 2nd December 2008 10:04 AM

Thank you. sreten.

I have this question, because from some photos seeked from internet showing that inductors in original machine seems like as a resistor with a solid core, and the inductors' size seems very small. If an air inductor would be very much much bigger size than that one in photo.

Experienced men including you and pintur believes these inductors should be air one without magnetic core. But how to explain its size in original machine?

I try to make inductors by very similar smaller size as per original photo, with size diameter 0.7mm wires and inside induct diameter 6mm. After 20 circles, it only gets 2uH at most.

So I just wander if the original inductors in Quad405, uses magnetic cores. And to see if somebody has the original 405 amplifier can help me. :whazzat:

Ouroboros 2nd December 2008 10:41 AM

The original 405 used simple inductors with a ferrite-rod core for these. (At least my two QUAD405 amplifiers had). The inductors do need to be as linear as possible, so air-cored parts are better if you have the room.

sreten 2nd December 2008 12:45 PM


The original circuit is in this article :

There seems no point at all in used cored inductors for the 3uH.
The distortion caused by the core would ruin the amplifiers figures.
The inductor needs to be "perfect".


edit : L2 part is "inductor anco 440/d" if that helps ....

Ouroboros 2nd December 2008 12:51 PM

Maybe, but every original 405 board I've seen uses a cored axial-lead inductor!

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