Why is the Aletheia DACs Power Switch on the back panel?

Last week, a customer who had just purchased an Aletheia dac1 asked why it only had a mains power switch on the back, and nothing on the front to switch on the mains power? (Thanks to Ruud in The Netherlands for a good question).

Well it’s like this for design reasons that we think are important for the DAC (and probably any hifi unit).

In the Aletheia DACs we bring the power in through the IEC inlet on the back panel then the internal wiring goes immediately to the switch and the fuse. Then the main feed is taken to the power input acoustic absorption module. But also at this point we connect the (low current) parallel wiring to the shunt RFI filter module. The important principle here is that these two modules form a barrier to incoming vibration and RFI.

Now, with respect to vibration, the mains socket, switch and fuse are attached to the chassis. And your mains lead plugs into the socket. So we consider these items as ‘dirty’ with regards to vibration. And by the way, this is why all the other input and output connectors are fixed on their isolated mounting plates – so that those delicate circuits do not have a direct hard contact with these dirty parts.

With respect to incoming RFI, we have kept the wiring short from the rear of the IEC socket, to the switch and fuse holder, then to the shunt filter module. The shorter this wiring is, the less wiring there is to radiate RFI inside the box.

Now lets consider if we were to fit a front panel switch. Well, first, if we took the feed from the output of the acoustic absorption module where it’s mounted now, and then re-connected it to the chassis again with a front panel switch, we re-introduce vibration again from our ‘dirty’ parts – we’ve bypassed our barrier.

And Also of course, if we run long leads to a front panel switch before we take it to our RFI filter, we have now introduced much longer lengths of mains wiring inside the box that would radiate far more RFI inside the box.

When we worked on the critical overall layout within the DACs, where these mains modules were mounted also had an impact on other important considerations. And we could clearly see that keeping the mains modules close to the rear chassis enabled a much better layout relationship between the mains transformer (mounted in its decoupled box) and the first stage regulator – an area now where things are becoming very significant for sound quality.

I didn’t mention using a mains relay switch. with a low-power switch and a mains relay switch mounted internally at the rear. But with this option we fall again into significant problems with RFI and vibration being taken into the box and close to all the processing circuits.

Oh, and of course, we recommend you leave the DAC switched on in between your listening sessions, and only switching off when you go away on holiday. So you shouldn’t need to use the switch often anyway.

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