Addicted to Fun and Learning
- Sep 4, 2021
They are touching along all the way, so the area is as minimal as possible to reduce the antenna effect. I use velcro to keep my cables tidy. The only close AC line I have is the one at the back of the preamp.
Apart from that, every other power line is as far as I can get them. The RCA cable is quite thick and the length is exact, if I twist them they won't reach.
I've been reading your 8KHz noise issue with interest through this thread. In a previous life (before retirement) I worked professionally developing variable speed drives: Electronic devices that chop mains AC into variable frequency AC (VIA a DC Link) to turn AC motors at variable speed. A typical application for these devices is elevator control. And a typical "switching speed" (They use PWM to generate the variable AC) is 8KHz.
I would be almost certain the noise you are experiencing is 8KHz (and harmonics of) switching frequency conducted back from the drive and down the mains. This can be quite nasty because the 8KHz output from the drive is high voltage (probably 800V+ Peak to peak) with fast switching edges - so has lots of harmonics. In a proper installation these should be filtered out. I'd be very surprised if the companies statement that they are within limits is true. But your only recourse would be to try to get the power companies involved, and the effort would be disproportionate, especially as you seem to have got the noise under control.
As others have said, it will be magnetic coupling, especially into ground loops. To minimise, you should have all your power cords connected to one location (eg power strip) and have the individual power cords as close to each other as possible to minimise "swept area" of the ground loop.
Ideally you shouldn't have ANY loops in your signal interconnect (point to point or star arrangement). - eg if you have multiple devices connected to your amp or dac, avoid connections between the devices. If a loop is unavoidable - tie the looping cables together to minimise area between them. (From your description, you are already doing this)
The reason your turntable is suseptible will be the high impedance high gain input of the pre-amp. Minimising noise there, and on the low signal connections from the TT is critical. If you can put a filter on the power lead to the preamp that would be worth trying. It doesn't need to cost $1500 though . As a test, it might be worth trying to power your pre-amp from a battery, eg a power bank (if the pre-amp is normally powered from a low voltage wall wart).