Line Level Audio Cables

[adila]

Hi guys,

I have an Onkyo amp, and I need to connect a turntable to the unbalanced phono input. The turntable is an Unitra G602C with moving magnet cartridge. What needs to be the specs of these cables? The regular RCA cables are very thin and I don't trust them very much. Is it OK to use 75 ohm coax cable? I don't have any device to measure the loss of the cable. Attached is the turntable diagram.

Thanks,
Adrian

 

[JonSnell Electronic]

75Ohm coax is not screened very well and may give you hum issues. You will get good results by cutting the jack plug off this 2m SHIELDED OFC 3.5mm Jack to 2 RCA Audio Cable Twin Phono Plugs Stereo 6ft | eBay and run a flexible earth cable from the deck arm/chassis and preamp chassis.

 

[adila]

Thanks Jon for you quick reply. I have a coax cable with a good screening I would say. The screen has around 1.5-1.6 mm2, so for both channels I have at least 3 mm2. The core diameter is 0.7mm (0.384 mm2). I decided to give it a try with this cable. The cables are 1m long.
Tests:
1. Amp volume at max, any digital or analog input selected, but no source connected: There is no hum present.
2. Amp volume at max, phono input selected, no source connected: There is a hum present.
3. Amp volume at max, phono input selected, turntable connected: There is a different hum present, less annoying I would say.

What can cause the hum for the phono input when no source is connected? Is it the amp/preamp itself the issue or the hum should disappear completely if a turntable is connected with a proper cable?
Thanks
Adrian

 

[DF96]

What can cause the hum for the phono input when no source is connected?

Probably capacitive pickup. That is why the cable has to be screened, but no cable is perfect. Adding a cartridge should reduce this a little, as the cartridge will have low impedance at low frequencies.

I don't have any device to measure the loss of the cable.

Not a problem at audio frequencies.

75Ohm coax is not screened very well and may give you hum issues.

It all depends on the quality of the coax. Good coax has a good screen.

 

[Speedskater]

If the 75 Ohm cable is the type that the cable TV companies use, the yes the low frequency shielding/screening is poor. They are designed for 100 MHz to 1 GHz signals.
But these days, most coax cables are 75 Ohm cables.

 

[Pano]

I like JonSnell's suggestion. If you want to get fancy, but some Canare or Mogami cable. Very well built, low microphonics, excellent shielding. You can use the 75 ohm vidoe coax, or use the guitar cable. Either will work well.

The Canare has a tightly braided shield that isn't easy to work with, but does its job well.