Re: Re: Re: Re: Re: DIY Power cable
jneutron said:
Hey... I agree...
Unfortunately, there are far too many unbalanced input audio deeevices around. No pin one.
Unbalanced stuff is just a pain in the ****, but there are reasonable things you can do even there....
If you use a phono socket that is isolated, you can use a differential input stage between the pin and the (isolated) sleeve to at least minimise the pain on the input side, and it is possible to implement impedance balance at the output, which will at least get you some resistance to earth loops.
Agreed. Unfortunately, on this side of the pond we have the NEC which requires a bonded chassis to ground, and we end up with currents in places that are not really well controlled.
We have CE which includes bonding requirements somewhat more rigorous then the NEC does (mostly because our domestic small power is 230V), but you miss the point:
For a balanced IO device, the internal signal reference trace is strictly a local reference, it never leaves the case, and as long as it is bonded to the case at A SINGLE point, it really does not matter what voltage the box ends up at as long as it remains within the common mode range of the IO drivers. Shield currents have very little effect as long as the shield is terminated to the case and does not enter it (Tests indicate that SCIN is only a factor for a stupid amount of shield current or where the input stage CMRR is exceptional).
Currents flowing due to earth loops will flow on the outside of the case, but so what?
Loop currents are harmless unless they have some way to couple to something inside the box.
For some fascinating work on this (including some measurements) see
www.tonywaldron.co.uk and his papers on interference free audio and on screen bonding.
Searching the 'theatre sound' mailing list for some very extensive discussion featuring Tony Waldron, Neil Muncy and Bill Whitlock on this subject may also be interesting.
SimontY said:
Hehe, this is the wrong forum to ask about power cables, there are lots of people on here who don't like things that don't affect harmonic distortion measurements.
Something of a straw man argument don't you think? I don't think anyone here has claimed a measurement as crude as THD can be used to completely qualify an audio component. For certain no engineer would.
However, as power input quality (within reason, and there is nothing a power lead can do about anything that would exceed 'within reason') should NOT effect the performance of an audio component, we can state with some certainty that any design where changing a power cord DOES effect the sound, must be a poor design at least in engineering design terms (it might meet some other criteria, say low parts cost or minimal components or something, but as an audio component of reliably good and consistent quality it fails).
In the UK we have a fuse in the power lead, which can be bypassed with a solid copper bar.
Do NOT do this.
That fuse is there for a reason, as without it that cable could pass 32A without causing anything to trip. This is horrible advice to give any newbies reading this thread, and is a blatant safety problem (not to say completely lacking in common sense).
There are a few very special cases where using an unfused plug is acceptable, but audio is never one of them (Think portable Xray machines where the maximum duty cycle is tiny and where other protective measures are in place).
The RFI reduction and loop impedance can be measured but really, what's the point? Just listen & enjoy!
Some of us would like to measure such effects so that we can design gear that does not need funky power cords just in order to work right. You cannot do that without measuring the basic cause of the problem and its effects as otherwise you are not doing audio engineering you are doing audio voodoo, and I am a poor witchdoctor at best.
Regards, Dan.