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FenderBender11 17th August 2011 03:40 AM

What type of wire for signal in?
 
So I'm FINALLY (only took 6 months) wiring up my amp. Everything is done. Phew.

I already have a decent 3.5mm to RCA wire. The question I have is regarding the wire inside of the amp. So from the RCA jacks to the amp boards. I've seen some tiny wire used. Looks like 28awg or thinner. Is there a reason for this? Stranded or solid?

I'm not looking for hi-fi silver wire. That's nonsense and you know it.

Just looking for general characteristics. I have some Solid 24AWG. Granted it's not very flexible, I do have a lot of it.

Any ideas?

Thanks.

gootee 17th August 2011 04:12 AM

Since it's a low-level signal, you'll probably want to use small wire, to keep the inductance low.

Much more important is the physical configuration. If you can't get shielded twisted pair and ground the shield to the chassis only, at the input end only, then twist two wires tightly together. The idea is to have the smallest possible "loop area" between every natural pair of conductors, everywhere, to prevent any loop from having a time-varying current induced in it by any time-varying electromagnetic fields in the air (and vice versa), avoiding receiving or transmitting hum, for each pair.

Tom

VictoriaGuy 17th August 2011 07:57 AM

Quote:

Originally Posted by gootee (Post 2675149)
Since it's a low-level signal, you'll probably want to use small wire, to keep the inductance low.

This was a new idea for me, so I did a quick bit of 'web research'.
I found this inductance calculator.

Suppose the wire is 20cm long (longer than in most amps, probably).
AWG 26 wire will have an inductance of 270nH, AWG16 wire will have an inductance of 227nH, according to the calculator.

What effect will such tiny amounts of inductance have at audio frequencies?

VictoriaGuy 17th August 2011 08:05 AM

Quote:

Originally Posted by FenderBender11 (Post 2675128)
I have some Solid 24AWG. Granted it's not very flexible, I do have a lot of it.

Solid core wire is better for most amp wiring IMO because it is stiff enough to 'stay put' where you route it. Just be careful not to nick it when you strip the insulation.

Whether you 'need' shielded wire depends a lot on your chassis layout. I usually use shielded wire to get from the inputs to input selector&volume control. If it's just a 'short hop' from your RCAs to the V1 grid, you can probably get away with unshielded wire.
The ideal situation IMO is for the RCA to be close enough to the preamp/driver tube so that the lead of the grid stopper resistor can make the connection.

sofaspud 17th August 2011 08:31 AM

If you have an old VCR for a donor, you might find some decent shielded wire inside, albeit shorter lengths. I usually use the recycles, or I have spools of "Type CL2" I believe it is.

Mark.Clappers 17th August 2011 10:39 AM

take a look at RF cables
 
I personally use RG179 (very thin) or RG59 (standard 75Ohm) coax cable in my amps for line level signals. It's cheap, flexible and works fine!

DF96 17th August 2011 11:10 AM

Quote:

Originally Posted by gootee
you'll probably want to use small wire, to keep the inductance low.

I thought 'small' (i.e. thin?) wire has more inductance than thicker wire? But except for long lengths, completely irrelevant at audio frequencies.

Screened coax, or screened/unscreened twisted pair, should be fine. Decide which end you are going to ground: at the chassis socket or at the valve input. The latter is almost certainly better.

Mark.Clappers 17th August 2011 11:25 AM

Yes thin (or small ) wire has more pure resistance (for RG179 coax, 802Ohm / KM), but this doesn't matter for short lengths. Also inductance and capacitance are different per meter but again for audio frequencies it simply doesn't matter. It's a whole different story if used for RF or long cable runs, but that doesn't apply here. For audio signals usually the bandwidth is limited to 0 - 100KHz and pretty much all types of coax cable can handle this with ease. Same goes for a twisted pair though... I guess it all depends on your personal preference and availability of materials.

VictoriaGuy 17th August 2011 12:57 PM

Quote:

Originally Posted by DF96 (Post 2675416)
Screened coax, or screened/unscreened twisted pair, should be fine.

If you have excess coax with good copper braid shielding, you can pull a length of shield and put twisted pair (or several twisted pairs) inside, making up your own shielded cable. Ungrounded end can be tidied up with heat shrink. Very 'old school' looking, and effective. (That said, I'll be working on my bucket of salvaged shielded cable from dismantling old gear for a few years more.....)


Quote:

Originally Posted by DF96 (Post 2675416)
Decide which end you are going to ground: at the chassis socket or at the valve input. The latter is almost certainly better.

+1

gootee 18th August 2011 02:21 AM

Quote:

Originally Posted by VictoriaGuy (Post 2675281)
This was a new idea for me, so I did a quick bit of 'web research'.
I found this inductance calculator.

Suppose the wire is 20cm long (longer than in most amps, probably).
AWG 26 wire will have an inductance of 270nH, AWG16 wire will have an inductance of 227nH, according to the calculator.

What effect will such tiny amounts of inductance have at audio frequencies?

Probably not too much. So 26 AWG should be OK, eh? (Silver-plated might be OK but I'd stay away from gold unless you are very familiar with the associated problems of trying to mate or solder dissimilar metals.)

Actually, I was probably thinking more in terms of speaker cable lengths, where there are audible and measurable effects from the still-extremely-small inductances.

ON the other hand, considering that almost all amps are competent-enough to be indistinguishable when attempting the vast majority of audio reproduction tasks, the highest-quality amplifiers might therefore be differentiating themselves based on better performance at the extremes of "does it matter?", aka attention to minor details.

At some point, high-enough inductance would be "a bad thing". So why not minimize it, since you have the choice (and are not sure how much it will matter)?

Tom


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