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.
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.
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
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
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?
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.
I thought 'small' (i.e. thin?) wire has more inductance than thicker wire? But except for long lengths, completely irrelevant at audio frequencies.gootee said:
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.
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.
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.....)
+1
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
I don't feel like arguing about this tonight. Sorry!
Roger that. Lately, I'm brain-dead after about 4 pm, anyway.
I'm curious about why grounding the shield at the tube-input end would be better than at the chassis end. I know why it would definitely be better if using unshielded twisted pair. But would it also be better for just the one-end-only ground of the shield of a shielded pair?
Yeah that all sounds about right (except 100 kHz should be 200+ kHz, in general). But we also don't want to give the impression that we are "only" working with audio frequencies and can completely disregard RF. RF is everywhere and all inputs (and internal stage inputs), all outputs, and all power inputs and outputs, should have RF filtering, unless we are SURE that they don't need it (which might very well be the case, here; but you have to think about it at least once, for every design of every type of circuit).
Wow I forgot about this thread and I'm not sure why! I'm sorry and thank you to everyone.
Well inside the amp, the signal is actually going have to travel quite a bit. After coming in through RCA, it's being sent down the side of the case to 4-pole switch on the front panel which I'm using as a ghetto output selector. (It will switch the signal to go to either the headphone amp or the two power amp boards.).
After that, it has to go all the way to the back of the enclosure to the amp boards (or 3 inches if it's going to the headphone amp).
Also to note, as the signal goes to the output selector switch, it goes right past a huge toroid transformer and a small 6VCT split bobbin type transformer. Torroids don't generally make as much noise or interference, but they still DO and the standard split bobbin type makes a bit of stray junk.
So I guess I should use some shielding of a sort. Needless to say that I will probably not notice a thing...but to be safe. Any recommendations on some good online sellers. My poor excuse of a Radioshack is usually lacking in the wire department, so somewhere online might not be a terrible idea.
Also, I'm wiring two potentiometers to the front panel for volume control. I already wired them and used 3x 24AWG solid and kind of "twisted" them together. Any words of advice for wiring up those pots. The wire runs are about 1.5ft per pot. Kind of long, yes.
Well inside the amp, the signal is actually going have to travel quite a bit. After coming in through RCA, it's being sent down the side of the case to 4-pole switch on the front panel which I'm using as a ghetto output selector. (It will switch the signal to go to either the headphone amp or the two power amp boards.).
After that, it has to go all the way to the back of the enclosure to the amp boards (or 3 inches if it's going to the headphone amp).
Also to note, as the signal goes to the output selector switch, it goes right past a huge toroid transformer and a small 6VCT split bobbin type transformer. Torroids don't generally make as much noise or interference, but they still DO and the standard split bobbin type makes a bit of stray junk.
So I guess I should use some shielding of a sort. Needless to say that I will probably not notice a thing...but to be safe. Any recommendations on some good online sellers. My poor excuse of a Radioshack is usually lacking in the wire department, so somewhere online might not be a terrible idea.
Also, I'm wiring two potentiometers to the front panel for volume control. I already wired them and used 3x 24AWG solid and kind of "twisted" them together. Any words of advice for wiring up those pots. The wire runs are about 1.5ft per pot. Kind of long, yes.
Belden #8451. 100% foil sheilding. Used miles of it in high power transmitter / studio location. Now being superceded by Belden 110 ohm digital audio cable. But 8451 is hands down the DC-3 of the broadcast audio world.
Doc
www.belden.com/techdatas/metric/8451.pdf
Doc
www.belden.com/techdatas/metric/8451.pdf
Depending on where you live you might find a bunch of it fairly cheap. As I said with the move to digital everyone is shiffing (has shifted) to 110 ohm digital audio cable. The PBS station I worked for a few years ago yanked out a pile about 18 feet in diameter by 4 feet tall. It went of to University surplus. You might try such places or even give some local radio stations a call. Ask for the engineer and tell him you're looking for some surplus one pair beldfoil cable. Might get lucky.
Doc
Doc
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