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Old 19th February 2009, 03:54 PM   #1
neazoi is offline neazoi  Greece
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Default line impedance

hello,
what is the impedance of the line output of a preamplifier? I am talking about the cables. RCA connectors do not are impedance matched for audio I think, but what the nomimal impedance should be in a system between the preamplifier and the amplifier?

Could I use RF connectors of 50 or 75 ohm for audio?

see http://www.nationmaster.com/encyclop...inal-impedance
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Old 19th February 2009, 04:12 PM   #2
Tweeker is offline Tweeker  United States
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You could use coax. The lines your driving arent really acting as transmission lines. What you are driving is the input impedance of the amplifier + the capacitance of the lines.

The source resistance of the preamp line out varies by amplifier.


Ideally amp Zin > 10:1 preamp Zout

Pay no mind to line "impedance" for audio.
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Old 19th February 2009, 09:59 PM   #3
DcibeL is offline DcibeL  Canada
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Characteristic impedance has no effect at audio frequencies.

The relation between characteristic impedance and the signal is frequency and transmission length. Look at visible light for an example, when the light hits a surface, some is absorbed and some is reflected back.

This same principle applied to electronics. If the cable impedance equals the load impedance, then all the power is absorbed into the load. However, if the cable impedance does not match the load impedance, then some of the signal is reflected back.

So when does the reflected signal become a problem? Well if you think about it, the higher the frequency, the shorter the rise and fall time of the waveform, so the more a reflection will affect it.

Electricity travels at near the speed of light, so the wavelength of a 20kHz waveform is...very long. Something around 15000 meters I think. I'd worry about cable impedance if my cable was 15000 meters long, but really if I had a 15000 meter long cable I think that a reflection would be the least of my problems.

If you want to know output impedance of your preamp put a multimeter on it. Generally though output impedance is low (<100 Ohm), and input impedance is high (>10KOhm). To answer your question, yes you can use RF connectors and cable for audio.
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Old 20th February 2009, 07:40 AM   #4
neazoi is offline neazoi  Greece
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1. so in conclusion, none of the preamplifier-amplifier combination is REALLY impedance matched, but this does not matter for such low frequencies right?

I like RF connectors (especially some high quality steel/gold SMA used in microwaves) for their connections repeatability and rigidness.

2. I suppose I could use 50 Ohm coaxial cable between the power amplifier and the 8 Ohm speakers? Good quality (aircom plus 2.5GHz) 50 Ohm RF coaxials have a much closer impedance to my speakers and output of the amplifier, than parallel wires used for audio. RF shielding will be strong with coaxial too.

Are my considerations right?

Thank you very much for your help.
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Old 20th February 2009, 09:19 AM   #5
Tweeker is offline Tweeker  United States
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Shielding is good, as are solid connections. Bear in mind that line-line capacitance can be much higher with coax than many other cabling types. Teflon insulation etc helps losses at UHF, but it doesnt change the characteristic capacitance of 50ohm coax.

The load your preamp have to drive is the input impedance of the amp in parallel with any cabling capacitances.

Example 600 ohm ladder line might actually perform better than 50 ohm coax. Rather harder to shield if RFI is an issue, but easier to drive. Impedance matching is not an issue here. You'd need many miles of cable to have an SWR issue at audio...

If preamp and amp are close and/or preamp Zout is low (it surely is if preamp is solid state), a little more capacitance to drive in the lines wont matter.
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Old 20th February 2009, 11:24 AM   #6
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I personally love BNC connectors for audio. Not for any impedance qualities (has been mentioned as irrelevent), but the fact it makes a solid connection, a ground-first connection and dont pull free when you move things around.

Also being designed for a weatherproof connection from DC to 4GHz, you don't need geewhizbang, audiophile grade rare earth metals to make them work

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Old 20th February 2009, 12:02 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Geek
I personally love BNC connectors for audio.................connection from DC to 4GHz, you don't need geewhizbang, audiophile grade rare earth metals to make them work
I do wish my equipment was fitted with them. How do you buy/assemble cables cheaply?
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Old 20th February 2009, 03:12 PM   #8
DcibeL is offline DcibeL  Canada
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Quote:
Originally posted by neazoi
1. so in conclusion, none of the preamplifier-amplifier combination is REALLY impedance matched, but this does not matter for such low frequencies right?

I like RF connectors (especially some high quality steel/gold SMA used in microwaves) for their connections repeatability and rigidness.

2. I suppose I could use 50 Ohm coaxial cable between the power amplifier and the 8 Ohm speakers? Good quality (aircom plus 2.5GHz) 50 Ohm RF coaxials have a much closer impedance to my speakers and output of the amplifier, than parallel wires used for audio. RF shielding will be strong with coaxial too.
1. Yes. SMA Connectors are excellent, but you have to screw them on. BNC will work equally as well, as will MCX if you want something really small.

2. For speaker connections, a high gauge wire is important to carry large currents without loss. If you want to use an RF cable for this, try RG-6. Realistically any heavy gauge wire (like a 16AWG extension cord) will work equally as well.

Quote:
Originally posted by Geek
I personally love BNC connectors for audio. Not for any impedance qualities (has been mentioned as irrelevent), but the fact it makes a solid connection, a ground-first connection and dont pull free when you move things around.

Also being designed for a weatherproof connection from DC to 4GHz, you don't need geewhizbang, audiophile grade rare earth metals to make them work
BNC are excellent, however note that only a 50 Ohm BNC is rated to 4GHz. 75 Ohm BNC is really only good to 2GHz.

Quote:
Originally posted by AndrewT
I do wish my equipment was fitted with them. How do you buy/assemble cables cheaply?
Work for a company in the RF field. cable, connectors, and crimpers are just a few cubicles away...
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Old 20th February 2009, 11:00 PM   #9
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Quote:
Originally posted by AndrewT
I do wish my equipment was fitted with them. How do you buy/assemble cables cheaply?
Of all the cables I have tested for audio, the best bang for the buck is RG8X, aka RG8 mini-foam. These tend to fit the more common and cheaper 75 ohm BNC connectors perfectly since RG8X and RG59 are close enough in outer diameter.

The connectors I buy surplus and grab them where I can. If you're good with an iron, get the solder type. The twist-on are tolerable, but don't like the stranded core of the RG8X, unless you make a single wire centre with a clipped resistor lead or something. Doable but tricky.


Quote:
Originally posted by DcibeL
BNC are excellent, however note that only a 50 Ohm BNC is rated to 4GHz. 75 Ohm BNC is really only good to 2GHz.
I stand corrected


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Old 20th February 2009, 11:24 PM   #10
neazoi is offline neazoi  Greece
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So I can see you almost all agree BNC seems to be a good solution.
Although I like to stuck with SMA for line level because of their smaller size and their more rigid clipping, screwing is more painful than just inserting and turning of the BNC.
For the Speaker level think of the N-type too. They are very solid and can withstand easy the stress of moving around speakers as you listen to music, to find the best place. They can also accept big diameter cables like Aircom plus, which excibit a relatively low capacitance level of 82pF/m and have a single big diameter central core (although I do not know if single or multiple cores are better).
I tested both SMA and N-type and they seem to be of ¨Earth-first¨ construction. That is an important parameter I haven thought! When live connecting or disconnecting audio cables you could burn your speakers (or ears) from the un-earthed signal hum if the volume of your amplifier is high. But you do not do that, do you?

I think since impedance is of no great consideration at audio frequencies, we could improve other things, like shielding, stress, easyness etc. what do you think?

Last year I bought a pair of RCA interconnects for 30Euros cause I thought they would be better. The fact was that they were too tight and they scratched almost all my RCA female plugs especially the silver plated ones. Since then I do not like to give a fortune on such promising products and I prefer building my own solutions as strange as they could be, like using RF cables and connectors in audio. What is your thoughts about this?
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