Found this quote from the late Al Margolis, who was either an EE or a physicist and who was in any case brilliant, on the "cables" forum in 2009:
"Characteristic impedance, being part of what are called "transmission line effects," is (at least for typical interconnect lengths) generally considered to be utterly inapplicable to audio frequencies. Note that I limited the statements in my first post above to cables carrying analog audio, not digital signals, video, or rf."
And found this quote from Atmasphere (Ralph), posted in 2008 on the Cables forum. Here he is speaking of speaker cables, but it's the same parameter, characteristic impedance shortened to "impedance" when speaking of ICs that are 50, 75, or 110 ohms in their characteristic impedance. I put his significant words in bold italics:
"The second thing to understand is that all speaker cables have a property known as ’Characteristic Impedance’. This is not the resistance or impedance of the cable, rather it is the property of the cable such that it will have its best performance when terminated by a specific impedance. For example, a cable with a characteristic impedance of 8 ohms will perform best when terminated by an 8 ohm load. Its actual DC resistance will likely be quite low as will its AC impedance, however.
Case closed, so far as I am concerned. You need not be concerned about cable (characteristic) impedance. I did not steer you wrong.
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They are talking about "characteristic impedance". Whether that matters at audio frequencies and with typical 3 to 6 foot long ICs is the question. The more I read about it, the more I am convinced it does not matter. Here is a practical rationale: If the characteristic impedance of the cable matters in calculating the source impedance at audio frequencies (less than 20KHz), then a typical LOMC with an internal resistance/impedance of 10 ohms would be seen by the phono stage as a 110 ohm source impedance (assuming a "100 ohm" cable). Many load such cartridges at 100 ohms. If the cable characteristic impedance is important in that calculation, you have a 110 ohm source driving a 100 ohm input. This would result in about 50% of the signal voltage going to ground. Maybe that’s OK, but when you then increase the input Z of the phono stage to, say, 1000 ohms or even 47K ohms, which many do use even with such LOMC cartridges, there should be very obvious increase in gain that goes with that change from 100 ohms to 1000 ohms or 47K ohms. In my own experience that does not happen, and I have never read that anyone else has had that result. This is just speculation on my part.
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Pro-ject answered me that the impedance of the XLR output was of 100 Ohms. So assuming that the impedance of the cable itself is between 75 and 110 Ohms, we should be fine with a 1,66 kOmhs input impedance.
Regarding the way we should count the impedance of the cable in the system, I can't answer at all but I would be curious to learn if someone can help.
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The jury is still out on whether the "impedance" of the cable, which when we say it is 50 or 75 ohms for a single-ended cable (with RCA terminations) or 110 ohms for a balanced cable terminated in XLRs, should be added in, in calculating the ratio between the input and output impedance of two devices connected to each other by cable. Lanx is absolutely right to say that cables have "impedance", but when we use that term for cables, we are really referring to a quantity more properly called "characteristic impedance". Characteristic impedance is the square root of the cable inductance divided by its capacitance (L/C). We express this fraction in "ohms". By this definition, cables with high capacitance have low characteristic impedance. Obviously, if you just take an ohm meter and measure the resistance across any IC, it will be only a few ohms, which can be ignored in our calculation of the ratio of the input to output Z. It is or was my belief that characteristic impedance only becomes significant at frequencies way higher than the audio spectrum, e.g. at MegaHz or GigaHz frequencies OR if the cable is very long, much longer than what you are likely to be using. But maybe I am wrong. Maybe we will find out.
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@lewm, don't worry about that, I'm grateful for your help so nevermind if it happens to be not 100% exact. Thing is, I'm gathering some informations and will draw conclusions later.
I've read that XLR cables have an impedance of 75 Ohms and DMX cables used for light shows have an impedance of 110 Ohms.
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"How can you not know this and still spout all kinds of technical details here?" Because I only write what I think is correct. However, I am not an engineer, not infallible, and I can make mistakes. When I am in the wrong, I like to be corrected, because I will have learned something. Your point about cable impedance is making me think again about what I thought I knew. I thought cable impedance enters into the calculation only at RF level frequencies. I thought that at audio frequencies, we need only worry about cable resistance, which in this case is only a few ohms, at most. Cable resistance (not impedance) can be ignored in this instance, or so I thought. If I was putting out incorrect information, I apologize to the OP, and thanks for correcting me. I'd like it if someone else can comment on this particular point.
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Thanks a lot for those explanations, guys.
I'm reaching my limits in terms of understanding resistance and electricity but I will slowly learn and then understand what you're talking about.
As far as the sound quality is concerned, it's fine the way I'm going now. But you know, it's always better to match what you hear with the good figures so you are sure that everything is fine. Want to be sure that I made nothing wrong that could have an undesirable effect later...
What I was concerned about mostly is: I wondered if there could be damages if input Z is too low and output Z too high. From what I read it seems that it's not the case, but better to be sure.
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How can you not know this and still spout all kinds of technical details here? The nominal impedance of an XLR cable used in professional studios is typically 110 ohms, while an RCA cable has 75 ohms. Also, when determining effective voltage gain, damping factor, and so on, the cable impedance is considered on the source side, not the load side.
If you are not familiar with Denafrips gear, I suggest you avoid making assumptions, such as "driving on the voltage mode," that you need to verify first before continuing the discussion.
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Where on earth do you get the idea that "an XLR cable" has a 110 ohm impedance? And even if it did, do you count that as part of the output Z or does it add to the input Z? That choice would drastically affect the calculation. Of course, the higher the ratio above 1:10, the better the numbers look, but it usually makes no audible difference so long as the gain structure is sufficient. What may matter is input capacitance, which when the impedance match gets much less favorable (much below the 10X "minimum", approaching 1:1) begins to cause a high frequency roll off in the audible range. At a ratio of 1:8, that should not be a problem, either.
Re the Denafrips DAC, about which I know very little, if its output Z is 5000 ohms and if it is driving in voltage mode, no amount of current is going to help if the input Z of the driven component is much below the 10X rule. No matter what, if the input Z of the driven component is 5000 ohms, equal to the output Z of the Denafrips (1:1), then you would lose 50% of signal voltage to ground. The calculation for voltage remains the same regardless of current, in voltage drive. In a DAC, even that poor impedance match may not be noticeable, absent an effect of C on frequency response, because DACs make so much signal voltage, they can usually drive an amplifier even without a linestage. All you need is an attenuator.
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+1 @boozendormi Now knowing you are in Europe, I could relate to your situation.
+1 @lewm but forget not that the 110-ohm impedance of an XLR cable also plays a role. Suddenly, you have 1660/(1660 + 200 + 110) = 84.3%. No big deal? I’m afraid not. I believe this should have some impact, at the very least, on the conversion quality of an ADC.
For impedance matching, I usually aim for a much higher ratio than 1:10—often by another order of magnitude. That said, there are exceptions in the market, such as Denafrips DACs, which have a relatively high output impedance. Even with an output impedance of 5000 ohms, their strong current delivery allows them to drive most (pre-)amplifiers effectively. Additionally, their transformer-coupled output stage provides a degree of impedance matching.
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@lanx0003, I considered the Skoll indeed but Schiit products are hard to find here in France so I had no opportunity to listen to it first. I was offered a good deal for a demo DS3 B so I went for it as I was very satisfied with sound and flexibility.
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1.66K divided by 200 = ~8/1, so not quite 10/1, but the difference is trivial. There is nothing magic about the 10X rule of thumb.
The fraction of signal voltage transferred is defined by the fraction Zi/Zo+Zi. When the ratio of output to input Z is 1/10, that fraction says 92% of signal voltage is available to the driven device. When it’s a ratio of 1/8, about 89% of the signal voltage is transferred. No big deal.
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I am not sure why you prefer Pro-ject phono box over other brands. The xlr output imp on Schiit Skoll is only 10 ohms. Fully discrete, true balanced output and more dynamic. The SINAD from the 3rd party (asr) rated 78 for DS3 but 81 for Skoll.
Oh, you save a bunch of green.
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@lewm I can raise it a bit for 6.7Vrms / 2.46kΩ input impedance but then the dynamic range is less satisfactory. So if output impedance is low enough, I would rather keep the 4,5 Vrms / 1,66 kOhms setting. I'm trying to figure out if the ratio of output Z to input Z is about 1:10 as you have guessed.
I emailed Pro-ject first but they read my email too quickly and sent a specs sheet without the information I needed. Will try again on Monday!
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Lanx, The OP wrote above that he is driving a 1.66K input impedance. And I think that's no problem, even if the output impedance is 200 ohms.
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AI says 200 ohms for xlr and 50 ohms for rca. Write an email to Pro-ject for confirmation if you want to be sure. So as long as the preamp xlr has 2k input imp. or higher you should be good to go otherwise use rca.
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If it's 1.66K ohms input Z, then you are probably OK, but I don't blame you for wanting to be sure, because unless the output Z of the DS3B is under 200 ohms, the ratio of output Z to input Z might be a shade lower than 1:10. On the other hand, if the ratio IS only a little below 1:10, I doubt you would hear a problem. You suggest above that you can adjust input Z; can you raise it a bit?
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Thanks for your thoughts about that. Yes it's the SS Phono Box (not the Tube Box).
I'm using an E1DA Cosmos ADC Iso to rip LPs. A cheap little device made for measurements but many people use it to rip audio.
You can set the input range / impedance with a dip switch; mine is set on 4,5 Vrms / 1,66 kOhms. Input sensitivity is matching very well with the gain of my Phono Box but input impedance may be too low (though I didn't notice any trouble such as voltage drop or distortion), depending on whether or not the output impedance is high.
Hence my question about output impedance. I was only able to find the output impedance of the Phono Box RS, which is of 50 Ohms.
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I can’t find it, either, but assuming you own the all solid state DS3B, the output impedance ought to be very low, in the range of 100 ohms give or take, which means it can drive any linestage you’d care to use. Is there some reason you need to know exactly?
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