Could I cheat the James Randi speaker wire challenge with ultra high impedance? - Page 1
Forum section: electronics / general

author: sfs1
author_rank: Contributor
timestamp: March 08, 2021, 10:26:47 am
content: I understand (understand might be an overstatement) that in headphone amplifiers, you want to have a ratio of output impedance to headphone impedance of about 8:1, and that a low ratio will change the frequency response. Presumably the same is true for amplifiers and speakers. So I had this thought that if I used an ultra high impedance cable to connect the speakers to the amp, and it changes the frequency response, I would be able to reliably tell it apart from the coat hanger and claim the prize.I guess I'm not sure if the impedances just add up in the way I'm imagining, i.e. if a 10Ω output impedance amp with 450Ω impedance cable looks the same as a 460Ω output impedance amp. Also, if the idea could work, how much of an impedance would you need to get a noticeable change.

author: exe
author_rank: Unknown
timestamp: March 08, 2021, 10:33:34 am
content: Quote from: sfs1 on March 08, 2021, 10:26:47 amWhy?I'd think the smallest output impedance is the best. Change my mind

author: sfs1
author_rank: Contributor
timestamp: March 08, 2021, 10:40:24 am
content: Ugh I meant to say ratio of headphone impedance to output impedance of 8:1

author: exe
author_rank: Unknown
timestamp: March 08, 2021, 01:04:39 pm
content: Well, the statement "if a 10Ω output impedance amp with 450Ω impedance cable looks the same as a 460Ω output impedance amp." looks mostly correcy, except for the part that impedance is a complex number. So, this holds true for resistive impedances. If this is what you want, then you can use a low-impedance cable and add a 450 Ohm resistor with it.If you want a cable with 450 Ohm characteristic impedance (in the same way as, say, 50 Ohm coax), then it will probably not work because the "apparent" cable impedance is only "valid" at high frequencies. At sound frequencies you'd need many kilometers of wire, the length should be comparable to the wavelength. Like, hundreds of kilometers (but then it will have huge losses). Otherwise it will look just like a small-walue resistor.PS I googled what "James Randi speaker wire" challenge is. I'm not sure what you want to achieve here. For sure you can distinguish a coat hanger and a shielded cable. Just use a siggen and oscilloscope, or something. I think what he asks for is to prove that an expensive cable is better than a hanger, that's different.

author: sfs1
author_rank: Contributor
timestamp: March 08, 2021, 01:08:11 pm
content: Right, the challenge is to be able to detect your supposedly better cable. The cheat would be to design a cable that completely messes up the sound and so you can reliably distinguish it just by listening. If anyone asks you can say it sounds "warm" or "musical".

author: exe
author_rank: Unknown
timestamp: March 08, 2021, 01:24:32 pm
content: well, I'm pretty sure that if you take many kilometers of cables, they will sound differently "thanks" reflections and discontinuities in impedance.Actually, afaik, it was a real problem with long phone lines and telegraph. It's described here:https://www.voip-info.org/causes-of-echo/(the part where they are talking about plain old line telephone service (POTS): "An improperly balanced hybrid won’t correctly filter out all of the transmitted signals, and will reflect some of it back down the trunk. Imbalance may be from poor design (common) or unpredictable impedance conditions on the POTS line (very common)". ("hybrid" seems to analog-to-digital converter at the end of analog phone line when when the signal is converted into a digital form for VoIP)

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2021, 09:50:43 am
content: Quote from: sfs1 on March 08, 2021, 10:26:47 amAs mentioned above, the output impedance of the amplifier should be as low as possible to give a good damping factor, otherwise the earphone transducers can ring, which will emphasis certain frequencies.QuoteThe impedance of the cable makes absolutely no difference, at the the lengths used in a home hi-fi system. What is really meant by cable impedance, is characteristic impedance, which only applies when the cable length is significant, compared to the wavelength of the signal. It's not a problem for home audio installations, where the DC resistance of the cable just needs to be a tiny fraction of the speakers or headphones.

author: sfs1
author_rank: Contributor
timestamp: March 09, 2021, 09:56:55 am
content: Quote from: Zero999 on March 09, 2021, 09:50:43 amOkay wow. It's a bit of a silly topic but I am trying to seriously learn about characterising amplifiers and this was a major misunderstanding. I will get my books out and take this more seriously because I'm still at the stage where the more I learn about impedance the more confusing it becomes, and I wanna get over the crest. Thank you for pointing me in the right direction.