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Naim Lumina - 300K resistor connecting + and - wires ? Snake oil or zobel ?

tomek

tomek

pfm Member
In the Lumina speaker cable there is 300k resistor connectig + and - wires.



sorry for the bad pic quality ...

What is the purpose of this resistor ?
According to some reviews Naim is "tuning" the cable and using different values depending on cable length.
Is it snake oil, or just some kind of "zobel" stabilising the amp ?
 
According to Hi Fi Critic, a terminating RFI damping resistor is situated at the amplifier end of the cable, to drain away a proportion of any environmental RFI induced in the cable. The value of the resistor is apparently dependent upon the length of the cable.
 
wylton said:
Well, that it may be, but the guy asked a question & that's what Martin Colloms said; I'm making no claims or endorsements here!
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It kinda surprises me (but shouldn't) that not everyone has yet caught on to the fact that, by and large, what you read in magazines, are told by dealers or see in manufacturers advertising is to be trusted about as much as a tory excuse for a party!:)
 
Arkless Electronics said:
It kinda surprises me (but shouldn't) that not everyone has yet caught on to the fact that, by and large, what you read in magazines, are told by dealers or see in manufacturers advertising is to be trusted about as much as a tory excuse for a party!:)
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True! I don't buy hi-fi mags these days (I googled the Lumina speaker cable resistor to get that answer) & in any case, I wouldn't pay that kind of money for a loudspeaker cable, though admittedly, my Atlas Bi-wire cables aren't cheap!
 
If you wanted to terminate the cable at RF, you would use a resistor close to the characteristic impedance of the cable. This mostly depends on cable geometry, and to a lesser extent on the dielectric constant of the insulator. Coax constructions are typically somewhere from 50-80 Ohms, while spaced pairs range from maybe 100 up to perhaps 300Ohms, depending mostly on the ratio of conductor diameter to spacing.

300KOhms does not correspond to the impedance of a physically realisable cable, and as Arkless says, it is pure snake oil.
 
I was already aware of this feature of these cables. While a bit above my level of technical knowledge, I recall reading on PFM some way back an exchange where JimAudiomisc hinted that with a solid state amp, one way of mimicking some of the aspects of a valve amp sound would be to insert a resistor in the speaker cable run. Apologies if I’m mistaken on that, or barking up the wrong tree, but if that’s the case it might account for an actual difference in sound due to a cable. Possibly might be cheaper to use an equaliser, or an amp with tone controls? I think there was also another brand of speaker cable which featured a resister box, but I can’t now point to which one.
 
nicetone said:
I was already aware of this feature of these cables. While a bit above my level of technical knowledge, I recall reading on PFM some way back an exchange where JimAudiomisc hinted that with a solid state amp, one way of mimicking some of the aspects of a valve amp sound would be to insert a resistor in the speaker cable run. Apologies if I’m mistaken on that, or barking up the wrong tree, but if that’s the case it might account for an actual difference in sound due to a cable. Possibly might be cheaper to use an equaliser, or an amp with tone controls? I think there was also another brand of speaker cable which featured a resister box, but I can’t now point to which one.
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I didn't see the thread you mean, but perhaps he was suggesting you could add some series resistance (maybe 1 ohm or so) to a loudspeaker cable to simulate the higher output impedance of a valve amp? But a 300 kohm resistor across the cable is a different thing altogether. MIT and Transparent cables used to use network boxes on their cables, (maybe still do), but they had more than a single resistor in them.
 
It's an old old idea and it doesn't work. OK yes theory states that damping factor is reduced etc and indeed everything does what theory suggests electrically. What it definitely does not do is make your SS amp sound like a valve one. It in fact just reduces sound quality somewhat, a little muddier, less resolved etc etc.
Interestingly some amps (Naim do it) use a small resistor like this as a permanent part of the circuitry, for different reasons to do with stability, which IME can only be a bad thing.
 
Arkless Electronics said:
It's an old old idea and it doesn't work. OK yes theory states that damping factor is reduced etc and indeed everything does what theory suggests electrically. What it definitely does not do is make your SS amp sound like a valve one. It in fact just reduces sound quality somewhat, a little muddier, less resolved etc etc.
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Bang on.

I've a pair of 10w WW resistors of 0.68ohms with inline banana skt/line plug I made up decades ago to play with / demonstrate such.
Yes, I find the effect really is on a scale from fairly obvious to 'blatant', depending on 'speaker - and never ever an improvement.
 
Someone at Naim once told me that little series resistor had a bit of inductance and actually formed the [missing] output inductor. Right...
 
martin clark said:
Bang on.

I've a pair of 10w WW resistors of 0.68ohms with inline banana skt/line plug I made up decades ago to play with / demonstrate such.
Yes, I find the effect really is on a scale from fairly obvious to 'blatant', depending on 'speaker - and never ever an improvement.
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Same here but using "chocolate block" connector. I've tried values from 0.1 to 1R over the years and yep always a bad idea.
I guess what it will do is mimic various speaker cables!
 
MJS said:
Someone at Naim once told me that little series resistor had a bit of inductance and actually formed the [missing] output inductor. Right...
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Hmm not quite.... It will have a tiny bit of inductance but it mainly works due to the resistance.

It reduces the Q of the resonance of the tuned circuit formed by the inductive output impedance of the amp and the capacitance of the speaker cable/load and somewhat shelves the associated phase shift at the output, from where the NFB is taken.

It is interesting to experiment with this on prototype power amps and helps give you a "feel" of whats going on stability wise with the amp. I've known 0.01R be enough to make it stable and also 1R to not be enough!

Obviously a damped inductor is a better idea as it can give the required impedance where you need it at say 50KHz without much loss at audio frequencies.

If you make the amp fast enough and a short enough feedback path then you can sometimes manage without any Zobel or "Thiele" network at the output!
 
I think the point I was making in the past wrt 'valve sound' referred to adding a *series* resistance of a few Ohms. The point being to mimic the high output impedance of many valve amps with booger-all feedback or crappy outout trasformers. *Not* a shunt termination. Quite different animal.

Given that speakers have impedance vs frequency behaviour that is like the alps I can only guess that a 300k shunt might be useful for a case where:

1) The amp has an output series cap - i.e. single rail design

2) The speaker *also* has a series cap - i.e, nominally infinite impedance at dc.

A shunt might then allow the resulting power-on dc on the cable to gradually wibble away. Avoid your cat getting a belt if it nibbled the cable because it hated your taste in music. 8-]

The idea that a 0.3MOhm shunt will reduce the RFI seems, erm, barking to me.
 
Arkless Electronics said:
Hmm not quite.... It will have a tiny bit of inductance but it mainly works due to the resistance.

It reduces the Q of the resonance of the tuned circuit formed by the inductive output impedance of the amp and the capacitance of the speaker cable/load and somewhat shelves the associated phase shift at the output, from where the NFB is taken.

It is interesting to experiment with this on prototype power amps and helps give you a "feel" of whats going on stability wise with the amp. I've known 0.01R be enough to make it stable and also 1R to not be enough!

Obviously a damped inductor is a better idea as it can give the required impedance where you need it at say 50KHz without much loss at audio frequencies.

If you make the amp fast enough and a short enough feedback path then you can sometimes manage without any Zobel or "Thiele" network at the output!
Click to expand...

A difficulty with your initial comments is that you really need transmission line theory to deal with this rather than assuming lumped elements. Oherwise you may end up trying to fix one problem at the expense of another cropping up.

The impedance of the speakers at RF is generally unknown, even by the makers I suspect. I doubt they're all well above 0.3MOhm across the RF though.

WRT amps being stable unconditionally at RF without a zobel/thiele it comes down simply to the gain and feedback details of the amp. Basically, the way the gain dies at RF will determine that. I think I did a page on this but have forgotten the URL. In practice though, an output network that doesn't get in the way in the audio range is more reliable and simpler. Then, no need even to tell people they have to have a cable with X microF because for some magic reasons output inductors are evil. 8-]
 
Jim Audiomisc said:
I think the point I was making in the past wrt 'valve sound' referred to adding a *series* resistance of a few Ohms. The point being to mimic the high output impedance of many valve amps with booger-all feedback or crappy outout trasformers. *Not* a shunt termination. Quite different animal.

Given that speakers have impedance vs frequency behaviour that is like the alps I can only guess that a 300k shunt might be useful for a case where:

1) The amp has an output series cap - i.e. single rail design

2) The speaker *also* has a series cap - i.e, nominally infinite impedance at dc.

A shunt might then allow the resulting power-on dc on the cable to gradually wibble away. Avoid your cat getting a belt if it nibbled the cable because it hated your taste in music. 8-]

The idea that a 0.3MOhm shunt will reduce the RFI seems, erm, barking to me.
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I believe everyone was pretty clear on it being a series resistance. As I said, it doesn't work ie it does not mimic the sound of a valve amplifier but just messes up the sound IME.

There is no case where the 300K may be useful and that's clutching at straws there jim! Most sensibly designed single rail designs have a resistor to discharge the output cap anyway. IIRC I used 1K2 in a recent single rail amp I designed as they were to hand.
 
Jim Audiomisc said:
A difficulty with your initial comments is that you really need transmission line theory to deal with this rather than assuming lumped elements. Oherwise you may end up trying to fix one problem at the expense of another cropping up.

The impedance of the speakers at RF is generally unknown, even by the makers I suspect. I doubt they're all well above 0.3MOhm across the RF though.

WRT amps being stable unconditionally at RF without a zobel/thiele it comes down simply to the gain and feedback details of the amp. Basically, the way the gain dies at RF will determine that. I think I did a page on this but have forgotten the URL. In practice though, an output network that doesn't get in the way in the audio range is more reliable and simpler. Then, no need even to tell people they have to have a cable with X microF because for some magic reasons output inductors are evil. 8-]
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If considering the OP's question then yes it would need looking at as transmission line but I see little relevance elsewhere. RF impedance would likely be in the range 30 Ohms to lets say 2K ish in practice so WAY different to 300K! And largely irrelevant as it's foo of course!

On your last paragraph I think were pretty much saying the same thing but from slightly different perspectives.
My point was merely that it is possible to make a stable power amp with no output networks that keeps messrs Bode and Nyquist happy.... it's not common but I've done it several times.

As far as the "evil output inductor" specifically goes well unfortunately it's one of those things where the beliefs of the average hi fi enthusiast are a million miles from reality (as is common!) and due to a company (Naim) telling porkies... which of course MUST be true 'cos well Naim say it is innit!

I have offered in line boxes containing the usual damped inductor and which then allow users of dodgy power amps like Naim to use whatever cable they like in whatever length they desire.... I advertised them on pfm several times and had only one enquiry! Amazingly the single enquiry resulted in a sale. The chap was very happy with it and reported back in a thread that it did what it says on the tin and with no audible down side... still no interest whatsoever.

Why the lack of interest? Pretty obvious really... those without electronics knowledge presume Naim MUST be right and I MUST be wrong cos they're the big famous Naim brand and I'm a one man band and that there has to be a sound quality downside... The more prosaic truth of course is that Naim cocked up the design and couldn't then change anything (a la Avondale) as it would be an admission of their original error and that it gives them a reason to tell people to buy Naim brand speaker cable:rolleyes:
 
Arkless Electronics said:
I believe everyone was pretty clear on it being a series resistance. As I said, it doesn't work ie it does not mimic the sound of a valve amplifier but just messes up the sound IME.
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I agree that it messes about with the sound. But that's my point. If you look at frequency responses of valve amps with significant output impedence into typical speakers you can see that the response develops a profile like the Alps due to the impedance interaction between the amp's output impedance and the speaker impedance. This should have a quite marked effect on the sound in many/some cases.

Of course, if you choose a well designed valve amp with low output impedance this effect becomes minimal. But amps with little or no feedback of a kind that get raved about in magazines for their 'sound' uusually show the effect. It becomes hard to credit that this *isn't* what they then bang on about as wonderful valve sound. :)

Didn't 'HH' once see a SS amp that had a 'creamy valve sound' knob the user could wind up? 8-]
 
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