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Speaker Cables, A Special Note (from ye olde Naim Audio)

Well, I regard needing specific cabling to avoid the risk of amp instability which might cause some damage as something to treat more cautiously than: "It is just a recommendation you do not have to abide by it"!

If I gave that "advice" and kit got damaged as a result the owner might feel I should pay them compensation, or at least apologise. So you might want to reconsider what you wrote.

Consider: Would you think it safe to say, "Fitting the specified fuse values is just a recommendation,,," ? I hope not.
 
To be honest I doubt it would blow up it’s probably just arse covering in their part. Of course they want you to buy their cables too.
 
I also think the probability of a "blow up" is low, statistically. The problem is not knowing the stability margin of the amps. Back in he early 1980s I saw an early amp show clear RF instability, but I assume later models are less prone to that.

However "probably" is fine in most cases, but if something is possible then someone may hit the spot and be 'unlucky'. If someone says not to worry about the advice, then they might find they get the blame when that happens.

On most car journeys you would have been OK not to wear a seat belt. But...
 
Amplifiers can blow up almost instantly and in some cases take the speakers out with them due to exactly this type of instability. Whether this applies to Naim amps is another matter...
 
Not seen a Naim amp blow up. But have seen bursts of ultrasonic oscillation at a specific voltage level on output waveforms. e.g. at the same point on each cycle of a test sinewave. This was, however, decades ago.
 
Jim Audiomisc said:
Not seen a Naim amp blow up. But have seen bursts of ultrasonic oscillation at a specific voltage level on output waveforms. e.g. at the same point on each cycle of a test sinewave. This was, however, decades ago.
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That's how tested them.
I saw many in repair/test that had blown up. By far the most common failure mode was a nearby lightning strike taking out the output stage and LTP. Unstable amps just tended to cook themselves and were easily seen in molten emitter resistors.
 
Yes i'd imagine electrical surges are a far more common cause of 'blowing' amps rather than using a non manufacturer spec cable lol...
 
Well, I can't recall any Armstrong amps being sent back for repair due to a mains 'surge'. Closest to that was the user who - for some reason of their own - decided the speaker outlets were for the mains input. 8-]

Hopefully, Naim amp failures are also rare, given that people may tend to follow the advice given by the makers.

But I've never seen any stats, either way.
 
Nearby lightning strikes can take out just about anything. Much depends on whats connected to what obviously...
I briefly worked at a place repairing DECT type landline phones when they first came out and there would be days when maybe a dozen all came in from one village/street where there had been an electrical storm! Usually all beyond economic repair.
 
Arkless Electronics said:
....there would be days when maybe a dozen all came in from one village/street where there had been an electrical storm! Usually all beyond economic repair.
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Cordless phones, fax machines and modems were a nightmare in Malaysia. The long phone line introduces massive surges referenced to the local mains. Always not covered by warranty.
I have only seen HiFi die from direct strike to the house as only the mains supply goes outside unless you had an outdoor VHF antenna
 
Arkless Electronics said:
Yep. Ideally one would open up the NAP250 (or whichever model) and remove the 0.22R resistor, replacing it with the Theile network.... You could call it something else then.... Avondale maybe:D

The 0.22R resistor wastes power especially with low impedance loads and reduces the damping factor to 16-18 when driving 4R speakers.
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Several years ago I found a thread on PFM from Les Wolstenholm, talking about installing a zobel network into a 250. It consisted of a about a dozen coils of Cu wire around a resistor, this being placed in series with the speaker. It looks very much like the zobel network on his Facebook page. The PFM thread no longer seems to exist, and I cannot remember the resistor value. However, I implemented it, and my 250 has performed perfectly ever since with about 1m of linn k20 cable. It remains cool, so I assume it's not oscillating nor any other nasty business is going on.
It's interesting to read, if I've got it right, that I could also remove the 0.22R resistor, (I'm about to recap the reg boards) but is there likely to be any benefit?
 
diybry said:
Several years ago I found a thread on PFM from Les Wolstenholm, talking about installing a zobel network into a 250. It consisted of a about a dozen coils of Cu wire around a resistor, this being placed in series with the speaker. It looks very much like the zobel network on his Facebook page. The PFM thread no longer seems to exist, and I cannot remember the resistor value. However, I implemented it, and my 250 has performed perfectly ever since with about 1m of linn k20 cable. It remains cool, so I assume it's not oscillating nor any other nasty business is going on.
It's interesting to read, if I've got it right, that I could also remove the 0.22R resistor, (I'm about to recap the reg boards) but is there likely to be any benefit?
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Did you read the whole thread?

The reason I ask is that former member (and amplifier designer) Jez Arkless of Arkless Electronics explained to us that Naim amps contain a Zobel network but lack a Theil network. I'm definitely not the expert here, however, I assumed that the 0.22R essentially forms the former (i.e. Zobel) and Wolstenholm was essentially adding on the latter (i.e. Theil, as did Arkless with his external plug-in 2 x Matchbox). According to Arkless, taking out the 0.22R will almost guarantee the demise of your amp and possibly the loudspeakers along with.
 
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I'm also no expert, but I don't think it's that clear cut. Les W. Suggested that a zobel network be added, so seeing as I implemented it, and it worked, it's safe to assume that the zobel network is absent in the original design? Let's see if anybody else can shed further light on this...
 
Naim amps have the series CR network on their output, sometimes called a Zobel network and sometimes called a Boucherot cell. What they don't have is the usual series inductor sometimes called a Thiele network. Normally this would 'isolate' the output of the amplifier from any load capacitance at high frequencies beyond the audible range removing a potential source of instability. JV reckoned his amps sounded better without it, but this means the cable itself has to provide a bit of inductance and must be low capacitance. The NAP250 pro model supplied to the BBC blatantly has the inductor in photos. Presumably the beeb wanted stability in all circumstances with all possible cables.

The Naim also had a 0R22 resistor in series with its output limiting the damping factor you'll get. Not sure of the reasoning behind this, but it was not uncommon in 70s designs.
 
I spy with my little eye...

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martin clark said:
Yes, exactly that - see schematic here http://www.acoustica.org.uk/t/naim/poweramp_pix/NAP250 schematic.jpg
There's a Zobel network, and then the 0.22ohm resistor (marked R14 on the linked image) in place of a damped inductor.
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Hi Martin, forgive my lack of knowledge here, but having re-read this post, is the resistor contained within the coils of an inductor, which I've fitted to my 250 as per an ancient post by Les W, (and visually similar to the inductor in his amp modules), actually a thielle network, and not a zobel network?
As I previously posted, I've added this resistor/inductor assembly to my 250 some years ago,and it seems fine, no overheating etc.
If this is a thielle network, should I theoretically be removing R14?
 
You are right - the damped-inductor you have added is the Thiele network ( the Zobel is the C+R from 'amp-output to spaker 0v' that is retained.)

Yes - you could remove R14 having done-so; but you do not need to.
Be aware that R14 adds a hairs-breadth of output impedance that (1) makes such Naim amps sound like they do and (2) enhances their sometimes-flighty stability.

No harm is soldering a wire link across R14 to try though, if you want to find out what removal/bypass yields in your setup. I think I probably would not, if the speaker is very low impedance/ known to be a bit of an odd load.
 
MJS said:
So, that's a conventional 250.2 with an inductor (finally!) and an LM317/337 supplied balanced front end. No DR here, just a bit of rail filtering.
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Thanks Martin
It's interesting that the resistor/inductor thielle network I've included is often mis described as a zobel network on many websites.
I'm an inveterate fiddler, and my speakers are kef R500, so not a difficult load as far as I know, but I'll do my best to leave it alone!
 
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