Speakers 30ft from amp - help please!

[Anonymous]

A great spec would involve double the power into 4ohm (300 W in this case) and a TIM measurement. The fact they do not offer that and pimp the tired, old and misleading THD is worrying.

That wouldn't be Transient InterModulation distortion, the '70s audiophile's favourite technobabble, would it? You sure have a funny understanding of the phrase 'tired and old'.

And as for your nice demonstration at maths: are you implying that a speaker has a uniform impedance across its entire frequency range? Your audience already knows that is untrue, so who are you trying to fool here? I would also be interested in how you get your V/I figures, because for 150Wrms into 8ohm, I get roughly 40V and 5A. The 1500W figure is completely uninteresting because that's mains power draw and hence includes operating power and transformer losses.

 

[Anonymous]

And as for your nice demonstration at maths: are you implying that a speaker has a uniform impedance across its entire frequency range? Your audience already knows that is untrue, so why are you trying to fool here? I would also be interested in how you get your V/I figures, because for 150Wrms into 8ohm, I get roughly 40V and 5A. The 1500W figure is completely uninteresting because that's mains power draw and hence includes operating power and transformer losses.

As I never claimed or implied a constant impedance your post is puzzling. I said show me a speaker that dips to 1.6 ohms - it's pretty rare isn't it? So the 1500W instantaneous power is never going to happen.

P = VI, I = V/R, so P = V.V/R, so V.V = P.R, so V = root(P.R) = root(150.8 ) = root(1200) = 34.6V

Those are RMS values, so the power rail will be 34.6 . root(2) = 49V

Of course current will always be governed by I = V/R so 40V at 5A is a distinct possibility into 8ohm, although speakers also have inductance and capacitance.

Of course TIM may be old, but it's still a better measure than THD which is totally worthless. A spectrum analysis graph would in fact be far far more useful as we need to know how the harmonics behave as the order increases. GNFB is useless for most harmonic distortion above about the 3rd harmonic anyway so it's a very revealing graph as it gives a picture of the actual, audible, quality of the amp. Still, I guess it's all technobabble to many.

 

[Anonymous]

Ah yes, I was applying the RMS conversion before calculation, but to do that at the power level I need to use 2 and not its square root 🙂

As for the speaker impedance: you are still assuming that 1500W power draw means 1500W applied to the speaker, which is too simple. Most power transformers are only about 80% efficient, so you need to incorporate those losses as well. And the figure may have been given for both channels driven. Both of those assumptions put the minimal speaker impedance at around 4ohm for 8ohm speakers, which seems a reasonable margin.

What's GNFB?

 

[Anonymous]

From here: http://www.cyrusaudio.com/product/power-amplifiers/mono-x-200

it just says 'Burst Peak Power >1500W'

Which I read as an output burst but you are right - it could mean anything as it's unspecified.

Assuming it does deliver a 30A peak at that power it's interesting it claims 80A p-p (or 40A peak) current.

If we pretend it has 20,000uF of reservoir (per rail), and can drop 5V on the peak, it has Q=VC = 0.1C reserve, and C = IT so T = C/I = 0.1/30 = 3.3ms of that power level.

Then of course at the speaker it meets a lovely 'wodgey' inductor for the bass driver so that sharp current does very little but get impeded, and a small fraction of the power reaches the tweeter. Most tweeters rarely see even 1W so in the real world these figures are particularly meaningless and should be therefore dismisses as 'sales patter'.

The zero feedback is interesting though - that indicates an alternate topology which is always interesting. Of course there will be masses of feedback in places (a 114dB S/N ration (confusing quoted as an absolute 114dBa ?!) doesn't come naturally), but it matters where and how it is applied - not all feedback is bad at all!

GNFB = Global negative feedback. When a part of the speaker signal is fed into the input of the amp 180degrees out of phase.

Pros: Low noise, low output impedance, correction of frequency response (no amp should need this!).

Cons: You couple the speaker to the input of your amp, you multiply the harmonics, you court instability, the more you need it;- the less good it sounds.

Lots of debate about GNFB, in general applied to a linear system it's fine, applied to a non-linear system it's bad. I expect most of the 'rhythm' of the old Naim amps was due to the 0.22R output series resistor and the slightly inductive mandated speaker cables just pushing the influence of the speaker away from the amp to let the amp do it's thing. I.e. to make sure the dog was always wagging the tail.

 

[Anonymous]

Interesting. I would never have considered including the speaker's response in the feedback loop. As you say, any nonlinearity in the speaker's response will wreak havoc on the amp's accuracy. That would explain why manufacturers consider damping factor such an important characteristic, I think.

Thanks for the info. Now let's get back on-topic.

 

[Anonymous]

Well I hesitate to get back to something as mundane/boring as my original post - but thanks for everybody's input. I'm now trying hard to find a way to

a) retain doemstic harmony

b) move the amp closer to the speakers

When I asked the question I didn't expect to have my choice of power amp criticised so roundly - especially as I struggle to understand the reason. All I do is listen to the music....

 

[Anonymous]

The best advice is from the record spot.maplin speaker cable.will do the job perfectly. In professional recording studios across the world,they have massive runs of all types of cables and they seem to work ok. Too much focusing on measurements and not actual sound produced on hear i think.

 

[Anonymous]

I use Mogami 2972 in 6.5m runs to my speakers. ~£150 a pair terminated?

 

[Anonymous]

When I asked the question I didn't expect to have my choice of power amp criticised so roundly - especially as I struggle to understand the reason. All I do is listen to the music....

Sorry to digress, your amp sounds like it's designed far better than most out there.

I'd stick with Maplin 12AWG cable, and buy some banana plugs for the ends like these:

http://www.ebay.co.uk/itm/6PCS-Gold-Plate-Audio-Banana-Speaker-Plug-Plastic-Shell-/280717977726#ht_2049wt_907

It's cable, it needs to be conductive, it's not a high cost item that's going to affect the sound unless they are too thin or have too much capacitance. Those Maplin ones are perfect.

 

[Anonymous]

Thanks for all the help/comments gentlemen - I've now ordered some Van Damme Blue 6mm cable.