Technical question!

quote:

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Originally posted by Reto D:
Hi guys,

As a non-technical freak I'd like to know what exactly a heavy load is. Is it about the amps ability to deliver enough power for a specific loudspeaker? It is often said that Naim amps tend to have problems with heavy loads, isn't it? So, apart from personal taste, which are those demanding loudspeakers (which could be difficult) that should better be avoided? A question of impendance (Ohm's)?

Any help appreciated....

Reto

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Naim amplifiers employ a minimum number of matching resistors for the output stage to maintain the integrity of the musical signals. They use super fast/high current 007 resistors for the amp power regulation and the output stage. However, there is a limitation on how much current the amp can provide with a given number of (pair) resitsors. Other makers like Mark Levinson, Krell, etc use a lot more resistors which provide more power but "less clean" signals. It is the philosophy "belief" that Naim Audio still maintain till now. The new Naim amp designs have very high transient swings (compared to the older design) but for the same (or less) money you can buy more powerful amps from other makers. Matching speakers and amplifier is required for any HiFi system. However, more powerful amp allow more flexibily on the speaker selection. I personally think that Naim amp (NAP250 and above) have enough current for most but "most difficult = high demand current = hungry speaker designs which have a lot drivers to be powered and operate at low impedance (below 4 ohm).

What does below 4 ohm mean? (< or > 4 ohm). I thought that more ohms need more power...

My conclusion is: 8 ohms are more difficult to drive than 6, 4 or 2 ohms.

Maybe I'm dead wrong......

Still need help.

Reto

For a same power requirement, lower impedance speakers (2,3,4 ohm type) require higher current from the amplifier. Power = R x I x I where R is the resistance and I is the current. As you see, for a given power required, if the value of R is low (low impedance) the amount of current that is needed is higher. in HiFi term, heavy load or difficult speakers mean speakers that need lot of current because of low impedance or too many drivers (super-tweeter, tweeter, mid-range, bass).

On the subject of difficult speaker loads the usual situation is that the quoted impedance is misleading. Most speakers are quoted to be 8 ohms.
This says nothing about how the impedance varies with driver requirements to deliver different sound frequencies effectively. For the lower frequencies the "piston" movement of the speaker gets pretty big. Under these circumstances the voice coils consume significantly more current to drive the speaker against the air resistance aswell as the damping that is applied in the speaker design to prevent the cone from flapping around at the lowest frequencies.
It is quite common for the speaker impedance to drop to 4 ohms in the lower frequency region which is considered quite normal. Speakers which are often referred to as "More difficult loads" quite often drop to 3 ohms or even lower which means they will call upon the amplifier for more and more current as the bass content of the music increases.

Another issue is the efficiency with which a speaker turns electrical energy into sound. The "benchmark"value for this, which seems to be desireable is 90dB. Again a speaker is referred to as harder to drive as this value (quoted for standard signal input) goes down. So a speaker with a value of 85dB is a lot harder to drive to the same sound level as a 90dB rated speaker(remeber dB is a logarithmic sacle).

Taking my speakers as an example. I have Totem Mani 2's. These are definitely difficult speakers to drive!

They are rated at 84dB sensitivity
They drop below 3 ohm impedance in the low bass
And to add to that they are isobarik which means there is a second driver inside the cabinet which is driven in time with the one facing the outside world to extend the bass frequency responce. This means to get the sound level you normally get from one driver you have to drive two (double the load).

I can drive them fine with my 250mkII , but when I had only a 150 though I thought they sounded pretty good the amplifier would go into "distress" if I tried to get the volume up too far.

regards
GEOFF

Thanks all of you,

Then I could conclude that in general a Nap 300 manages to drive a 4 Ohm 90db loudspeaker (2way) without problems!


Reto

Tuan, after reading your post It must be true,
How is the "NEW Job"?
If so, how is your new desk?
How is the coffie?
When will the Nait 500 start to ship?
Will the Nait 500 be 125 W/C, 250 W/C 4 omhs , 500 W/C 2 ohms, 1000 W/C 1 omh ?
Also please make it so you can order it two ways ,with or with out phono.

Steve

quote:

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Originally posted by hi fi fo fum:
Tuan, after reading your post It must be true,
How is the "NEW Job"?
If so, how is your new desk?
How is the coffie?
When will the Nait 500 start to ship?
Will the Nait 500 be 125 W/C, 250 W/C 4 omhs , 500 W/C 2 ohms, 1000 W/C 1 omh ?
Also please make it so you can order it two ways ,with or with out phono.

Steve

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hahaha Steve, one day it will come to that. May be I should send my job application to Linn. It can be done with the switch mode power supply...

Tuan,

I like the sound of those super fast/high current 007 resistors, can I get me some

I guess the induction training is proving a struggle

One thing to bear in mind w.r.t. amp power is it's dependant upon two things, the voltage across the load and the current through it (which is dependant upon the voltage and the load's impedance).

Naim amps don't have lots of voltage headroom - the supply voltages are lower than some competing amps, they do have lots of transient current available, but how much of this is used, depends largely on the load. The 500 would be the primary exception to this, since its topology is different and would allow greater voltage swings.

This is why with 'easy' load, high impedance speakers Naim amps may sound quieter than others, whereas on difficult loads they come into their own.

The thing that makes the biggest difference to volume though is speaker efficiency - big amps are a wasteful and inefficient way to get extra volume.

Andy.