Alternative switching amps

I was just looking at the XFM website, and among the ads for Brinkmann and 47 Labs (Shurely shome mishtake? You'd expect sony perhaps!) was an ad for Nu-Force digital amps..
Anyone heard them?
At $2400 for a pair of their top model it seems quite reasonable for very well reviewed monoblocks..
They're HERE.

 

someone mentioned them a while back - can;t remember much about the thread though...

 

Currently appraising a pair (the bigger ones)

 

Tony, do you mean the Ref9? Let us know what you think of them. They are claiming that their amp is an analogue switching one instead of a digital one... I thought most of ClassD is analogue anyway. Their control loop principle sounds OK, I think similar to the UcD one that was developed by Hypex. I read lots of good reports but it sounds like the product is more in the final development phase than full production. Since the beginning of the year they have made several revisions to the Ref8s. There is a huge thread on AudioCircle.

 

It also uses a Switch Mode power supply instead of the traditional linear one. They just released the Ref9 monoblocks which has a beefed up SMPS (350W) compared to the 100W of the Ref8. Anybody like to comment on the merits of using an SMPS instead of a linear PSU?

 

The ref 9s do look like great value for money...
My problem with them is based on their weight!
Being an old fart, an amplifier has to weigh enough to permanently put your back out - in much the same way as a motorbike has to have a capacity of at least 750cc. Anything less is kid's stuff (I know, head in the sand - smaller bikes frequently outperform bigger ones, sometimes the same with smaller amps but it's just plain wrong!)

 

I'm still not sure on the SMPS I reckon its one you can buy from RS for a quite meager sum.
I'll be experimenting with these a little, they arn't bad in all honesty though Len

 

whats xfm?

 

I too have succumbed to temptation and bought a Ref 8. It's second hand though and only the 75W version. Waiting for it to arrive. Being only 3 lbs and having an international voltage convertor makes it quite easy to ship.

Btw, that AudioCircle thread was massive.

JJ

 

Switch Mode Power Supplies (SMPS) and Switching Amplifiers (Class D and variations) are extremely similar. They use the same principle in that a steady input voltage (rectified mains in the case of SMPS, PSU DC voltage in the case of an amplifier) is rapidly switched (by MOSFETs) such that the output waveform after smoothing (Usually a simple LC filter in the case of a switching audio amp) is that required... Audio signal or DC or AC or whatever. If your SMPS switching frequency is similar to your audio switching frequency then your output filter will take out the noise from both. Any objections to the principle of SMPS seem ludicrous in the case of a switching amplifier.

A class D amplifier will typically break down to 4 stages:

Modulator

This is where your signal (analogue or digital) is converted to PWM. This can be done with an op-amp (for example, LC Audio's Zappulse) or a dedicated modulator chip (available from many semiconductor manufacturers). Essentially an analogue modulator is a comparator comparing the audio input with a triangle wave. The speed of the triangle wave dictates the resolution achievable.

Gate Driver

This section may not be required. A modulator chip or op-amp may be able to drive small mosfets for low-power output stages. However, where you have very large devices or many of them in parallel (to reduce output impedance in an open-loop architecture) then the gate capacitance of your output devices is significant. In order to drive them with appropriate speed they have to be supplied with quite a belt of instantaneous current. This section can take the form of a gate driver chip (again available from various sources) or discrete circuitry.

Output Devices

Invariably MOSFETs. In theory you could produce a valve (or even relay) based output stage. Some very low power solutions from Tripath, TI and Philips have the output devices integrated with the modulator as a one-chip (or "monolithic") solution.

Output Filter

Typically this is in the form of a first-order LC. This filter takes out the high-frequency switching noise from the output signal. It is there for a variety of reasons... without it the switching noise would carry a significant amount of energy which would be dissipated in the voice coils or crossover components of the speakers, and would also cause the amplifier to fail any applicable EMC standard miserably. It could also lead to interference with AM radio bands (as the switching frequency can easily fall in the LW band).

 

Good description.......smps are normally cheaper to make..no hulking great transformer being needed. As to which is better ....I'd go with traditional types but keep an open mind.

 

XFM is an indie/rock radio station in London, now owned by Capital FM.
Good source of new music, if you like that sort of thing. I tend to listen over the net, though it's also available on various satellite packages.

 

Thanks - and they advertise high end hifi kit?

 

Bizarrely - if you click "listen live" a new window opens with ads in it, for variously, KR audio, Nu-Force and Singapore Audio! I would have thought your average XFM listener was a scruffy oik with a micro system.
But then I used to listen to nowt but XFM when I was in London..