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Review and Measurements of NuForce STA-200 Power Amp

amirm

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#1
This is a review and detailed measurements of Optoma/NuForce STA-200 Power Amplifier. It is on kind loan from a member. I see it listed for USD $499 including Prime shipping. Looking elsewhere, it appears to have a list price of $1299.

This is one attractive enclosure that does not usually come in a $500 package:

Optoma NuFroce STA-200 Power Amplifier Review and Measurement.jpg

The finish is slightly textured and looks pretty nice.

Feature wise, there is nothing there but a set of RCA inputs and speakers. The RCA inputs are paired with speaker terminals on each end of the unit so you better have RCA cables that separate that much.

Somewhat out of character is a traditional extruded aluminum heatsink poking out the back. I guess this avoids having to put the heatsink inside and having to vent the enclosure. The heatsink is pretty small though and it was cooking even when testing the unit at just 5 watts/channel. I worry about its ability to run OK at full power for any extended amount of time.

Rated power is 80 watts/channel with 8 ohm load. My testing was at 4 ohm however.

Let's get into measurements and see how she does.

Measurements
As with my previous amplifier tests, let's start with a dashboard view of the unit while it was outputting 5 watts:

Optoma NuFroce STA-200 Power Amplifier Dashboard Measurement.png


Boy, this is disappointing. We have tons of mains and power supply components (which I reduced somewhat by grounding one leg of the speaker terminals to my AP analyzer). The dominant distortion components are the cause of poor SINAD though with the second harmonic peaking up to nearly -70 dB. So even if the rest were not there, the SINAD would remain the same.

Company states THD+N of < 0.03% and we are essentially there so our testing is correct. The write-up brags about response up to 1 Mhz and such. I suspect instead of using fair amount of feedback to bring the distortion down, they have traded for wider bandwidth.

Looking at the power rating versus distortion, the STA-200 doesn't bring much to the party over the much cheaper Topping TP60 ($199):

Optoma NuFroce STA-200 Power Amplifier Power Measurement.png


Yes, there is less noise at lower power levels but by the time we get to 1 watt, they equalize in performance and at limit, the Topping TP60 (red) actually has less distortion.

The spec says the signal to noise ratio is 100 dB. My measurements fall somewhat short of that:
Optoma NuFroce STA-200 Power Amplifier Signal to Noise Ratio SNR Measurement.png


Likely the mains intrusion is the cause (which would vary from installation to installation).

Sweeping input level and measuring IMD we get:
Optoma NuFroce STA-200 Power Amplifier IMD Measurement.png


As with THD, the STA200 has good bit less noise than Topping TP-60 but then suddenly clips and clips badly. Looking at the gain we see why:
Optoma NuFroce STA-200 Power Amplifier Gain  Measurement.png


This is a lot of gain for a low power amplifier. No wonder sensitivity is spec'ed at just 0.45 volt (versus nominal 2 volts). This means if you may have to significantly turn down the volume in your pre-amp/dac+pre. This would make for an amp that plays loud due to its high gain and perhaps that is a conscious choice.

Frequency response is reasonable to 40 kHz (0.3 dB down):
Optoma NuFroce STA-200 Power Amplifier Frequency Response Measurement.png


Anyway, I think we have enough here to know the merits of this amplifier.

Conclusions
I was pretty depressed when I reviewed the Topping TP60 and FX Audio FX502SPro and had high hopes that the NuForce STA200 would easily outperform them. Such was not the case. The Topping TP60 at less than half the price essentially matches its performance. It also runs substantially cooler than this unit. It doesn't look quite as nice though although it has a more manageable desktop form factor if that is how you use it.

The NuForce ST-200 seems to cater the audiophile myth of "fast" and high-bandwidth amplifiers, sacrificing fidelity to get there. This is a poor trade off in my book when just about all the content we play is quite band limited.
 

amirm

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#3

helloworld

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still dont know how benchmark implented the thxaaa technique in power amp, I searched in the THX web and found the technology is only for preamp or headphone amp.
 

Sythrix

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#6
Well that was depressing. I hope we see some good amps at some point. Thanks anyways!
 

RayDunzl

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#7
What's the handicap gonna be for an amp powering speakers from across the room vs a DAC/Headphone Amp that's almost or actually in your ear?
 

Sythrix

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#8
What's the handicap gonna be for an amp powering speakers from across the room vs a DAC/Headphone Amp that's almost or actually in your ear?
If it's needed, there could be a disclaimer for people who don't understand the implications behind powering speakers versus headphones. I don't think precision measurements should be given a handicap though. That would possibly produce backlash... tampering with published results and whatnot.
 

Thomas savage

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#9
Seems amps are not the ‘solved problem ‘ many see to assume. I think these kind of measurements will be even more helpful than the DAC’s amirs measured . After all many of the discrepancies in those measurements where,, academic , these amps tests already look like they are turning up some real world issues.
 

restorer-john

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#10
still dont know how benchmark implented the thxaaa technique in power amp, I searched in the THX web and found the technology is only for preamp or headphone amp.
It appears to be feed forward and/or direct distortion cancelling. Nothing new to see here.

Companies were doing that in the very early 1980s. Denon, Sansui, etc. With the right devices (ring emitters/LAPTs etc) we had amplifiers with specifications and performance to die for.
 

miero

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#11
@amirm looking to these power supply harmonics ... is your AP immune to ground loops? For example does it have input/output galvanically isolated from each wire in the power outlet and also isolated audio input ground from audio output ground?

This happens to be an issue when measuring equipment using soundcard in PC and I wonder if AP has this solved. Thanks.
 
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restorer-john

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No wonder sensitivity is spec'ed at just 0.45 volt (versus nominal 2 volts).
Most domestic audio power amplifiers have been specified at nominal 1 V for full rated power output for many decades. There were a few that had lower sensitivity (1.5V) and a number of UK made amplifiers with higher sensitivities of .5V (500mV for full rated power out). Quad were a proponent of such and their preamplifiers had matching 0.5V rated outputs.

2V 'nominal' was an output level established for the release of the Compact Disc in 1983.
 

restorer-john

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#13
These pictures below I found on the internet show internals. There appears to be several versions and cable dress changes between them. There also appears to be dual vs single transformer (perhaps a deluxe higher powered version) and ones with half the number of driver and output devices populating the PCB, discrete paralleled diode bridges and monolithic bridges in other images.

If the photos below are an accurate representation of the unit you tested, I would certainly expect wild variations between the channels with transformer/psu induced harmonics due to the proximity of the input cabling and the mains/transformer wiring. The right channel would logically be the worse performer based on the photographs.

nuforce01.jpg


nuforce02.jpg


nuforce03.jpg


All in all, I can see a $299 maximum retail as being reasonable, but at $1299, it's utter theft.
 

stunta

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#14
they defended it by saying that they "design by ear."
Great! I can sell them these measuring tapes I make that are marked by feel. You see, thermometers are designed by touch. The speedometer in our cars are calibrated by sight.

Seems amps are not the ‘solved problem ‘ many see to assume
I hope this is not the case and we'll start to see better measuring budget amps in Amir's lineup. But I fear you might be right :(
 

stunta

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If the photos below are an accurate representation of the unit you tested, I would certainly expect wild variations between the channels with transformer/psu induced harmonics due to the proximity of the input cabling and the mains/transformer wiring. The right channel would logically be the worse performer based on the photographs.
What is the general guideline here in terms of proximity? In other words, how far is far enough? Can shielding prevent/mitigate this issue?
 

Grave

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#16
It appears to be feed forward and/or direct distortion cancelling. Nothing new to see here.

Companies were doing that in the very early 1980s. Denon, Sansui, etc. With the right devices (ring emitters/LAPTs etc) we had amplifiers with specifications and performance to die for.
Why is this not used all of the time then?
 
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TimW

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#18
@amirm I see Grados on a measurement rig in the background of your photo. Is this a hint of things to come?
 
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#19
What is the general guideline here in terms of proximity? In other words, how far is far enough? Can shielding prevent/mitigate this issue?
I haven't encountered a proximity rule of thumb for mains isolation, other than as far as you can get without unacceptable compromise - so it varies. Emitted field strength drops with the square of the distance, so diminishing returns are approached quickly.

Twisting the source/return pairs about 180 deg/in helps a lot to reduce emitted electrostatic fields from the mains. Specifically, L/N (blue/brown) and the switched L/L (blue/blue). The secondary (red/yellow) potentials and resultant electrostatic field strengths are lower, but it wouldn't hurt to twist them a bit.

As for the rectifier harmonics on the PCB: generally you would want spacing between the P/S and signal copper of at least 5 times the thickness of the PCB dielectric interposed between the source/return copper (signal/GND, L/N). Judicious routing of source/return on the PCB is key. The energy is in the fields, and the copper is the guide.

[edit]: I'm reminded of an encounter I had with Bill Whitlock (of Jensen Transformers) at a CEDIA Expo back in 2004. I brought up the notion of mains coupling in the wall to adjacent analog cabling, and he very firmly stated that electrostatic fields from mains wiring was a "non-problem" in that application, despite the presence of hum bars in many wall-mounted component video installations that were mitigated using non-NM twisted pair L/N wiring.
 
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amirm

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#20
@amirm looking to these power supply harmonics ... is your AP immune to ground loops?
Yes and no. Ultimately unbalanced connection in audio is a broken thing so you can't completely sidestep it. Here, the speaker output is measured using balanced connection to AP. The signal feed though is unbalanced since that is the only input on this amp. There, AP keeps unbalanced floating unless you manually ground it yourself. I play with and without such ground connections to see if I get improvements. In this case, I managed to get the SINAD up from 63 to 68 or so.

The challenge is that it is hard to tell if the problem is induced by measuring the unit or is internal to the unit itself.
 
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