Fosi M03 / Aiyima 3001 / Nobsound G2 Pro mono sub amp review

[mcdn]

One amplifier I recently bought states 750 Joule of energy storage for example. It has a Vpk of 185V

that choice of Joules would be classic spec fiddling by the marketing team I think . 750J at 185V is 2*750/(185^2) = 47000uF. Huh, look at that, the amp likely has 47000uF of capacitors in it - can you name the model or open it up and take a photo? If that is true then probably only 25% of that is usable as the voltage drops sharply with initial discharge, so instead of the implied 750 watt-seconds, we're looking at more like 175, which isa still useful 1kW for 100ms.

What's interesting is that it suggests the real difference between "Pro" and "Consumer" power amps is the use of higher output voltages in the pro gear to get to higher peak outputs.

 

[mcdn]

I see these used often for driving tactile transducers, which can create usable output well below 20 Hz. Any other measurements or considerations that could be beneficial, to characterize the amp for this type of use case?

No inherent problem with 10Hz, to say any more would require knowing the impedance and sensitivity of the bass shaker.

 

[restorer-john]

If you want to measure dynamic headroom, you'll have to record the signal the amplifier is generating and prod the recording manually (or use an AP/Klippel routine; I'm not sure if either of those has a method to output power over time graphs).

You're making it too hard, and potentially confusing amplifier clipping headroom with amplifier dynamic headroom.

You do have an oscilloscope, right? For dynamic headroom, use the standard EIA 1kHz toneburst (20 cycles 0dB, 480 cycles -20dB). Wind up the level with your loaded amplifier until the 20 cycle bursts hit the wall, back it off and read the peak voltage. That's how it has been done for 50 years or more.

 

[peniku8]

That's how it has been done for 50 years or more.

Amplifiers today are fundamentally different from amplifiers 50 years ago. A 20 cycle burst (20ms duration) will tell you dynamic headroom for a single 50Hz cycle (20ms duration), which is typically where the kick drum sits, so it's a useful stat in the practical sense, but again less precise and less information than the output over time graph I proposed and showed examples of earlier in the thread (scroll down from the post you replied to and you'll see some screenshots). Providing 20ms burst power ratings is nice tho and I'm happy when I see them alongside long term power output figures. I'm not sure what you mean by "clipping headroom", I've never heard that term in any kind of technical context.

 

[peniku8]

that choice of Joules would be classic spec fiddling by the marketing team I think . 750J at 185V is 2*750/(185^2) = 47000uF. Huh, look at that, the amp likely has 47000uF of capacitors in it - can you name the model or open it up and take a photo? If that is true then probably only 25% of that is usable as the voltage drops sharply with initial discharge, so instead of the implied 750 watt-seconds, we're looking at more like 175, which isa still useful 1kW for 100ms.

What's interesting is that it suggests the real difference between "Pro" and "Consumer" power amps is the use of higher output voltages in the pro gear to get to higher peak outputs.

14 of these equates to 42mF:

In my tests the amp delivered 4x2600W (4 Ohm) for 40ms and then dropped to 4x700W at 400ms. The 4x8Ohm test held 4x1300W for 250ms and then also dropped to 4x700W at 400ms (which is quite possibly the breaker protection limiter, which was set to 15A, which is the maximum it can be set to and you can't turn it off).
That equates to a little over 400 Joule of energy released in the first 40ms burst, which likely first drained the caps, then spiked current draw on the PSU side, which subsequently triggered the AC mains current limiter. I have access to like 6 different limiter types in the dsp and there are at least 4 internal (protective) limiters (peak limiter to prevent hard clipping; breaker protection limiter; temperature control plus another limiter which keeps long-term rms output current under control). It feel like looking at a fine-tuned engine nowadays: everything is optimized so that the device doesn't damage itself, but every parts is working near its limit and no potential is wasted. 40 years ago the solution would've been "have you tried making it heavier"

 

[scanspeak]

Thanks for that review. Note that the filter response is 2nd order (12dB/octave) not 1st order as mentioned in your review.

 

[gaussianmist]

View attachment 404214

Are the frequencies between 60 and 200 for the "SUB FREQ" control continuously selectable?

 

[mcdn]

Are the frequencies between 60 and 200 for the "SUB FREQ" control continuously selectable?

Yes they are

 

[mcdn]

Thanks for that review. Note that the filter response is 2nd order (12dB/octave) not 1st order as mentioned in your review.

Oops! I’ll correct that.

 

[gaussianmist]

Thanks for the review.​

A difference I notced among the 3 is that the Nobsound has gain control switches. Also, the 3255 is variously referred as the 3255D. Is this an important distinction?
Wouldn't the Frequency Control be better assigned to the back of the unit, since after optimizing the setting of the Frequency Control (using something like REW) it could be accidentally changed from the front?

 

[mcdn]

Thanks for the review.​

A difference I notced among the 3 is that the Nobsound has gain control switches. Also, the 3255 is variously referred as the 3255D. Is this an important distinction?
Wouldn't the Frequency Control be better assigned to the back of the unit, since after optimizing the setting of the Frequency Control (using something like REW) it could be accidentally changed from the front?

The Aiyima has the switches on the board, but they aren’t exposed through the case so you have to take it apart to change them. As for the “d2” designation, it only seems to appear in a diagram on page one of the datasheet, there’s no separate chip of that name.

 

[Kimbrough Xu]

I have a Nobsound G2 Pro, it has gain selector DIP switches underneath. Does the Fosi have those, too?

I'm using the amp to drive a 12" Velodyne sub with dead plate amp (6 ohm driver). Works wonderfully.

What do you do with the gain selector when I have a volume control?

 

[mcdn]

What do you do with the gain selector when I have a volume control?

If your DAC/Preamp has the ability to output high voltage you can lower the gain of the Fosi, which will improve the signal to noise ratio. Or if your source has a low output voltage like some AVRs, you can increase the gain to get a more useful range in the volume control of the Fosi. It's not particularly relevant to the intended use as a passive sub amp, but nice to know it's there if needed.

 

[Kimbrough Xu]

If your DAC/Preamp has the ability to output high voltage you can lower the gain of the Fosi, which will improve the signal to noise ratio. Or if your source has a low output voltage like some AVRs, you can increase the gain to get a more useful range in the volume control of the Fosi. It's not particularly relevant to the intended use as a passive sub amp, but nice to know it's there if needed.

I see, thank you.

 

[Rottmannash]

I have one of these, trying to get it to run a sub w/ a fried amp. At first blush I'm not impressed but perhaps I have something not set up properly. As for the gain switches, does one have to remove the case to access these? And if one does how does one know what values they are selecting? Their tiny owners manual isn't all that informative.