This is a review and measurements of the
Fosi Audio M03 mono TPA3255 amplifier. It's designed to drive passive subwoofers which is why I bought it as I have an old sub I want to put to use in my games room. It appears to be identical to the Aiyima 3001 and Nobsound G2 Pro amps, and costs $90 on the Fosi website. It's a dedicated mono amp so only has one set of speaker outputs, and comes with an adjustable low pass filter in "sub mode". It ships with a 36V 5A supply brick, so we can expect max power to be 150W or less. It does claim it can be used with a bigger power supply, so I also tested that with a 48V/350W supply - see the results below.
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Build and components
Before we get to the measurements let's talk about build and component quality. For a $90 amp the case, switches and knobs are all solid, with positive switch movement and good soldering. The power supply cabling on the DC side is marked as 20ga/0.5mm^2, but I checked and it's more like 22ga/0.3mm^2. Either way it is marginal for the rated 5A. The internal cabling from the amp outputs to the speaker terminals is also too thin, but I didn't take it apart to measure it. The board itself seems reasonably well put together. To increase power you can use up to a 48V supply according to the manufacturer. But the power supply capacitors are only 50V rated, which leaves no margin and could decrease the lifetime of the unit if operated at high power outputs. The heatsink is also only connected to the amp chip not the case, and in soak testing it hits thermal cutout after
60 seconds 5 minutes at 100W continuous output, or 1 minute at 150W (with a 48V supply). The good news is that the built in protection works and after power cycling it works fine again.
Inputs, switches, gain and filters
I need to explain the inputs and switches because the manual isn't easy to interpret. There are two RCA inputs on the back, one set of speaker outputs, and a switch on the front marked "PBTL/SUB". Then there's a "VOL" knob and a "SUB FREQ" knob. In either "SUB" or "PBTL" modes both inputs are summed. Gain in PBTL mode with both channels driven and the VOL knob at its midpoint is 32dB. Switch to SUB mode and gain doubles to 38dB. So one input driven in sub mode, or two inputs driven in PBTL mode give you the same overall gain. I don't show it below but distortion is the basically the same with the VOL knob at midpoint or max, so if you have a low output source you can use the volume knob with confidence.
In PBTL mode the "SUB FREQ" control does nothing. In SUB mode it gives an approximately second order rolloff depending on the setting. -6dB points are 45Hz at the lowest setting and 200Hz for the highest setting. All measurements are made with sub mode off.
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Let's go to the measurements.
Unless otherwise stated, these measurements are taken at 5W into a 4ohm load using the stock 36V/5A supply in PBTL mode and an AES17 40KHz filter as is usual for class D amp measurements. The software is REW, the DAC is a Topping D10 balanced, and the ADC is an E1DA Cosmos ADCiso.
A frequency sweep looks good although for a sub amp the drop at 20Hz isn't ideal - maybe it uses PFFB as there seems to be no hump after 20KHz?
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The 1KHz 5W FFT is good for noise at -110dB, but not great for distortion for a TPA3255 implementation, where we would expect closer to a 94dB SNR. I suspect the issue is with the front end where the filtering is taking place. Even though filtering is not enabled we see a bit of excess 3rd harmonic. This will be irrelevant for the intended use as a sub amp though.
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19&20KHz dual tone is what you would expect given the above performance, with IM peaks around -50dB vs best-in-class TPA3255 at -70dB. Not audible or important for the usage as a sub amp, but worth noting.
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The 32 tone multitone test isn't very pretty either at 12.8 bits, but is consistent with similar amps and fine for a sub amp - we are paying less than $100 here!
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Now let's talk about power. Using the stock supply into a typical 4ohm load at 20Hz we only get about 100W at 1% THD (the result is the same at 20Hz or 1KHz):
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If we switch to a bigger supply (Mean-Well 48V 350W) we can get to 230W into 4ohms:
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But if we use a 2ohm load, even with the bigger power supply we only get to 100W.
I suspect the inductors are saturating, or some other internal limiter is kicking in. [EDIT: doh! The bigger supply is 350W = 48V*7.3A, so I^2*R = 7.3^2*2 = 106W. Lots of these power results are simply current limited by the power supply].
Conclusions:
If used as intended with the stock power supply it will give 100W of clean power to a 4ohm passive sub, while summing both the inputs and providing some useful low pass filtering options. That's what it's meant for and I'll be keeping it in my games room system for that purpose. I'm not so impressed with some of the other claims made by Fosi regarding upgrading the power supply and so on, but for the money it's hard to complain.
. 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.
