ZK-AS21 TPA3255 Bluetooth 5.1 Power Amplifier Board HiFi 2.1 Channel 220Wx2+350W Digital Audio Receiver AMP Board with Treble and Bass Control for DIY Speakers

Amazing, powerful little amplifier. The app isn't the best, but considering it's $30, it's pretty good. It's able to power a subwoofer, 2 tweeters, and 2 midranges with no problem. Its built in cooling fan keeps it ice cold, so there's no need to worry about overheating. Its size is also very compact, making it easy to take with you. Its bluetooth is amazing, with absolutely zero noise whatsoever, compared to its little brother ZK-MT21. Now, onto the CONS: 1. For some reason, the AUX input is delayed, and it's obvious when you try to play it with other speakers. 2. The main volume control isn't measurable, as it's built for the app, so you have to eyeball it when setting the volume (it never stops spinning). 3. From its size and power, it requires an external power supply. I used a 36v 10a power supply, which is PLENTY for it. 4. Since the terminals are so close together, be sure to turn off power before adding speakers to prevent shorts, and make sure there aren't any loose strands.

I’ve got this powered by two Makita 18v batteries equaling 36v. I connected two Boss 6x9” coaxial speakers that are 4ohm each and a Skar 8” subwoofer that is 2ohm. I must say this amp is awesome! I ran it for 5 hours bumping hard in my shop that was 60 degrees and it didn’t even get warm. My shop was rattling. I ran it through a bass sweep and it does 30 to 50hz very good. It only used one bar on the battery indicator so I’d say it works well and is not a knock off. It connects to Bluetooth easily and reconnects if I go out and then back in which is a big deal to me. The connection voice is English and is pleasant. Definitely worth it.

The fan is loud, so I disconnected it. I don't have overheating issues, because it's only use when I'm using the projector in the HT room. I connected a Klipsch surround system and it sounds awesome.

Too much distortion at low 1-5 watt power level. Not power supply but something in the design that makes the class d chip get really distorted. Worse, it comes and go. Power supply at 32 volts dc, 3a. Max amp draw at relatively high power is <.5amp. Distortion come and goes. This is my fifth class d amp and the first with this issue and first based on tpa3255.

This little amp rocks even at only 24 volts. Amazing tech to this old timer. One star off for "Thud" noise on power off.

Only thing is the fan has gone out on it but this thing is amazing tons of power to push 6 speakers 2 highs 2 mids and 2 subs

I love this thing it has been what all my attention is focused on for over a month! Building and modifying a jaw dropping bluetooth speaker! there are many great and interesting uses for these amps but you will have to do the work to find out yourself, Just know when done correct these little guys will produce a sound that will laugh in disbelief! They are that good! Ive been running 40v DC to mine since it was purchased it works great the only thing I found that is a minor problem is that when the bass is loud enough you cannot see out of the mirrors on your vehicle sometimes something happens where it seems to lose all of the mid and high range noise but the low keeps going as loud as ever? but switch it off and back on and immediately the problem is resolved surely a value for the money. they are roughly 4 inch x 4 inch by 3/4 inch thick.

This review is for the ZK-AS21 (2.1 TPA 3255). Good chipset, fun and easy DIY process and aesthetic (it's very easy, takes about 5 minutes, tool included), good features for the money. The challenge will be finding a power supply to take advantage of the amplifier. I went with a Douk Audio one because, despite the fact that it's not reaching the full potential, it's enough and Douk has a pretty good reputation, so their power supply was likely built with noise and transients etc. in mind. I hear no significant noise on mine, I'm happy with my decision. There are a few things that some people might want to take note of, and ones that are honestly pretty disappointing: 1) The right and left channels are high-pass and it can't be defeated. I'm running it at 2.0 right now because I just opened and assembled it a little bit ago and it's definitely not powering the bookshelf speakers I grabbed (Definitive Technology with passive radiators, they kick pretty well by themselves) to their full potential. By my ear, it has a mid-to-low order high pass (soft roll-off) around 100Hz. This can not be defeated, at least that I've found. It is unaffected by the gain and low-pass settings on the subwoofer channel. 2) There is an atrocious pop noise when powering off the device. Most (but not all) of these DIY amp kits that I've had have done this, but this one is the worst by quite a bit (presumably because it's considerably more powerful than the others I've purchased). This is pretty disappointing to me because I plan to use it on my desk where it's easy to access and therefore easy to turn off when not in use to save power. This unit would be better suited for stowing out of site, or if you don't mind a noisy environment (getting to that shortly), where it can be left powered on indefinitely. I'm sure it has a low idle power draw, but I haven't tested it to see how low. If I think to bust out my Kil-A-Watt, I'll report back with my findings. 3) The fan noise is kind of annoying. This is why I just mentioned stowing it away. It sounds like a laptop fan at 75%, and it'll run at this speed for as long as the unit is on, regardless of temperature or load. I like that it has a fan, but I wish they had included some logic to it. 4) The volume nob is digital. There are no hard stops, there's no way to tell how high it is set. For devices that don't have remotes, I vastly prefer range-limited dials. The included dial has a notch on it, but trust me, it's pointless. On the plus side, the detents of the knob are very nice and tactile. I imagine the decision to use this type of volume control has to do with being able to use USB as an input, and bluetooth absolute volume. I won't knock it for this because it's kind of just a preference thing. Just something potentially of note. On the plus side, the sound quality seems very good (undefeatable high-pass aside), there's virtually no signal noise (for a device of this type), and this has a tremendous amount of gain potential. When I first powered it on, I was startled at how loud it was, and then realized my Windows volume was only set at 20%. As I mentioned, I don't know how high the gain was set, but even if it was at 100% out of the box for some reason, it is still capable of WAY more volume than the others I have. This thing seems like a beast. I have little doubt that when I incorporate a passive subwoofer (Dayton Audio VS8, nice little unit, temporarily being used in my kitchen) this thing will feel like a beast for desk use. This would probably make a great little 2.1 TV setup too. --- So my experience so far is kind of love-hate. I'm hesitant to recommend it because of the terrible pop noise on power-down, and the fact that you'll spend a fair bit of money on a power supply that won't totally fail to live up to the TPA3255's capability, the price ends up much closer to the assembled amplifiers (e.g. Kinter) you can find around, some using the same great chip, but likely with more refinement. In truth, I'd steer most people toward that route. As for my use, I honestly wouldn't be surprised if this ends up in my garage at some point. That amp runs 24/7 (hooked up to an old Echo dot before they got rid of the audio out) and the perpetual fan noise would be drowned out easily.

Best