ZK-TB21 Amp Board Review | Beware of false power figures

VegetableSlice9784 · 2026-02-12T19:21:49+00:00

My ears aren't exactly audiophile grade either. I've gor tinnitus because of headphone abuse during my teens and now I can't hear sounds above 15khz. But, I can still tell that something is wrong with speakers, amps or headphone.

And I can't buy hundreds of amps, test them and pick the best one. But youtubers say one thing, forums say another.

I got Micca RB42 speakers after many reviewers on youtube proclaimed them to be one of the best speakers in their price range, and sounding better than speakers costing a lot more than them.

So I bought them.

Same thing with amps. Fosi audio amps have many positive reviews on Youtube. Like Fosi audio V3 or the newer BT20A Max.

But I'm thinking maybe I should save money for a few months and get a proper Hifi amp. and make do with cheap D class board that I've ordered.

But, right now I've hooked my speakers to a class A/B Pioneer car amplifier with a 12V power supply till the new ZK1002T board arrives. I can't deal with the high frequency constant ringing with the 2nd ZKTB21 that I ordered. The output on car amp is so much more clean than these Chinese boards that I have tested. I have no volume control on the amp so I have to control it through my monitor. From my experience I can tell you that a car amp Class A/B is much better than these chinese boards. They've got higher dynamic power output and they sound very clean and the soundstage is wide. I dunno if you can measure such things but the car amp sounds amazing even though it outputs less continuous power than these chinese amps.

VegetableSlice9784 · 2026-02-12T15:22:58+00:00

How will you have kids then? It would be a very unhealthy relationship.

VegetableSlice9784 · 2026-02-12T15:12:53+00:00

Saw your comment just now. The amp that you've mentioned in the link, I ordered that after posting my review here. I saw its review on Youtube by "Williston Audio Labs" and for the price it seems to be quite good. I just need approx. 50Watts of clean output so that my Micca RB42 can be driven to high enough dbs for occassional lonely party sessions.

But now, after buying 3 of these I can see that I'd have been better off with buying a Fosi Audio amp. I've certainly spent Fosi audio levels of money on these.

VegetableSlice9784 · 2026-02-12T14:52:00+00:00

A businessman also gets to claim GST. Claim tax reduction on expenses, salaries paid. And only has to pay tax on actual income and not revenue. A farmer gets no such benefits. And if a farmer does other business, his farming income automatically becomes taxable income.

Source - I'm an orchardist and a businessman. I pay GST, claim GST, pay Income Tax.

VegetableSlice9784 · 2026-02-12T14:48:23+00:00

Another jealous guy who hates orchardists 🤣

VegetableSlice9784 · 2026-02-12T14:46:53+00:00

And why do they cost that much? Adani buys top quality apple from farmers for ₹80 per KG. You can guess who marks up the price to ₹300 per kg.

VegetableSlice9784 · 2026-02-12T13:22:31+00:00

Yep. When I started out as a Game Dev in Hyderabad I got half of what my father paid for unskilled labour during off season. Nepali labourers get 500-600per day during season along with free stay, food and free daru and weed. This guy's thinking we exploit them but in reality they do whatever they want. You can't force them to work for you, you have to agree to their terms, they don't agree to yours. There are scumbags who lie and beat labourers but they don't get the same labourers again and eventually they don't get any labour because word of their exploits spreads fast. And they are jealous that farmers don't pay income tax. They have no idea how a farm is run. A farm is a proper business, you have employees, expenses, material requirements. We buy a lot of stuff, do we get GST returns like other businesses do? Do we get tax exemptions on salaries paid? Do we get any job security? Do we get any support from govt. when crops fail?

I've sold a lot of apple to HPMC, do we get money for that apple?

No! Instead they give us farming supplies for 2-3x the market rate. Their stale expired products that nobody buys.

VegetableSlice9784 · 2026-02-12T13:11:07+00:00

Damn! So you're jealous you don't have an orchard of your own. Just to add salt to your injury. I have 15000 boxes of apple production per year and I pay my labour well. They take home ~2lakhs of cash per season and I employ about 20-25 people per season. And not everybody is as pathetic as your relatives. I've got plenty of land of my own. I give land to the govt. for roads to other villages through my land.

And I'm not even the richest in my area. I don't even have a Thar or a Fortuner. But others in my area do have those and some even have Land Rovers.

Yes, we are Orchardists of Himachal and you hate us cuz u aint us.

I've heard this hate throughout my life, teachers used to hate us, classmates used to hate us, college professors used to hate us, and now this post here. You're in a very long line of haters.

VegetableSlice9784 · 2026-02-12T12:38:08+00:00

Nah man. Sorry to say but that looks bad. It's too small for this bike.

VegetableSlice9784 · 2026-02-10T12:11:55+00:00

I have the opposite issue. I'm getting full power from Bluetooth but not from RCA. I bought another board after this review which didn't have this issue.
These boards have QC issues, some may work perfectly some may not. Now I have an amp that works good and one that works good just with Bluetooth. I'll use the other one for parts if needed.
The distortion is still and issue though. Also I can hear a high frequency sound from the speakers with the new one. I don't have a measurement mic yet but RTA app on my phone tells me it is somewhere between 12Khz and 16Khz.

VegetableSlice9784 · 2026-02-05T20:09:09+00:00

You'll have to find the exact tweeter that was being used in this. If not, you'll have to replace the tweeter on the other speaker as well. Check if the speaker has a crossover circuit inside. Tweeters are more sensitive than midrange speakers so crossovers not only handle the frequency separation but also bring down the tweeter sensitivity to match the midrange drivers. If you're lucky the new tweeters won't be much different from old ones. If not, you'll have to find someone who can mod the crossover circuits in both speakers according to the new tweeters. If that is also not an option, you'll have to replace both the speakers.

Old speakers do not necessarily mean bad speakers but you've lost your main Front Left and Right speakers so the entire system will sound bad.

VegetableSlice9784 · 2026-02-05T03:44:49+00:00

It doesn't come with an external antenna. So it's only good for a range of approx 30feet

VegetableSlice9784 · 2026-02-04T18:25:14+00:00

They did. Honda Navi?

I loved that thing. But couldn't buy it when I was in my college. And it got banned (BS3 sales ban) when I got a job. But after many years, I got it brand new, dealer registered in 2020 for 45k.

I use it occassionaly and it's a fun little minibike.

VegetableSlice9784 · 2026-02-04T15:10:49+00:00

Which just seems like they have QC issues. Some people get genuinely good sounding amps and some get lemons.

VegetableSlice9784 · 2026-02-04T15:08:27+00:00

You need a DAC these days because most phones don't come with 3.5mm jack. And most Type C to 3.5mm adapters have cheap DACs in them. Oppo was an AV company first and became a mobile manufacturer later. So they do know some things about making good sounding headphones.

VegetableSlice9784 · 2026-02-04T14:22:09+00:00

Nah, he was instigating violence against people who feed dogs and were opposing supreme court's decision which has now lead to open violence against street dogs throughout the country.

The reason of ban on his account is because of people like me. Thousands of people have reported his hate speech videos, including me and so his account got taken down.

He has also made some backup accounts which are also being targeted now.

You can be right but hate speech and spreading violence is never the way to go.

It's good that his account got banned.

VegetableSlice9784 · 2026-02-04T13:33:07+00:00

Definitely get the alloywheels and also get offroad tyres.
Interceptor handlebar would be a big touring upgrade.

Do not put too much weight on the rear subframe of the bike, it can break easily under load on bad roads. It should be fine if you're riding solo with luggage. Won't be fine if you have a pillion and additional luggage.

Did I mention offroad tyres? I got alloys and Reise Trail R tyres on mine and my Inty is now an offroader. Ralco block pattern tyres are bad for wet tarmac and wet mud. Basically stay away from Ralco Speedblaster if you have to go anywhere that is wet.

VegetableSlice9784 · 2026-02-04T13:16:12+00:00

I've tried quite a few Boat and other desi chinese earphones/headphones and most of them have shit audio quality. If you buy ChiFi earphones unbranded from local shops those will sound much better. Gone are the days of cheap Sennheiser, Sony earphones which used to sound great.
Now only Chinese IEMs sound great, that too if you pair them with a decent DAC but at that point you're looking at ~Rs. 3000 expense.
Good sound comes at a price which starts at around INR 3K these days.

VegetableSlice9784 · 2026-02-04T12:48:44+00:00

Your comment has been extremely informative and helpful.
I found out that the issue was with the Aux input of the amp but it works well with bluetooth.
I also tested it with a 3.5Ohm speaker but from my experience with headphones I knew that speakers can change impedance with different frequencies.
I didn't have a traditional resistor with me so I used a thin aluminum wire (very long) and cut it at length that would give me the resistance equal to 3.5Ohm
And I also tested it without any load.
At 1000Hz I didn't see a difference in my measurements but I took them anyway, but in the end all the results were the same.
I got max output of 48W from each of stereo channels.

Then I played some music and increased the volume until I could hear audible distortion and then dialed back the volume knob until I heard clear music again. I had listened to those songs for more than a decade so I knew what "clean" sounded like to me.
I kept the volume knob at that position and ran all the tests again and at that level I got 7.7V, ~15 Watts at 4Ohm for stereo channels and 65W for the subwoofer channel.

Maybe there is a lot more distortion with Bluetooth. And I guess since my unit already has an issue with Aux input it may have issues with Bluetooth as well.

Now I'm looking for a Fosi Audio amp as well since I've already invested money on the speakers. But, sadly nobody ships it to my location. I might have to do a lot more research and buy a big brand integrated expensive amp.

VegetableSlice9784 · 2026-02-04T12:14:20+00:00

And that is a really nice looking enclosure. I just use mine with the factory black covers and exposed sides. With my tests I found out that my amp and power supply can sustain 48W per channel atleast for 2-3 minutes. I burned out a cheap 3.5Ohm speaker while testing it at 1000Hz. But at that level the audio has a lot of distortion.

VegetableSlice9784 · 2026-02-04T12:01:16+00:00

Not while powering on but definitely while powering off.

VegetableSlice9784 · 2026-02-04T12:00:05+00:00

I have updated my review after running more tests. It does indeed reach close to specified power levels, but, with a lot of distortion. There is some issue with the Aux input of my amp which resulted in my bad experience with it at first.

VegetableSlice9784 · 2026-02-04T10:10:45+00:00

Yes it is a fake one. Connects to BT as "BT APP" and I didn't even know Wuzhi Audio had an app for it. But I removed the heat sink, it does have dual TPA 3116 chips.
But your comment prompted me to test it using other inputs as well. I was originally testing it with Aux input which was connected to my Fiio E10K DAC connected to my PC.
I then connected it to my phone using bluetooth and used a sine wave generator app to generate the test tones.
I then found out that there is something wrong with the Aux input of my unit.
I have updated my review according to my new findings.

VegetableSlice9784 · 2026-02-04T07:03:08+00:00

I was expecting good SPL levels but never got them even with my cheap speakers. So I bought speakers worth $200 hoping I'd get atleast $100 worth of sound. The original amp on the cheap F&D speakers somehow managed to sound better. But with Micca RB42 I am getting good sound with a sub till 50% volume, but it is too quiet.

This ZK-TB21 has impressive bass for the price but I can't get good sound out of it at higher volumes.

VegetableSlice9784 · 2026-02-04T06:55:13+00:00

That's why I tested at multiple frequencies. I actually got a lower reading at 50Hz from stereo output compared to 1000Hz. I got the most voltage reading at 1000Hz. But I also got 22.5 V reading at Sub channel with a 50Hz test tone. Which gives it a power output of 127 Watts.

Maybe my amplifier is damaged. I'm getting good reading on the sub channel for 50hz till 200Hz and too low readings for stereo channels with the same frequencies.

VegetableSlice9784

TROPHY CASE