The amplifier

A project log for 3D printed Bluetooth speakers - LittleJet!

My goal is to design a 3D printed standalone speaker based on transmission line theory - info: Martin J. King -

Davide ErcolanoDavide Ercolano 02/04/2020 at 16:581 Comment

Since this speakers are meant to be "stand-alone", I'm searching for a little amplifier with wireless connection and this features:

I used to follow some YouTubers working in this field then I knew which IC amplifier to search, two of them actually: the Texas Instrument TPA3116D2 and the Yamaha YDA138-E.
For the "budget" point, I'm searching on Aliexpress (a good platform indeed).
Anyway, almost 2 months are elapsed now and I have some informations to share with you.

FIRST BOARD: WUZHI AUDIO ZK-1002 - $13.09 shipped to Italy

Really good PCB layout, pretty decent capacitors and inductances on the RLC filter and power line, volume POT with ON/OFF, two TPA3116D2 in single-channel mode... "Perfect!" do you would say... NO.
The Bluetooth IC is a wild one, named AS19AP22702-25A4 from a company with a "greek Pi" as a logo (edit: is "JL" logo, by JieLi company); the input audio is managed by this chip, also the input from the AUX jack goes inside to this IC, then, is that bad?! Quite YES

Using the AUX input  the amplifier is NOT linear and the signal get smaller from around 2.7kHz becoming really silly at 22kHz.
Usually this behavior is caused by an incorrect RLC filter, but is this the case?

Connected on Bluetooth you get a linear output till 19kHz... What happen later?...

This! What is this? I really don't know, a noise I would say, but anyway it is crazy output therefrom 19kHz to 22kHz and up.

So the Bluetooth IC it's not reliable and the AUX input is junk.

Anyway, the TPA3116D2 in this single-channel configuration (two, one per channel) it's really good and actually listen to it by Bluetooth is nice, with hi-power and no distortion.

SECOND BOARD: HAZY AUDIO XH-A354 - $14.86 shipped to Italy

When I have chosen this board I was attracted by the YDA138 single-channel configuration, so to have more power on a 4ohm driver, like the one into LittleJet.

The first impression on this board when it's arrived was so terrible: the codes on the Bluetooth IC have been erased, also on the controller for the SD and two more wild IC that I think are some OP-AMP (do someone of you know??). Also the volume POT doesn't have the on/off switch and the worst part: the inductances are ridiculously small. Finally the AUX input goes inside the Bluetooth chip, like the Wuzhi, then... Let's see...

0.5 per division, almost 2v p/p and it is already REALLY distorted (1khz).
And by bluetooth is the same...
At full power the inductances get really hot and also the linearity (from 4kHz to 22) is a shame

Measuring along the source path I found that the signal is good later on the decoupling filter (RC) and also later on the volume POT, then till the BL input, but it was absolutely distorted on the output of the Bluetooth IC.
Listen to this amplifier is a pain, this board is simply garbage. DON'T BUY IT.

Anyway I want to listen to a good YDA138 board, so I'm waiting for a new one (without Bluetooth connection).


The actual integration of the standard by these boards (everyone you can find now) is wild and not reliable then I will adopt a different approach: amplifier board (TPA3116) + Qualcomm CSR8675 module. The budget will grow up a little but we will have more audio quality and reliability.
Anyway now everything is going very slow in China because of Corona Virus, so I don't know when I will have this configuration. Meanwhile I will design the integration for the Wuzhi ZK-1002.
See you soon!


claudio Silva wrote 07/22/2020 at 16:02 point

Hi Davide, regarding WUZHI AUDIO ZK-1002 board I have notice exactly the same issue on the line In input. yesterday I have analysed the circuit and found that the problem is due capacitor C41 and C42 that are connecting que input signal after pass through a resistor to a ground before connect to the bluetooth IC. I have just removed these capacitors and surprise, now the sound is full of bright.

Could you please remove the same capacitors and test again the circuit on the osciloscope?

I am curious how about the frequency response have changed after these changes.


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