510
Item nr.
Stereo set in speaker box.
| Production | The Netherlands, 2020.
Price was app. 8€. |
|---|---|
| Bands | FM (87.5-108 MHz). |
| Cabinet | Wood. Size 14x23x13cm. |
| Power | DC 5V. |
| Documents | Manual. |
This multi purpose stereo set was built experimentally from a bluetooth unit and PAM8403 amplifier, built in a speaker. The unit (obtained from AliExpress) acts as a bluetooth receiver, FM radio, MP3 player from USB stick or memory card, and amplifier (from front AUX input). It comes with remote control, has automatic scanning for FM stations, and has an equaliser. I first built the set with the following features.
(1) Bluetooth unit is at the back of the speaker box, because the front was too small.
(2) The amplifier can be switched to Mono, so you can use it with just one speaker.
(3) The internal speaker in the box itself can be switched to either the left or right output, or none. So, you can use it with the other speaker and then decide if the Master is the left or right channel, but you can also connect bigger speakers and not use the internal one.
(4) You can connect external speakers of 4 (preferrable) or 8 Ohms.
(5) There is an earphone jack, disconnecting the power amp.
(6) Whip antenna for FM.
(7) The set plays on 5V DC from a USB charger.
(8) The internal speaker is connected with screw connection for easy opening of cabinet.
(9) Power switch. A bit over the top maybe, because you can also switch off the set by removing the power plug or go to stand-by with the remote.
(10) Output amp of 2x3W is controllable and switchable.
(11) Tip holder for USB thumb drives (left of master volume).
The speakers 
were probably sold in combination with a small home stereo set (photo). I got them in the thrift shop, for just 2.50€, but without the stereo set. Maybe the owner had improved his sound with bigger speakers, reason to toss the original ones to the thrift.
| Obtained | 2/2020 from Homemade. |
|---|---|
| Condition | 8. |
| Value (est.) | 4,7€. |
I wanted to build the set with a TDA7297 amp without volume control, but changed to an amp with rotating volume control. The reason is, the bluetooth unit produces some soft humming sounds, and so it is pleasant to be able to turn the master volume down. After building a few units, I started to prefer the bluetooth modules with on board amplifiers, like used in my BigTooth speakers. | Mode | Watt |
|---|---|
| Off | 0.1 |
| Standby | 0.4 |
| AUX | 0.6 |
| FM | 0.7 |
| Bluetooth | 0.6 |
The table on the right shows the energy consumption (from the mains) in the various modes. Power supply is a Huawei USB charger, and the unit is playing at normal room volume.
The combination of the bluetooth unit and PAM8403 amplifier cause a soft constant hum when operated from a common power supply. This led to the addition of another connector and switch. I added an additional 5V DC jack to feed the amplifier, so running from two separate supplies, there is no hum. When just a single adapter is available (or wanted), or for demonstration purposes, the amp can be switched to run from the first supply too.
Conclusion of the experiment? When you don't really need single box operation, earphone, or mono switch, don't make things too complicated, and go for the AliExpress unit with on board amplifier. In June 2022, I rewired Dr Watson to have such a unit in the front.