50 hz mains hum in the speaker output of a Logitech LS21 LS 21 stereo sub-woofer combo.
The amplification circuit of this combo has the following problems:
-poor transformer placement (induces hum into the sub-woofer speaker)
-no voltage regulator in the circuit (mains 50 hz ripple goes amplified to the output of the amplifier circuit)
-case is glued together (had to use an oscillation tool to cut open the case).
the underside of the sub-woofer:
removing the sub speaker shows how close the transformer is placed to the speaker, inducing the 50hz mains hum.
the amplifier PCB. No voltage regulator included.
I un-soldered the rectifier diodes in order to attach my DIY voltage regulator + capacitor PCB.
This is what the audio output of the amplifier looks like when it is off.
When turned on. As you can see the 50hz mains hum is not smoothed out by the capacitors on the amplifier PCB.
So I decided to insert a (7809) 9V voltage regulator and a 25V 2200µF capacitor between the rectifying diodes and the rest of the amplifiers circuit.
The voltage regulator was getting a bit warm so I added an improvised heat sink.
The finished regulator/cap combo.
The backside of the sub-woofer was glued to the case, so I had to use force and an oscillation tool to open it.
The transformer was removed.
The regulator/cap combo attached to the rectifier diodes.
A DC power-in jack attached to the backside. I had to drill a hole in it and use hot glue to fixate the DC jack.
The transformer was mounted to a little piece of ply wood and latter fixated with 2 screws to the underside of my PC table.
ambinaut 
Tienes el manual de reparacion LS-21.
El problema que tengo es que solo un parlante suena y el otro casi no se escucha. El subwoofer funciona bien. Saludos
Hola, no, no la tengo.
MIRA ESO SE PARECE A MI PROBLEMA…. CREO NO ESTOY SEGURO SE TRATA DEL COMPONENTE DE VOLUMEN QUE TIENE LA ENTRADA DE AUDIFONOS, EL QUE ESTÁ AFUERA. LA SOLUCION ESTÁ EN CAMBIAR LA RESISTENCIA VARIABLE POR OTRA SIMILAR… TIENE 5 PINES VCC, VCO LH LR N. 50K….
Yo también tuve ese problema y al parecer es el POTENCIOMETRO de la cajita que va afuera donde se enciende el LS21. Es un potenciometro que al manipularlo mucho las base se desgasta muy rápido. En aliexpress encuentras el repuesto y el nombre del accesorio es: “B503” (50k, diametro 16mm, grosor 2mm; datos que te pueden servir tal vez)
hola se me daño la consola de volumen hay alguna forma de usarlo sin eso?
puedes enviarme en un esquema sencillo tu colocación y soldaduras del circuito extra que hiciste
Hi, I did this without schematics.
Qué tal, al mio no le funciona el subwoofer y cuando muevo el control de graves el sumbido sube y baja.
Le remplazo el integrado que tiene un heatsink?
Hi no I did not replace the amplifier chip, I only added a voltage regulator with an extra capacitor.
Hola! Tengo este parlante con el problema de que el SW no emite audio, solo un zumbido. Por donde me sugieres que revise? Gracias y saludos.
Hi, try measuring the voltage before and after the rectifier diodes.
Hi, I have one of these sets with blown satellites. I would like to wire in a set of the apple pro speakers that I have. They were made for older macs with a special port that did both audio and 10 w per channel amplification. My question is whether or not the small amp thats in the sub drives both the sub and the satellites, and if it does would it be powerful enough for the apple speakers.
Thank you
I don’t know what’s inside the satellite speakers, don’t know how to open them. In any case here are the specs: https://www.cnet.com/products/logitech-ls21-speaker-system-for-pc-wired-series/specs/
So it can only go up to 7 watts, might be a bit weak for two 10 watt speakers.
El amplificador se prende solo cuando lo conecto a una fuente de corriente o tiene un interructor porfavor digame
Hi, it has a on/off switch.
Hi, I have the same problem as you, I do not know how to feed the amplifier, I do not understand that from your 7809 to connect to the amplifier board, could you help me with a diagram please, and another question is that the amplifier is powered by 9v?
Hi, do you know how a four diode rectifier works? Do you have a soldering iron?
I don’t remember what is the exact AC voltage on the output of the transformer. I’ve used the simplest 7809 circuit from the datasheet with an extra 25V 2200µF capacitor added :
I don’t have a schematic diagram but the modified amplifier circuit works like this:
Transformer – Rectifier diodes (on the logitech pcb) – my 7809 circuit – back to logitech PCB.
hello, could you give me the values of capacitors C2 and C36
Hi, sorry I can’t find the high resolution pictures 😦