I got a Bose L1 compact system in for repair. The complaints are the level of (white)noise the unit produces when idle.
The user of the unit is an older musician that cannot hear the noise, but others complain about it.
The noise is present when both volume dials are completely off (instrument + microphone). The noise becomes more present as the volume is increased. Shorting the inputs make no difference.
Bose kindly provides complete schematics and a service guide on their website.
The schematic:
https://assets.boseprofessional.com/m/768a81365b2773ce/original/318882_SCH_R1.pdfThe service manual:
https://assets.boseprofessional.com/m/3be683fca0d937a0/original/318882-SM_Rev03.pdfOther documentation (ctrl-f for L1 Compact):
https://www.boseprofessional.com/102955-2The service bulletins that are listed for this model do not apply, the fixes are already applied on the unit that I have.
Things that I already did to search for the source of the noise:
- Short the inputs, no difference
- Remove the preamplifier board, leaving only the SMPS and the power amp, the noise is almost gone, which indicates the problem is the preamplifier.
- Check for obvious things: soldering, leaky caps etc.
- Voltages are OK to the preamp board
Now I am a bit stuck, what to do next? I wonder if the unit was noise free to begin with, the preamplifier is build with a ton of TL072's which all add noise. All the amplifiers that I personally own are all noticeable noisy when the volume is up. Any analog wizard who can say anything about this when looking at the schematics?