Bose L1 noisy - Page 1
Forum section: electronics / repair

author: aldi
author_rank: Contributor
timestamp: January 05, 2025, 06:44:25 pm
content: 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 boardNow 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?

author: andy3055
author_rank: Super Contributor
timestamp: January 05, 2025, 08:00:45 pm
content: I am not looking at the diagram but take a known good .01 mfd capacitor (the value is not critical) and ground one leg of it. Then connect the other end to a probe (may be from your meter) and start touching each input and output on each stage (check schematic). At some point, the noice will reduce or diminish. Then you will know where the problem is.

author: Swake
author_rank: Frequent Contributor
timestamp: January 05, 2025, 08:02:39 pm
content: Are you sure the noise is not 'by design'? This is Bose stuff after all. They had some good speakers, but the day they started selling 'electronics', boy.... have they sold serious crap.Anyhow, herewith some ideas- There is also that +/- 15 V power supply that is feeding everything pre-amp. scope it for ripple. desolder the 7815/7915 and connect a lab supply instead to bypass.- Noise in both high/low frequencies ?- You could short the audio path further down the input, like after the mic amplifier or after an EQ stage. as soon as the noise is gone you know it was 'before' the short.- you could also 'remove' or 'disconnect' parts of the preamp such as the equalization stages and the microphone input by removing a component and then bridging that section. You can't just bridge as that would keep the offending components in the loop.- Thoroughly clean input jacks/connectors- Thoroughly clean the pots- disconnect the 'LED control'- Can you visualize the noise on a scope? and then trace it back from the output to the input till it is does not show up anymore.- Earthing problem?

author: aldi
author_rank: Contributor
timestamp: January 06, 2025, 07:52:21 pm
content: Thanks for the suggestions. Did not have the time to test them all today, but did some scope probing.QuoteThe +/-15V looks alright, not much ripple at all (ac-coupled, 5mv/div, 20MHz bw):QuoteOnly in the higher frequencies. So the T+/- outputs leading to the PA.QuoteThis is the T+/- output to the PA:And here with the pre-amp gain at the maximum:Looks like its quite hard to visualize the noise, any tips on this? I guess when the noise is smaller than the noise of the scope, the scope is useless for these kind of measurements.QuoteI'm strongly leaning this way, but want to make sure before I draw my conclusions.

author: andy3055
author_rank: Super Contributor
timestamp: January 06, 2025, 11:26:58 pm
content: Try the capacitor trick. You will be able to hear what you cannot see.