50mv DC on the speaker of a subwoofer.  Likely cause (Supercube SC2000) - Page 1
Forum section: electronics / repair

author: gooseEL34
author_rank: Regular Contributor
timestamp: November 07, 2016, 09:17:17 pm
content: Powered Subwoofer.Definitive Technologies SC-2000 Supercube.Hey folks..I told a buddy I would take a stab at his woofer. What he complained of, was that the woofer was going on and off and the power light would go dark. This unit has no on/off switch and supposedly turns itself on when a LFE signal is present at the RCA jack input (only controls/connections on the back panel plate amp are the AC outlet for a standard computer style cable, the RCA jack (LFE in) and the volume control (rotary pot).So I do a quick components inspection and do not see any obvious blown caps. I plug the unit up to a signal generator (sine) at about .100vAC and 1k and get a nice sine wave through the speakers, however, when I turn off the signal source, the amp hums. This is the 50vm DC I have to assume. There is no on/off switch and the amp must have some type of detection to turn on when LFE is present. However, as soon as I stop the signal, I can clearly hear the hum.I cannot seem to find a schematic and this thing is full of SMD mini components.I build tube amps, can use a scope and have several Flukes. I can trace a signal path on a schematic and find the appropriate path on the circuit board (but in this case -- no schematic).Any idea where I should look? To be honest, I would not have thought 50-60 mv of DC offset would have created such a hum, but it is pretty bad...As additional info, I read that the heat sink in this unit was too small. I guess I am unaware of what causes DC offset, but I am guessing it has something to do with mismatched Output Transistors maybe?ThanksTim

author: gooseEL34
author_rank: Regular Contributor
timestamp: November 10, 2016, 08:51:52 am
content: Wow...no one???

author: krivx
author_rank: Frequent Contributor
timestamp: November 10, 2016, 09:09:39 am
content: 50mV isn't huge. DC shouldn't cause hum either. Have you probed the signal at the speaker? Look for 50/60 Hz mains noise. If you find it, works backwards to see if you can find where it starts. Check that's it not on the power rails.

author: CJay
author_rank: Super Contributor
timestamp: November 10, 2016, 10:03:08 am
content: 50mV is nothing as Krivx says.It's not possible to provide any kind of fault finding information other than extremely generic check supply rails for hum when we have so little to go on.Board photos, device types, component markings from the major parts are all useful info to have or we are just stabbing in the dark.

author: Kjelt
author_rank: Super Contributor
timestamp: November 10, 2016, 10:08:37 am
content: Too little info to go on, how is this with a brand new one? Some still have this hum.However on a different brand of subwoofer (The Velodyne DD series from 2005 onwards) on another forum we found devices failing, starting with hum and eventually failing totally which was deduced to failing smd capacitors from some vague brand. So I would suggest desolder some of the small smd elcaps and measure the ESR to see if this could be a root cause.

author: MosherIV
author_rank: Super Contributor
timestamp: November 10, 2016, 10:38:40 am
content: QuoteWhat cable are you using to connect the sig gen to the sub woofer?Is it sheilded ?Is the hum mains freq?What does "turn off the signal source" mean - did you unplug the sig gen from the woofer ?Or does the sig gen have a switch to turn off the signal ?Have you checked if the sig gen is passing mains hum?

author: Assafl
author_rank: Frequent Contributor
timestamp: November 10, 2016, 01:28:45 pm
content: Disconnect the input and short the RCA (or XLR) inputs - does it still hum? If so the problem is some power supply hum or leaky cap.Make sure that when testing the power is still on (usually, Subwoofers have a timed "turn off" - a few minutes after the last signal).If it doesn't, then the problem is up link. Perhaps ground loops (probably the single main reason to run balanced to all components).

author: madsci
author_rank: Regular Contributor
timestamp: November 12, 2016, 08:30:19 am
content: I generally like to see big power amps with <15mV of DC offset but a few designs do have 50mV offset as a normal condition. (Harman Kardon amps and receivers from the 1980s come to mind with 60mV being in spec!)Filter caps can dry out and that can introduce hum. This is usually accompanied by an inability to drive rated peak power into the load however. If one filter cap fails, you will get hum and asymmetrical clipping at maximum output. (This happened on a KLH sub, I've documented that here:https://www.reddit.com/r/audiorepair/comments/57qj8n/klh_powered_subwoofers_common_problems_fixes/)Even with higher quality electronics present in the SuperCube, the KLH troubleshooting steps may help. You will need to follow the signal path and look for the hum along it.The output stage is probably good. Class D rarely fails gracefully. The output waveform usually goes totally wonky or, more frequently, the mains fuse is blown and a hand full of MOSFETs have exploded andtaken out a good many passives in the output filter network. I'd focus on the input buffer and active crossover since it still mostly works.

author: cvanc
author_rank: Frequent Contributor
timestamp: November 12, 2016, 09:45:55 pm
content: Some nice clear photographs of the internals would be helpful here.Agree that 50mVDC at the woofer, while not wonderful, is probably not your problem. You wouldn't hear DC as hum - it would just generate a slight position shift of the woofer cone (in or out, depending on polarity).Good luck with your fix.