Amplifier hum - Page 1
Forum section: electronics / repair

author: timelessbeing
author_rank: Frequent Contributor
timestamp: December 06, 2016, 01:52:15 am
content: Hey guys.I've got a little 30W stereo for my office desk. It's not exactly hii-fi, but I like it because the main unit and speakers have a very slim profile, which allow me to put them right against the wall and use very little precious desktop real estate. Anyway, I need help because it produces a prominent mains hum. It's loud enough to be annoying when there's no music playing. I know it's not being coupled in through the aux input, from my laptop for example, because it's still there when I unplug it. It is present in all modes (tuner, tape, aux) however in CD mode it is very faint. The volume setting has no effect on it.Are there any hacks for making the 60Hz hum go away?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: December 06, 2016, 02:20:52 am
content: Here are some photos of the guts. The shell says it was manufactured Feb 2005, but from the inside it looks like it could have come from the 80's.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: December 06, 2016, 02:22:51 am
content: The speaker terminals are mounted to the board with the big heat sink, so that must be the amplifier module. Incidentally the mains transformer outputs go directly into that board too, so perhaps it's doing double duty as the power supply as well (poor design decision?).Maybe if I measured the main rail voltage, I could rip out the transformer (there is only one tap on the secondary) and use an external switcher power brick to feed it the required DC?

author: cvanc
author_rank: Frequent Contributor
timestamp: December 06, 2016, 02:39:54 am
content: Wow, that thing claims a 2005 build date? You're right - it looks thirty years older inside.What is the model number? Low chance of finding schematics online for Jensen stuff but worth a try. Almost certainly one (or more) (or all!!) electrolytic caps on the power supply/amp board are failed...What test equipment do you have? How far down the rabbit hole do you feel like going here?

author: BMack
author_rank: Regular Contributor
timestamp: December 06, 2016, 06:30:29 am
content: Change the filter caps on the output portion of the board. You won't find a schematic on that. The majority of the caps will be fine.Easy way to do it is to trace your speaker out back to the output transistor(s) then change the caps in that portion of the circuit. I'm surprised you aren't having tact switch issues.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 06, 2016, 12:03:50 pm
content: +1 on the caps.Most of this type of this cheap radios, also apply to tvs, were made using dirt cheap Chinese components. In the close up of the power supply looks like the top right hand side cap is melted (maybe). You may spend more time and money repairing this thing that what the radio is worth.

author: dcac
author_rank: Frequent Contributor
timestamp: December 06, 2016, 01:51:18 pm
content: Quote from: timelessbeing on December 06, 2016, 01:52:15 amAre you sure it’s 60Hz hum and not rectified 120Hz ripple, 60Hz would not be very audible with small speakers.But either way you could always check if any cables run too close to the transformer. Did the hum i.e. change when you run the amplifier with the backpanel open.

author: cat87
author_rank: Regular Contributor
timestamp: December 06, 2016, 02:27:37 pm
content: Try bodging a mains filter on the thing. Something like a common mode choke and some X-rated caps across the lines.If that doesn't do it, the culprit can be either the hum is being coupled somehow to the rails of the amplifier (the suggested re-caping might do it) OR bad design in the stage before the main amplifier (if it has something like that) With that monolithic am there's not much you can do...

author: CJay
author_rank: Super Contributor
timestamp: December 06, 2016, 04:22:06 pm
content: Could also be mechanical hum, I've a DAB radio which has an audible hum in standby until you pick it up or press on the 'right' bit of the cabinet.But, I'd replace the caps anyway, they look cheap and nasty.

author: mzacharias
author_rank: Frequent Contributor
timestamp: December 06, 2016, 08:01:51 pm
content: Just about has to be a bad filter cap associated with the bridge rectifier or perhaps with a regulator transistor. Recommend replacing all such caps seen along the top right in pic 2, and especially the larger one seen partially enclosed by that heat sink.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: December 09, 2016, 12:18:51 am
content: I got it out onto the bench today. No caps are bulged or leaking. I probed the rectifier output with my scope and it's pretty clean. I hooked up a speaker to examine the output on the scope, and sure enough the hum was gone. I leaned the stereo in closer to the transformer (which was loose on the bench at this point) and the hum came back.I think I'll go ahead with my idea of hooking up an external power supply.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 27, 2017, 04:54:30 am
content: No good.I finally got around to modding the stereo. I hooked up a DC laptop supply, and now the hum is REALLY LOUD.I don't get it though. I tested the supply and there was very little ripple. Maybe a few hundred mV under load. Certainly not significantly more than what the old transformer had.I'll try adding some capacitance and see if that helps.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: January 27, 2017, 05:12:50 am
content: Is the hum dependent upon the volume control setting? Or is it a constant level regardless of volume?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 28, 2017, 01:17:39 am
content: The hum is not affected by the volume dial.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 28, 2017, 01:38:45 am
content: How is the pitch of the hum tone (usually right at 120Hz in my experience for mains hum with 60Hz cycles)? Higher frequency noise would probably point to something else. If you can pitch match (singing or whatnot), just put on a tone generator and give it a listen, you should at least to get an idea of what frequency it's at. 120Hz is low, think lower end of an average male singing voice range. 60Hz can only really be reached by very low voices (somewhere around a B natural an octave under the bass clef staff, if you're familiar with music), so it's often easy to rule it out unless it sounds really low.Can you see the hum at the speaker and can you trace it back through the signal path?It could be that a dried up cap is making something oscillate which is coupling into a signal line... the chassis looks metal so it's unlikely that it's just outside interferance... but since you said moving the speaker affected it, it could be something radiating inside the case. This again could be a dried out cap or something, but maybe test a layer of shielding between the speaker and the rest of the unit and get it close like before. If that bit of shielding reduces the hum, it may be a viable solution, or at least may point towards the component that is introducing the noise.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 28, 2017, 01:46:35 am
content: It's still 60Hz.I never mentioned speakers. I said moving the transformer got rid of the hum, which is why I got rid of the transformer.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: January 28, 2017, 02:58:03 am
content: No speakers? Are we talking about physical vibration hum? That sounds like a transformer design issue, and possibly a physical mounting issue leading to some type of resonant enclosure.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 28, 2017, 09:43:27 am
content: You guys aren't listening.Of course the sound is hum through the speakers. I was responding to DaJMasta, who said "since you said moving the speaker affected it ... shielding between the speaker and the rest of the unit". The speaker is far away and has nothing to do with it.As I've said a few times now, the transformer is gone.I'll try powering it up from my bench supply. That should tell me if the source of the buzz is the power source, or the stereo circuitry. It sounds bang on 60Hz so my money is on the supply.

author: Lucian84
author_rank: Newbie
timestamp: January 28, 2017, 09:48:28 am
content: I guess the transformer is poorly designed, cheap, and it vibrates on the chassis. Maybe replacing the transformer with a more powerful one and putting some silicon glue will fix the hum. It's strange that you still got the hum with the external PSU though.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 28, 2017, 08:49:11 pm
content: WTF.The transformer doesn't vibrate!

author: mzacharias
author_rank: Frequent Contributor
timestamp: January 29, 2017, 06:18:48 pm
content: Just replace the larger capacitors and you should be fine. If you have a scope and are familiar with using it to troubleshoot, observe the DC voltage at each leg of each larger cap with respect to ground (mostly the positive leads) and one of them will show a large, unfiltered ripple component. Replace that one. It's pretty obvious from looking at the pictures that the caps are "el cheapo" so it would be a good idea just to replace them all on general principles.

author: dan3460
author_rank: Frequent Contributor
timestamp: January 30, 2017, 01:42:22 pm
content: I think we all are tuning to different satiations. I'm a little confused. Can we reset?- Tell us what is the setup that you have right now. Seems to me that you have bypassed the power supply and powering directly from a bench power supply.- What kind of instrumentation do you have, can you determine the frequency of the hum? If you have a smart phone there are some apps that can pick up a sound and tell you the frequency. This is crucial to determine if the hum is power related of may be some kind of feedback.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 30, 2017, 10:33:48 pm
content: Sure. You can see a photo of the stereo in my first post. That's the main unit. The speakers are separate and connect via long leads to terminals on the side.As I said earlier, I opened up the main unit to investigate. I found a mains transformer mounted to the inside of the case, and fairly close to the amplifier board. I found that the hum would disappear when the mains transformer sat on my bench, away from everything else. Thus, I concluded that the audio signal was picking up interference from the transformer, so I removed it completely.The setup now:I am powering it with a cheap chinese laptop power supply (20V, 90W), which I have connected where the transformer used to be, ie rectifier input. (I put it here to utilise the diode drop to bring the voltage down to what I measured on the rectifier output in the factory setup, around 19V).

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 30, 2017, 10:43:04 pm
content: Here is some info on the buzz right now.I am attaching an MP3 file of the sound itself, and photo of the signal on my CRO.Vertical scale is 10mV/div. Timescale is 1ms/div making the period roughly 8ms, which equates to a frequency of 125Hz (probably actually 120). Though, If I listen to a generated 120Hz tone it sounds higher to me, but that could be the distortion in the waveform throwing it off.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: January 30, 2017, 11:00:59 pm
content: I'm a lot closer to the source of the problem now. I was able to get rid of the buzz completely. Here's what further investigation revealed... It gets much weirder!I think it's some kind of ground loop effect. It only happens a bunch of conditions are met. Sorry, it gets a bit complicated, but here goes...Condition #1:Stereo is plugged into a certain outlet. If I move it to a far away outlet, buzz disappears.Condition #2:I have a laptop, which I connect via DisplayPort to a monitor. Monitor takes digital audio signal from DP and converts to analog audio for output on a 3.5mm jack. When I connect this analog audio to the AUX input on the stereo (+Condition#1 is met) then I get buzz. Even if I connect the laptop headphone audio to the AUX input there is buzz, butonly while the external monitor is connected.Once you take the monitor out of the equation, laptop headphone audio is fine. For any other isolated audio device (ie smartphone), the audio is fine.Since this is a cheap chinese power supply, it may have some poor grounding design. In the stereo, it's a cheap 1-sided phenolic PCB, but the traces did look like they used star grounding. I might try probing around with a grounded lead to see if that makes any difference.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: January 31, 2017, 03:07:56 am
content: Connect a ground wire from every involved instrument to a single grounding point in a star configuration. All the grounds should meet at a single grounding point, not separated by a conductor of any length.Alternately you could try an isolator between the stereo and your computer.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 06, 2017, 07:44:53 am
content: I connected a naked speaker between the grounds on the two household circuits, and I got the same buzz. That's not supposed to happen right?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 06, 2017, 10:46:34 pm
content: Well sure enough ...When I plugged the stereo into the same outlet as the monitor ... complete silence. The hum/buzz completely disappears.It's not ideal, but it will get me going until for now. In the meantime, I'll have to look into where that mains earth noise is coming from.I think I'll test the old transformer again. It had no earth reference, but the neutral line goes back earth. If that works I'll go back to the old design.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: February 07, 2017, 02:03:50 am
content: Glad it is working.

author: CJay
author_rank: Super Contributor
timestamp: February 07, 2017, 09:37:10 am
content: Nasty, I've sat in an office of around 150 people and argued with the management that they should turn off and unplug all their PCs because there was obviously an electrical fault, it'd caused several PCs and monitors to go bang rather spectacularly. By the time they agreed with me they'd lost around 40 PCs and a similar number of monitors, a very expensive Avaya IP phone system, the UPS for the servers and phone system and managed to trash a disk array that, fortunately, was fully backed up but took a day and a half to restore (it was *big*).All caused by a fault which sounds similar to yours, a poor neutral connection which had started to arc in a conduit.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 07, 2017, 06:49:26 pm
content: Wow that does sound nasty. My problem is trivial by comparison. The current was barely enough to light up an LED.

author: CJay
author_rank: Super Contributor
timestamp: February 07, 2017, 09:04:36 pm
content: It was, with the exception of the Avaya, not actually expensive in hardware terms, a couple of thousand pounds to rectify, the major cost was downtime, a day until the building management got a generator in place and then two days to get their storage array up, running and restored.It did have the nice side effect for us of selling them a full disaster recovery plan with additional testing days.My point was that when you have a fault like this, it's probably best to find it before it becomes a real problem, could be as simple as a loose connection in a socket somewhere. .

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 11, 2017, 04:41:13 am
content: I tried the transformer again, and it still causes interference. It's just a bad design. So back to the DC power supply again, and it sounds great as long as I keep it on the same circuit as audio source. I thought I'd post some pics in case somebody wanted to make a similar mod ...Lenovo 20V 4.5A 90W power supply from eBay: $ 8 US (shipped from NJ)DC jack from eBay: $1 US

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 11, 2017, 04:49:38 am
content: There was a weird looking hole where the fm antenna came out (just a thin piece of wire). Since I never use FM, I enlarged the hole and mounted the jack into it. Two stiff bits of wire from some clip were used prevent the jack from getting pushed in. The slotted right into some grooves on the sides of the jack.Then I just routed some wires from the jack over to the power supply board.