Tracking down the source of amplifier hum.
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- Опубликовано: 26 окт 2018
- My technics amplifier has a very slight mains hum when the volume is set to minimum and listening through headphones.
This video is how the fault is tracked down. 
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Thank you very much for sharing this! I was working on a guitar amplifier with the same symptoms and nothing worked until I saw this video. I've tried adding filter caps, adding an improvised shield, etc. After watching this video, I started experimenting with extra points from the PCBs to GND using alligator clips. I found some points that needed better GND. I soldered some wires there and now the amplifier is much quieter.
i guess I am kinda off topic but do anyone know a good website to watch newly released movies online?
@Izaiah Johnny I watch on flixzone. Just google for it =)
@Patrick Angelo Definitely, I have been using flixzone for since april myself :)
@Patrick Angelo thank you, signed up and it seems like they got a lot of movies there :) I really appreciate it !
@Izaiah Johnny You are welcome :)
I recently restored an old 8-track player from the early 70's. Everything was working nicely apart from a 120Hz hum, I couldn't get rid of, no matter what i did, until, by chance, I came across Your video. So I disconnected the ground wire that came from the volume and balance pot (ground went from balance to volume and then to a ground point right next to where the tapehead had its groundpoint, going to a star-groundpoint, using a pcb trace) and connected it directly to chassis ground... The hum was finally and totally gone! The 8-track player, just being turned on, without a cartridge inside to mask the hum, was so quiet, I at first thought I plugged things in wrong *lol* Yay! Finally! Thanks! :D
Thank you for letting me know. Glad it worked.
In the 70s I worked for Cambridge Audio on the P40/50 we had a few customers complaining of hum but only on phones. I spent many hours trying to track this down. In the end I found that some kinds of headphone jacks were slightly larger than others and it would short the body of the jack to ground making an earth loop. On later production the hole was simply made larger to stop this happening. Funny the stuff you remember!!
Hats off to your troubleshooting abilities. Thanks so much for sharing.
Yabadabadoo!
Hi. This was the suggestion I need the day I needed it. Thank you for sharing. I learned a lot.
This is where an old school signal tracer would be invaluable! With a simple 100hz low pass filter in series with the input of the probe on the signal tracer would show up the 50hz hum wherever it is hiding! Just connect the oscilloscope to the output of the signal tracer to see the 50hz hum, Simples! If you don't have a signal tracer on hand, just use the phono input from another amplifier and diy a probe. There's plenty videos about DIY signal tracers and probes on RUclips... With a good signal tracer, you can hear how some resistors are just noisy due to internal arching. It's fascinating what you can hear with a high gain amplifier! I have seen a video on a channel called Kreosan, where he used the cassette head from a Walkman as a probe! By simply removing the head, he was able to hear both the digital and analog signals from a cd Walkman without having to open it! That's pretty amazing... So yeah! A signal tracer is indispensable...
make a carlson super probe
The LP140 tt comes with the tonearm shield grounded to mains earth, if used with earthed amplifier with ground lead, loud hum, no ground lead no hum, I got in there and separated the audio ground from chassis in the turntable and have the ground lead do it's assigned job- tonearm grounded to phono stage
Thanks for this video Simon. I know it was posted 18 months ago, but I have a similar problem with a Sansui A80 amp that I just replaced the power supply caps on. However the hum here is volume dependent so here goes...into the vortex.
check your filter caps are in series and not in parallel, some people make this mistake when refitting and forget to check the orientation. Also check to see if any screws that might ground certain circuit boards to chassis ground might be missing.
Thank you for this - it's helpful.
Hi Simon, an unusual problem, well done for tracking it down :) I need to read up on earth loops. Do you have a new camera? The audio was quite quiet.
Very helpful. Thank you.
I will have to watch this in a few hours, I will be live streaming in 45 minutes and have to get ready for that.
Vary the input AC to the amp with a Variac as you look at the scope to see if the ripple changes, this will tell you if the amp has good voltage regulation or not. Then add additional parallel capacitance and see if the power supply becomes more stable with low or varying input AC boltage.
You're using a pair of red & black hook clips to the BNC connector on your scope it looks like. I'd find that more convenient myself than the actual probe for working on stuff like this. Did you have to make those up yourself? With the clips, some coaxial cable, a BNC connector and something like a 1M resistor?
I think i have a similar problem on a Cloud cv 1000. i am trying to repair my last 2x cv 1000 so i an sell them to a friend. Still two boards to fix. One seems to work fine other than a nasty hum, thinking it's a bad ground loop somehow. looks like a sawtooth wave on the speaker output sitting high around 0.4V.
That's interesting how you can add or subtract the hum Simon. Had you thought of increasing the headphone attenuation as I guess 8 ohms cans used to be more common than it is now.
Yes that did cross my mind as the attenuation in the headphone socket does seem to pass a high level.
Another possibility for causing residual hum (not necessarily in this particular unit) is that somewhere in the unit are a pair of mismatched +/- supplies where just one side of the supply has an additional current-supplying function such as powering a relay, or a pilot lamp ( or two or three or four), or a circuit that only needs the plus + supply, maybe +12 or +18 or whatever. A phono preamp, for instance, might not use a minus ( - ) supply....even a stereo indicator lamp in a tuner or receiver might also unbalance the supply a little. Anyway, you might encounter a situation where, say, a +/- 18 volt supply is actually +18/-17.2 or something like that. Another clue could be that, when you shut the unit off and check the stored filter cap voltages to see if the caps need to be drained before doing any parts-changing, one half of a +/- supply might be several volts higher than the other because some additional, tacked-on circuit is upsetting the balance of the complimentary supplies.
Yes good point.
this is a very helpful comment for my problem with one of my amps , thank you .
Nice bit of detective work Simon. A hum can be very annoying
I have the matching ST3500, which I have just tweaked to get the stereo to work, as it was only mono output.
Yes I have one too. A great duo.
Cheers, I have simillar issues with an old Sony str receiver - volume pot going to pcb. If I understood right you haven't found the reason for the different potential, we are just guessing that power supply circuit is not perfect. You just reduced the magnitude of the loop, by using the chassis as small resistor.
Howdy. Nice work.
One point, however. Never just crimp Abiko cable shoes. Always solder too after crimping. Will save one a lot of headaches through the years.
Regards.
I'm a fan of 10R series ground resistors for low current signal input returns. Minimal attenuation, ground loop gone.
Interesting idea.
Yep, this is how I was taught to do it too... signal ground should always be separate from power ground, via a 10 ohm resistor bypassed by 100nF for RF coupling. Dead quiet on everything i've used that technique on.
@@jaycee1980 , some Adcom and NAD amps isolate the signal ground from power supply ground, and power supply ground from the chassis, with resistors, 10 ohms for one and 100 ohms for the other, though I can't remember which is which. We've seen the resistors go "open" from ground loops or voltage surges, which is better than having to repair burned foil traces.
This is way above my knowledge lmao
Can I ask a question? Was your ground loop hum static/fixed or did its volume increase and decrease with the volume of the unit?
Yes the hum was drastically reduced. The hum was unaffected be the volume control.
@@SoddingaboutSi Hey thanks so much. That is bad news for me as I was hoping my hum was related to my outlet. Looks like it could be internal as well.
I have access to a scope but don't know how to use it... maybe I'll visit a pro. ha.
@8:00 I can see the frequency is 50Hz, not 100Hz, that made me wonder if it has dropped a rectifier diode, this would double the ripple.
That's a good point Scott! I might have to double check that.
So after the rectifier we see 100hz hum? I never thought of it like that but obviously it's folded in half so doubles.
Can you repair my Vestax sound mixer?
Did you have problems posting this video? When I xlixked on it a couple of times earlier YT said it had a problem with it. Now seems ok.
No it seemed to upload fine.
My workshop is next to the bedroom, I sometimes here a strange humming or vibration coming from next door when my wife’s in. I don’t know what going on, perhaps you could come round and take a look for me.
Well of course Simon you know why the amp was humming to begin with right? Because it doesn't know the words! So sorry, just couldn't resist! ;-)
Hhah Dino! Well it's knows the words now because it's stopped humming along!
My Rotel amp just started humming. Most likely it's the caps, eh?
Check the input connections first. If it's caps it's usually takes time to get louder and louder
Check your audio input cables for bad condition and check they are plugged all the way in then also check for bad solder joints where the phono plugs solder to the board. It might be some thing simple.
can I get you to fix my microphone preamp it have some hum in it to.
Pm me
@@SoddingaboutSi it's bellari rp-220 it sounds good but that hum want go away, I don't what Pm stand for but I will pay you. do you have a email
Connect it's chassis or ground lead to your amplifiers chassis.
Are you in the UK
I am indeed