Author Topic: two humming audio amps mystery (solved)  (Read 5567 times)

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Offline g.costanza

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two humming audio amps mystery (solved)
« on: September 23, 2017, 04:32:02 pm »
UPDATE...
I finally broke down and hacked an extension cord and reversed the wires. I plugged the subs' power cords into the reversed polarity extension cord. Result = the voltage coming off of the subwoofer plate amps went from 118vac to 1vac and no more hum! The wall outlet IS WIRED CORRECTLY. I even went out earlier and bought one of those fluke non contact voltage testers. That, one of those cheap 3-light outlet testers, and a DMM showed outlet was wired correctly. I also measured the voltage coming off of the chassis of the denon in both powered polarities and a Sony receiver I had lying around. The Denon measured around 50-60vac in both polarity configurations. Oh well :( . The Sony was 1vac in factory polarity and 70vac in reversed polarity. I'm now curious what other Denon x3300 receivers measure???
Thanks everyone for your help!


ORIGINAL POST>>>

Wondered if u guys could help with these humming subwoofer amplifiers that apparently doesn't know the words
I'll stick to the facts I know...

I bought two new JAMO powered J110 subwoofers that are near clones of the Klipsch R110/112/115 family. Same plate amps between JAMO and Klipsch versions depending on woofer size. They have 2 prong power plugs.

Denon Receiver has a 3 prong power plug.

I previously had a pair of DIY subwoofers on a single parts express BASH amp (3 prong plug) in the same spot as these new subs. Zero noise issues.

Using same single RCA coax cable connection on new subs as old setup.

JAMO subs do not hum until I connect the RCA cable going to the Denon.

JAMO subs also do not hum if I connect the headphone out jack of my iphone using a 3.5mm to RCA plug adapter.

I moved Denon and connected it to same wall outlet as the subs. No help. Still Hums.

I put a ground loop isolator on the RCA audio connection. No help. Still hums.

I tried a different RCA cable. Still hums.

I don't have "cable TV", so that can't b a hum source.

I used a FLUKE meter to check wall outlet. I get 120v from narrow HOT slot to wide slot, ground hole, and center screw. Normal.
I get 0v from wide COMMON/NEUTRAL slot to ground hole and center screw. Normal.

I used continuity setting of FLUKE and got "tone" between outer/ground ring of sub amp's RCA audio inputs JACKS to the screws, etc on the EXPOSED metal plate amps on subs.

I was then surprised to find that if I touched the screws, jacks, etc of the metal plate amp with a probe of the fluke on V~ setting, and the center ground screw on the wall outlet, I got 118v. Is that Normal? Touching the metal plate amp doesn't result in a shock. I didn't try touching the center wall outlet screw with my other hand, lol. Not sure what would happen.

The hum does increase or decrease depending on subwoofer amp volume knob setting, if that matters. When the RCA plugs are removed from the amps, I get no noise, regardless of volume setting.

Any ideas?
« Last Edit: September 27, 2017, 10:02:39 pm by g.costanza »
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #1 on: September 23, 2017, 04:45:04 pm »
Plug your RCA into the amp and then short with a copper wire, the other end of the Pin and the shell. Does the hum disappear? Wriggle the RCA cable about, does the hum appear?
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #2 on: September 23, 2017, 04:58:18 pm »
Just to b clear, plug rca cable into sub amp. On other end of cable, connect center pin and outer ring together? I do know already if i touch the tip of the end of the rca cable plugged into sub amp, I get a very very loud hum.
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #3 on: September 23, 2017, 05:05:52 pm »
Just to b clear, plug rca cable into sub amp. On other end of cable, connect center pin and outer ring together? I do know already if i touch the tip of the end of the rca cable plugged into sub amp, I get a very very loud hum.

Yes, connect center pin to outer ring together with a jumper cable. The purpose is to detect the location of the problem. Do it I will tell you.
You hand is a signal source my friend, induced from environment static, RF etc... Please don't touch with your hand you may damage your speakers.
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #4 on: September 23, 2017, 05:08:36 pm »
Ok. What u r suggesting is confusing me. I stated earlier that I get no hum from the sub amp at all unless the RCA is connected to the receiver. I do get a very very loud hum if I place my finger on the tip of the rca cable that is no longer connected to the receiver. If I slip my finger down and also touch the outer ring as well as the tip, the very very loud hum is not as loud.
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #5 on: September 23, 2017, 05:12:09 pm »
I will try the wire method and report back :)
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #6 on: September 23, 2017, 05:14:38 pm »
I will try the wire method and report back :)

Don't forget to wriggle the cable [while the jumper wire is connected] to detect hidden breakage. Hope the cable is good.
« Last Edit: September 23, 2017, 05:16:39 pm by Armadillo »
 

Offline technogeeky

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Re: two humming audio amps mystery
« Reply #7 on: September 23, 2017, 05:21:01 pm »
Detecting something like ~115 volts on the case or mounting hardware of a subwoofer may or may not be indicative of a problem. If the voltage goes away when you disconnect all of the audio inputs (in other words, a 115 volt but very low current signal is being carried along the audio wire), this is certainly a problem.

It would be a huge help to have an oscilloscope to make more definitive statement about the source of the hum (although you might be able to use the frequency detection of a multimeter).

I just repaired two different subwoofers. In my case, both had hum from mains interference. Both were ultimately caused by bad soldering joints.

One of them, on a very old Bose 3-2-1 subwoofer, was on the amplifier's power/ground pins where they connect to the PCB. Mechanical stress due to the amplifier chip being clipped to a large heatsink with the pins soldered to a PCB broke them free. Then, either the noise was capacitively or inductively coupled into the amplifier.

On the other subwoofer, the Phase 0° - 180° switch was found to cause the hum. There were no signs of obviously bad soldering, but when rubbing the solder pins with a plastic probe, it was easy to cause a signal which had large 60hz spikes and 20-30mV p-p of other noise.

I only mention this because there are a lot of mechanical ways for a subwoofer to damage itself, so it's important to keep an eye out for this. Having said that: for me, in both cases the hum was not increased with higher subwoofer gain (unlike your case). The hum was present with no signal input at all (unlike your case). It was a 120Hz (rectified AC, plus harmonics) hum for the Bose, and a 60Hz hum for the other subwoofer (with no harmonics).

Three facts in your case lead me to believe the problem can't be with the subwoofers:

  • the hum does change volume with subwoofer amplifier gain
  • the hum is absent at all gain levels when no input signal is present
  • both subwoofers are new (are they?, and they both have this problem

To me, and assuming that wiggling the RCA cable and connectors around does not cause the hum to appear or disappear (meaning a mechanical->electrical problem at the inputs of the subwoofer), then the problem is surely with the receiver. The primary counter-evidence to this is that your older, 3-prong-plugged subwoofers did not have this problem. Again, to me, this just implies that your old subwoofers hid this problem by tying both devices to the same ground reference.

Some tests I would run:

  • With only power and nothing else connected to the receiver, powered on, with the volume as low as possible but not zero/muted, check the audio output (RCA jack?) of various outputs (left/right analog audio, subwoofer pre-out, etc) for voltage and check for frequency with the multimeter. If there is no signal into the receiver and you see 50/60Hz (or 100/120Hz) signal output, then you can confirm it's the receiver. Where exactly and how? Who knows.
  • If the above multimeter frequency check fails, increase the volume little by little until you either see a 50/60/100/120 Hz detection or no detection and you are sure it's loud enough to have heard it.
  • If that check fails, hook up a function generator or a computer and generate a ~ 70 or 80 Hz tone, and make sure you can detect this with the frequency and possibly voltage settings of the multimeter.

If the problem does prove to be the receiver, it's probably going to be difficult to diagnose and fix without an oscilloscope. If you think it is the receiver, please tell us the model number.
« Last Edit: September 23, 2017, 05:23:59 pm by technogeeky »
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #8 on: September 23, 2017, 05:26:05 pm »
OK.

With nothing plugged into sub I get zero hum at all sub amp volume levels.

I use a small 6" Y adapter to connect single rca to both left and right sub rca inputs. Plugging only it in to sub rca inputs, I can get the very slightest hum with sub amp volume at max. This is with my ear up to woofer cone in a dead silent room.

If I then attach the longer rca cable that would normally go to the receiver, but is left unconnected, the hum is ever so slightly louder at full sub volume, but still very low and nowhere near as loud as when the cable is connected to the denon audio receiver.

If I attach a jumper between the center pin and outer ring on the loose end of the rca cable, I'm back to having zero hum with sub amp volume at maximum.
« Last Edit: September 23, 2017, 05:27:43 pm by g.costanza »
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #9 on: September 23, 2017, 05:28:39 pm »
OK.

With nothing plugged into sub I get zero hum at all sub amp volume levels.

I use a small 6" Y adapter to connect single rca to both left and right sub rca inputs. Plugging only it in to sub rca inputs, I can get the very slightest hum with sub amp volume at max. This is with my ear up to woofer cone in a dead silent room.

If I then attach the longer rca cable that would normally go to the receiver, but is left unconnected, the hum is ever so slightly louder at full sub volume, but still very low and nowhere near as loud as when the cable is connected to the denon audio receiver.

If I attach a jumper between the center pin and outer ring on the loose end of the rca cable, I'm back to having zero hum with sub amp volume at maximum.

And you forgotten to wriggle that cable while the ends is connected. !
 

Offline technogeeky

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Re: two humming audio amps mystery
« Reply #10 on: September 23, 2017, 05:28:59 pm »
OK.

With nothing plugged into sub I get zero hum at all sub amp volume levels.

I use a small 6" Y adapter to connect single rca to both left and right sub rca inputs. Plugging only it in to sub rca inputs, I can get the very slightest hum with sub amp volume at max. This is with my ear up to woofer cone in a dead silent room.

If I then attach the longer rca cable that would normally go to the receiver, but is left unconnected, the hum is ever so slightly louder at full sub volume, but still very low and nowhere near as loud as when the cable is connected to the denon audio receiver.

If I attach a jumper between the center pin and outer ring on the loose end of the rca cable, I'm back to having zero hum with sub amp volume at maximum.

All of these sound like the noise floor of the amplifier in the subwoofer. Everything listed in this particular reply is normal.
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #11 on: September 23, 2017, 05:44:10 pm »
Ok.
I get 120v on fluke meter when 1 probe is on plate amp and 2nd probe is on center screw on wall outlet when sub is powered on and with no other connections on the subwoofer amp.

I placed the fluke meter on the end of the rca that is connected to the receiver's subwoofer output that I'm getting hum from when connected to the subs. One probe clipped to center pin. One probe clipped to outer ring. I left all the sources connected to the receiver. I measured 0vdc and 0vac on the meter even if denon volume was turned up to max. I then removed the rca attached to the subwoofer output of the receiver. I switched fluke to continuity and shorted the receiver end of the rca cable and got a tone. This was just to make sure that the ends of the fluke probe attached to the rca cable plug weren't touching in the previous test.
« Last Edit: September 23, 2017, 05:51:08 pm by g.costanza »
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #12 on: September 23, 2017, 05:45:29 pm »
Sorry . Nothing changes from moving the wire. I've tried a second rca cable going to receiver from sub amp. Both cause hum
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #13 on: September 23, 2017, 05:50:54 pm »

JAMO subs also do not hum if I connect the headphone out jack of my iphone using a 3.5mm to RCA plug adapter.


iphone no hum. So we suspect easy problem first [other than ground loop hum]. The contacts of your RCA or the receiver is poor thereby changing the designed impedance of the input.  I don't know how bad is the rust or the mechanical spring contact of the connector. So you try to use contact cleaner to clean it first. Swop other music source as input source is a alternative idea.    ;D
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #14 on: September 23, 2017, 05:58:28 pm »
the denon receiver is 3 months old. i had no hum issue with older sub setup that i took down to install this new setup. the only difference with new subs is that they are not 3 prong powered. and i find it odd that i get 118v from the exposed plate amp "chassis", but that's y i came here to get yall's advice :)
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #15 on: September 23, 2017, 06:03:44 pm »
Is it safe to connect a wire from the plate amps exposed chassis (one of the many screws, for example) to the center screw of the wall power outlet to see if that eliminates the hum?
Remember, this sub plate amp has a 2 prong power plug. There's no 3rd grounding pin.
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #16 on: September 23, 2017, 06:05:43 pm »
the denon receiver is 3 months old. i had no hum issue with older sub setup that i took down to install this new setup. the only difference with new subs is that they are not 3 prong powered. and i find it odd that i get 118v from the exposed plate amp "chassis", but that's y i came here to get yall's advice :)

You should simply use a "test pen" to touch the chassis than using the fluke to touch centre pin of the outlet cause in some places, the earth and the neutral are connected. Also try to connect the 2 prong 180 degree around, to see if this occurrence disappear.
 

Offline technogeeky

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Re: two humming audio amps mystery
« Reply #17 on: September 23, 2017, 06:09:24 pm »
Ok.
I get 120v on fluke meter when 1 probe is on plate amp and 2nd probe is on center screw on wall outlet when sub is powered on and with no other connections on the subwoofer amp.

I placed the fluke meter on the end of the rca that is connected to the receiver's subwoofer output that I'm getting hum from when connected to the subs. One probe clipped to center pin. One probe clipped to outer ring. I left all the sources connected to the receiver. I measured 0vdc and 0vac on the meter even if denon volume was turned up to max. I then removed the rca attached to the subwoofer output of the receiver. I switched fluke to continuity and shorted the receiver end of the rca cable and got a tone. This was just to make sure that the ends of the fluke probe attached to the rca cable plug weren't touching in the previous test.

Some quick questions just to fill in information:

1. What Fluke multimeter do you have?
2. What Denon amplifier do you have?

A multimeter will be able to pick up even a tiny 120v leakage current. So being able to see 120v by touching the chassis of the subwoofer and the ground pin of a socket (through the screw) is no big deal. We know the subwoofer isn't fully grounded (no 3rd pin). This is, overall, probably a red herring.

I messed up my advice a little. It's possible (plausible) that the receiver will shutdown the output of any RCA jack that it senses is unconnected. Evidently the way these line-level systems work is:

The output impedance of the LINE OUT audio source (Denon) is usually somewhere in the 100-1000 Ohm range. The input impedance of the LINE IN audio sink (subwoofer) is usually somewhere in the 10k Ohm and up range. It's possible the receiver tries to sense this and turns off the output. Your multimeter's input impedance is probably 1M Ohm, which is so high that this sense process may fail, generating no reading.


So the test procedure would be:

1. Find out the input impedance of the subwoofer's audio input (use the resistance mode of the multimeter; connect to the center and outer pin of the input RCA jack; you may or may not need for the subwoofer to be powered; you may or may not get a sensible reading anyway; if this fails assume 10k)
2. Sacrafice a crappy RCA cable and solder a resistor of the above value (10k or whatever) between the inner and outer conductor of the cable.
3. Plug this into the Subwoofer Pre-Amp LINE OUT on the Denon
4. Take the measurement across this resistor. (Voltage is interesting to note, but really I would want a frequency measurement to see if there is 60 or 120 hz on this line)

 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #18 on: September 23, 2017, 06:16:27 pm »
The subwoofer 2 prong plug is polarized. Won't go in the other way. Will I damage something If I used a "cheater plug" on the subwoofer power plug that I can put in the outlet "backwards"?
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #19 on: September 23, 2017, 06:18:58 pm »
Meter is a fluke 115. Denon x3300w

Can't I just measure the output of the subwoofer rca from the receiver while it is playing something to see if it is muted or not?
 

Offline technogeeky

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Re: two humming audio amps mystery
« Reply #20 on: September 23, 2017, 06:20:41 pm »
Is it safe to connect a wire from the plate amps exposed chassis (one of the many screws, for example) to the center screw of the wall power outlet to see if that eliminates the hum?
Remember, this sub plate amp has a 2 prong power plug. There's no 3rd grounding pin.

There are two options:

1. Temporarily defeat (to test, be careful, do not touch the metal chassis of the Denon) the ground of the receiver by using a ground separating plug or taping up the  ground pin with thin but electrically insulating tape e.g. kapton.

2. Try and tie down the chassis of the chassis of the subwoofer to ground, as you have suggested. I don't think we can say for sure if this is safe or not. It should be safe for you, but if the subwoofer's power plug is unpolarized (e.g. you can reverse it), this has a 50/50 chance of being very bad for the subwoofer (and tripping the breaker). If it's polarized, then assuming it's designed correctly it should be fine.

 

Offline technogeeky

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Re: two humming audio amps mystery
« Reply #21 on: September 23, 2017, 06:23:18 pm »
Meter is a fluke 115. Denon x3300w

Can't I just measure the output of the subwoofer rca from the receiver while it is playing something to see if it is muted or not?

Yep!

Your meter also has a LoZ impedance option. You can just do the same measurement (both with and without music playing) with that setting, as the input impedance of the multimeter is about the same as the subwoofer should be (about 10k Ohms).

Your meter also has a frequency setting, so if there is indeed hum you should be able to see its frequency.
 

Offline technogeeky

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Re: two humming audio amps mystery
« Reply #22 on: September 23, 2017, 06:25:01 pm »
The subwoofer 2 prong plug is polarized. Won't go in the other way. Will I damage something If I used a "cheater plug" on the subwoofer power plug that I can put in the outlet "backwards"?

If you have a cheater plug, use it on the receiver instead (do not touch the metal chassis of the receiver, in case it is really malfunctioning).

There is no need to defeat the polarization of the subwoofer, and it can only lead to bad things.
 

Offline Armadillo

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Re: two humming audio amps mystery
« Reply #23 on: September 23, 2017, 06:29:13 pm »
The subwoofer 2 prong plug is polarized. Won't go in the other way. Will I damage something If I used a "cheater plug" on the subwoofer power plug that I can put in the outlet "backwards"?

Better to try cause I heard some places get the live and neutral the wrong way around unless you have the socket outlet tester.
 

Offline g.costanza

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Re: two humming audio amps mystery
« Reply #24 on: September 23, 2017, 06:36:15 pm »
I have to go to work now, but I used the continuity setting of the fluke on the ground hole and the wider NEUTRAL slot on the wall power outlet, and got a tone, which implies they r connected.

So the 2 things I want to try to test to see if the hum goes away are:
1 - Connect plate amp chassis to center screw of wall outlet with a jumper wire
2 - Reverse the power plug of the amp using a cheater-plug adapter that has the wider pin shaved down so it will go in backwards
 


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