unusual hum from MC2102

[TOGA]

Hi,
Today I noticed a very slight hum from the woofer of JBL L100
which connected to MC2102 tube amplifier.
I disconnected everything off the amp, the only cables that
remain attached to the amp were speaker cables. That was strange.
I measured 0.000 VAC from that speaker terminals while speakers
were still humming.
the level I heard was very low in quiet room, have to put ears within
a foot from woofer to hear that. But with amp switched off and unplugged ??

Any idea ?

[Charles]

Toga, how does the volume level affect the hum? I wouldn't be concerned unless as you increase the volume, the hum becomes audible. Audible hum at the sitting position is not good. Hum can be weird. I hope you get it figured out.

Do you have any hum in your reference system?

Best

Charles

[TOGA]

Originally Posted by Charles

Toga, how does the volume level affect the hum? I wouldn't be concerned unless as you increase the volume, the hum becomes audible. Audible hum at the sitting position is not good. Hum can be weird. I hope you get it figured out.

Do you have any hum in your reference system?

Best

Charles

Charles,
It was not VOL level dependent. This was nuts. It was not about MC2102 at all.
I tried to think, unplugged for many hours, all charge in capacitor would drained out , but it still hum!
I disconnected speaker cables from the amp side and it still hum.
I disconnect speaker cables from the speaker it still hum !
Only when I removed jumpers that the hum stopped.
The hum was heard from the woofers.
This mean the HF terminals were sending something down to the LF terminals and caused a hum, all by itself without external power source.
With the pictures attached, the L100 still hum in that configuration.
I will try to run Biwire where LF and HF terminal will meet electrically at the amp terminals.
IMG_5259.jpgIMG_5260.jpg

Toga

[TOGA]

Connecting true biwire did not solve it.
Whenever the HF and LF meets electrically, the hum started.
I am confused. This occurs on both sides of speakers.

I shorted the speaker’s red and black terminal, the hum stopped for as long as I keep them shorted. But when
Connected jumpers back in place, the hum started again.
It was like part of the crossover induced some EMI, RFI
And generated very small voltage. I will try rotate speakers to see if the crossover networks really worked as antenna.



Toga

[TOGA]

Rotating the speakers did not change the hum sound.

So the only way is the do the bi-amp ??
It is the only way to isolate HF and LF terminals. It s weird that I did not
Detect this before. I blamed the amp for a day, until I removed all cables and even tubes and the hum did not stop.

Toga

[cleeds]

Originally Posted by TOGA

... disconnected speaker cables from the amp side and it still hum.
I disconnect speaker cables from the speaker it still hum !
Only when I removed jumpers that the hum stopped.
The hum was heard from the woofers.
This mean the HF terminals were sending something down to the LF terminals and caused a hum, all by itself without external power source ...

Something here just does not seem right. We know from the first law of thermodynamics (conservation of energy) that energy can be neither created or destroyed. So whatever is causing the hum in your woofer must be created by energy elsewhere. Perhaps the caps in your amplifier can hold a charge long enough - that's not farfetched.

You say you measured 0 VAC - did you measure that across the speaker terminals? The hum may well be caused by DC voltage, so that's something you might also want to measure.

[TOGA]

Originally Posted by cleeds

Something here just does not seem right. We know from the first law of thermodynamics (conservation of energy) that energy can be neither created or destroyed. So whatever is causing the hum in your woofer must be created by energy elsewhere. Perhaps the caps in your amplifier can hold a charge long enough - that's not farfetched.



You say you measured 0 VAC - did you measure that across the speaker terminals? The hum may well be caused by DC voltage, so that's something you might also want to measure.

I did checked DC and AC. Result was 0.
But this was not the amp anymore. As you saw from pictures, the speaker alone hummed by itself.

Some JBL such as DD67000 has a 9V cell in its crossover.
I am not sure the L100 has it or not.
But I wrote to JBL . I will let you know when I get response,

Toga

[cleeds]

Originally Posted by TOGA

... Some JBL such as DD67000 has a 9V cell in its crossover.
I am not sure the L100 has it or not. But I wrote to JBL. I will let you know when I get response

That's interesting! I'd never heard of such a thing!
Just Googling around, it looks like the battery is used to bias caps in the DD67000 xover. So it's not difficult to imagine that a problem with it or the circuit could be your issue if the L100 xover is similar. So please do keep us posted on this puzzle!

[Stahle]

It sounds like this thread should be titled “unusual hum from JBL” because if I’m reading this correctly, the amplifier is not causing this. What a strange issue. Definitely keep us posted.

[W9TR]

Do you live near any high power transmitters? Ex. Cell tower, TV or Radio Station? Do you have a WiFi access point nearby? Throw the big breaker and power down your entire house. Hum still there?