F3 - Small humming noise, how to get rid of it

[Nicola]

The left channel of the F3 has a light humming. When the F3 runs without any RCA / input connected, it is dead silent on both channels. If I connect the active crossover, the left channel has a light humming. The right has none, dead silent. The active crossover allows to switch on / off the channel. If I switch off the channel where the F3 is connected, the humming disappears. If i swap the left and right input on the F3, the humming remains on the left. I also changed the cable thinking it was the culprit, but not. The crossover output voltage specs are 2.5v / 50ohms with a minimum load impedance of 600ohms. Would any of you be kind to advise me in which direction i could look. Many thanks

 

[Nelson Pass]

As usual, I have to ask the question - is this a commercial F3
or a DIY version?

 

[Nicola]

F3

It is #547, built by the master (i.e you)
[Best

 

[Nelson Pass]

In that case, it is very likely a ground loop with the RCA
input connectors, since the commercial version has a
regulated supply.

I would start by checking the noise for each channel with
only a shorting plug on the input, which will eliminate an
external loop.

 

[Nicola]

Humming - what tests were done

Thanks for the advise, this is exactly what I did
-the left channel has a slightly more audible humming than the right
- if I unplug the left RCA from the left input there is almost no humming, comparable to the right side
- if a shorten the left input the humming becomes slightly more audible than when not shortened
- i connected the left input channel into the right output channel of the xover the noise on the left channel is exactly the same (i suspected it could be the xover)
- i changes the connecting cable suspecting one could be faulty, but nothing
-whenever i connect / disconnect a RCA on the left channel there is a smal 'clac' in the loudspeaker, phenonenon which is not present if i do the same operation on the right channel. Nothing that would risk damaging the driver.
When everthing is silent, at night, i can here the light humming on the left channel. I've been trying to find out where it comes. Of couse i could easily putr an lpad on the left amp, about 4-5db

Regards

 

[Nelson Pass]

My records indicate that you have had this amp for about
3.51 years. Has it always done this, or is this a new noise?

 

[Nicola]

You're right. It had it from the beginning, but I thought it was normal and since the driver is a 106db you proposed me to address it with a lpad, which worked fine. Now with the arrival of the J2 and the F5 and your upgraded F2J i revisited the whole set up. I wanted to see how far I could lower the value of the Lpad. And this is how it came that the left channed was making slightly more noise than the right. And I started wondering why.... I can perfectly carry on with a lpad, although i would rather before see if there are other was to address it. Thanks.

 

[Nelson Pass]

The noise figures when they leave my bench are about
100 uV, and it is not unusual for one channel to have more
noise than the other, as the transformer field radiation is
somewhat random.

That said, 100 uV is still a very low noise figure. 105 dB
drivers will certainly bring that out. We could lower the
gain of the amp, easily done by inserting resistance in series
with the positive input. I would try 4.7K to 10K values,
which your tests indicate will take the noise down further.

Another option would be to isolate the grounds of the two inputs
by driving the input with an line level isolation transformer.

 

[Nicola]

F3 - light humming, solutions

Thanks, i guess I'll try first the resistance in serie with the left input. I guess i'll try with a variable one in order to determine the minimal value.

Since I'm by far not a knowledgeable in this field, i would need a bit more explanation for "Another option would be to isolate the grounds of the two inputs by driving the input with an line level isolation transformer."
Would that mean on the left input line a transformer would need to be inserted between the RCA input and the print ? Are there specifications for this line level isolation transformer ?

From a specifications perspective what would be the most prefered approach ?

Thanks

 

[larryho]

Hi, Nicola,

Just want you to know, although my F3 doesn't have the hum problem, My speaker is 104db. Install a new pair line level input transformer actually brings a sonic improvement. You can even do the balance input and totally noise free.

FYI.