Line Magnetic LM-216 -- Annoying Hum

[mrkaic]

Colleagues, maybe you can help me with a noisy amplifier.

I bought a Line Magnetic LM-216 integrated amp. Overall, I like it, but it is quite noisy with an audible 60 Hz hum. Measurements show around 3 mV rms 60 Hz hum. It is also clearly visible when I measure THD+N -- a strong spike at 60- Hz.

A funny thing is that the noise goes down if I increase the volume (with no inputs) to about 3/4 of the max and then increases again.

I bought an isolation transformer, but that did not help. I replaced all the tubes, but that did not help either.

Could someone please tell me what to do with the amp? Many thanks.

MM

 

[BinaryMike]

Could be hum pickup on the signal input wiring, possibly from the heater power circuit. Post pictures of the wiring and a complete schematic.

 

[nigelwright7557]

A funny thing is that the noise goes down if I increase the volume (with no inputs) to about 3/4 of the max and then increases again.


MM

Sometimes hum on the input can cancel hum in the pre amp itself.

60Hz hum is often heater related unlike power supply hum which is usually 120Hz as it is rectified.
I would check heater wire layout.
Make sure no heater wires are close to the valve besides direct heater connections.

Sometimes magnetic fields other equipment above or below the pre amp can migrate into the pre amp.

 

[rsheptak]

Check your line level sources. When I first plugged my LM-216 in, I heard a hum, too, but quickly was able to isolate it to one of the line level devices I had connected to it (a Tivo Bolt). Unplugging it cleared up all issues and my Line Magnetic 216 is quiet, with no hum.

 

[mrkaic]

Check your line level sources. When I first plugged my LM-216 in, I heard a hum, too, but quickly was able to isolate it to one of the line level devices I had connected to it (a Tivo Bolt). Unplugging it cleared up all issues and my Line Magnetic 216 is quiet, with no hum.

I tried the same approach and unplugged everything to see if input devices were to blame. But the hum was still there, even with all inputs unplugged.

 

[mrkaic]

Sometimes hum on the input can cancel hum in the pre amp itself.

60Hz hum is often heater related unlike power supply hum which is usually 120Hz as it is rectified.
I would check heater wire layout.
Make sure no heater wires are close to the valve besides direct heater connections.

Sometimes magnetic fields other equipment above or below the pre amp can migrate into the pre amp.

Thank you, will check the heater wires.

 

[DF96]

Sometimes hum on the input can cancel hum in the pre amp itself.

Yes. More commonly, people report hum which has a broad peak at mid-volume settings. This means that hum is getting in to the grid circuit immediately following the volume pot, probably due to stray capacitance from a heater circuit. In your case, it seems this hum is partly cancelling hum from elsewhere.

Sadly, you appear to have two hum problems. Fixing one may make the hum worse until you fix the other one too!

 

[mrkaic]

Sadly, you appear to have two hum problems. Fixing one may make the hum worse until you fix the other one too!

Thank you for the clarification.

In addition to the hum the amp also exhibits very pronounced output transformer high frequency oscillations -- a square wave overshoots by 26%!! Please see the attached picture.

It appears I bought a bad amplifier. 🙁