Tube Amps and EMI/RFI(?) Noise

Tube amps can be vulnerable to external noises and they can be difficult to sort out.

A few years ago, I had an open tube amp which picked up odd noises that even varied in intensity. I moved it around the room and the intensity changed but could find nowhere where it completely disappeared.

Then I took it upstairs to the bedroom and the noise had gone. Only in one place!! Ultimately, I put a cover over the tube to help, but the source of the noise turned out to be a synthesiser that was plugged in and left on in another room. It was connected to other devices via midi leads and boy did it generate noise!!!

At least if it's coming from the power supply, it is a bit more controllable, but airborne noise is a pain and an open tube amp can be pretty good at picking it up.

Funny thing is that my later amps didn't pick noise up from my synth. Bother my Ember and Polaris remained quiet.

They do reject noise well, but the nature of tube amps; especially in an open frame could mean noise. The foil cover would help decide if it's airborn or mains born. Frans is good at getting cures. He might well have some ideas.

I'm not sure what a Homeplug is. When I got the noise, I was using a plug with a mains filter.

Tubes have rather large plates (anodes) that stick out like antenna's. These antenna's are great at picking up high frequency signals. Tubes also are great at picking up magentic fields. CRT TV's use magnetic fields to 'bend' the electron beam and this is what magnetic fields of nearby transformers will do hear as well. They will modulate the signal with 50/60Hz.

Old radios also have lots of tubes stciking out but nobodgy complained because there were no switching regulators, no cell phones and no wifi...

Also these all tube devices used high voltages. A higher voltage = higher current and higher signal voltages. Small signals induced thus gets 'drowned' by the large voltage swing.
So these high voltage devices thus are reasonably insensitive.

The problem with tubes on lower voltages is that the currents and signal voltages are very small. This means very high impedance circuits have to be used. That high sensitive circuit is connected to the plate.
The part you can see looking at the tube. That part thus is the antenna on the input of a very sensitive part (input of the following opamp) which thus easily picks up all kinds of Electro Magnetic fields.
The EM part of EMC.

A higher tube voltage will also give higher currents and thus the circuit's impedance will drop and thus will be somewhat less sensitive. As this amp runs on 12V it is very sensitive. 24V or 48V will be better in this aspect.
Also it depends on how the PCB was designed and whether or not counter measures against RFI are used.
This PCB has no counter res at all.
The used opamp's input transistors will act as 'rectifiers' = 'RF detectors' for any signal above 0.5V.
This rectified signal is what you hear.
These designs are NEVER tested for EMC so they will most likely NOT pass any EMC RFI tests either.


When you are using a small transformer based wall wart (the heavier ones) the magnetic field of those may induce hum.
When using those lightweight SMPS power supplies close by these will certainly emit RF (wideband) noise.
And even though most of these devices MAY actually comply to EMI/RFI criterea UNDER TEST CONDITIONS, these may emit MORE when not connected 'optimally'.
A filter in front of such a device may actually INCREASE airborn signals but could decrease emitted signals INTO the mains.
Another filter may well lower emitted RFI noise depending on HOW it is constructed and what mains it is connected to.

Also any given device may well be sensitive or insensitive to say a GSM signal but may or may not be sensitive to other frequency bands.
(Cheap) SMPS often have a very wide RFI spectrum that may exceed

IMPOSSIBLE to predict in each situation IF or not any 'filter device' will have the desired effect... the effect desired by the device(s) connected that is.

So the short answer is ... yes, possibly RFI related.

Solution: Either screen the entire amp in a sealed and grounded metal enclosure (pereferably tin metal/steel, not aluminium) or keep it away from sources that emit RF signals.



See above explanations.




Combining filters MAY or may not be beneficial, it may even make RF emission worse.
NO way to tell unless one measures it (with specialist and VERY expensive EMI receivers and antennae in special rooms.

Mains filters are designed to attenuate signals present on the mains.
This may mean that RFI (signals emitted through the air, like radio) may actually become worse but may also become less.
This depends on the SMPS build and filter build in each case.

A bit of voodoo...

Darkarn, perhaps the Polaris might be a better option? No tube. Finding noise sources can be tricky and tubes are very prone to interference. Let alone higher noise floor and sometimes a bit hissy. If you have doubts about noise, I'd go ss rather than tubes personally. Or else you may end up with a juggling match to get noise down.

That's a bit odd Gordon. Neighbours got a new electrical present? I had a really hard time with the Sunrise a while back now and I never fully understood what was going on with it. The synth caused really bad problems, plus other gear in the room so that all went off. I was then left with a low level hum type noise which altered with the proximity of my hand or where the amp was placed which kind of fixed itself for some reason.

Never got a thing with the Horzon, which I loved. Then moved on to Ember and Polaris. The Ember is noticeably more noisy than the Polaris and I don't use anything with low impedance on it. The Polaris is fine for more sensitive headphones.

Ah. I thought so. Low impedance Gordon. Best not to use them without an attenuator. Jeremy is making a lovely one if you want to use low impedance with something like the Ember, but you probably already know.

I might get one myself eventually for an MSR7. Sounds delicious from the Polaris but too much power there for it really. The Polaris does seem to be a good match soundwise. I'll have to save up being tight arse old pensioner now!!

I found it way more obvious with low impedance on the Sunrise.

The Senns are extremely good at picking out mains hum but not so good with hisses and crackles. The msr7 picks the hiss with ease on the Ember. (Probably also my cheap tubes)

Thanks for the explanation! Took me a while to read up and look around a bit more, and it made sense. Let's see if my understanding is right:



1. My tube amp is by design more sensitive to such noise due to its low voltage (12V) and the lack of countermeasures unlike more expensive/better designed amps

2. The power supply that I am using may not have the required components that can help negate such noises

3. The Belkin Mastercube happen to have a filter that helped decrease emitted signals from the mains that are created by the HomePlug, but also did not increase airborne signals, thus leading to an overall improvement of sound but not solving the problem entirely.




Thanks for the suggestion, I have heard good reviews of the Polaris too! Just that I prefer tube amps for their general sound signature, especially after listening to a modded (with the help of solderdude!) Bravo V2 years ago (too bad it died due to my carelessness); I want to see if I can solve this since I have already pinpointed the source of the noise which is the Homeplugs. Also, I have some 12AU7 tubes (look like RCA 60's/70's grey plate) that came with the Bravo V2 that I want to make use of too!

Let me think this through a bit... (I am having a hard time deciding between Sunrise, Horizon and Ember... They are all that good to begin with!)




Thanks Gordon and Ian, I didn't realise that low impedance can be a factor too. I forgot to mention that the noise is slightly more obvious when I was listening from my Beyerdyanmics Custom ONE Pro (closed headphones, 16 Ohms) and even then, it is pretty soft; it was obvious only when there is no other audio being played or during more silent passages of an audio file.

On my Swans M10 (powered speakers, 22K Ohms(?!)), it is inaudible unless I crank up both the speakers and the amp to maximum volume. By then, there will be other bigger problems to worry about. (e.g. unhappy family members, my poor ears e.t.c.)
On my modded Superlux HD668B (open headphones, 56 Ohms), it is almost inaudible unless I crank up the amp to maximum volume (which will hurt my ears too).

Those powerline internet things are notorious for noise. The one I had made listening to FM radio a unpleasant experience to say the least. That's the biggest reason nobody offers internet over powerlines here in NA. I've read it can make HAM radio unusable.

Another thing, check the bias on your tubes. My P1 amp (bravo clone) get very noisy if the bias gets too high.