My subwoofer notes

Started 13June2018, updated 21Sep2018

This page is in group Technology and is a blog note about my experience with two subwoofers. One had trafo and membrane hum, the other magnetic induced iron hum.

Standard Disclaimer. Plus I am no expert on audio.

I grew up at a house close to a transformer station, where my father worked. My room was filled with 50 Hz hum 24/7. My brain had to adapt. But since long, when it became far away in all aspects, I tend to detect 50 Hz whenever there is any.

MicroPod by Podspeakers

My speakers (MicroPod by Podspeakers) here at my home office did not completely fulfil my idea of full sound. After all, one of them has an analogue 3.5 mm (mono) subwoofer output, so thinking about a subwoofer was (perhaps) just the next point.

BASS8 by Argon Audio

I went down the street and picked up a BASS8 by Argon Audio. A quite nice speaker to look at. It sounded good as well. But I did hear 50 Hz hum from it. It was coming from the speaker, even with nothing connected. It must have been picked up in the preamplifier stage since both the crossover and input volume potmeters when turned down seemed to lower the hum. (Not sure about that, in the shop one of them seemed to increase the hum! Hmm.)

I tried a grounded power receptacle and the hum seemed worse. All subjective, nothing measured! So I added a cable switch in the power line to switch it off whenever I didn’t want to have hum in the background. This worked until I decided to play Piano by Benny Andersson. The music is so soft, has so little bass when played at a low volume that the hum to me was always present. Piano plus hum is nothing for me.

I talked with the people at the shop and the unit was opened. There is a laminated core transformer in there, and an analogue amplifier plus cooling fins in the back. I think it’s got capacitive coupling to the speakers, non-tube low voltage class-AB amplifiers I guess won’t need an output transformer. When I built a scope when I was a youth (here), I could see the 50 Hz field from the transformer on the CRT if the iron were not good enough, or the transformer was mounted at a wrong angle. When I built amplifiers and could not afford a toroid trafo with lower spreading field, then hum was almost impossible to get rid of. No matter how good I was at grounding trying to avoid ground loops (one ground!) and using shielded cables. And I didn’t have a subwoofer to amplify the problem. Argon Audio knows all this plus lots more. Within the budget they had for this box I guess hum was unavoidable(?) Or better: low hum was considered acceptable to most customers. (But they could have made the Auto off/on better, it switched on and off too often. That’s where my easily accessible power switch came in.)

SUB C-8 D by DALI

But not acceptable for me. So I brought it back and swapped it for a SUB C-8 D Subwoofer by DALI. It cost twice the price. But I could hear no hum in the shop. None. From the downwards speaker. The man in the shop said I should take it home and try it, there may be a different background sound level there.

There was. This box has a Class-D amplifier (it’s “switching”, not “digital”). Plus a switched-mode power supply. A rather large one, hundreds a watts. I didn’t have to open it to find out. The box has no external heat fin (they use the flat metal in the back) even if DALI says there is 30% loss. I think this is including the loss of the power supply, since the Class-D could in theory go up to 90% efficiency. I guess it’s easier to allow this kind of power supply when there is a Class-D amplifier. Besides there would be very low 50 Hz background field in the box. As I have indicated already I could hear the switched-mode switching! The idle switching from these is rather non-periodic with a high frequency pitch, so it’s easy to distinguish. When I put my ears to the box: I didn’t like it. From under my table: barely. Or a little more. Or perhaps none. Besides, the Auto off/on works much better. I don’t seem to need my table-top line switch much, do I? Yes, I did have to! There was (some other?) noise coming from it that irritated me. Stay tuned.

I was rather impressed with both boxes, how I could not hear any (or not much) additional noise when the analogue cable was connected. So my ground level(s) must to be rather cooperative, too.

By the way, electrically connecting the mono output from the MicroPod to both Right and Left RCA connector inputs of the DALI doubles the voltage and quadruples the power (about 6 dB, see Is it worth using a Subwoofer Splitter Y-Adapter Cable? and Wikipedia). I’d like to see the equivalent circuit for this to believe it goes for the circuit starting at the RCA connectors ending in the speaker coil. It should be ok if the switching amplifier acts as a current source: delivering into the coil (4 Ohm?) a current like 1 A from the Left RCA and 1 A from the Right RCA sums (which a current output amplifier does) up to 2 A. This is four times the power, from 4 Watt (W) to 16 W. I believe this makes sense since the filter capacitors do deliver current to the speaker coils. Ohm’s law is W=R*I2=U*I and it spells out as Power in Watt = resistance in Ohm (Ω) of the coil * current in Ampere * current in Ampere = Voltage * Current. In the example above: 4Ω*1A*1A=4W vs. 4Ω*(1+1)A*(1+1)A=16W. When I soldered a subwoofer splitter Y-adapter cable from parts I had then I think I could actually feel, with a finger, the membrane getting more movement! But the DALI box has more gain so it isn’t necessary. As an engineer I would say it would also help with the signal/noise ratio, but the thermal noise would be rather low at those frequencies.

The box is rather nice because I have curious grandchildren popping into my home office; or me and them laying on the floor doing Märklin my world train layouts. No visible membrane to easily punctuate! Or even I could kick my foot into an unprotected vertical membrane. I like it!

However. The SUB C-8 D was almost returned because yes, the noise is audible and yes, most of the time it kicks me out of flow. I seldom listen to music when I am in flow, but if I do and the music stops, or even if I play it on low volume, there is a humming that has me reach for the off switch.

Nothing is perfect when it comes to non-wanted sound. Even the fan in my iMac was somewhat problematic. But I was able to dampen it enough. See Dampening iMac fan noise. And my aquarium’s air pump with the open transformer that kicks a rubber membrane 50 times a second, I placed in the basement room underneath the aquarium. 50 Hz gone! This would have annoyed me much more that the sound from bursting air bubbles in the aquarium. However, it’s always a matter of measurable level and psychological acceptance threshold. Plus fighting the tinnitus level up there.

My DALI subwoofer top-of-box help sheet

In order to avoid having to drag the box on the floor into the light, or sit under the table with a torch to read the rear side panel text, I have made a sheet that I printed out on an A4 paper and placed it on top of my SUB C-8 D box. It’s here. Now I can just reach my hands back there and find the right buttons to turn. I scanned the graphics from the manual that came with it, as I could not find that picture on Dali’s pages. I may have broken some copyright, but for a good cause..

My DALI SUB C-8 D hack “repair”

The day before I was going to return it to the shop (“just come with it and we’ll have a look and listen to it”) I decided to take the chance to void (..?) the guarantee. So I unscrew the 10 screws and listened as I withdrew the electronics with power on. (I have worked with electronics all my career, so it didn’t make me sweat). (I had to loosen the speaker cable first). I could hear the noise disappear!

My hypothesis is that the large transformer has a pulsing magnetic field that makes it or the iron in the speaker vibrate, or both – when they are close. It didn’t sound like a switched-mode regulator pattern, it was more steady than that. In other words, they seem too close, with no magnetic shielding iron in between. When I moved them only 1 cm away I could hear the hum almost gone. And at 2,7 cm my ear had to be almost on the panel to hear it. So I added a list frame 27 by 15 mm (the 15 mm width was too thick, so I had to make some more room for the electronics). The frame has loose joints in the corners to make it 100% practical to place it. I had to check the new screws thoroughly so that they didn’t hit the speaker. And they must hold the electronics as firmly in place as the original screws.

Disclaimer: This may not necessarily be a general problem of all DALI SUB C-8 D. Transformers of the same type may generate more or less hum depending on how the laminated steel core’s individual steel plates come out from the production. Any air gap will cause more or less leakage flux. As you see on the picture both larger trafos have been wrapped in tape, I assume as an attempt to keep it quiet. Nice. I hypothesised that the problem was with the leakage flux also being picked up by the steel in the (too close?) speaker element, and my fix addressed it. I did not try to intervene by doing anything with the trafos or having the board replaced with a “better” one. After all, I wanted to void the guarantee as little as possible!

Press picture to see fullpixel:

Fig.1 – DALI SUB C-8 D: Minimising magnetic field induced hum by moving the electronics away from the speaker element. Disclaimer: opening the electronics probably voids the guarantee! Observe new screw length! (Press for fullpixel)

It sounds like the wood is small, “dead” and the joints tight enough to make this hack work. Also, it looks like the electronics gets rid of its heat loss through the aluminum plate, not the box. It’s actually damped with some rubber foam towards the box. However, the box’s internal volume is increased by about a liter. It does not sound like the box is tuned so accurately that this should be any problem.

While I write this it’s on and I don’t play any music. Yes, it’s on. And I can’t hear it!

Norwegian summary: During i basshøyttaler pluss lyd fra svitsjet strømforsyning og fra magnetisk indusert during. Reparert!

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