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Old 8th January 2004, 04:54 PM   #1
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Default Singing MOSFETS (basic question)

Ok, I apologise for asking this very basic question, but I haven't found a simple answer anywhere.

I have a simple MOSFET amp (Maplin kit) using Exicon MOSFETS (I am ordering PCBs for the Aleph-X!). When I run the amp through a dummy load, I can hear the TO3 transistors "singing" to the input, not a resonant mode, but complete reproduction. Is this an example of magnetostriction, are the silicon dies lose or is there a perfectly simple explanation?

Once again, I am sorry if this is a rediculous question.

Old 8th January 2004, 07:33 PM   #2
sam9 is offline sam9  United States
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Do you mean the TO3's are mechanically vibrating? If so that's too wierd to think about.

If you just mean that the input network seems to picking up signal from the output, then it is "just" a matter of shielding the input wiring and keeping the small signal stage away from the output stages. This is a common problem with a simple (but not always easy to implement) solution.

Or do you mean something else -- if so try restating a bit more descriptively and precisely.
Old 8th January 2004, 08:05 PM   #3
sully is offline sully  United States
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I've also heard this..not heard of someone hearing it, but actually hearing it myself.

Within the package is a pair of wirebonds. The source will be anywhere from a 5 to 20 mil thick aluminum wire...the gate can be anything from .7 mil gold up to 5 mil aluminum.

If the source wire is vibrating, I'd be afraid of the reliability of the device, as the wire would tend to fatigue. (But, perhaps that is only an ultrasonic resonance issue, one I've been bitten by.)

My gut feeling is that it's not an internal thing, but the wires connecting the source or drain moving in response to the current within, and the to-3 is larger surface area to radiate it.

Neat, isn't it? who needs speakers??

BTW..if the silicon were loose, it would blow in less than a second as there would be no heat path keeping it from vaporizing.

Cheers, John's not a ridiculous question..
Old 8th January 2004, 09:01 PM   #4
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That is totally off in the outer space region of our Galaxy.

You should use IR MOSFET, IR240 or IR244, or IRP244s in the output.

If the source wire is vibrating, I'd be afraid of the reliability of the device, as the wire would tend to fatigue. (But, perhaps that is only an ultrasonic resonance issue, one I've been bitten by.)

The will sing when music is applied.
Jim W.
Old 9th January 2004, 12:22 AM   #5
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thank you for the replies. It would appear that it is not an especially common occurance then. Both of my amp channels exhibit this behaviour, and in both N and P channel devices.

The only thing that I can think of, is that these are "lateral power devices", would this have any bearing on the noise output? I don't have the circuit diagram to hand here, but I have included the details of the tranistor below (the N and P are identical in these figures).

Ptot(max) 125W
VDSmax 160V
VGSmax 14V
IDmax 8A
IDdiode 8A
I/P cap 500nF
IDSSmax 10ma

One last thought is that these MOSFETS have some kind of diode inside them, could it be this that is producing the audible noise? It is "quite" loud, in that at about 100W from the amplifier (dummy load) with the lid off, I can hear it about 1m away.

I am not too worried though, a quality amplifier will follw (I hope).

Thank you for all your help,
Old 9th January 2004, 12:26 AM   #6
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Oh, and sam9, it is a vibration from inside the TO3. I wasn't going to put my ear on the case because there are a few volts flying around, but you can localise it to them by getting close and shielding the noise with your hand. There is nothing else close enough to produce the noise.

As long as it isn't a normal occurance that simply isn't mentioned because it is so obvious, I am not worried. See no evil, hear only the music!

Old 9th January 2004, 01:45 AM   #7
jcx is offline jcx  United States
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i have read about this phenomena in the past and the given explanation was differential themal expansion, perhaps your mounting pressure/flatness is way off and mechanically distorting the can as this really shouldn't be occuring in well built modern transistors
Old 9th January 2004, 03:56 AM   #8
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I have a very hard time believing this, however you can test this out with out putting your ear even close to the device that is making this noise. You need say about a 26mm x 26mm x 400mm about piece of wood and press it against the part, then press the other end against you ear, if you can physically get to the part. The sound energy will conduct vibrations through the wood or some other dense material to your ear.

The bonding wires being so short could only vibrate at some high frequency. In addition, if were doing such a crazy thing the bond would loosen over time causing a failure.
Jim W.
Old 9th January 2004, 04:03 AM   #9
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Actually this is a pretty common phenomonon and I used to hear this noise all the time when I used to repair Phase Linears, ESS, and Crown amplifiers back in the mid 70's. The higher power the amp the more you would hear it sing. I believe the noise is caused by current induction in the wiring itself... somewhat similar to the buzzing noise that is heard in a power transformer. I've found that almost all high power amps sing to some extent...what they sing just depends on what you feed them. I've even been able to feed an amp from an FM tuner and actually hear dialog eminating from amplifiers by running an amp near wide open into a static 8 ohm load.
Perhaps someone like Nelson Pass would be able to better explain it.

Old 9th January 2004, 04:21 AM   #10
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Mark Gulbrandsen,

What have you been smoking, send me some. I've worked on a truck load of amps even some of those crapy Phase Linears, MAC's Crown DC300 to Ampzillas and never hear this. Maybe its the amps crying not singing, I could understand a Carver amp doing that.

Do you hear this under load or no load?
Jim W.

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