Hi all,
While testing my latest 3-stage design into a 9R dummy load, I noticed at high powers (50W plus) that the amplifier circuit itself is generating a sound which tracks the frequency as well as amplitude of the signal being handled. The sound is evident only for fairly high powers and is fairly low in volume. The sound is evident for a wide range of frequency, ie. hundreds up to thousands of Hz.
I wonder where this sound comes from and how to eliminate it? I suspect it must be from mechanical vibration in an inductive coil, but I do not have any inductors in my circuit - perhaps the wire-wound output resistors are the source of the noise? Strangely it did not appear to be coming from the wire-wound resistors in the dummy load - only from the amplifier PCB itself!
The toroidal transformer is quiet, and the amplifier performs well at all tests, ie. slew, overshoot, linearity, gain, freq. response, output impedance etc. stability seems fine with no oscillation.
Cheers,
HPT
While testing my latest 3-stage design into a 9R dummy load, I noticed at high powers (50W plus) that the amplifier circuit itself is generating a sound which tracks the frequency as well as amplitude of the signal being handled. The sound is evident only for fairly high powers and is fairly low in volume. The sound is evident for a wide range of frequency, ie. hundreds up to thousands of Hz.
I wonder where this sound comes from and how to eliminate it? I suspect it must be from mechanical vibration in an inductive coil, but I do not have any inductors in my circuit - perhaps the wire-wound output resistors are the source of the noise? Strangely it did not appear to be coming from the wire-wound resistors in the dummy load - only from the amplifier PCB itself!
The toroidal transformer is quiet, and the amplifier performs well at all tests, ie. slew, overshoot, linearity, gain, freq. response, output impedance etc. stability seems fine with no oscillation.
Cheers,
HPT
I suspect semiconductor packages are vibrating in sympathy with the signal they are passing. I think the ClassAB pulses in the driver and/or output stage may vibrate more than the devices passing ClassA signals.
I certainly cannot imagine the mechanism by which a transistor could vibrate in sympathy with the signal, save perhaps by magnetic force against the heatsink? I would have suspected this force to be truly insignificant - even more so because naturally the heatsink is aluminium, not ferrous. The transistors are mounted against silicon rubber pads and bolted down tightly.
Is this a common phenomenon? A design flaw? What is the exact mechanism by which this occurs? Does this phenomena have a particular name? I cannot find any references to this occurance on the internet despite what I search; nor have I come across references in any of my literature.
Alex - I did suspect perhaps the caps in the Zobel networks, but I could not locate the noise to one or the other cap.
I could not locate the sound with my ears; however if it is generated by all output devices against the heatsink, then it would make sense that the sound source is difficult to locate beyond simply being in the vicinity of the amp board.
Is this a common phenomenon? A design flaw? What is the exact mechanism by which this occurs? Does this phenomena have a particular name? I cannot find any references to this occurance on the internet despite what I search; nor have I come across references in any of my literature.
Alex - I did suspect perhaps the caps in the Zobel networks, but I could not locate the noise to one or the other cap.
I could not locate the sound with my ears; however if it is generated by all output devices against the heatsink, then it would make sense that the sound source is difficult to locate beyond simply being in the vicinity of the amp board.
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A good case for MKTs or polys?
I have 0.047uF monolithic ceramics in the Zobel network, and 39pF ceramics for compensation.
I will O/C the Zobel networks and see if this changes anything!
(EDIT: I have found a reference which suggests the higher the dialectric constant, the greater is the piezoelectric effect for such materials.)
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High value small size ceramics are best used for RF decoupling and kept well clear of audio signals. Low value ceramics are fine.
Thanks DF96, this seems to be the most likely source of the mechanical noise.
I suspect my Zobel caps are quite high as far as ceramic caps are concerned. I shall replace these with MKT if neccesary. First to remove Zobel and confirm it's reponsibility.
Thanks for all the replies.
I suspect my Zobel caps are quite high as far as ceramic caps are concerned. I shall replace these with MKT if neccesary. First to remove Zobel and confirm it's reponsibility.
Thanks for all the replies.
Lenz's law is about changing magnetic fields. So are induction motors. Unlikely to be an issue here. You need lots of turns around a ferrous material to get a big enough field to do anything noticeable in nearby non-ferrous metalwork.
I've heard TO3 devices "sing" before under high power signal conditions particularly at higher audio frequencies. I presumed it was the bonding wires vibrating in free air.
In any case anything that causes an electromagnetic field or eletrostatic charge can cause vibration so using gmphadte's suggestion you should find the source or sources. You might also find that it's normal. I can sometimes hear my amps when powering into a dummy load.
Cheers
Q
Thanks guys. I'm not new to electromag; my suggestion was that the force would be insignificant, not that it doesn't exist. As quasi has pointed out, I can see how this is a possibility in TO-3 packaged devices, but not solid packages.
Looking at the information available, I think it is very likely to be the zobel capacitors in my situation. I am opening up the amp for repairs this afternoon (somehow managed to pop the OPS devices during testing) so I will follow up all the suggestions and report back! I have some nice MKTs to put in place of the ceramics.
When I get some time, I'll place up the schematic for scrutiny.
Amazing!!! I think you must be right.
Looking at the information available, I think it is very likely to be the zobel capacitors in my situation. I am opening up the amp for repairs this afternoon (somehow managed to pop the OPS devices during testing) so I will follow up all the suggestions and report back! I have some nice MKTs to put in place of the ceramics.
When I get some time, I'll place up the schematic for scrutiny.
Amazing!!! I think you must be right.
An induction motor can be done without any ferrous metal. You can build a linear accelerator ('coil gun') for example with the stators being copper coils, the 'rotor' just a piece of aluminum pipe, all supported with a plastic 'barrel'.
The effect for a single winding coil (as with current through a power transistor) is very small, but can be significant at higher harmonics and bigger currents.
Not sure it can be heard with a typical amplifier though...
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