I am bench testing a Skar lp-80.4 and when I turn up the gain to max I can hear music from the pcb. I have isolated it to channels 3/4. How is this possible. Something has to move 20times per sec to create sounds correct? I can clearly hear vocals! Someone help me please!!
Many amplifiers (not just class D) can produce audible sound from various components such as power transistor etc. A Class D will have coils and filters on the output stages, all can 'sing'.
It could be an inductor or a class II ceramic capacitor that acts as a loudspeaker. Inductors and transformers can produce sounds due to an effect called magnetostriction (see Magnetostriction - Wikipedia ), ceramic class II capacitors due to piezoelectricity ( Piezoelectricity - Wikipedia ).
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With the amp on and no speaker hooked up with music playing into the rca inputs it is playing the music I am inputting from the pcb on the output section of the amp. The amp is a class ab with no inductors on the output section of the amp.
An externally hosted image should be here but it was not working when we last tested it.
It's a common phenomenon and I've noticed it myself. Power transistors do this but whether it is the junction itself or something mechanical* and perhaps related to how its mounted I don't know.
* ultimately something mechanical has to be occurring somewhere to move the air in relation to the electrical signals and thus produce the sound that we hear.
It is common though.
* ultimately something mechanical has to be occurring somewhere to move the air in relation to the electrical signals and thus produce the sound that we hear.
It is common though.
I'll vote for transformer as well. This often happens when there are loose windings. It also seems to be more common in amps with regulated power supplies. Sometimes, touching the windings can make a difference.
Audio, especially vocals, is higher than 20Hz. It's more in the range of 400-1kHz.
Audio, especially vocals, is higher than 20Hz. It's more in the range of 400-1kHz.
I had something similar with this tube amplifier, http://home.kpn.nl/verwa255/esl/amp.zip , the output tubes seemed to produce sounds corresponding to the input signal all by themselves - although I wasn't suicidal enough to put my ear really close to the amplifier to hear precisely what component made the sound. I guessed it had something to do with the forces due to electric fields slightly bending the electrodes, but I'm not sure about that at all.
In a power transistor that carries a large signal current and that has iron in its case, I could imagine that the changing magnetic field of its emitter bondwire would cause it to bend with the music. However, in this case there is no loudspeaker and no dummy load connected, so there is hardly any signal current.
Touch around the board while listening to see if you can damp it out. Does it do it with a load connected (power resistor).
We sometimes get it in mobile phones, usually it's the GSM bumblebee or a switched mode supply, but occasionally get other sources. To locate the offending component we use a stethoscope with a thin pointed tube on the end, probe the board and components and listen for the buzzing or singing component. One DIY version of this was a plastic cup from the vending machine with a needle poking through the bottom, same principle as two cups and a bit of string.
We sometimes get it in mobile phones, usually it's the GSM bumblebee or a switched mode supply, but occasionally get other sources. To locate the offending component we use a stethoscope with a thin pointed tube on the end, probe the board and components and listen for the buzzing or singing component. One DIY version of this was a plastic cup from the vending machine with a needle poking through the bottom, same principle as two cups and a bit of string.
It source around the speaker terminals, way far away from the transformer the only things around are aluminum electrolytic caps(smd), resistors (smd), and some zeners. There are a couple of 2x transistors but I changed them out just to rule them out. The amp is functioning properly now so what the hell, but I just can't let this go I have to find out the answer!
On the photo, I see these relatively big 4.7 ohm SMD resistors in series with what appear to be class II ceramic SMD capacitors. Are those Zobel networks? If so, it is probably the piezoelectric effect of the capacitors that makes the sound.
MarcelvdG you are correct it was a ceramic class II capacitor! https://www.google.com/url?sa=t&sou...FjADegQIAhAB&usg=AOvVaw0-NjT9sm3w2tajiflrxgGC
I once built a DAC that after lots of debugging and modifying worked well, except that its PCB would sometimes produce a high-pitched tone and sometimes not. I thought it had something to do with several switched-mode regulators on an FPGA module until Jan Didden pointed out my mistake: I had decoupled the output of an LM317 with only ceramic X5R capacitors, and those were not lossy enough to reliably keep it stable. On top of that, they worked as loudspeakers when the LM317 oscillated.