On 25/03/2018, Simon Hobson <li...@thehobsons.co.uk> wrote: > Edward Bartolo <edb...@gmail.com> wrote: > >> Lately, I have experienced an amplifier input failure that was >> supplied audio signals from a laptop powered with an AC mains power >> supply. > > 2pin or 3pin mains input to the power supply ?
It is a three wire mains input which means there is an earth connection. But this seems to be causing more trouble rather than being a solution. >> As you know, these AC power supplies are of the switching type >> that use an inductance to transform voltage. The problem with these >> power supplies is the fact that energising an inductance, also >> energises parasitic inductances and capacitances that give rise to >> transient interferences that may cause an external circuit to fail. > > It's not so much the transformer, but the input filtering and lack of earth > on 2 pin mains inputs. In effect, the output of the supply will naturally > float to around 1/2 the mains voltage (so around 115V AC on a 230V mains) > with a weak driver via the filter caps. With my (metal cased) MacBook Pro I > can sometimes feel this voltage as a "sensation" in my palms if using a 2 > pin adapter* > What you wrote reminded me of a dangerous filter that consists of two high voltage series-connected capacitors connected in parallel with the mains with their middle point earthed. Since these capacitors are almost certainly the same value they will devide the mains voltage by two. This fact can be verified with a high impedance voltmeter connected with the disconnected earth connector and one terminal of the mains supply. I want to avoid using a dangerous circuit like this. With it, my death would only be a dielectric failure away. One way to solve this problem may be to use a pair of isolating transformers for the audio signals but these transformers are often very expensive. Although expensive, using an audio transformer introduces its own ingredients (harmonic distortion). So, I am looking for a way to use small ferrite toroids to use for the transformers, wind two coils on each with around thirty turns and connect their primaries in series with a resistor to swamp their impedances to negligibility. This to minimize the ingredients introduced by using transformers. The small output signal would be amplified with an opamp. The only issue I see with this setup is that the audio signal is first attenuated with the transformer setup, then it is amplified again. This is the best recipe to introduce white noise although many electronic setup precisely do that. _______________________________________________ Dng mailing list Dng@lists.dyne.org https://mailinglists.dyne.org/cgi-bin/mailman/listinfo/dng
