Post by thread:[EE] PSU interference with Velleman 4001 audio amp

Hi Im building a custom built all in one PC for use as a museum touch screen kiosk. Im trying to use 2 Velleman 4001 analogue audio amps (one for L one for R) to boost the audio output from an Intel motherboard so it can drive 2 1.8Ohm speakers at a reasonable volume. Ive fitted a 47k log pot on the 2 amp inputs as recommended by Velleman to adjust the volume. The rest of the PC components (motherboard, hard disk, LCD screen, LCD controller board & LCD CCFL inverter) are powered from a Traco Power PSU with a 10Amp 12v DC output. When I power the 2 amps off the PSU also, it causes interference through the speakers, but if I power the amps off a separate bench 12v supply the interference problem goes away. The connections from motherboard to amps and amps to speakers is unsheilded, unbalanced twisted pair of fairly hefty guage (~20AWG). I have considered replacing it with shielded, but as far as I can tell, the bulk of the noise is coming through the power leads not from the audio cables. Any ideas how I can clean the supply, given that physical space inside the enclosure if fairly limited. I did think about a voltage reg, but the amps need between 8-18v and so to use a 12v reg I would need an input supply > 12v which I dont have off the Traco Power PSU. Thanks Andre

On Mon, Jan 23, 2012, at 08:10 PM, Andrew Wood wrote: > The rest of the PC components (motherboard, hard disk, LCD screen, LCD > controller board & LCD CCFL inverter) are powered from a Traco Power PSU > with a 10Amp 12v DC output. When I power the 2 amps off the PSU also, it > causes interference through the speakers, but if I power the amps off a > separate bench 12v supply the interference problem goes away. I suspect you have a ground loop. The easiest thing to do is run the audio amps from a dedicated power supply. Cheers, Bob -- http://www.fastmail.fm - Send your email first class

On 23/01/12 20:26, Bob Blick wrote: > I suspect you have a ground loop. > > The easiest thing to do is run the audio amps from a dedicated power > supply. > > Cheers, > > Bob > Thanks Bob, looks like you might be right. A separate PSU isnt really an option though. What would be a suitable isolation transformer

Since the signal source and the amplifier share the same DC and audio ground, you would need to isolate the audio. Good quality audio coupling transformers are expensive, so you might do better coupling them with a differential amplifier (you can make one with an opamp). But getting it right can be fussy unless you've done it before. That was why I suggested powering the amplifier off its own dedicated supply. Best regards, Bob On Fri, Jan 27, 2012, at 09:13 PM, Andrew Wood wrote: {Quote hidden}> On 23/01/12 20:26, Bob Blick wrote: > > I suspect you have a ground loop. > > > > The easiest thing to do is run the audio amps from a dedicated power > > supply. > > > > Cheers, > > > > Bob > > > Thanks Bob, looks like you might be right. A separate PSU isnt really an > option though. What would be a suitable isolation transformer? -- http://www.fastmail.fm - Does exactly what it says on the tin

On 27/01/2012 22:37, Bob Blick wrote: > Since the signal source and the amplifier share the same DC and audio > ground, you would need to isolate the audio. Good quality audio coupling > transformers are expensive, so you might do better coupling them with a > differential amplifier (you can make one with an opamp). But getting it > right can be fussy unless you've done it before. That was why I > suggested powering the amplifier off its own dedicated supply. > > Best regards, > > Bob > Wouldnt one of these between the motherboard and the amp do the job? http://uk.rs-online.com/web/p/audio-transformer/2106481/ Regards Andre

If you don't need excellent fidelity, yes, it would do the job. Transformer impedances should ideally match the source and load impedance. And a transformer with good specs from 20-20000 Hz doesn't cost that little. You could give it a try, it may be adequate. Cheerful regards, Bob On Sat, Jan 28, 2012, at 02:34 PM, Andrew Wood wrote: {Quote hidden}> On 27/01/2012 22:37, Bob Blick wrote: > > Since the signal source and the amplifier share the same DC and audio > > ground, you would need to isolate the audio. Good quality audio coupling > > transformers are expensive, so you might do better coupling them with a > > differential amplifier (you can make one with an opamp). But getting it > > right can be fussy unless you've done it before. That was why I > > suggested powering the amplifier off its own dedicated supply. > > > > Best regards, > > > > Bob > > > Wouldnt one of these between the motherboard and the amp do the job? > > uk.rs-online.com/web/p/audio-transformer/2106481/ > > Regards > Andrew > -