Topic: Remove switching power supply noise

[vinietje]

Hi all,

first of all, BIG thanks for the eleborate forums, help and other info regarding the world of DIY dacs!

I build my own laser in the first phase, ebay galvo's (20pps), and an RGB-brick with analog modulation + showcard. Works great, however as i found the stock figures boring, I build the sound-dac.

This is were the problems arises:
It works very nice (used the tl042 op-amps, 50k gain pots, and the negative voltage regulator + the 5v-> 12 +- converter. The only problem i have, is that It picks-up the noise of my mac-book power adapter. If the show is running, and i disconnect the power adapter, clear images, no pixalited lines between two non-intersecting shapes, but if I add it to charge my battery, it does render more lines than i care fore.

I added the 3 10uF caps (tantalum), added an additional one to the 5v->12v+- converter (also 10uF). But still it pickup noise. the signal doesn't strike me as an 50hz distortion (eu), though. And as my battery isn't great anymore, I rather run in dc-connected.

Anyone has an idea where to put more caps? I power the converter by taping into the test-points on the sound card, as I rather not add more electronics to the dac, like an external powered hub or similar.

any tips are very appreciated!

[James]

Hi vinietje! Welcome to The LaserBoy Forum!

Since you used a DC/DC converter to get from +5V to + & - 12V, did you connect the ground from the input ground of the DC/DC to the output ground? Typically these are isolated.

In other words, the DC/DC converter has an input ground and +5V and an output ground and a + and - voltage. The input and output grounds are not connected inside the converter. Try connecting them externally.

Other than that, maybe your laptop power supply is noisy. Can you try another one?

James.

[vinietje]

Hi vinietje! Welcome to The LaserBoy Forum!

Since you used a DC/DC converter to get from +5V to + & - 12V, did you connect the ground from the input ground of the DC/DC to the output ground? Typically these are isolated.

In other words, the DC/DC converter has an input ground and +5V and an output ground and a + and - voltage. The input and output grounds are not connected inside the converter. Try connecting them externally.

Other than that, maybe your laptop power supply is noisy. Can you try another one?

James.

Hi James,

Thanks for the reply,
My input (gnd from the 5v) is indeed bridged with the 0volt of the output. This had to be done anyways, otherwise the dac's output voltage was very weird. So unfortunately that I already have done. I will try my other macbook adapter, but I highly doubt if that will solve the issue, as these are produces in very large quantities.

Perhaps you could advice me whether it would help to add big elco's to the 5-volt input?
Thanks again

 

[vinietje]

Hey All,


for future reference, i've decided to tap into my 12Volt supply which was present in my projector anyways.
I've added an 7805, an elco of 220uF at the input, and a 10UF + 22nF on the output, now my signal is Crystal clear:), with or without laptop-powerbrick attached.

[James]

I'm glad you found a working solution.

The only problem I see with that is that now you are limited by the length of a USB cable. If the DAC was powered by the laptop, you could run 10 meters or more on the ILDA calble.

Oh well.

BTW The power supply for your scanner amps is almost definitely differential; probably + & - 15V. So you could just use that to power the op-amps with no extra regulator or DC/DC converter.
 
James.

[meandean]

Hi vinietje!

Welcome to the forum- glad you found a solution to your problem.

Just to share experiences, I've found that mixing and matching various powerblock devices can sometimes result in 60Hz interference.

When I connect my laptop to a pair of powered speakers (with their own independent AC adaptor) I get humming noise even with
the volume turned all the way down, but only when the laptop is on AC power... The problem goes away when I connect the system
common to a real ground (like the screw on an AC outlet cover).