NEED ADVICE: controlling Mini2x4 volume when using digital inputs - Page 2 - diyAudio
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Old 1st May 2013, 10:34 AM   #11
gpapag is offline gpapag  Greece
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Charlie
Every digital circuit generates HF EM fields. These can couple to everything in the vincinity, on board and to it’s connections to the other circuits.
E.g. The clean psu I wrote above, when probed while feeding the working miniDSP shows a nasty waveform 100mVpp whose lowest frequency is at ~1MHz -despite psu very low internal impedance- thanks to the 50cm connecting twisted leads. Local decoupling and filtering at the board’s dc entry then is a must, particularly when one psu is feeding other boards too. Same effects and treatment is to be thought of for all connections, these for analogue signals too.
This is not a problem with only this DSP unit. You can open the cover of a CD unit or a digital radio and probe around.
To your question: The single pole 30-50kHz LP filter at amp input, will reduce the 15mVpp 1MHz noise into around a 0.5mVpp one (-30db), much lower for the higher harmonics. Not bad at all.
A two pole filter will reduce this noise to a theoretical 10uV (-64db).

George
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Old 1st May 2013, 02:56 PM   #12
Davey is offline Davey  United States
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Here's a noise measurement (from a few years ago) showing miniDSP 2x4 (red) versus 2x8 (blue) output noise. This plot spans a 300khz bandwidth, but I also checked to 5Mhz. The output noise of the 2x4 unit does not rise above -91dbV in the 5Mhz range.

I don't see any issue with low-pass filtering the outputs, but there isn't much to filter...with the 2x4 unit.

Cheers,

Dave.
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Old 1st May 2013, 03:30 PM   #13
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Thanks to both of you for the useful info. I tend to forget how noisy some of the digital circuitry can be at certain frequencies...

-Charlie
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Old 1st May 2013, 04:07 PM   #14
gpapag is offline gpapag  Greece
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Dave, thanks for sharing.
I find the figure impressive! -91dbV is ~ 28uV. This ruins my theory
Was the unit I measured defective? I fried it early this morning but I have another one in my living room playing music. I will measure it too.

George
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Old 1st May 2013, 04:52 PM   #15
Davey is offline Davey  United States
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Fellas,

Here's a better look with a measurement I just made.
This is my miniDSP 2x4 with all processing disabled, all gains set to 0db, and input shorted. (blue trace.)
The red trace is the other channel of my (el-cheapo) USB spectrum analyzer with a shorted input.....for reference.

The analyzer auto-scaled to its lowest input range to yield this measurement. The miniDSP noise is highest in the 1.5Mhz range but only approximately -88dbV worst case.

The miniDSP 2x8 is a different animal with completely different noise characteristics.

Cheers,

Dave.
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Old 2nd May 2013, 10:56 PM   #16
gpapag is offline gpapag  Greece
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Dave thank you for the new plot.

I looked at my other miniDSP board. Output noise is of the same amplitude as previously reported with a bit higher frequency components.

The huge difference compared to your measurements makes me wonder what is going on. It is not a ground loop issue, it is not a psu issue. I tested any feasible combination of these two.

Using my 10mm x 10mm one-turn H-Field probe, I looked around the board and I spoted the troublemaker. It is the L1 (56uH) coil.
What this probe sensed close to L1 coil is a square signal 0.52 us repeating every 0.74us with an amplitude of 30mV. This spreads out frequencies of 1.92Mz, 1.35MHz and 50MHz (the steep edges having a rise and fall rate of 24mV/0.008us).
Probing the DC input which is very close to L1, I saw a strong 48mVpp footpring of this squarewave with less steep edges and well damped 50MHz ringing.
On the 3.3Vdc probed at pins 34, 36, 48 of ADAU1701 the squarewave ghost is 8mVpp and the 50MHz ringing is 3mVpp.
On the 1.8Vdc probed at pins 13, 24 of ADAU the squarewave is 16mVpp and the 50MHz ringing is 12mVpp. (*)

All these with 12Vdc supply voltage. L1 field strength and the amplitude of the coupled square wave pattern increase with voltage, becoming almost double with supply voltage of 24Vdc (the 50MHz coupled wavetrain retains the same amplitude being the result of rise time which doesn’t change with voltage).

When I was moving the H-field probe around L1, the current consumption of the board was fluctuating due to the stray coupling of the probe with the magnetic field. This means that a screen around L1 will affect it’s (unknown to me) operation.

I’ll try to repair the dead board and use it for working around L1.
Meanwhile, is it possible for some of you to probe with a scope the analog out of a 2x4 miniDSP (no signal on input) and report here?

George

(*) The amplitude of noise at 3.3V and 1.8 V pins change with the position of probe’s ground (this is how I fried the other board. The ground wire slipped...)
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Last edited by gpapag; 2nd May 2013 at 11:05 PM.
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Old 3rd May 2013, 12:25 AM   #17
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Quote:
Originally Posted by Davey View Post
[snip]This is my miniDSP 2x4 [snip]
Dave, which 2x4 board were you testing (unbalanced or balanced) and was it a bare board in the open, something you assembled (with PS, in an enclosure), or was it the 2x4-in-a-box with a wall wart PS?

-Charlie
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Old 3rd May 2013, 01:44 AM   #18
Davey is offline Davey  United States
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It's a standard 2x4 unbalanced board with RCA connectors. Not mounted in an enclosure (just laying on my bench) and powered with an el-cheapo 12VDC wall-wart.

Dave.
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Old 3rd May 2013, 03:17 PM   #19
gpapag is offline gpapag  Greece
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I bet a couple of beers that L1 is part of a voltage step down (ďbuckleĒ) circuit. All the necessary elements are around, itís location on the board and the increase of the magnetic field with the increase of the applied dc, all hint to that. Iíll check it in a couple of hours.

George
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Old 4th May 2013, 03:50 PM   #20
gpapag is offline gpapag  Greece
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The above is verified. But where is the switcher?

As a by-product, here is the current demand of the miniDSP v1.8 board in relation to the external psu DC voltage. This is the current after the momentary turn-on peak. This peak is ~ 3x higher.

George
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