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Old 16th February 2014, 10:13 PM   #1
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Default Minidsp 2x4 noise floor

I've just received a minidsp 2x4 and I was interested in measuring its noise floor.
As a first thing, I powered it through USB, set to 0dB gain, shorted the inputs and looked at the 2-way advanced crossover plugin VU meters.
The two inputs show values that jitter between -73dB and -86dB, independently and randomly. It looks like noise (see attached)
Is it normal or is my minidsp defective? I would have expected a stable value similar to the noise spec of the AD chip used by the board. The minidsp datasheet doesn't seem to specify a noise floor value.
So far I'm a little disappointed, should I?
Anyone sharing his findings, any comment from the DevTeam maybe?
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Old 3rd March 2014, 04:41 PM   #2
Davey is offline Davey  United States
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Do you notice excessive noise from your speaker drivers with no music playing??

Dave.
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Old 5th March 2014, 08:12 PM   #3
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Default Noise with inputs shorted

Yes.
i tried two different setups:
1. amplifier with gain of about 16, input impedance of about 47Kohm, speaker connected to output, inputs shorted, no miniDSP; I can hear no noise even with my ears at a few cm from speaker cone
2. miniDSP with inputs shorted, connected to the above amp. MiniDSP is set with 0dB gain, all filters bypassed. This time with my ears close to the speaker cone I can hear a distinct hiss.
Minidsp power supply is 12V, filtered with 2200uF + 10uF tantalum + 0.1 uF
If I power the miniDSP through USB, of course the hiss get louder and more "random", so definitely a better PSU reduces the hiss, but I couldn't manage to eliminate it, even with a "good" PSU like the one detailed above

Any hint, is there anything I could try to eliminate or at least further reduce the hiss, which is really annoying?
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Old 5th March 2014, 10:26 PM   #4
Davey is offline Davey  United States
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A bit of hiss/noise would be normal......relative to low-noise analog circuitry.
If your speakers are fairly high efficiency and you still need to put your ears close to the speakers, I'd say you're doing okay and your miniDSP is operating nominally. Is it audible from 3-4 feet away?

With some gadgets you could pad the outputs to lower noise, but since the 2x4 unit maximum output voltage is 0.9VRMS you probably can't afford to do that.

Dave.
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Old 6th March 2014, 03:13 PM   #5
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Davey, thanks.
Still, I'm a little disappointed as all my other commercial audio equipment is essentially noiseless
You probably have used a number of other miniDSP products. Do the 2x8, 8x8 or the miniSHARC maybe have less hiss?
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Old 6th March 2014, 03:18 PM   #6
usul27 is offline usul27  Switzerland
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-73dB looks a bit much, but -86db is already very good for this chip. That's the limitations of the DSP used here. However, if you don't use digital volume control, this is usually good enough.
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Old 6th March 2014, 03:55 PM   #7
Davey is offline Davey  United States
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Quote:
Originally Posted by SunShade View Post
Davey, thanks.
Still, I'm a little disappointed as all my other commercial audio equipment is essentially noiseless
You probably have used a number of other miniDSP products. Do the 2x8, 8x8 or the miniSHARC maybe have less hiss?
IME the noise from the various miniDSP units is pretty low relative to other DSP gadgets. However, if low noise analog circuitry is the reference then you might be disappointed.
Much depends upon the voltage gain of power amplifiers and the efficiency of the speaker system. If either (or both) of those is pretty high you might have some audible noise at distances greater than a few feet.

Are you sure you've eliminated other variables in your system that might be exacerbating the issue?

Cheers,

Dave.
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Old 7th March 2014, 09:45 PM   #8
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MiniDSP has a max 0.9 Vrms output, so to get any bare decent usable wattage out, an amp needs to have a gain of at least 20, I hope you agree
But, again, the problem is the miniDSP noise. I did the following additional tests:
- turn the output volume to min using the plugin: no effect on background noise, still there
- mute the inputs using the plugin: no effect on background noise, still there
- switch from analog to digital source using the plugin (no digital source connected): no effect on background noise

As switching between analog/digital inputs has no effect on noise, do you think it's possible that the problem is in the internal DAC, or maybe the output filters? Has anyone seen the schematics maybe?

If I'm not mistaken a user reported noise issues that were fixed by replacing a diode, does anyone know which diode was that?

I'm wondering if all other DSPs really have the same issue. Does anyone have experience with the DCX2496 or the Hypex DSP or the miniSHARC+DAC maybe?
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Old 8th March 2014, 02:37 AM   #9
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Default Reduction of Noise.

Why don't you try running the DSP from a 12 Volt SLA battery, or better for an even lower noise floor 4 x Lithium Iron Phosphate Batteries.
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Old 8th March 2014, 02:52 AM   #10
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Default Reduction of Noise.

Why don't you try running the DSP from a 12 Volt SLA battery, or better for an even lower noise floor 4 x Lithium Iron Phosphate Batteries.
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