Minidsp 2x4 noise floor

[SunShade]

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?

 

[Davey]

Do you notice excessive noise from your speaker drivers with no music playing??

Dave.

 

[SunShade]

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?

 

[Davey]

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.

 

[SunShade]

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?

 

[usul27]

-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.

 

[Davey]

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.

 

[SunShade]

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?

 

[Davey]

Yeah, that's a good suggestion to try (temporarily) with a battery. That would eliminate any power supply issues you might have.

If the noise is still too much for you, then I think this is not the gadget for you.

Dave.

 

[SunShade]

Yeah, that's a good suggestion to try (temporarily) with a battery. That would eliminate any power supply issues you might have.

If the noise is still too much for you, then I think this is not the gadget for you.

Dave.

I just tried using a 9V battery. Any residual 50Hz noise that was still there (just a faint) is now gone, but the white noise hiss is still there and I can hear it if I place my ear at about 5-8" from speaker (mine is a very quite environment).

Overall, I don't think there is a way to further improve and I have to say I'm disappointed, not sure the miniDSP can qualify as a "Hi-Fi" gadget

Thanks to everyone for the help, though

 

[Davey]

I'm not sure what you're expecting for "hi-fi" noise performance. 🙂 If you have to put your ear within 12 inches of the speaker driver to hear noise, that's actually pretty darn good noise performance for many digital gadgets like these.

Of course, there's much more to the miniDSP capabilities than (marginal) noise performance. Maybe you're weighting this aspect too high? When you have music playing you won't hear it anyway.

Cheers,

Dave.

 

[Davey]

For a non-subjective reference, here's a spectrum analyzer look at the noise from my 2x4 unit with nothing programmed and inputs shorted. This is a 625khz linear window...blue is the output of my 2x4 unit and red is the other channel of my spectrum analyzer with input shorted for a baseline reference. The miniDSP output noise is equivalent to approximately -86dbV across the measured bandwidth.

It's possible to shunt the output with an appropriate value capacitor to create a low-pass filter using the miniDSP output resistance. This will reduce the above audio band noise which could be causing some sort of modulation into the audio band with certain power amps. I haven't found this to be necessary with any of my power amps, but the option is there.

Dave.

 

[SunShade]

For a non-subjective reference, here's a spectrum analyzer look at the noise from my 2x4 unit with nothing programmed and inputs shorted. This is a 625khz linear window...blue is the output of my 2x4 unit and red is the other channel of my spectrum analyzer with input shorted for a baseline reference. The miniDSP output noise is equivalent to approximately -86dbV across the measured bandwidth.

It's possible to shunt the output with an appropriate value capacitor to create a low-pass filter using the miniDSP output resistance. This will reduce the above audio band noise which could be causing some sort of modulation into the audio band with certain power amps. I haven't found this to be necessary with any of my power amps, but the option is there.

Dave.

yes the background noise is not much, but in my application I'm using six 2-way speakers, which means that the noise will be x12, most probably audible from a few feet when no signal is played through.

The spectrum analyzer screenshot is interesting. I wish I had such an instrument.
In your opinion, what is causing the ~470KHz ~570KHz noise, which seems to be very very narrow band and not related (maybe a harmonic ?) to any clock/frequency used by the board?

Also, as you have the instrument 🙂, may I ask what is the typ noise floor of an amp (with gain 20 or more, average efficiency speakers) that is silent even to ears placed very close to the speaker cone?

If you maybe have it to test, what is the noise floor of Curryman's DAC?

 

[Rullknufs]

I found this thread through googling minidsp background noise. I am experiencing the same problems as SunShade. I use a pair of 4" CSS EL70 full-range drivers in a near-field setup. The driver sensitivity is ~84-85dB if I remember correctly. I use a Harman Kardon HK3490 amplifier connected to my computer with a 10m optical cable. Without the MiniDSP I can't hear any background noise at all in my listening position but if I put my ear to next to the drivers I can hear a faint white noise. If I connect the MiniDSP I can clearly hear a white noise at listening position which is really annoying when listening to classical music a low volumes.

I had found another thread a while ago suggesting trying a 9v battery to eliminate noisy USB power supplies. I tried this out tonight and I have written more in detail about it at my blog. Using a 9v battery didn't reduce the noise at all, so I suppose it is the MiniDSP itself making the noise and not the power supply. Has anybody found any solutions to this?

 

[Davey]

I'm confused by your setup. You are using the volume control of your Harmon Kardon receiver as your main system volume control? If so, that should effectively reduce the noise produced by the miniDSP unit.

Dave.

 

[Rullknufs]

I'm confused by your setup. You are using the volume control of your Harmon Kardon receiver as your main system volume control? If so, that should effectively reduce the noise produced by the miniDSP unit.

Dave.

Yes, I do. The HK has pre-out and main in connectors which are normally bridged, see bottom left in this picture

I connect the MiniDSP in between those two and the background noise is the same regardless of what volume I chose on the amplifier. Therefore it is no problem when playing loud music but it is very audible on lower volumes.

 

[jan.didden]

Yes, I do. The HK has pre-out and main in connectors which are normally bridged, see bottom left in this picture I connect the MiniDSP in between those two and the background noise is the same regardless of what volume I chose on the amplifier. Therefore it is no problem when playing loud music but it is very audible on lower volumes.

That proves that its not the preamp that causes the noise.
If the noise is also gone when you connect pre out directly to power in, that proves that the minidsp causes the noise.

Do you need to turn the vol control down a lot when using the minidsp?

Jan

 

[Davey]

Yes, I do. The HK has pre-out and main in connectors which are normally bridged, see bottom left in this picture.
I connect the MiniDSP in between those two and the background noise is the same regardless of what volume I chose on the amplifier. Therefore it is no problem when playing loud music but it is very audible on lower volumes.

The only (potential) solution would be to attenuate the output of the miniDSP unit before it enters your power-amp inputs, or switch to a power amplifier with less voltage gain.

Since your setup is near-field you most likely are not using a significant range of the miniDSP 0.9VRMS maximum output. So, either of those solutions might be successful.
It's also possible a low-pass filter just above the audio band might improve your situation. See my comment in post #14.

Dave.

 

[Cask05]

Overall, I don't think there is a way to further improve and I have to say I'm disappointed, not sure the miniDSP can qualify as a "Hi-Fi" gadget.

Everyone that I've talked to about using miniDSP on another forum has had severe noise issues. No one has come up with a solution. I've been recommending other solutions, due entirely to the reports of noise with what some call "high efficiency loudspeakers", which includes sensitivities as low as 99 dB/1 W at 1 m. This is 10 dB lower sensitivity than my TAD TD-4002 drivers, so I would neither use nor recommend use any miniDSP product until the company moves to address the issue and post the test results.

So, I agree with SunShade: miniDSP apparently isn't a real hi-fi device, but rather a low cost DSP kit that might work on much lower efficiency loudspeakers.

I instead recommend scouring used commercial digital crossovers (Xilica, EV, Ashly, Yamaha, etc.) that will do the job without that noise and with a much lower price than list.

YMMV.

Chris

 

[Rullknufs]

That proves that its not the preamp that causes the noise.
If the noise is also gone when you connect pre out directly to power in, that proves that the minidsp causes the noise.

Do you need to turn the vol control down a lot when using the minidsp?

Jan

As I said, with the MiniDSP not in the loop there is no audible noise. The volume is the same with or without the MiniDSP.

The only (potential) solution would be to attenuate the output of the miniDSP unit before it enters your power-amp inputs, or switch to a power amplifier with less voltage gain.

Since your setup is near-field you most likely are not using a significant range of the miniDSP 0.9VRMS maximum output. So, either of those solutions might be successful.
It's also possible a low-pass filter just above the audio band might improve your situation. See my comment in post #14.

Dave.

How do I physically implement any of these things you mentioned? What components are needed? Sorry, I am new to this.