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miniDSP Low cost, modular Digital Signal Processor (DSP) kits for the DIYer from miniDSP.

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Old 28th October 2015, 03:04 AM   #41
Davey is offline Davey  United States
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Well, you edited your post, but I think you might be confused by my previous post (#14) regarding the noise level.

Here's another measurement of an output of my miniDSP 2x4 unbalanced unit with no programming (green) and with my soundcard input shorted (yellow.)
ARTA in this mode displays voltages relative to one volt RMS in a 1Hz bandwidth.

This is pretty darn good noise performance.....relatively speaking. This 2x4 unit connected to my 29db gain power amplifier attached to an average sensitivity speaker driver yields audible noise only if my ear is within a few inches of the driver. I have some DSP gadgets from other manufacturers on my shelf that exhibit worse performance than this.

Cheers,

Dave.
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Old 28th October 2015, 02:54 PM   #42
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by kouiky View Post
This noise issue has been a bit of an ongoing behavior for some users for years, along with the absense of power on-off muting. These obvious problems still persist today and need to be addressed in a revision.
I would disagree on the noise there is nothing to address as Davey shows. Consider a CD player with DAC at line level out and a PA with no volume control what do folks do? What I do is use a Goldpoint passive attenuator at the output and of course the amp now may not be able to be driven to full power. I don't edit the music files to use fewer lsb's so I can run the amp full on. There are plenty of CD players and pre-amps with ~90dB SNR's and people do enjoy LP's on their high sensitivity speakers it's all in the gain structure.



EDIT - Davey could you clarify a previous comment, what is the SNR 20-20kHz. I got the impression there was some out of band noise included in the number quoted.
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Last edited by scott wurcer; 28th October 2015 at 03:22 PM.
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Old 28th October 2015, 05:42 PM   #43
kouiky is offline kouiky  United States
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We have to agree to disagree for the moment. I wasn't replying to Davey.

But, since it's posted, his noise performance (shown in that image above) appears better than anyone else's measurements of the miniDSP by order of magnitudes. I am curious of the particulars as to why his approaches -130dB (I'm skeptical), while others are closer to -73 to -83dB in reported similar tests?
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Old 28th October 2015, 05:55 PM   #44
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by kouiky View Post
We have to agree to disagree for the moment. I wasn't replying to Davey.

But, since it's posted, his noise performance (shown in that image above) appears better than anyone else's measurements of the miniDSP by order of magnitudes. I am curious of the particulars as to why his approaches -130dB (I'm skeptical), while others are closer to -73 to -83dB in reported similar tests?
You need to compute the total rms, the little red text at the lower left.

Think about it a second take Davey's setup and without changing anything substitute 103dB speakers for 85dB ones, it gets too loud. Attenuating at the amp preserves the original SNR. Attenuating the digital words keeps the noise and attenuates the music. Listening to CD's you are already at -93db or so at best using the whole full scale of the DAC.
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Old 28th October 2015, 06:06 PM   #45
Barleywater is offline Barleywater  United States
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I had a noise complaint from a friend using miniDSP HD 10 x 10. He was using Carver PM175 amplifier with low efficiency drivers. Carver amplified has input attenuators, and no setting with this resolved the issue; tweeter noise was unacceptably high. He shipped me the miniDSP to see what I could find. I had no problems with my various amplifiers.

Davey's wide-band noise measurement in post #14 is quite telling. miniDSP uses single pole low pass filter, and passes significant wide band noise. Carver PM175 has poor low pass filtering as well.

I shipped the miniDSP back to my friend, and had him purchase a Behringer A500 amplifier, and the noise problem went away.

Apparently miniDSP wide band noise modulates with power amplifier, placing audible noise into output.

Davey's suggestion of placing small capacitor acrosss outputs of miniDSP is step in right direction, but in many cases a two pole low pass filter may be required dependent on low pass filtering/bandwidth limiting of power amplifiers used with miniDSP products.

Yes, it would be nice for miniDSP to own up and revise output filters.
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Old 28th October 2015, 06:13 PM   #46
Davey is offline Davey  United States
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Kouiky,

The "others" are either not measuring correctly and/or misinterpreting the measurements/numbers. I suggest to perform some measurements yourself.

I've switched to my E-MU Tracker-Pre (lower noise than my other soundcard.)
Here's another noise floor test with 96khz SR. Same graphing scale and setup as the one in post #41.

I believe Barleywater is correct and that, in some cases, out of band noise modulates with certain power amplifiers to create noise within the audio band. This probably accounts for the differences of opinion regarding audible noise of the miniDSP 2x4 unit.
If switching power amplifiers is not an option, another might be to couple the outputs via Jensen Iso-Max transformers.

Dave.
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Last edited by Davey; 28th October 2015 at 06:19 PM.
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Old 28th October 2015, 06:21 PM   #47
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by Barleywater View Post
miniDSP uses single pole low pass filter, and passes significant wide band noise. Carver PM175 has poor low pass filtering as well.
This is a different issue, I would think some RFI/EMI protection would be present in the amp. I would think it would be hard to get a sense of broadband white noise from this but who knows. Anyway this was not the described problem, -86dB device directly into 30dB PA = -56dB SNR. The gain structure issues are real and there are numerous resources.

It is very easy to find any number of folks with 100dB or so speakers for whom the simple attenuation works.
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Last edited by scott wurcer; 28th October 2015 at 06:26 PM.
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Old 29th October 2015, 02:41 AM   #48
kouiky is offline kouiky  United States
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Quote:
Originally Posted by scott wurcer View Post
You need to compute the total rms, the little red text at the lower left.
Thank you Scott. Now that I know where to look for that, Davey's value is within range of everyone else's measurements.

Quote:
Originally Posted by scott wurcer
Think about it a second take Davey's setup and without changing anything substitute 103dB speakers for 85dB ones, it gets too loud. Attenuating at the amp preserves the original SNR. Attenuating the digital words keeps the noise and attenuates the music.
I did, actually.
I had written the same thing regarding gain sequencing and speaker efficiency prior to condensing post #40, part of which remains; Attenuation should be after the DSP's outputs and at the power amp's inputs. This assumes a scenario where there's no way to reduce the amp's inherent gain. I'm not sure why you mention shortening word length though, I made no reference to such digital domain attenuation.

Quote:
Originally Posted by scott wurcer
Listening to CD's you are already at -93db or so at best using the whole full scale of the DAC.
There is quite a leap from miniDSP's -78/-83dB to a hypothetical Compact Disc player's -93dB noise floor. A CDP is always attenuated prior to the amplifier through a preamplifier or passive attenuator. It seems that the DSP should be treated similarly as part of the gain structuring sequence, thereby attenuating noise along with the audio signals, ie: post #40.
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Old 29th October 2015, 02:40 PM   #49
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by kouiky View Post
I'm not sure why you mention shortening word length though, I made no reference to such digital domain attenuation.
I know that was just my way of saying that's what you do when you attenuate before the DSP, it runs with only some of its bits not the best idea.
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Old 29th October 2015, 02:47 PM   #50
Rullknufs is offline Rullknufs  Sweden
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Minidsp 2x4 noise floor
Quote:
Originally Posted by scott wurcer View Post
I know that was just my way of saying that's what you do when you attenuate before the DSP, it runs with only some of its bits not the best idea.
So, in my case I should connect my O2+ODAC combo between my computer and MiniDSP and do the DAC there instead of in my HK amp, run full volume there and then connect from MiniDSP to normal input of HK amp. Will give that a try, I think I have enough cables for such a setup.

Edit: I did indeed have enough cables for it and it was a very quick setup. Just removed my headphones from the O2+ODAC, plugged in a cheap 3.5mm->2xRCA straight to the MiniDSP. Connected the MiniDSP to a normal RCA input on the HK. Set the volume to the second last step on the HK before it mutes totally. Slowly turned the volume up on the O2. I'm not sure though, but I think I hear some distorsion if I trn the volume on the O2 all the way up but it's hard to tell, can only play on low volume at the moment because I don't want to disturb my flatmates who are studying. Is there any danger to the O2 by running it on full volume?
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Last edited by Rullknufs; 29th October 2015 at 03:03 PM.
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