TOPIC:

miniDSP has finally made my dream a reality! For more than a decade, I've been trying to find the equipment needed to build a multi-amped surround sound system with fully digital signal stream (up to the DAC) and programmable DSP on every channel. Years ago I posted several topics to the miniSHARC forum on this subject, including a wish list of capabilities. At that time, the miniDSP products were not quite capable of what I needed, primarily due to shortcomings in master/slave I2S capability. A firmware update eventually addressed the issue, and I ordered the needed modules in October of 2014.

The system is now fully designed, built, tested, and ready to support my custom loudspeaker system. Here is a brief list of features, followed by some pictures. I hope they inspire you in your own efforts!

• USB digital audio input for stereo or surround sound formats up to 7.2 channels
• Mac and Windows compatibility
• Two independent O2 headphone amps available on the front panel
• Loudspeaker music channels are split into 3 separate channels for subwoofer, woofer, and tweeter
• Each output channel and headphone amplifier has a dedicated DAC
• Subwoofer signals are summed to drive a single left or right subwoofer as appropriate
• Careful attention has been paid to turn-on sequencing in order to eliminate startup and shutdown transients
• A custom microcontroller board manages power supplies, power sequencing, mute, headphone amplifier gain, and remote amplifier triggers
• A custom motherboard provides I2S line drivers, and replaces much of the necessary wiring for connecting the miniDSP cards
• Each motherboard accommodates 2 miniSHARC cards and up to 5 Curryman DAC cards.
• Each motherboard provides 8 rear panel output jacks, and the boards can be daisy-chained (there are two in this project)
• The output buffers on the DAC boards have been depopulated. They are power hungry and my testing indicated no performance benefit
• The motherboard accommodates op-amp output filter/buffers (sallen-key topology), but they aren't currently used
• In its present configuration, the output jacks are driven directly by the DAC outputs with no intervening buffers

There are many, many, careful design decisions behind these choices, including countless hours of measurement and testing. Yet for me, the fun part is only beginning. Now I have the tools to really make my custom loudspeakers shine!

Oh goodness! David, where have you been spending your summer?
Your box is part of the top designs we've seen to-date... 4 sharc hey?, I guess you like Horse power... hehehe
All running 4x8 plug-in we're guessing?

We can only imagine from your statement:
"There are many, many, careful design decisions behind these choices, including countless hours of measurement and testing."

that it is a true representation of how much work you've been putting into this project.. We're really impressed and will give you quite few KARMA points for that..

Thanks again for sharing and we can't wait to hear more of your setup.. if you're spending as much time on building the speakers + filters, the result will be out there..

Devteam

Oh goodness indeed!! What a setup!

I have one question. I know it's a detail compared to the rest of your work, but it's one of the many blocking point of my own project.
I guess when you plug a headphone on the front panel, it disables the speakers' outputs. How did you manage that functionality?

Thanks for your ansewer, and thanks again for sharing your project!

Actually, the speakers don't automatically mute when you plug in a pair of headphones. One of my design requirements was to allow listening to speakers and headphones at the same time. My father is hard of hearing, and now he can blast his headphones while the rest of us listen to the speakers at a comfortable level.

The speakers are easily muted by pressing the front panel button above the speaker icon, which turns off the +12 VDC trigger controlling the external amps. This functionality is handled by the custom control board.

If you really want an automatic mute, you could panel-mount the O2 controls (rather than board mount) and use a switched stereo jack to detect the insertion of the headphone plug.

Ok, i see how to implement that on my project! thx!

devteam,

Thank you for your kind words. I'm recently retired, so now I get to spend all of my time doing what I love to do.

As you presume, I used the 4x8 plug-in (48 kHz, more on that later). This project probably doesn't need the full horsepower of 4 SHARCs, but I have 8 source channels of I2S data and each plug-in accepts 2 channels, therefore 4 miniSHARCs. I used the SPDIF channels to pass the left and right subwoofer data from one miniSHARC to the next. BTW, I'll be posting my plug-in wish list soon .

For those of you new to DSP, the miniDSP team has done a fabulous job of shielding you from the pain of programming DSP chips (which I've been doing professionally for 25+ years) and allowing you to concentrate on the fun part: designing the filters that will enable your systems to live up to their full potential. The real-time plug-in interface is a unique and wonderful aspect of their products. My heartfelt thanks go out to the miniDSP team for making this project possible. I wouldn't have done it without you!

Just for fun, I've attached the system block diagram that I used to convince myself that the project was feasible.

Here are some early results from my Prism loudspeaker system using the 7.2 surround sound processor constructed from miniDSP products. The system is named Prism because of the shape of the loudspeakers, with the woofer baffle at a 45 degree angle to the tweeter baffle (more on this at a future date). I've spent a few days designing and testing the DSP for my main left and right speakers, and have attached figures of the resulting frequency response, impulse response, and step response. These speakers cross over to a subwoofer at 90 Hz. All data was recorded using a miniDSP UMIK-1 microphone. Note that the oscillations in frequency response that occur between 100 and 1000 Hz are due to a comb filter effect of the direct wave to the microphone interfering with the reflected wave from the floor. This is entirely expected and should not be equalized or the power response will not remain flat. The oscillations have nothing to do with speaker performance, and everything to do with the placement of the speaker with respect to reflecting surfaces. You will see that I use 1/12 octave smoothing for all of my frequency response plots because it is precisely the width of a single half-tone, e.g., the difference between C and C#.

Great feedback David and thanks for sharing your updates!

DevTeam

your sound Q could be greatly improved by replacing your IC Opamps with these discrete analogs from BURSON.... I am using them in my PC 7.1 analog sound card and the sound quality is extremely open and coherent... be aware that that require a significant burn in period of 30-40 hours but the results are amazing....well i now see your opamps are bypassed

Burson Opamps

rts100x5,

Thanks for the suggestion. As you observed, however, there are no op amps in the speaker signal path of this project. I even removed the JFET buffers from the Curryman DACs. I could not measure or hear any audible benefit to the buffers, and they require additional power supplies to drive them. The only op amps in the design are in the O2 headphone amplifiers with which I am well pleased. You can find extensive documentation on the O2s at the designer's web site (search for NwAvGuy).

Note that I am not fundamentally opposed to op amps or transistor buffers, but they should serve a purpose and provide a benefit. Otherwise they are just burning power and adding noise/distortion. In my application, I could detect no benefit. Every application is unique, however, and you should always evaluate (i.e., test) the pros & cons for your own situation.

Oh my what a brilliant build David. I have been thinking about doing a similar thing but was unsure of the parts needed for the build. Are you able to use a minisharc and input all surround sound chanels into it, and then chose which outputs match to which inputs?

EG. input. i2s 1 lr (suround FL FR)----output to i2s1LR, i2s2LR and i2s3LR. (am using 3 way active speakers for front left and right.
input i2s 2 lr (surround rear R and rear L)----output to i2s4LR
input i2s 3 lr (Centre and subwoofer.1)

---

output to i2s5LR.

outputs then to some dacs then to amps then to speakers.

Hi nissman. The 4x8 plugin is the only one available for the miniSHARC. It has 4 input channels, but 2 of them are dedicated to I2S signals and two to S/PDIF. My source signals are all I2S, so I can only bring two input channels into a single miniSHARC. Take a look at my post #21168 (above) for more details. You will also find a block diagram of my system design that illustrates how I used the I2S inputs for source material and the S/PDIF signals for bass management.

I find the input configuration of the miniSHARC very limiting, since most users will have either I2S or S/PDIF source signals, but not both. I don't know if this is a limitation of the hardware or the plugin. I've sent a query to the design team to get this question answered, but haven't heard back from them yet. I have found hints in other forum postings that it may be a hardware limitation.

With regards to outputs, the plugin allows for routing of input signals through your signal processing chain to any and/or all 8 output channels. It also supports input mixing of multiple inputs.

Best of luck with your project!

Thanks for the reply dave. So it looks like the only way I can do what I want would be to do something such as you did and use multiple minisharcs. I was hoping it may be able to do using only 1 minisharc. looks like I may need 3, 1 for front channels, one for rear channels and one for center. and use spdif for bass chanel as you have done. Am using 5.1 here.

Cheers

Simon