MiniDSP 2x8 w/phono pre build pls offer suggestions - diyAudio
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Old 5th January 2014, 02:27 AM   #1
DWP is offline DWP  United States
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Default MiniDSP 2x8 w/phono pre build pls offer suggestions

I've built myself my little dream unit based on a MiniDSP 2x8. After years of reading about the benefits of a fully active crossover system (Rod Elliott in particular among others) and hearing a couple nice systems I decided to convert my home built 3 way speakers in my studio. My system has gone through many incarnations in the past 25 years, this is the most dramatic. My only sources in recent years have been vinyl and computer digital but I wanted to keep my options open.

However, this is the first time I've built anything aside from a small tube buffer pre so I'd like some of you folks to take a look and if you see anything glaring that I should address particularly with grounding and power, please let me know.

inside_layout.jpg
The chassis is an old Aiwa CD player, 14" wide and 10.5" deep x 3" high. If you enlarge this image you can see the labeled parts squeezed into the box.
components used:
MiniDSP 2x8 (12V 1.5A)
Vol-FP add-on board
Digi-FP add on board
NuForce U192S usb-digital
kit phono pre board (12V 500mA)
USB hub
external regulated linear power supply, 12V 3A
scavenged parts


inside1.jpg
In this view you can see the Nuforce U192S screwed to the chassis (coax cable in the mail), the back side of the phono pre and my rats nest of cabling which I know I need to clean up some. Under the selector shaft on the left is the 4PDT switch to feed a data signal to the Digi-FP.

inside2.jpg

inside3.jpg

back_panel.jpg
Still room for a S/PDIF pass-through on the back panel if I ever decide I need a digital out or replace the Nuforce.

front2.jpg

Since I decided to put the Nuforce in the box I needed USB power and data running all the time but since the Vol-FP/miniDSP is unresponsive when USB is connected I ran an unpowered hub inside off from the back panel connector and hacked some cables together and put a 4PDT switch on the front panel so I can toggle when I want the computer to talk to the 2x8.

The Vol-FP takes care of attenuation, source selection and provides a IR for remote. The Digi-FP takes the digital input toslink, S/PDIF or AES-EBU and also provides output in any of these forms.

The phono pre-amp is a mediocre board and I plan on replacing it but for now it works just fine. I soldered a 4 wire cable with an inline quick connector onto the analog IN pads on the bottom of the 2x8 and brought it to the rotary switch. Analog is controlled by using one of the 4 DSP configurations, of course you've got to manually turn the knob to select the analog source. The external power supply might help some with the miniDSP but definitely helps the phono pre.

The miniDSP pre amp runs a set of massive (6.5 cubic foot sealed enclosures) speakers with 15" drivers, 6" mids and 4x10" horns that are suspended from a 10 foot ceiling. All drivers are high efficient 95+ db_SPL sensitivity. The room is a 12x16' box, no room treatments to speak of (yet) but plenty of books on the walls.

Driving it all is a pair of old NAD 2200PE amps, an AMC 2445 2/4 channel amp (temporary) and I use my little home built tube buffer with the horn tweeters to attenuate non-digitally and perhaps give a bit of warmth to the high end. I'm considering making another one for the mid drivers just to keep the experiment going - as if it ever ends.

system.jpg

So please, any suggestions are welcome.

Last edited by DWP; 5th January 2014 at 04:55 AM. Reason: added parts list
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Old 6th January 2014, 10:34 PM   #2
DWP is offline DWP  United States
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Is there another place on the forum where this post should go? It's kind of an amalgamation of tech but essentially DSP and computer based so I thought this was the best place for it but really I'm looking for confirmation of my building skills.

No hums or anything, works as it should but I don't want it to short, fry or fail due to stupidity.
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Old 23rd January 2014, 09:18 AM   #3
chakija is offline chakija  Serbia
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This is a very interesting build. Congratulations!
I to have same problem - I have digital source that I like (buffalo 32s) and I also want to play vinyls through my pro-amplifier (crown cts4200). My loudspeakers are MLTL boxes with beyma 15cxa400Fe pro coaxials. I tried to set it with passive crossover with single amp per channel, but I found bi-amped version lot better.
I am currently listening buffalo 32s which feeds 4 balanced channels of crown, and than to passive X-O into the loudspeakers. I think that this unit that you have made is just what I need to integrate turntable into my current system.

Last edited by chakija; 23rd January 2014 at 09:36 AM.
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Old 23rd January 2014, 04:06 PM   #4
DWP is offline DWP  United States
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Thanks for the response chakija, I'm overall pretty pleased with the miniDSP products. Though their tech support is sometimes lacking, there is a good community in their forum. For your set up the 2x4 model would work just fine, the software front-end to set up XO, polarity and time alignment are pretty intuitive.

Since I put up this post I've changed things up a bit, now the Nuforce feeds the miniDSP via a coax connection and I have another digital source coming in to the miniDIGI via toslink. Also added dampening to the enclosure and some shielding around the phono preamp.

Overall I'm still happy with the design but I'm looking for a different phono board that'll fit in my case.
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Old 4th June 2014, 01:28 PM   #5
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Hi DWP,

Awesome project. I don't have much to add on the electronics part, but I'm interested in the speakers end of your system. Can you share what drivers are you using for the 15" bass, 6" mids and the horn?

How is the crossover setting on the mini DSP, do you have to add a lot of PEQ filtering, in addition to the crossover slopes for the bass, mid and tweet? How does it sound?

Any plans on trying a SEOS waveguide for the highs?

Cheers!
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Old 4th June 2014, 08:54 PM   #6
DWP is offline DWP  United States
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rhapsodee,

My old Eminence bass drivers finally needed replacing this past late winter when the old foam surrounds started falling apart. I tried a replacement kit but wasn't happy with the results. I had to match up the new driver specs to the old enclosures so that limited my choices as well. What I've ended up with are some pro audio drivers from a newish company PRV Audio out of Brazil, spec sheets can be viewed online at Parts Express.

PRV 15W700 (27-2,800Hz, 97dB sensitivity) XO @ 27 & 467Hz with a Butterworth 36dB filter
PRV 6MB300 (sealed backs, 310-6000Hz, 93dB) XO @ 467 & 4000Hz with 36dB BW filters

The published curves looked good on these and they fit my needs for the enclosure. The 15's actually would like a bigger box closer to 8 cubic feet for a sealed enclosure and this is why I went with the sealed back mids so I wouldn't have to detract any more volume. I stuffed the boxes with some acoustic stuffing and have got good measured results and bass response sounds tight to me. I'm using 5 PEQ filters on the 15s with only one as high as 6dB, pretty good curve right out of the box. The 6's needed considerably more work with EQ and include a high shelf fliter 10dB gain to even it out, probably because of the sealed backs.

All measurements were done with a EMM-6 mic and RoomEQWizard software through a Focusrite Scarlett 2i2, see graphs below. The first one is the individual drivers EQed and overlaid and the second graph is the room measurement.

The tweets were always the weak, or at least unknown, link in these speakers as they were unbranded drivers mounted in some old metal Magnavox horns from the 60's. Tomorrow the new tweets should arrive from Parts Express - some 7x10" Tractix horn factory buyouts from Klipsch (4000-22000Hz, 99dB). These should be a big step up, the Tractix horn is designed for long axis vertical orientation which is what I need in my configuration. I had been thinking of using some other sort of waveguide for some time now but for the price of these @ $45 each with drivers I'm going to give it a try.

As far as sound, I think it's pretty damn good. Compared to the old passive 3 way crossovers there is no comparison at all in nearly every aspect. The sound is very clean at all volume settings and no audible distortion up to around 110dB on my SPL meter. All the well documented information out there on active systems is not snake oil and the benefits are many. I'm a tweaker though and have now added another tube preamp based on one of Chinese 6N3 kits from Fleabay and use it to attenuate the mids and hope to replace the second NAD and the AMC amps currently in the setup with a couple Class D chip amp kits later this year for the lower power demanding mids and tweets.

My room is actually 16x20' (I mis-typed in my original post) and it's the main work room in my studio. The "sweet spot" exists but one of my goals is for the sound to be pretty even throughout the room. Adding the tube pre in the midrange drivers seems to be focusing things too much perhaps and I'm not sure if it's going to stay in the system but it's only been a week and there's tube rolling experiments to be done and that sort of stuff.

It never ends...
Attached Images
File Type: jpg measure2.jpg (60.6 KB, 92 views)
File Type: jpg system.jpg (38.0 KB, 92 views)
File Type: jpg speakers.jpg (40.1 KB, 204 views)
File Type: jpg room_measure.jpg (94.6 KB, 98 views)
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Old 5th June 2014, 02:50 AM   #7
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Looks great! If you are modifying the box for the new horn, may i suggest that you orientate the mid and tweeter in the vertical axis? With the current horizontal layout, there will be a limited sweet spot like you said, due to the crossover and vertical (now horizontal) lobing off axis between the mid and the horn. Orientating both vertically will widen the horizontal dispersion and widen the sweet spot. The woofer is still ok beside it, because the lower XO frequency between the woofer and mid will have less lobing.
Also, you may want to experiment with toe in of the speakers, after orientating the mid and tweet vertically. There is some literature on horn toe in, up to 45 degrees. It may seem counter intuitive, but the wider dispersion of the horn will allow the sweet spot to extend almost the width of the room. You will not hear one speaker louder as you move sideways at the listening area.

Cheers!!
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Old 5th June 2014, 03:24 PM   #8
DWP is offline DWP  United States
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rhapsodee,

Thanks for the reply and tips. I had planned on reorienting the speakers but you got me to thinking of doing it in a completely different way. The old L-pads are still present from the old passive XO days and they need to come out to keep the box tight anyway. When I was young and foolish I built these giant boxes (they served double duty as stage speakers way back then) I dadoed the baffles into the box a full inch so adding a new " high density plywood baffle over the top of the old one will allow me to recess the 6 and 15" drivers instead of the current surface mounting. The speakers are already toed in about 15 degrees but I'm sure everything is going to change with the box modifications and new tweets so I'll have to remeasure everything once the construction is done.
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Old 6th June 2014, 04:46 AM   #9
DWP is offline DWP  United States
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I'd really like more input on the redesign of the front baffle for these boxes as I've not kept up on speaker design theory for about 20 years.

I'm going to gather my building notes from the "way back" files and move this discussion over to the Multi-Way speaker building forum and reference any advisers back here for background - hopefully soon.
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Old 8th June 2014, 07:31 PM   #10
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