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TOPIC: My miniDSP 2x8 pre-amp with phono stage

My miniDSP 2x8 pre-amp with phono stage 5 years 8 months ago #11723

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About a year ago I got my 2x8 and set myself to work converting my studio sound system to an active 3 way set up. I was a bit nervous as the only building I had done before was a little tube buffer but I've been a stereo junky for over 20 years now on the quest for the elusive "perfect sound" and while I've had a few bumps along the way (smoke too) I've learned a lot and have no regrets.

For years now I've only utilized two sources - vinyl and digital via computer and I knew the weak link was the computer, the DAC in the system was about 10 year old tech. So for a year I contented myself learning, reading and drawing plans and was content to keep feeding the 2x8 an analog signal coming from the old USB-RCA DAC. Then I ordered the DIGI-FP and VOL-FP to see if I could cobble together what I really wanted and had some time over the holidays to fabricate. So without further ado, some pics and then if you're still interested some more explaining:



The chassis is an old Aiwa CD player, 14" wide and 10.5" deep. If you enlarge this image you can see the labeled parts squeezed into the box.



In this view you can see the Nuforce U192S screwed to the chassis, 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.





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.





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 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 phono pre-amp is a pretty cheap board and I plan on replacing it but for now it works just fine. I soldered a 4 wire cable with an inline connector onto the analog IN pads on the bottom of the 2x8 and brought it to the rotary switch. All 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. I also replaced the little switching power supply with a regulated linear external beast which might help some with the miniDSP but definitely helps the phono pre.

The power switch was salvaged from an old Nikko Gamma tuner and the face plate is an old intaglio engraving of mine from loooong ago until I make a new fancy plate with labels. The curly maple was salvaged baseboard from an old victorian home. If anyone else is interested I've made an editable pdf file that is to scale of the vol-FP which I plan to use to have the new face plate made. For those of you in the USA, check out Owosso Graphics and what they can do if you're into magnesium, copper or bronze in 16 gauge or ΒΌ inch. In my other life I'm a printmaker and make fine books so for the finish stuff I cull from my art background. I'll probably foil stamp labels for the back panel on thin leather or book cloth and adhere it as one piece.

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. Here's a pic of most of the kit which resides at my work desk in the studio.



And finally, the current working configuration measurements from Room EQ Wizard

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My miniDSP 2x8 pre-amp with phono stage 5 years 8 months ago #11724

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Sorry about the extra posts, must have hit the wrong button when images were uploading or something. Hopefully a moderator will take care of the extra incomplete ones.
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My miniDSP 2x8 pre-amp with phono stage 5 years 8 months ago #11770

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The REW measurement I put in the first post was for refining the individual drivers in the speakers as described in John Reekie's excellent tutorial here. I've also set up some configurations with room measurements but need to work out a good house curve as things are a bit boomy right now. The room measurement gives a perfect sound within a very small "sweet spot" but I'm looking for a more general "all over" sound as this is in my work studio and I move around. For now I'm mostly using the driver correction config with just a few tweaks to the base driver.

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My miniDSP 2x8 pre-amp with phono stage 5 years 3 months ago #14559

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A little update to the project. I've gone ahead and built and added in another little tube pre based on one of the Chinese 6N3 kits on flea-bay. I tried it first in the mid drivers but wasn't happy with the imaging and switched over the little 12AX7 to handle the mids.

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.

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. It has been suggested and I will be following the advice of redoing the front speaker baffles so that the tweet and mid are arrange together on a vertical orientation. The L-Pads shown in the pics are leftover from the passive XO days and aren't connected to anything so a new baffle is in order anyway.

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 couple weeks and there's tube rolling experiments to be done and that sort of stuff.





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Last Edit: 5 years 3 months ago by DWP. Reason: duplicate info
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My miniDSP 2x8 pre-amp with phono stage 2 years 9 months ago #29213

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I thought I'd update this a little since it's been almost 3 years with the 2x8 in my system.

I now use a Schlit Modi2U DAC from my digital sources as well as a Schlit Mani for a phono preamp. New faceplate to cover up the now unused 4PDT switch hole since the Digi-FP is fed by optical coax now and the Nuforce U192S and internal USB hub are unused.

Amps now consist of some home built boards - class AB for the tweets and D for the midrange. A Crown XLS 1500 is used to power the 15" PRV woofers and I've added a 15" Dayton RSS390HF subwoofer to the system in a 4 cubic foot sealed box which is powered by one of the NAD 2200PE amps in bridged mode. Fills out the bottom end nicely and I've still got 1 channel left on the 2x8 to add an additional sub to balance out the room more in the future. As things are now the current XO mostly used is as follows:

sub @ 20 w/ 36LR to 50 w/ 36LR
bass @ 50 w/ 36LR to 600 w/ 24LR
mids @ 600 w/ 24LR to 4800 w/ 24LR
tweets @ 4800 w/ 24LR to 20,000 w/ 12BW

I replaced the sealed PRV mids with some Eminence Alpha 6As and have on order some Dayton ribbon tweeters to see what that's like so I'll post some pics when I get that project together.

I've mostly given up on this thing being a preamp and feed it with an old PS Audio 6.2 passive pre. Much nicer analog switching and manual volume control experience. It is my opinion that the selection of preamp used in this fashion is quite important and audibly noticed. I tried quite a few in auditions, anything from a digital HLLY AMK-2, NAD 1300, Apt-Holman, Kenwood C2, & McIntosh C38 - the PS Audio just sounds the best in my particular system. I don't use the little tube preamps anymore at all so effectively have removed all possible tone controls from the system and the PS Audio pre doesn't have any either.

I'll admit that it's been awhile since I fired up REW and took any real measurements and things are mostly tuned by ear. Once the new tweets are installed it'll give me the push needed to set up everything for measuring once again.

Mostly what I've learned from using the 2x8 during the past 3 years is that for sound, the room is everything followed closely by speaker driver selection and placement for that particular room and room treatments that can realistically be done. DSP is an incredible tool for an active XO and has certainly revealed many new layers of detail to the music as well as a far better understanding of why & how it sounds better. The only drawback is now MP3 compression is quite noticeable on quality recordings but good vinyl mixes have never sounded better.

Cheers to the MiniDSP crew, thanks for the ride.
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My miniDSP 2x8 pre-amp with phono stage 2 years 9 months ago #29234

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@ DWP

Thanks so much for the update on your setup! We're really sorry that somehow we missed in the past to compliment how great your setup is! You also provided some great pics. Thanks so much for sharing all your experience.

We look forward to your new measurements with the new tweets! :-)

DevTream
MiniDSP, building a DIY DSP community one board at a time.
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