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Old 16th April 2014, 12:41 AM   #1
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Default Need more gain from your MiniDSP?

Hi all! I have posted here a few times before, but want to share a "find" that may help some of us. I have been a happy (for the most part) user of a 2x4 (the EQ, not the lumber) for a few years. However, I have often wished more output was available than the 0.8 V (if I remember correctly). First, let me note that (1) this is the unbalanced version and (2) my situation may be unusual because I use my MiniDSP in place of a Bose EQ (there is an older thread precisely about that) and combined with room EQ. It works fine, but the problem is my curve is around 20-30 dB between its min and max. In practice that means most of my music is playing at least 30 DB below probably more like 40-50 dB, below the max output of a MiniDSP. This problem is compounded by my using "pro" level amplifiers (Behringer iNuke NU3000) which will take unbalanced or balanced inputs.

A partial solution is to use balanced (XLR). You get more signal level. I may try a MiniDSP with balanced outs next time. Or...

Use a line amplifier stage. I've tried conventional stereo pre's (unbalanced) but they seem to add too much noise. I am going to try something new (and cheap!):

Behringer: ULTRALINK PRO MX882

For about $100 (delivered, USA) this box does all kinds of things. I had to read in the manual but it will do up to six channels "level matching", in/out can be unbalanced/balanced, attenuate or gain up to +15 dB. I am not sure of the max signal levels but I think it can put out +20 dBU which I think is pretty hot. "In theory" almost no noise of its own should be added, nor (if you use balanced in's) will any from the input ... except what comes from the MiniDSP 2x4.

I will report on how this works. But I am eager to share it, because this is by far the most practical and cheapest solution I've found. (Also, worth noting I am bi-amping so I will run the sub channels through the unit too.) This may give me a chance to try the 2x4's external volume pot.

Yours truly, Soldermizer. Shameless Behringer shill (actually, no commercial ties, just a happy user.)
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Old 16th April 2014, 03:36 PM   #2
Davey is offline Davey  United States
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Charlie Laub was working on some add-on boards that would allow much higher voltages from the unbalanced 2x4 outputs. That would seem to be a much simpler solution.
Not sure where he is with that project.

Dave.
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Old 16th April 2014, 04:11 PM   #3
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That Behringer could work fine. I would be concerned that you would not be able to get good level matching between channels. If you did some careful measurements and set it once so that all controls gave +10dB that could work I guess.

As Davey mentioned, I designed some analog filter boards that can be configured as many different types of functions, including a gain block. They look like this:
Click the image to open in full size.

This topic has come up before in this forum. I built up one of my board to have 20dB gain and did some testing of the distortion and FR. See below:
Click the image to open in full size.

Click the image to open in full size.

The advantage is that you can set the gain using 1% or less tolerance resistors and it will be consistent from channel to channel.

Each board has two independent channels, so you would need four of them for 8 gain stage channels. Feel free to contact me if you want to buy a couple of these. You can use a simple LM317/LM337 power supply that you can get on Ebay, assembled for about $25 IIRC. I used this to power the board while doing the testing shown above, with everything just sitting on my desk next to my computer. In a metal case the noise floor should be lower still.

-Charlie
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Old 18th April 2014, 02:32 PM   #4
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The MX882 has arrived. Of course I had to set it up as soon as possible

First impressions are favorable. I don't hear any added noise. I am using unbalanced inputs from the 2x4 and balanced (XLR) to the iNukes. My application they call "level matching" and it is on page 14 of the pdf manual. You gotta love the 21st century: the paper manual supplied says to read or print the online full manual

Thank you for the DIY tips guys, but you are dealing with a Soldermizer who is (1) lazy (2) inept and (sometimes) (3) practical* At $100 the MX882 is hard to beat for what it does. I declare it the Swiss Army Knife of preamps At some point I will do a level matching. Test tones, multimeter and db meter I have.

* I am not 100% practical. I have actually built a few kits in the past several years (they even worked!). My speaker antics are findable here and at The Other Forum That Shall Remain Nameless but is known for being a REW peddler
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Old 21st April 2014, 01:55 AM   #5
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Hi again. Still having good results with the MX882. Tonight I did a "test" with the input muted (WD media streamer), and cranked up the gain on all three MX882 channels in use and same on the NU3000s. I do hear noticeable noise but only from the Bose 901s ("highs"). However, the 901's NU3000 normally has its gain set at maybe 1/2. And for the MX882, I get more than enough loudness below 0 dB gain (and they go to +15 dB). Sorry I don't have precise figures but I don't have an o-scope. Can readings be done with just a DVM?

Lacking hard data, I continue to be satisfied with the MX882 as an inexpensive and no added noise "preamp" with 6 (8? not sure) usable channels. of which I only need 3 for now

I may try the 10K pot on the 2x4 for volume control and then I would not need to adjust three channels on the MX882.

Here is a shot of most of my current system. It changes from week to week. As you perhaps can tell from the decor, appearance is not a priority. The green object is a goat skull I got in Mexico and spray painted green because -- why not???
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