Thursday, September 8, 2016

Setting up the new Bass crossover

Stack of DEQ units on right is new Crossover
On Labor Day weekend I finished the last part of the new living room system crossover design I have been working on for the last couple years.  I removed the old Behringer DCX 2496 digital crossover which has only analog outputs and installed the new Behringer DEQ 2496 for the bass section.  The DEQ has digital inputs and AES digital outputs which are connected to a new Emotiva DC-1 DAC which drives the two subwoofers through balanced cables.  So now my 3 way system uses all Behringer DEQ 2496 digital processing and connections for the crossover section and 3 high quality specially chosen DAC's for each way.  The Emotiva DAC is the only Sigma Delta type DAC in my system because I feel that Sigma Delta--which I believe corrupts high frequency timing integrity and thereby loses or fakes high frequency information--is appropriate for and only for the subwoofer section.  I was hopeful that the DC-1 might be direct coupled, but I see that direct coupling become unpopular recently (Oppo went from direct coupled to capacitor coupled in their latest TOTL player, the BDP 105 because of compatibility issues, so I suspect Emotiva has followed similar reasoning) and Emotiva consistently never says whether it is direct coupled so it probably isn't.  Anyway the Emotiva is small and very high quality (very good, though not the very best, noise and distortion specs--which are given quite completely) and had key features I need (AES input and balanced output) at amazingly low price for an audiophile grade DAC and the digital volume controls have been useful.  I determined in a previous experiment (not yet described here) that the volume control on the DAC has less noise than the DAC itself because lowering the volume equally lowers the (very low, but still detectable) output noise level for the first 20dB or so, and the noise is already lower than my Onkyo DAC at 0dB (but not as low as the inputs of my Parasound power amplifier or my Meguro noise meter--making it easy for me to test and make this finding).  The Emotiva has balanced AES inputs and balanced audio outputs which are useful for the subwoofers.  I like it a lot but I could have gotten a truly direct coupled DAC of similar quality from Ross Martin audio for slightly cheaper but the Ross Martin version would not have had the useful digital volume controls which I figured I was going to need for now.

New Emotiva DC-1 DAC on top for Subwoofers has all balanced connections, Onkyo 1704 DAC below for supertweeters

Actually the crossover change was slightly complicated by the decision to put the newest DEQ on the top.  The newest DEQ had a slight chassis warp which I fixed partly by readjusting the rack handles and partly by setting on only 3 feet.  Each DEQ unit sits on rubber feet.  I stick together three 1/4 inch high feet very much like the old Radio Shack feet (which Radio Shack no longer sells, but Newark has an almost exact replacement) to make each foot for each unit and I normally use 4 feet .  But on the top DEQ unit I use two feet in front and only one in back, which eliminates any wobbling.  Well clearly this unit sitting on only 3 feet needed to go on top so as not to endanger the others.  And I figured I wanted the unit on top to show the tweeter response simply for intuitivity.  (BTW, unless you jiggle the stack of DEQ's very hard, you wouldn't realize it was anything other than perfect, it looks perfect.  And that perfect look took a little extra work on my part that I'm describing here.  When I started, the warped unit made the whole stack look bad.)  And since I decided the top DEQ should also be the supertweeter unit, I had to copy the supertweeter adjustments from the #2 DEQ into the newest one before copying the subwoofer adjustments from the DCX being removed to the new #3 DEQ.

For the AES cable to the new DAC, I used a 12 foot Mogami Gold which was plainer looking but actually more expensive than the Geistnote cable (which is made using slightly less expensive Mogami balanced AES wire) I am using with the Lavry AD10 ADC.  Actually I would have gotten another Geistnote cable but they don't make one in 12 foot length, only 10 and 15.

After setting everything up, the real fun begins.  I have to set up all the parametric EQ's, and I use parametric equalization extensively in the subwoofer bass, in addition to the crossover function itself which is realized using parametric EQ's.

It's only when you get to doing this that you realize some limitations.  The Behringer DEQ units are highly flexible, but their only gain control is stereo-only (there is only one gain control, in the Utilities Menu and it only works in 0.5dB steps...I can live with that because I don't think my adjustments are ever more accurate than that anyway, but 0.1dB would be best).

Fortunately, the SVS PB13 Ultra subs have their own gain controls.  I primarily use the left one which has the replacement amplifier with a digital gain control.  I like digital gain controls because the settings are reproducible.  With analog controls, if you accidentally turn the control (it happens to me all the time) the adjustment is lost, and you now may have to go through a complicated "adjustment" process to restore it to the previous setting and it may take hours until you are happy it is done correctly.  So I don't use the gain controls on the back of my supertweeter powering Parasound HCA-1000A amplifiers--I have found that to be especially dangerous because reaching behind the amplifier you might think the volume controls are the speaker binding posts.  In that case, where the gain is much too high for my intended use...and results in excess noise audible with ear to speaker...I use 6dB gain reducing RCA adapter plugs.  Now 6dB isn't quite enough, I could use as much as 12dB though 9dB might be optimal.  I have 12dB reducing plugs, which I had been using in the application, but I ultimately found they added distortion.  I'd love to make my own 9dB reducing plugs someday, or maybe just mod the amplifier.  With this 125W rated amplifier, full power is reached with 1.1V input.  By comparison, my 300W rated Krell requires 2.25V input for full power.  I feel this is too much gain in a lot of cases.  In mine especially since I don't need more than 25W for the supertweeters.  John Curl told me his hands were tied because of THX requirements.  A real nice amp would have control with discrete steps, which could then switch in a potentiometer as one of the steps.

I keep the analog volume control on the right subwoofer in the middle, pointing straight up, because that is a fairly easy position to reset.  The corresponding level on the other side is around -9dB.  It might be better to chose a different level on the right and "block" the control so it naturally reaches that level when turned up.  I'm thinking -3dB or -6dB might be better.  But it probably matters little whether I attenuate at the subwoofer itself or in the Emotiva DAC.

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