Tuesday, May 20, 2014

System back together, DAC's sound different?

A weekend earlier I took the pile of components on the left side of the living room stereo apart and moved the left Acoustat speaker out of the way so I could replace the plate amplifier in the SVS PB 13 Ultra subwoofer.  The original BASH amplifier had died after 6 years of usage.  I had already purchased the replacement Sledge amplifier, and already had it on hand, but it could not be connected to the speaker wires of my PB 13 because the connectors on my wires were too small for the new amplifier.  Disappointed about being unable to put everything back together, I moved on to other household issues, like cleaning the master bedroom floor with Scooba for the first time in 5 months.

I emailed SVS about the dilemma and they apologetically shipped me the "Bash Adapter" during the next week, so last weekend (May 18)  I was finally able to replace the subwoofer plate amplifier and put my system back together.  I noticed that the new Sledge amplifier has hiss that you can hear putting ear next to speaker.  I had noticed funny noise on that side of the room before but didn't remember that the subwoofer had been the cause.*  I checked the right subwoofer, and it also has some hiss, but about 4dB lower and with a lower frequency spectral balance.  This convinces me that I should be using external audiophile grade Class AB amplifiers for the subs, but it will take some time to develop and deploy an external amplifier solution with the required high pass filter and limiting.  (*What I remember now is that many times I heard faint noise or hum on the left side of the room, and couldn't figure out what it came from.  Perhaps one or more of those times I actually figured out the subwoofer was the cause, but that never registered permanently in my memory before.  It wasn't easy to disconnect the subwoofer from the crossover because of the deep corner location.  t also determined falsely or not many other presumed causes, such as an echo from the refrigerator in the kitchen, or the neighbor's AC compressor.  Now I have proven that the subwoofer makes audible hiss, and I don't think that is good, though a vintage amp with hum--as many of my older amps do--could be worse.  The hiss can't be measured because the background noise level in the room doesn't get low enough.  But it can be heard unambiguously with ear close to the speaker.  So there--measurements don't show everything!)

Saturday I went to a meeting of the River City Audio Society, had dinner at Earl Abel's with my lady friend, heard Beethoven and Prokofiev at the San Antonio Symphony, went to a reception at the Plaza Club, and then had an extended conversation with my lady friend mostly about my money borrowing and spending habits--I believe in borrowing and spending money on good things, within reasonable limits, it's good for me and good for the economy, and I will not be shamed or feared to worry about my small debts--that only adds to the problems of others through a deleveraging spiral.  We had some wine but she wanted to go home, but then 30 minutes later she called back and the conversation continued until 1:30am.  A wonderful long day, steam letting and all, but no energy left for working on subwoofer or stereo.

On Sunday I slept late, mowed the lawn, trimmed around the house, lady friend helped with dead tree branch, and then I had dinner until about 9:30pm, felt exhausted again, so took a nap until 12:30am.  By 5am I had bolted in the new subwoofer amplifier and had the system back together in a new way, though I had to cheat on one of the new digital connections.  For some reason, the optical connection from Tact preamp to Behringer DEQ (now being used as crossover for the Acoustat panels) didn't work, so I used an XLR-to-RCA adapter to permit connecting the DEQ via SPDIF coax.  The sound was obviously very detailed, but somewhat dry sounding.

On Monday the AES/EBU splitter had arrives, so I used that to more properly connect both the Behringer DCX and Behringer DEQ to the XLR AES/EBU output of the Tact.  With that configuration I was ready to do some serious listening to Pink Floyd "Wish You Were Here" over Sonos.

The sound was much higher resolution than I have ever heard before!  I heard shadow vocals, and multiple vocalists, where I had never done before.  There's a lot of stuff going on in the background on this album too.  I've listened to it hundreds of times, but this time was far more interesting.  I wasn't planning to listen to the entire album but couldn't stop.

At the same time, I wouldn't say I liked it better in every way.  There was still a peculiar dryness to the sound.  I wouldn't say it was harsh at all, just dry, but perhaps dryness is the first level of harshness.

Some of this may be that I really haven't gotten things set up yet.  Since I went a week without subwoofers, I turned off all my time alignment delays in the crossover.  (Normally, the panels are delayed the most since they are the closest.)  Now, with the new DEQ serving as crossover, I need to do the time alignment differently because the DEQ doesn't have the precise delays possible in the DCX.  Instead of having zero delay on the subs as I did before, I'll have to make a bigger delay on the panels and then do the fine adjustment with the delay on the subs.

But my guess now is that some of the dryness of the sound with an R2R DAC is unavoidable.  I think this is very much as with scaling old fashioned NTSC TV (480i) to HD (1080p).  The upscaled image often seems much smoother, and has no flicker.  But the upscaling can't possibly add resolution.  What it really does is reduce the resolution, just slightly.

Sigma Delta DAC's work by a process much like rescaling.  The input is oversampled to a much higher frequency, 64x minimum I believe.  In that way, sigma delta DAC's achieve great linearity on test signals.  But just as with rescaling video, some resolution from the original recording is unavoidably lost.  The result is a smoother sound, as if you had run a spline through the original 44.1kHz samples.  But that covers up the fact that there is less resolution still there.

Now I believe my Onkyo RDV-1 does do some oversampling anyway, just as most 1541 based CD players from the 1980's used oversampling too, typically 4x.  (Even Sony's first CD player used oversampling.)  But it can't possibly be doing 64x oversampling (otherwise it would have been DSD friendly).  Probably more like 16x or 32x.  Mainly, the Burr Brown 1704 chip achieves a large range of linearity the old fashioned way, with R2R ladder.

I still hope that as I re-do the time alignment, or make other changes, I will get some of the smoothness back.  But even if I don't, I am already hooked on the increased resolution.

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