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What happened this time?! EQ With a Healthy Side of Hum.



What happened this time?! EQ With a Healthy Side of Hum.

So I was on this job site the other day, with the classic set of challenges. The client had bought a high-end 2-channel system from the integrator: Wilson Duette speakers, Linn preamp, Moon/SimAudio power amplifier. After it was all up and running, the client complained there was "not enough bass". So, the integrator sold him a couple of JL Audio subwoofers and asked me to come along and make the whole thing work - setting crossovers, time-aligning the speakers, balancing levels, equalizing the response, and voicing. We used a trusty Ashly ne4400 DSP audio processor; it’s a pro-grade digital audio processor with EQ, crossover, delay, level - the works.



When I got to the location, things were a far sight worse than just "not enough bass". The installation of the EQ hadn't gone quite right; they had no audio signal, but a *lot* of hum. Client was steamed. So in I dove.

In cases like this, there's rarely just one thing wrong. First up the hum: The subwoofers were on different circuits. One sub was at the back of the room, and one at the front of the room to help with modal distribution, so we couldn’t magically get them all on the same circuit. My AC polarity voltmeter told us that one of the subwoofers had a power outlet wired in reverse, with the hot to neutral and neutral to hot, so we quickly remedied that. There was still hum in the speakers, but we decided to get the signal flowing before tackling that monster.

It seems in all the confusion that the audiophile cable company who custom made the interconnects to and from the EQ either didn't understand what we needed or didn't know how to wire a single-ended to balanced connector. They got some pins wrong and wires shorted, which led to the whole no-signal thing. Further complicating matters, someone thought the Ashly had XLR connectors, but it has Euroblock/Phoenix. So connectors had been hacked off and maybe there was some incorrect tie-down of the wires to the contacts. Regardless, signal wasn't flowing due to badly made wiring. We fixed that and got the signal back.

But there was still hum... In pro audio, the shield of a balanced cable is usually not tied to ground on both sides as a matter of practice to avoid hum loops. It is usually tied to the source side only. However, on these mixed signal systems, I typically leave the shield drain connected on the Ashly EQ side for both input and output. We did that, and it helped, but the hum was not gone (Like I said, there was a lot of hum).

At this point it had been a few hours, and the client poked his head in to inject, "WTF is going on in here? You guys are still at this?" Yes, sir; it's complicated!

After some more head-scratching, we discovered that the Linn was not properly earthed through the power connector. Attaching a ground wire from the Linn chassis to a ground pin on the Ashly Euroblock took care of the remaining hum.

OK, now I could start running some speaker and acoustical measurements. The room was big and reverberant (0.8s of reflection decay time), the speaker spectral balance was wrong and the soundstage imaging wasn't even close. The pair of speakers on their own displayed no significant bass below 400Hz (See the chart). The client was right; there was no bass, even in the speaker range, above the subwoofers!




EQ can do wonders for spectral balance in live rooms, and it did here. See the resulting chart:




In the end, we got it sounding pretty good. There still wasn't strong imaging, but it was improved, and at least there was nice, strong bass. We played some known program material with good quality bass, and confirmed that the system had all the bass we wanted. So we expected a happy client (finally).

Nope. The integrator got basically the same complaint a short time later. Not enough bass. Except this time the client was really pissed because of all the extra money and time we'd spent (not) improving the bass.

Back to the site. We put on some food music for the client, and asked if he heard the bass. Yep, he loved that. "So what's the problem?" we asked. Then he pulled out what he'd been listening to...some old rock 'n roll records from the early 70s. *Sigh*. Sorry, but James Taylor recordings don’t have the same bass levels as Cold Play or Daft Punk!

Talk about compounding errors! No bass from the speakers due to room acoustics; no bass in the music to start with; and an expectation of thunderous dynamics from having spent so much money. With some education and some tuning, we fixed all that.



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