It's My Party and I'll Rant If I Want To

Jul 1, 2005 12:00 PM, By Eddie Ciletti

BIRTHDAYS, EAR HEALTH AND DC POWER DISTRIBUTION

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This month kicks off and wraps up with two birthday celebrations — both national and my own, the latter marking a half-century of heartbeats, breaths, strange sounds and geekish behavior. So before addressing a most important health-related issue and zooming in on an SSL dynamics module's power distribution, I'm going to indulge myself.

figure 1: standard transistor and voltage-regulator case styles

In the spirit of celebratory chutzpah, so many things come to mind: semi-mental wanderings, “advertising” for that dream job (Hey, Pixar, you really want me?) or getting a few political rants off my chest. Audio and politics seem to be like oil and water, so 'nuff said 'bout dat, but my summer reading schedule will include Thomas Friedman's The World Is Flat.

Relative to Pixar, Sarah Vowell's vocal contribution to The Incredibles DVD makes me want to nominate her for the Queen of Geeky-Cool Award. (I understand that she considers herself a nerd and that I should, too, but geek works for me.) One final indulgence and then down to the real biz.

FORBIDDEN FRUIT
Would someone please explain why Fiona Apple's album, Extraordinary Machine, has not yet been released? After reading a “bootleg review” in The New York Times, I confess to downloading it before Sony closed down all the fan sites. I've even passed it along, guilt-free, to a few friends. It's a great record that should be released with this warning: “Contains Music. Has Potential to Stimulate Brain Cell Activity,” unlike 50 Cent, who seems focused on activity below the waist. Candy stores just ain't what they used to be.

CANARY IN THE COAL MINE
We all value our hearing, and nothing is more disturbing than temporary hearing loss due to colds or, worse, permanent damage due to high SPLs. Consider that most hearing tests focus on the upper midrange (speech intelligibility and not high fidelity). From here, extrapolate the medical profession's prioritization that your ability to hear well is tertiary to your ability to hear at all — heart health being its primary concern.

Some studies have linked elevated cholesterol and triglycerides with high-frequency hearing loss. In food — as well as in the body — most fat exists in chemical form as triglycerides. If these studies are correct and considering our preoccupation with hearing well, perhaps we can be of service to the medical profession by being the auditory canary in the coal mine. So if you are having odd or intermittent hearing issues, I suggest getting a blood test and making note of cholesterol and triglyceride levels.

Most of the time, my hearing is fine. A sympathetic audiologist went beyond the normal speech intelligibility range, and while I don't hear 18 kHz the way I did at 18, I was thrilled that my hearing was symmetrical. Bandwidth does not necessarily translate into knowing what to listen for; there's a lot to be said for educated ears.

SSL'S NOT WHISTLING DIXIE
And now, on to this month's topic. Twenty years ago I went freelance to help elevate the fledgling project studio environment to technically relevant level. Sounds better than “jumping on the bandwagon,” doesn't it? I dreamed of having my own miniature studio, and now I do, so you could say my belief and passion in a concept paid off. This is why I'd never seen the soft underbelly of an SSL. I never had the opportunity to track, overdub or mix on one, until recently, when I began teaching at the Institute of Production and Recording (www.iprschool.com).

IPR's on-call technical wizard is Julie Gardeski. A recording engineer by trade and a technician out of necessity, Gardeski is a very intuitive troubleshooter because she understands signal flow and can follow the AC signal path on a schematic. We're exchanging specialties — she knows the SSL maintenance protocol, and I know circuits — so when asked for troubleshooting assistance, I jumped in.

DIGGING IN
All channel strips can be interrogated via the patchbay, and with a little practice, anyone can become familiar with op amp-based circuitry. Even with variations, a mic preamp, EQ and summing amp are recognizable in most mixer brands and models. It's a great triumph to successfully find and replace the right IC.

Most dynamics processors have DC control circuitry in the sidechain. But don't be intimidated by unfamiliar parts or circuitry. When none of our dynamics problems responded to op amp swapping, I got out the 'scope and found a more fundamental problem that was power distribution-related. That's reason enough to start at the source and explore circuitry that's common to just about all op amp-based audio circuits.

POWER DISTRIBUTION
A power supply converts the AC line into DC, and a large-format console such as the SSL requires several power supplies — the exact number depends on frame size and online/backup. Most console supplies are of the “linear” variety, while computers and more modern gear have switch-mode supplies, the latter a topic for a future column. Either type is designed to run 24/7, even under less-than-optimum conditions, but that's no reason to ignore such a critical piece of gear.

Heat greatly reduces electronic component life, so if fans are an essential part of the design, confirm that they are still turning. And because fans make noise, the supplies are typically kept in a separate space, preferably with its own cool air connections: supply and exhaust.

As with any sophisticated piece of audio gear, the logic and audio circuits require an assortment of regulated voltages. The obvious benefits of voltage regulation are circuit protection from spikes and surges, and tolerance of brown-outs (when the line voltage falls below normal). Typical op amp-based audio circuits run on bipolar power — that's plus and minus — typically about 15 to 20 volts (30 to 40 volts total).

These two power “rails” are referenced to a common center point that is tied to and called “ground” (or Earth, if you live in the land of Greenwich time). Bipolar voltage regulation ensures that an op amp's no-signal output “lives” at the center of the voltage swing (a virtual zero volts) so that headroom is maximized and clipping is symmetrical. If the supplies become unbalanced, then the op amp's output is “offset,” resulting in switches that will pop, along with pots and faders that sound scratchy even if they are clean.

LIFELINES
With a remote power supply location, the increased distance must now be considered. All cabling has a DC resistance and an AC impedance — both are variables that increase with current demand and temperature — plus the added complexity from console point-of-entry to the motherboard and from there to the modules. Together, the length and number of interconnections contribute to potential noise problems.

Regulation reduces noise — power supply hum and HF noise — and for many lesser consoles, the buck stops here. More sophisticated power supplies have remote sensing: They can “see” what's happening at the other end of the cable (a voltage drop, for example). Adding local regulation at each module or subassembly lowers power supply impedance — and with it high-frequency noise (including the potential for oscillation) — and improves amplifier isolation (cross-talk reduction). All of these must be pushed as far down into the noise floor as possible or risk the potential of being amplified by high-gain amplifiers such as a mic preamp. Individual channel noise (hum and hiss) may seem insignificant until 48 channels' worth are summed together.

Power requirements for large-format consoles tend to rely upon discrete regulation components. The regulated portion of a linear power supply is a zener diode (for voltage reference) and an op amp (discrete or integrated) driving one or more power transistors. For low-power applications, these parts have been integrated into standard transistor-sized packages, such as the TO-3, TO-220 and TO-92 case styles as shown in Fig. 1. This brings us full-circle to the local regulation of the SSL subassemblies.

Each SSL I/O module is a large PCB populated with plug-in cards (“subassemblies” such as the dynamics module), each with their own local regulation; in this case, the 78Lxx and 79Lxx regulator series for the positive and negative rails, respectively. The “xx” denotes the regulation voltage. The 78L15 and 79L15 parts are 5-volt regulators; the “L” denotes a TO-92 package. (See “Voltage Regulators” sidebar on page 128.)

DIAGNOSIS
The SSL dynamics modules were behaving in odd ways, so comparative analysis is your friend. Starting with the schematic and a 'scope, there are test points at the very beginning of the sidechain where the AC signal is rectified into a DC control voltage. It was here that I noticed the problem: a high-frequency oscillation above the range of hearing that also showed up at the op amp power input pins. As best as I could determine, the oscillation was coming from the power rails and being amplified. (High-frequency oscillation tends to be a squirrelly beast to trace, even down to where the 'scope probes are grounded.)

I then swapped the dynamics subassemblies to “good” I/O modules to prove that the problem followed the card (it did) and then suggested capacitor replacement around the local regulators. (See Fig. 2 and the sidebar for an explanation.) Once installed, the repaired channels sounded better than their better functioning neighbors. On a 48-input console, that translates into a desoldering party invite!

READING THE FINE PRINT
The ability to translate schematics into real diagnostic information comes with practice. The AC signal path through an op amp is relatively easy to follow. DC circuits are more of a challenge, but an oscilloscope can reveal so much more than the presence of HF oscillation. No dynamics processor's metering is as revealing as a 'scope. So if you're looking to understand DC circuits in general and sidechain circuits in particular, then get a 'scope and start probing.


Eddie would like to thank Todd Hokenstrum and Brooke Krieger for the SSL schematic scan and Julie Gardeski for sharing her knowledge of the SSL. Visit www.tangible-technology.com for more stuff.

VOLTAGE REGULATORS
All op amp-based regulators, whether discrete or integrated (IC), can amplify AC and DC, so it is very important to address the potential for high-frequency noise and oscillation. Download the application notes for any IC-based regulator, and you will see a typical schematic along with the components that are essential for stable, quiet operation.

Figure 2: The local voltage regulation circuitry for the SSL dynamics module, “raw” power input is at left. Note that the three quad op amps (T5, T6 and T7) are fed before the local regulators by R100 (10 ohms) and D24 (a 5.1-volt zener diode). These ICs are used to drive the LED (gain reduction) ladders. Switching LED current on and off has noise-making potential. Powering them pre-regulator keeps the noise away from the audio signal path. Also note that capacitors C19, C3, C17 and C4 are on both sides (input and output) of each regulator. T3, a quad op amp used in the expander circuit, is isolated (post-regulators) to keep expander/gate control signal noise out of the audio signal path. The 15-volt regulator (TR5) directly feeds T3’s positive power input, while its negative input is fed by R18, a 100-ohm resistor in series with TR6. Downstream, series resistors R102 and R104 are clamped to 5 volts by zeners D23 and D26. These provide a 5-volt reference for the expand threshold pot and a section of T3 used for the gate.—Eddie Ciletti

Regulators should have bypass capacitors on either side (input and output) to keep them from oscillating, and this is just what the bad channels were doing, so it is likely that after years of operation, the original caps were no longer doing their job. For this application, I would choose Panasonic's FC Series (or equivalent switch-mode-grade caps) because they are designed to tolerate high frequencies well and are rated for 105 degrees centigrade (long life under difficult conditions).
Eddie Ciletti






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