XDR: Esoteric Microphone Preamplifier Sound Quality is Now Affordable.
Room ambiance so detailed you can practically hear the carpet pattern... high frequency resolution that defines cymbals, triangles and bells down to the molecular level... midrange that's as warm, fluid and comfortable as a warm bath... and steel-fisted bass that's intoxicatingly rich in harmonic texture.
1. A mic preamp should be able to protect itself from destructive external forces.
2. A mic preamp should not be prone to Radio Frequency Interference (RFI).
3. A mic preamp's response should not vary with different mic cable impedance loads.
4. A mic preamp should be accurate, sound good, and have very low noise and wide bandwidth at any gain level.
Many high-end outboard mic preamps achieve Goals 3 & 4, but lack protection and good RFI shielding. Conventional mixer preamps often meet Goals 1 and 2 at the expense of good sound1. XDR represents major advances in all four critical areas.
Slow degradation (and eventual failure) of mic preamps resulting from hot patching is one of the mixing console manufacturing community's dirty little secrets. If you route a phantom-powered mic through a patchbay, it's roughly equivalent to shorting out a cable every time you patch it. The mic preamp's input transistors progressively break down (called zenering), while the mixer channel gets noisy and can eventually totally croak.
Mackie engineers weren't interested in any "solution" that even slightly affected the preamp's sound. A lot of research and considerable, old-fashioned trial and error listening resulted in the addition of ultra-high-speed, large-geometry input diodes to the front end of our input circuit. It completely protects XDR mic preamplifiers from the consequences of hot-patching and direct short circuits in cables carrying phantom power. Yet it has no affect on sound quality.
A mic preamp has to be able to amplify faint one-millivolt2 input signals up to a thousandfold (60dB). In the process, its rectification components can also pick up radio frequency interference (RFI) from AM and FM stations, cell phones, and pager transmitters – even microwave ovens – and amplify them to audible levels.
The more capable of conveying fine sonic details and wide bandwidth a mic preamp is, the more prone it is to picking up the nearest trucker's CB or All Sports Talk Radio station.
We assaulted RFI on three fronts. First, we incorporate bifilar wound DC pulse transformers with high permeability cores that reject RFI but don't attenuate the sound at 20kHz and above. Second, we carefully matched high-precision components for critical areas of the XDR mic preamplifier. Third, we direct-coupled the circuit from input to output and used pole-zero-cancellation constant-current biasing (which also avoids increased intermodulation distortion at high common mode signal levels).
Bottom line for the non-technical: you can use our new XDR mic preamps at the end of extremely-long cheap mic cables in an RFI-saturated urban environment while talking on your cell phone and microwaving coffee... without hearing a trace of RFI.
A microphone's frequency response (and thus how it sounds) is a function of the load presented to it. That load is the impedance characteristic of the mic preamp it's connected to. If a mic preamp isn't designed right, it will actually sound different depending on the impedance of the microphone and the cable load3!
What we've done with the XDR design is create a Controlled Interface Input Impedance system that's able to accept an enormous range of impedances without compromising frequency response. The graphs at left show XDR frequency response with 50-ohm, 150-ohm and 600-ohm loads. Note that the charts are calibrated in TENTHS of a dB. As you can see, at any load, frequency response is virtually unaffected down to just one tenth of a dB at 20Hz and 20kHz no matter what the load.
We call our new design "XDR" because it has over 130dB of total dynamic range. That means the XDR mic preamps on our VLZ Pro mixers can handle inputs from 24-bit, 192kHz sampling-rate digital audio workstations without added coloration. It means you can generate 145dB sound pressure levels at the diaphragm of a suitable microphone and get a signal that doesn't break up.
But wide dynamic range is valuable only if the noise floor stays low at normal gain levels.
Equivalent Input Noise (E.I.N.) is a common mic preamp noise spec. But this oft-touted specification is traditionally measured at maximum gain levels. Normal operating levels are most often between +20 and +30dB of gain. Thus many mic preamps that brag about their -129dBm E.I.N. at +60dB generate far more noise at +30dB where you're going to end up using them much of the time.
The graph at right charts E.I.N. versus gain level for three mic preamps that boast -129dBm at +60dB. The measurements represent: a) a high-profile competitor's mic/line mixer preamps; b) our new XDR design and; c) a "status" outboard mic preamp retailing for about $2,000. As you can see, our XDR design maintains lower noise levels in the critical +20 to +30 range than either competitor. XDR mic preamps can handle input signals from a full +22dB line level (thanks to extended dynamic range) down to microvolts without a corresponding increase in noise.
While specifications are only one measure of how a microphone preamplifier sounds, XDR mic preamps match esoteric outboard designs number for number:
0.0007% Total Harmonic Distortion. A THD figure this low has never before been offered on affordable mic/line mixers (and only rarely on $500,000 mega consoles, for that matter).
Near "DC-to-Light" bandwidth – without RFI side-effects. Not only are XDR mic preamps flat within a tenth of a dB across the bandwidth of any known microphone, but they're also only 3dB down at an astonishing 192kHz, a figure that should satisfy anyone in search of elusive upper harmonics soon available with emerging ultra-high sampling rates.
Super-low intermodulation distortion at high operating levels. Or to put it another way... total independence and stable operation no matter what signal level you send it. This is due to our instrumentation-style balanced differential architecture, linear biasing, and use of DC-coupled pole-zero-cancellation constant current that frees the mic preamp from induced dynamic intermodulation distortion.
We can sling Audio Precision® graphs around 'till the cows come home and still not convince the truly critical listener. When it comes to mic preamps, the proof is in the listening. Plug a really, really good condenser microphone into an XDR mic preamp and do some critical auditioning.