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*** The Technical Details ***

for the Testing Setup for the 3D Pre-LP


The signal path for this recording was absolutely minimal. It was mic > preamp > trimmer > Mytek ADC (analog to digital converter) > hard disk. Each preamp did nothing but gain, using the most direct output path.

Lynn Fuston's job was first deciding on the mic choice, mic positioning and appropriate gain, by listening to each source, either female vocal, male vocal, acoustic guitar or snare drum. Once those decisions were made, Dan Kennedy of Great River Electronics trimmed the preamps for identical gain, within .02 dB of each other using a Fluke 8060A meter and a Tektronix oscilloscope. The acoustic calibration source was an Auratone in front of the microphone. It was driven with a 400 hz waveform signal, which was clipped to check for uniform polarity. The Auratone was taped onto a large boom arm and swung into place when it was time to re-cal. The speaker to mic distance was repeated by using a standoff taped to the speaker's front. (For pictures, see the PreLP Pictures page.)

Each mic was connected to only one preamp at a time. This assured that each preamp saw only the input impedance of the preamp we were listening to. The output from the preamp went to the switcher/trimmer box, built by Dan Kennedy with design help from Tim Farrant. The trimmer was required to get the levels precisely matched.The box had passive trim pots for each of ten inputs, followed by a buffer amp. Multiple inputs allowed us to listen in groups, expediting the switching for meaningful comparisons. The switcher output went to the DDA AMR-24 console at Classic Recording where we conducted the listening tests and the digital recorder. For the live listening, we used the preamp output into a Channel Line Input and fixed the level on the Channel fader and Mix fader. The monitoring system was Tannoy DMT-10 MkIIs driven by Harman-Kardon Citation 16 amplifiers. The monitoring volume was 92 dB SPL on peaks, measured by a Radio Shack Sound Level Meter sitting on the console (A weighted, fast response).

The switcher output was also sent directly to the +4 XLR input of a Mytek 8X96 ADC 24-bit/96k A-D converter, set to 44.1 kHz, 24-bit resolution. The 44.1 kHz sampling frequency was chosen to avoid sample rate conversion between the preamp and the listener. This helped assure the cleanest path possible from mic to CD. The AES/EBU output from the ADC went to an Alesis AI-1, to change from AES to SPDIF. The SPDIF output from the AI-1 went to a 24-bit AudioMedia II card in a Radius 8100/110, recorded with Sound Designer II (SDII) software at 24-bit resolution. We also monitored the analog output of the AMII card on the console. We compared the "live" output from the preamp to the ADC/DAC (digitally converted) chain and found the differences we heard between the preamps were also apparent in the recorded chain.

The soundfiles were edited in SDII. No processing was done to the original soundfiles (no level adjustments, compression, limiting or EQ) The recording level was set for reasonable headroom in the analog path. The signal never clipped or touched 0 dB FS (full scale). The assembled master was transferred to a PMCD replication master using MasterList CD 2.2, and was dithered from 24-bit to 16-bit in a single-speed burn. Absolutely NO processing was done to the original soundfiles. Editing was the only step between the studio recording and the signal you hear on this CD. The disc was manufactured using the JVC K-2 sutting system. The technical setup staff included Dan Kennedy of Great River Electronics, George Cumbee-owner of Classic Recording, Phillip Cooper and Kenny Fairris.


"Can YOU Hear the Difference?"


The 3D PRE CD-The easiest way to hear $75,000 worth of preamps in your own studio.


To order your copy of the 3D Pre CD, Volumes 1 and 2, click here.


3D Pre CD Ordering Info | Pre-LP Press | Pre-LP Pictures | Pre-LP Testing Setup |

Main Pre-LP Page | Preamp Lineup | Mic/Pre Shootout Sample | Let's Discuss Preamps-Your $.02


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updated 12:00AM CST on 1.1.2002

*All writing and content is Copyright 1996-2002 by 3D Audio Inc.