Equipment Review No. 2 November 2001
R. E. Designs SCPA-1 Six-Channel Preamplifier
[Photo also shows the separately-available SB-1 switcher on top of the preamp]
- Input impedance: 20k ohms nominal
- Output impedance: 100 ohms nominal
- Distortion: less than .002%
- S/N: -100 dB
- Channel trim range: 6 dB to 18 dB gain
- Tracking error: 0.1 dB max
- Stereo separation: 104 dB at 1KHz
- Output drive: 12 v peak
- Frequency response: R+10 k ohms; less than 0.1 dB deviation from 20 Hz to 20 KHz, driving 50 ft. to 100 ft. of cable
- Dim.: 3.47 inches H x 16.73 inches W x 12 inches D
- Rack mount handles available on request
- Power consumption: approx. 8 w
- SRP: $2700
- Sold direct mail from Shamrock Audio,
- 235 Coolidge St., Silverton, OR 97301
Since only the stratospherically-priced Accuphase SACD transport/processor system has appeared with digital outputs on all six channels, there is no six channel DAC as yet, and the record labels are all struggling financially and therefore even more fearful of music piracy, it may be a very long time before players for either of the high-res formats will offer FireWire or any other digital six-channel outputs. Some very high end processors and receivers still don't allow for six-channel analog inputs, and many of those that do suffer from poor channel-to-channel tracking.
This last point concerned designed Daniel Banquer and he vowed to build a straightforward analog six-channel preamp that would ensure precise balances between channels for superior results. The SCPA-1 is the result. It uses new perfectionist op-amps from Burr-Brown that equal the finest Class A discrete wired circuits. Each channel's pot has a concentric ring that allows level trimming to be done within .025 dB of the target gain. The rings can all be locked in individually after precise setting. The six smaller knobs are arrayed on the front of the unit, with the 24-step Master Volume pot on the right. Banquer uses a special "pseudo-balanced" grounding scheme which was originally published as a paper in the Journal of the AES. Grounding of the three wire circuit is central to this design, but the earth ground is not connected to the audio signal ground. An AC line filter is built into the preamp.
High separation channel-to-channel was another design goal of the SCPA-1. It was found that much of the crosstalk between channels on poorly-designed circuits is actually 90 degrees out of phase and can cause false imaging at the speakers. Such attention to precise level-matching between the five channels is very important to correctly reproduce the proper surround effects, whether on film soundtracks or surround sound for music. Especially when sounds are supposed to be placed on the sides instead of directly in front or behind, or when the rear surround channels are only providing natural ambience of a space, the precise balance of all channels becomes even more vital.
I did some experimenting with the two Opus 3 multichannel CDs that were made from two-channel Blumlein-miked originals. They are only 4.0 channel rather than 5.1 and the information in the surround channels is extremely subtle - primarily preserving the ambiance of the hall or church space where the recording was made. So if those two channels are even a single dB higher level than they should be, the whole naturalistic acoustic is compromised as the surrounds are too loud. If they are on the other hand a dB or so too low level, everything shifts toward the front soundstage and the recording sounds little different from the two-channel SACD version on the same disc. While the balance may be good enough at one level setting, at different settings of the master level control the tracking may get off the track with many receivers, processors and controllers. The SCPA-1 was built to correct these variations in level. An indication of Banquer's dedication to this point are the hand-entered figures on a special supplied sheet labeled Unity Gain Calibration Settings. It shows the specific gain settings set at the factory in order to give identical output levels on all channels
Since I was using five of the Genelec HT205 active speakers for reviewing the preamp as well as for the many multichannel format discs, all speakers were identical and I could carefully match levels and timbres using the DMP and Telarc multichannel SACD test tracks and a Radio Shack level meter. Using the meter I calibrated one channel at a time, which makes the task much simpler. I used the C-weighted setting on the meter. However, many users will have unmatched amps and speakers available for a 5.1 surround system, and the SCPA-1 can allow them to closely match levels all around in spite of that. I ran a variety of two-channel and multichannel programs thru the SCPA-1 and never heard a hint of overload, hum or noise. I did find the step from the lowest setting of the large master volume control to completely muted was rather large. I could image someone wanting to set it just for background music and having a frustrating choice between too loud and totally silent. Once the six small pots are dialed in perfectly and locked, one doesn't have to think further about their settings and can simply use the Master volume knob. It you are a SSfM [Surround Sound for Music] fan and want the cleanest and most gimmick free control of six channels in the analog domain, the SCPA-1 could be exactly the way to go.
- John Sunier
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