SCPA 1 Design and Setup
Unlike some esoteric preamps, the RE Designs SCPA 1 is a non discrete design using all high quality opamps (Burr Brown OPA671) in the gain circuits. Some audiophiles may frown on this, but these are usually the ones that believe in cable break in and other snake oil cable magic , with little to no understanding of electronics and how far opamps have evolved over the years, particularly for the duty of line level audio amplification purposes.
The OPA671 is a hard biased line drive op amp with very low noise & distortion. If you take a look at the spec sheet attached; on page 3 you will see a really low THD + N at gains of 1 and 10 into 200 ohms. The OPA 671 is also able to drive a lot of cable capacitance with minimal distortion, making it an optimal line driver for audio applications (driving long cables to powered speakers) Also due to it’s line drive capability output impedance can remain low for less noise pick up without the penalty of higher distortion. Feedback resistance can also be lowered for less thermal noise due to the op amps drive capability. The S/N at 12db gain of –100 dbv is very respectable for an unbalanced two stage amplifier.
According to Dan Banquer, Chief Engineer for RE Designs Audio, typical stereo separation at the listening position is usually somewhere around 8db to 10db. This alone would appear to nullify further investigation of electronics and stereo separation, but over the years Dan has made some interesting observations that he would like to share.
In designing and building a few line level units Dan has observed that the coupled signal when doing stereo separation testing has been 90 degrees out of phase from the input signal. It also appears that it is capacitively coupled and decreases as the frequency drops, and typically 6 db per octave. In reducing this coupling Dan has observed better noise immunity.
For Example: using “unshielded” wire to go from the back panel input to the board will degrade stereo separation. At the same time, due to the high input impedance of the input being connected too, would have higher susceptibility to noise due to pick up. If a piece of coax is used for the same connection an increase in stereo separation and lower noise pick up is realized. It would appear that a higher level of stereo separation could well be more indicative of better noise immunity in this application. Dan’s own personal observation is that reducing the 90 degree out of phase coupling gives a better “image” along with reduced noise. The RE Design goal is generally to achieve close to 70 db of separation at 20 kHz, Dan has found that audibility is very difficult above this. The above makes me ask the question; Is it stereo separation, or noise immunity, or both?
Inside the SCPA 1 notice the clean layout, copper clad steel coax, 6 Gang Analog Volume Control, tightly regulated power supply with toroidal transformer.
The use of the RG174 and RD174 in the SCPA 1 (a double shielded version of RG174) is for lower pick up noise in the unit. Consumer audio requires high impedance inputs, which are very easy to drive but also act as receiving antennas for unwanted noise. Using a coax with good shield coverage for point to point wiring reduces unwanted pick up noise. If you think about it for a minute; what has more shield coverage, a PCB etch or a coax with better than 90% shield coverage? The steel core of the center conductor of the wire helps ensure that the wire will not break under stress. The other reasons for the use of RG 174 is that it is a mini coax with a small O.D. and very flexible. In the world outside of audio it is commonly used as an inside the box wire, and scope probes.
There seems to be a bit of controversy on some of the cable cult forums regarding the merits of using copper clad steel. They claim copper clad steel exhibits distortion or diode rectification. Of course we know better based on our peer reviewed and repeatable measurements of cable distortion (or lack thereof) refuting such nonsense. It’s sad to see intelligent folks expend so much time on disseminating audio fallacies especially while overlooking the obvious and real impacts on fidelity of audio systems – the room. We shall not expand upon room acoustics here but make note that proper room acoustics and system setup will either make or break even the best of systems and associated gear.
Connecting and configuring the SCPA 1 couldn’t be easier. Simply connect 6 quality RCA’s from your player to the SCPA 1 and another 6 RCA’s to your power amps, plug in the power, and your done!
The volume pots are all analog, rotating the outer controls adjust with +/- 0.1dB max, 0.05 db typical, precision, while the inner controls allow for adjustability of .025dB along with a locking mechanism once you dialed in the optimal level. This type of precision is unheard of as even the best processors out there give you no better than .5dB precision. The SCPA 1 is 20 times more accurate in this respect. Dan and I have had disagreements about how much precision one really needs for channel trim adjustments. I feel .5dB is more than adequate; he thinks otherwise and can site studies of audibility of level changes as low as .20dB. I certainly won’t hold anything against a manufacturer offering higher precision than may be necessary and perhaps this type of precision is better suited for a professional audio environment such as mixing or mastering in a studio.
Calibrating the SCPA 1 was a bit of a chore since there was no remote control provisioning. Luckily I had a mic stand with a long extension cable to connect to my audio analyzer so I could sit near the SCPA 1 while reading the SPL meter and adjusting the channel trim levels. I was quite impressed with the level of channel trim matching I was able to achieve and noted the channel trim level balance maintained precision regardless of master level volume position confirming excellent tracking capabilities of the master volume control.