Woodside SC26 preamplifier
By Larry Greenhill   •   February, 1995 I had been sent a sample of the Woodside SC26 tube preamplifier during my June 1994 review of Woodside's MA50 monoblock amplifier (Vol.17 No.6). Although I used a number of preamplifiers during that review, I was most impressed with the MA50s' spacious, three-dimensional soundstage when driven by the SC26. At the time, I had an impression that the SC26's sonics combined a midrange richness with a good dynamic range. Although I had to return the Woodside MA50s to the importer after I reviewed them, I continued listening to the SC26.

Description
The SC26 looks functional and slim. The 19"-wide faceplate has a beveled top edge and four large rotary switches. The SC26P I reviewed was configured for moving-coil operation, but the preamp is also available in a MM version. For MC operation, two potted Sowter ribbon-core transformers on a moving-coil input transformer board are located adjacent to the phono input jacks (footnote 1).

The two left-hand rotary controls each have six switch positions: Phono, CD, Tuner, Video, Tape 1, and Tape 2. The Input selector controls which of the six inputs is routed to the SC26's line input stage; the Tape selector determines which of the six inputs is routed to the tape buffer amplifiers, which feed the two sets of recording Tape Out sockets on the back panel.

Between the front-panel rotary switches are three toggle switches labeled Mono, Mute, and Power. When the Mono toggle switch is flipped down, both left and right signals are mixed to give a monaural signal on both channels. The Mute toggle, which operates a mute relay across the line-stage output jacks, doesn't affect any signals to the two tape output jacks. And when the Power switch is flipped down, indicated by a power-on light located behind the front panel, the preamplifier enters a warm-up cycle, which starts with a timed mute operation while the internal circuitry is being stabilized. This prevents noise or voltage spikes from being sent to the amplifier when it's turned on.

Technical details
Arnold L. Weisenberg, who helped design the SC26, discussed its topology with me in a lengthy telephone call, and later mailed me a 19-page note full of circuit diagrams. The SC26's tube complement consists of nine ECC83s (12AX7s) and one ECC81 (12AT7), each of which has its own regulated power supply, these fed from a 312V regulated supply. This means that there are six regulated supplies on the phono board, and four on the line board. In addition to the ten regulated supplies for the B+, two more regulated supplies are employed: one for the filament, one for the mute relays.

Traditional source selector switches are plagued by noisy contacts and—unless shaft extenders are used—channel crosstalk and source-to-source crosstalk: the long leads or pcb traces have to be dressed near one another from the rear-panel sockets to the front-panel switch. Like the Mark Levinson No.26 and No.38, the SC26 uses a relay-switched arrangement to eliminate these problems. A group of 10 sealed-in-glass magnetic reed relays—two for each line input, two for the phono-board output—are located toward the rear of the line-stage printed circuit board. The front-panel selector switch directs 12V DC to energize the appropriate pair of relays, in accordance with the input selected. A timed mute circuit shorts the output to ground either during timed warmup or upon activation of the front-panel mute switch.

The circuitry of the SC26 is said to be very similar to that of the input stage of the MA50 monoblock amplifier. Two triodes are used in an "active plate load" configuration, with a 300V regulated power supply. This amplification module is repeated eight times in the preamplifier—two times for each channel of the phono stage, two times for each channel of the line stage. Thus, the phono-stage channel consists of two dual-triode 12AX7s and one-half of a dual-triode 12AT7 per side, for a total of five tubes. The line stage has two 12AX7s per channel, with another used to buffer the tape outputs for the second complement of five tubes.

In each case, the two triodes in each amplification module are halves of a single 12AX7. As the tube ages, it can be assumed that these two triodes will change more or less equally, and the performance of the module will thus remain stable. The second triode is used as the plate load of the first triode, and the output is taken from the cathode of the upper triode. This configuration results in a very low effective output impedance, which the manufacturer claims permits a very wide-band frequency response with minimum phase shift.

The phono preamplifier consists of two of these modules for each channel, followed by a shared 12AT7, which is the cathode-follower output of the phono preamplifier. The first phono stage is directly driven (not resistor- or capacitor-coupled) by the cartridge in the case of moving-magnet cartridges, or by the built-in transformers in the case of moving-coil cartridges. This input stage has no local feedback and is said to have an extremely fast risetime and wide bandwidth. This second stage drives the cathode-follower, whose output is taken to the selector switch via a DC-blocking capacitor. The cathode-follower also drives the RIAA feedback network, which is connected back to the grid of the lower triode of the second stage. However, there is a small shunt capacitor between the first and second gain stages to roll off the high end in the approved RIAA manner. The high-frequency gain therefore ultimately falls to zero. (With conventional series-feedback circuits, the HF gain drops to a minimum of unity, which results in an effective HF boost above the audioband.)

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Footnote 1: Sowter is one of the grand old names in British transformers, having a terrific, if local, reputation for the sound quality of their microphone transformers.—John Atkinson