Yamaha C-70 Preamplifier
The C-70 uses solenoid/relay switching with gold contact surfaces to minimize noise and keep the actual signal switches in the circuits to which they pertain. (Conventional mechanical switching requires that all signals be routed to the front panel and back again.) There are both inverting and noninverting outputs, so that absolute phase can be maintained with any power amp-a subject of considerable interest among some audiophiles, though its importance remains a matter for debate. And the handling of phono signals is considerably more advanced than it was in the C-6.
There are four sets of gold input jacks on the back panel. The first two are marked Phono 1 and Phono 2, respectively, and are selected by a switch on the front panel; the other two are collectively labeled direct phono-MM (fixed-coil) and MC (moving-coil), respectively. Yamaha says the direct phono inputs are intended to "ensure the shortest possible signal path with the minimum number of switch contacts [between] the cartridge and the amplifier." To this end, the equalizer/tone-control circuitry and defeat switch, the high filter, the mode switch, and the input selector are all bypassed by signals arriving via these jacks when you push DISC DIRECT just above the selector group. You can use MM and MC simultaneously (assuming you have two turntables and one pickup of each type), selecting the one you want to hear at the phono loading control next to the Phono 1/2 switch. Yamaha provides jack caps to cover the two pair that are not in use-at once keeping out dust and preventing incorrect connections.
The phono loading control provides three different capacitance options, in parallel with the standard 47,000-ohm resistance, for use with fixed-coil or high-output moving-coil pickups. (The latter, however, are insensitive to loading and will therefore work equally well with any of those settings.) As Diversified Science Laboratories' data show, the resistive loading is a hair lower than nominal (negligibly so), and the listed capacitances evidently are those of the shunt capacitors alone: Actual values measure 130, 250, and 360 picofarads, rather than the 100, 220, and 330 picofarads of the switch markings. This gives a good range of choices. In addition, there's the usual 100-ohm input for a moving-coil pickup and a rather mysterious mutant for which the manual suggests no particular use: a 100-ohm loading without the added gain built into the moving-coil position. A pickup requiring this last configuration hasn't yet come our way.
As you can see in the illustration, the loading switch and Phono 1/2 selector are among the controls that are found behind a flip-down door (which, incidentally, provides a control identification when you look down on it in the open position-a nice touch). There is also a recording selector, whose options include dubbing in either direction between the two decks for which there are connections.
But the most impressive of these concealable features are the parametric tone controls. One covers the low end of the spectrum, the other the treble. Each has a concentric control with elements for the bandwidth affected and the degree of boost or cut (the latter with a center detent at the "flat" position). A separate knob dials the center frequency of the band in which the other two will operate: anywhere between 800 Hz and 20 kHz in the treble and 31.5 and 800 Hz in the bass. DSL's measurements show that the center-frequency knob js quite accurately calibrated as such things go-as are the boost/cut dials, which have a range of ±12 dB. (The bandwidth controls are calibrated in arbitrary numbers.)
This three-way adjustability in each band gives the C-70's equalizer section a flexibility undreamed of in conventional tone controls. We also find it more useful than the multislider (though usually fairly simple) graphic equalizers built into some equipment by way of super tone controls, though the comparison is potentially unfair because of the usually significantly higher cost of a parametric design such as Yamaha's. We don't find this incarnation as easy to use as that in the C-6, however. The controls are all rotary, are smaller, and are recessed, making them both harder to get at and less clearly differentiated graphically. (The C-6 used a beautifully judged combination of one slider, one medium-size knob, and one small knob in each of the two bands; the C-70's knobs are easier to confuse.)
Because of its unusually low turnover frequency (15 Hz), the infrasonic filter has very little effect (-1-1/2 dB) at 20 Hz, but its fairly steep slope (12 dB per octave) makes it effective where it should be. The high filter also has a 12-dB-per-octave slope and an extreme turnover frequency-10 kHz in this case, which just takes the edge off some very-high-frequency effects. Evidently Yamaha doesn't expect this preamp to be used with any sources that rate less than good.
Frequency response through the direct-phono inputs measures extremely flat. It's also very flat through the high-level circuitry: Although we don't bother to show it among our data (the fact that it's only 'A dB down at 105 kHz tells the story), response does not drop by 3 dB until 422 kHz. Signal-to-noise ratio is also exceptional (thanks in part, apparently, to what Yamaha calls its Varigain volume-control design). As you might expect in a preamp of this quality, harmonic distortion is virtually unmeasurable, while the overload ceiling is more than generous.
What more can we say? Obviously, this is another superlative preamp from Yamaha.
