The Mod Squad Line Drive passive preamplifier Measurements

Sidebar 2: Measurements

The only measurements relevant to the Line Drive are the crosstalk between channels and its input and output impedances, which will affect compatibility with source components and interconnect cables. The input impedance on all inputs is a lowish 8200 ohms, which could lead to a lightweight bass with high–output-impedance, capacitor-coupled tube preamplifiers. The Line Drive's output impedance will depend on the volume control setting; the resistance of the conductive-plastic track between the wiper and ground and the wiper and the signal input will be in parallel, meaning that the overall impedance will be at a maximum when the control is set about halfway its travel (in resistance terms). I measured around 30 ohms at 1kHz for the Line Drive's output impedance with the volume control full up, to which must be added the source impedance of whatever is being used to drive the unit. With the volume control set to 12 o'clock, the output impedance measured 1825 ohms; while set to 9 o'clock, it was 800 ohms; to both of which must be added, again, the source's output impedance. The Line Drive's maximum output impedance I measured to be 2050 ohms with the control at 2 o'clock, which means that with reasonably capacitive interconnects such as Monster Cable M1000 or MIT 330, cable lengths of more than 2–3m are to be avoided if high frequencies are not to be rolled off early.

This is not a trivial point. I measured the 1m length of MIT's new Shotgun 330 CVT interconnect (which I initially used for the auditioning) as having an astonishingly high shunt capacitance of 1600pF—including the Hulk-Hoganesque locking RCA connectors—compared with Monster M1000's 160pF/m or so (footnote 1). With the Line Drive's maximum source impedance of 2100 ohms, and given a respectably low CD player output impedance of 60 ohms, this will give a –3dB point at just over 47kHz, assuming a high, 100k, power-amplifier input impedance. I actually measured the –3dB point with the Line Drive set to its maximum impedance and driving 1m of MIT 330 CVT as lying at 55kHz, being 1dB down at 11kHz and 1.75dB down at 20kHz with the 50 ohm output impedance of my signal generator. Dropping the Line Drive's volume control setting to 10 o'clock or raising it to 4 o'clock lifted the –1dB point to 25kHz, with then just a slight droop in the top octave of the audio band. These figures were taken with the Line Drive feeding the 1M ohm input impedance of the millivoltmeter.

With the input impedances of typical solid-state power amplifiers likely to be in the 10k–47k ohm region, and with source components having higher output impedances, the HF loss in the audio band will be severe (to say the least). Replacing the MIT CVT interconnect with Audioquest Lapis, which has a measured shunt capacitance of around 110pF/m including connectors, resulted in a response that was just an insignificant 0.2dB down at 20kHz with the Line Drive's volume control set for the maximum output impedance. It is understandable why I used the Lapis for all the serious auditioning of the Line Drive/Phono Drive combination.

Because of the highish impedances floating around a passive control device, intrinsic separation between channels and inputs has to be high. Crosstalk between channels of the Deluxe Line Drive was low, at –86dB at 20Hz and –83dB at 1kHz, dropping to –72dB at 20kHz, which is still excellent. Between adjacent inputs, it was –78dB at 20kHz, and unmeasurable at lower frequencies given the fact that the maximum voltage swing available to me for the driven input was 30V p-p. All these measurements were done with the chassis shorted to ground, and seemed unaffected by breaking the chassis/ground connection.—John Atkinson

Footnote 1: I say "astonishing," as this amount of capacitance will render the cable sub-optimum with many preamplifiers. Some will have too high an output impedance to maintain a flat response through the top octave of treble; some, particularly if they have op-amp output stages (like many CD players), will either be unable to hang on to their stability attempting to drive this much shunt capacitance, or will have difficulty delivering their rated output-voltage swing. Before you commit yourself to purchasing this $1100/m-pair interconnect, be sure to check with your dealer that your preamplifier or CD player will be not be fazed.—John Atkinson
The Mod Squad
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Starman's picture

Still use my WBT version every day.

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