The Mod Squad Phono Drive phono preamplifier Measurements

Sidebar 2: Measurements

The Phono Drive had a measured output impedance of 60 ohms from its direct-out sockets, and an even lower 50 ohms from its line stage. Neither outputs should have any difficulty driving long or highly capacitive interconnects. The input impedance of the line section I measured to be 11k ohms; again, no problem except with some antique tube components or the Mk.I version of the CAL Tempest CD player, which had a 5.6k-ohm output impedance.

The gain structure of the Phono Drive appeared to be around 20dB from the MC headamp, followed by 44dB at 1kHz from the MM section, which also applies the appropriate RIAA equalization. This should be sufficient to drive power amplifiers with up to 1.5V input sensitivities to full output with anything other than very–low-output MC pickup cartridges. The separate line stage is polarity-correct and boosts the signal voltage by a specified 15dB: I measured 16dB—must be the thin atmosphere here in Santa Fe.

Fig.1 shows the frequency response of the MM input, measured from the direct-out sockets. Apart from a slight boost in the 10–30Hz region, reaching a maximum of +0.75dB at 12.5Hz, the response can be seen to be effectively flat, at least within the tolerance limits of my inverse-RIAA network, throughout the audio band. The response drops below 10Hz, with a –3dB point at 5Hz reducing the level of warp-induced garbage, while the high-frequency response doesn't drop by 3dB until 110kHz. The line-stage response is slightly more band-limited, with a –3dB point at 85kHz. Our four cats report that the difference is not worth looking up from their bowls of Kibble for (footnote 1).

Fig.1 The Mod Squad Phono Drive, RIAA error (10Hz–50kHz horizontal scale) 2dB/vertical div).

You will have noted from the report on my auditioning that I found the background noise on the MC input to be a little high. With the MC inputs shorted, I measured the audio-band noise to lie at –41dB with respect to a 775mV output, this improving to –60dB with an A-weighting network switched in before the average-responding meter. This is about 6dB worse than average for high-end, solid-state preamps, but should not represent a particular problem except with such low-output MC cartridges as the Rowland Complement or the Ortofon MC3000. (I won't even mention the MC2000, with its 0.05mV output!) The MM input was very quiet, with an 88dB A-weighted S/N ratio, though this is academic for me, the only two cartridges to grace my system being MCs: a Linn Troika and a Koetsu Rosewood. (Us English have our standards, you know.)

I thought the measured noise levels worth further investigation, as I couldn't get rid of the vestigial hum when the Phono Drive was in use. Fig.2 shows the 1/3-octave spectrum of the noise when measured from the Phono Drive "Direct Out" sockets when connected to the VTL Compact 100s (middle trace), and that from the speaker terminals of one of the VTL amplifiers when driven by the Phono Drive (top trace) or by the Vendetta Research (lowest trace). (The measurements were done with an Audio Control SA3050A analyzer running from its internal battery, with the Troika cartridge feeding the appropriate preamp; all curves are raised by around 50dB referred to the onset of clipping on the VTLs.) The noise spectrum of the Phono Drive (middle trace) can be seen to slope gently down from low to high frequencies, with just a slight peak at the third harmonic (180Hz) of the mains supply apparent. When coupled to the VTL, however, the fundamental frequency of the mains appears at 60Hz, with its second harmonic noticeable at 120Hz. (There is also a slight accentuation of noise in the presence region.)

Fig.2 The Mod Squad Phono Drive, noise spectrum measured at speaker terminals when connected to VTL Compact 100 power amplifier; measured at Direct Out preamp jacks; noise spectrum of Vendetta Research preamp (all traces +50dB, 10Hz–50kHz horizontal scale) 5dB/vertical div).

There was no other mains transformer nearer than 3' from the Phono Drive's MC circuitry; the hum persisted whether one, both, or neither of the VTLs were grounded at the wall socket. It also failed to be eliminated either by separately grounding the Phono Drive or the Line Drive, or by flipping the latter's AGT switch upward. I had found a similar situation to exist when the VTLs were used with the Krell KRS2, though its audibility was a little lower than with the Phono Drive. At that time I had thought it was due to the mains transformers of the tube amplifiers injecting hum into the MC input. The fact that the hum was present with the Phono Drive but not the Vendetta, coupled with the fact that it is not present on the Phono Drive outputs even when connected to the VTLs, suggests that it is a compatibility problem of the VTL 100s, perhaps due to a less-than-optimum ground topology.

It was not present to anything like the same degree when the VTLs were driven by the Vendetta Research phono preamp. The lowest trace of fig.2 shows the spectrum of the noise present on the VTLs' output when connected to the SCP-2 under exactly the same conditions as the Phono Drive. It can be seen that there is still a 60Hz component present, but that this is now 28dB lower in level, with the overall noise from 400Hz upward below the –40dB measurement floor.

The other pertinent aspect of a phono preamplifier is that it should have high overload margins. Looking at just the MM input, a 20Hz waveform clipped at 6.3mV, 1kHz at 55mV, and 20kHz at 460mV, all voltages RMS and respectable, though ultra-high MM cartridges are best avoided. With its sensible 20dB gain, the MC stage should have even better overload-margin figures, however. The maximum line-stage output at 1kHz was 11.3V RMS, the negative-going peak clipping first with a burst of 1.4MHz oscillation every cycle as the slope of the waveform changed from negative to positive. This occurred at 2V RMS input, but for those who worry that this is the typical maximum output of a CD player, the line stage is preceded by the volume control, effectively increasing the overload margin to infinity—or at least to the edges of the home galaxy—when the volume control is turned right down.—John Atkinson



Footnote 1: Who says that you can't end a sentence in a preposition?
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