Tube Preamp Reviews

Sidebar 2: Measurements

Before I began measuring, I noted that the mute switch on the Premier 7A's left channel was defective. It would stick in one position. Pulling it out manually would release it, but it would then simply stick at the next engagement. Fortunately it stuck in the non-mute position.

The gain of the Premier 7A's line stage—from input to main output—measured 29.6dB in the left channel. The right-channel gain was 0.2dB lower. The line input impedance depended to a small degree on the channel and the setting of the volume control, but ranged from just over 18k ohms to just over 20.5k ohms. The main output impedance had a maximum value of 273 ohms (full volume), dipping to 177 ohms at lower volume settings (for the right channel; the corresponding values for the left channel were 247 and 166 ohms, respectively). A setting of 29 on the level controls gave the nearest approach to unity gain (within about 0.2dB).

The gain of the phono stage, with a 47k ohm load setting on the rear panel switches, measured at the tape outputs, was 39.3dB (L) and 38.5dB (R). With the input load reduced to 250 ohms—10 times the output impedance of the (Audio Precision) signal source—the gain was 35.7dB (L) and 35dB (R) (footnote 1). With the latter load, a 0.5mV input to the phono stage resulted in 1.1V (L) and 1V (R) signals at the main outputs.

Fig.1 shows both the line-level frequency response and the RIAA phono response of the Premier 7A. The former are the virtually straight curves; the latter show only a very slight rolloff at low frequencies and a broad but very minor dip in the 1kHz area—overall, a superb result. In fig.2, the bottom two curves show the line-level crosstalk at full output (level control at maximum; crosstalk measurements made with an input of 100mV). The upper curves show the crosstalk with the level control set for unity gain. In both cases, the topmost curve is the crosstalk from channel A to channel B, the bottom from B to A.

Fig.1 Conrad-Johnson Premier 7A, line-stage frequency response (flat traces) and RIAA deviation (right channel dashed, 0.5dB/vertical div.).

Fig.2 Conrad-Johnson Premier 7A, line-stage channel separation with volume control full (bottom traces) and at unity gain (top traces, 5dB/vertical div.). Measurement probably dominated by noise.

A further reduction in separation was measured at a level setting of 47 (roughly corresponding to 9 o'clock on a rotary control), giving a minimum of 66dB to 5kHz and 62dB from 5kHz to 20kHz (not shown). This suggests that the leakage from one channel to the other is occurring before the volume control. Note that the rise at higher frequencies—usually indicating capacitive coupling between channels—is less pronounced at higher frequencies than we often see. I also noted that the crosstalk varied somewhat with the degree of warmup, at its best when just turned on. The curves shown were taken after an "on" time of about 2 hours.

The phono crosstalk, assessed at the tape outputs (fig.3), is superb. Note that in this case there is almost no capacitive-coupling rise at higher frequencies. For all practical purposes, crosstalk from A to B is the same as from B to A. (The A–B curve is the one with a minimum at the 500Hz point.)

Fig.3 Conrad-Johnson Premier 7A, phono-stage channel separation (5dB/vertical div.). Measurement probably dominated by noise.

THD+noise is shown in fig.4. The second curve from the bottom is the THD+noise from the phono stage at an input of 5mV. The top curve is the line THD+noise at full volume; the second from the top curve is the THD+noise at the "47" setting, the bottom curve the unity gain result. The 1% THD+noise level—measured from the line inputs to the main outputs—was reached at 164mV input, producing an output of 4.93V. The 1% THD+noise level for the phono input—with an unequalized input and measured at the tape outputs—was reached at 79mV at 1kHz (7.19V output), 7mV at 20Hz (5.75V output), and 240mV at 20kHz (2.33V output). Recall that the change in overload voltage levels due to frequency, a natural consequence of the requirements of the RIAA playback curve, is common to all preamps we have measured. With the RIAA pre-emphasis taken into account, these voltage figures represent margins compared to the standard phono input of 5mV at 1kHz of 24dB, 23dB, and 13.6dB, respectively, which are excellent, excellent, and merely good.

Fig.4 Conrad-Johnson Premier 7A, THD+N (%) vs frequency for (from top to bottom): line stage at full volume; line stage at "47" volume setting; phono stage; and line stage at unity gain.

Finally, the Premier 7A inverted polarity from the line inputs to the main outputs, but did not from the phono inputs to the tape outputs. (It therefore inverted from phono in to main out). The DC offset fluctuated between 0mV and 1.6mV in the left channel, perhaps indicating the presence of very low-frequency noise in this channel, but was zero in the right channel.—Thomas J. Norton

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Footnote 1: A reader recently asked whether it was a rule of thumb that, as you lower the load impedance at the cartridge input, the dynamics and treble response deteriorate. No, it is not. If a cartridge has an output impedance (more appropriately known as the source impedance) of 5 ohms, then an appropriate load for this cartridge would be several times this amount; say, 30 ohms (these are actual source impedances and the manufacturer's recommended loading for a Dynavector XX-IL). It's unlikely that a small decrease or significant increase from this value (say, 20 to 100 ohms) will affect either the measured frequency response or the audible dynamic qualities of the cartridge—though it may have some impact on other factors (distortion, for one). A few years ago, with this question in mind, I checked out the effect of such a change in loading on the measured frequency response of a cartridge. The result? Zip, nada. I would not call this result a rule of thumb, only verification that the reality of the situation is a bit more complicated.—Thomas J. Norton