Conrad-Johnson PF-1 preamplifier Measurements
Sidebar 3: Measurements
The PF-1 is ostensibly a moving-magnet design with a confirmed 47k ohm, 260pF input impedance. Normally an input sensitivity of 1 or 2mV is available for an IHF standard output of 0.5V for such an input; the PF-1 delivered a 0.48mV sensitivity and an overall gain of 60dB. This was just sufficient for the loudest "low-output" moving-coils, and was well-suited to the "high-output" types which generally offer somewhat lower levels than the average moving-magnet.
Further consideration of the disc input is worthwhile to help understand the optimum matching conditions and to extract the best performance from the unit. Here we must also consider the input noise and input overload margins. Viewed as a conventional MM input, the overload headroom was marginal, to say the least. Though the figures of 16.6dB at 20Hz and 18.3dB at 1kHz are satisfactory, only 4dB is available at 20kHz. This means that referred to the standard maximum recorded level of 50mV at 20kHz (70mV peak), the PF-1 will only accept 111mV peak before its distortion reaches 1%. However, 40cm/s peak levels are possible from disc which could result in momentary peaks of 200mV from a high-output moving-magnet cartridge or a moving-coil, when used with a step-up unit of healthy matching ratio.
This explains why the disc intermodulation graph (fig.1) showed a rather high level for the 1kHz difference tone; this product was just 20dB down at the MM test level, corresponding to a full 10% distortion. When the input signal was reduced by 20dB, corresponding to a moving-coil drive, the distortion was then reduced by nearly 20dB, resulting in a satisfactory 39dB intermodulation figure. In the PF-1's favor was the "linear" character of its high-frequency overload and/or distortion; this should not be compared with a more audibly serious kind of feedback clipping overload, which produces severe "rattling" effects. At MC input levels, the other overload results correspondingly improved, there being now 36.6dB headroom at 20Hz, 38.3dB headroom at 1kHz, and 24dB at 20kHz.
Fig.1 Conrad-Johnson PF-1, phono input, HF intermodulation spectrum, DC25kHz, 19+20kHz at 0.5V output (linear frequency scale.
On a moving-magnet basis, the disc input noise was fine at 71dB, CCIR. However, this translates to only 51dB for a 0.5mV or MC input reference, which implied an audible hiss a little louder than average disc surface noise when using moving-coil cartridges of the Linn Troika rated output level; for example, 0.2mV for 5cm/s recorded velocity at 1kHz. Moving-coil cartridges giving an output of 0.5mV to 2mV for 5cm/s are most suitable. Additional input loadings may be applied if space is available in the phono plug, or better still, on the input terminal inside the preamp, installed by a good technician. 1k ohm metal film or a preferred foil type and a 1nF polystyrene in parallel are a good all-around loadings for MC cartridges.
The line input shows negligible distortion, that present of low harmonic order measuring typically 70dB or 0.032%. A fine 80dB 0.01% reading was obtained for high-frequency intermodulation (fig.2). Line-input noise was judged satisfactory at 80dB (CCIR-weighted) for a 0.5V input and 0.5V out. The "A"-weighted figure was 84dB, which corresponds quite well to the manufacturer's claim of 94dB unweighted, referred to a 2.5V output (this level 14dB above the 0.5V IHF level). These results are no more than satisfactory, except where high-sensitivity power amplifiers and/or speakers might be used, these imposing unusual demands with regard to very low system noise levels. With the volume control at zero, the unweighted output noise was 83dB ref. 0.5V or 97dB for full amplifier power (eg, MF-200). However, better than 105dB is needed to ensure a complete absence of audible noise with a high-sensitivity horn-type speaker.
Fig.2 Conrad-Johnson PF-1, line input, HF intermodulation spectrum, DC25kHz, 19+20kHz at 0.5V output (linear frequency scale.
Covering some basics, the disc section through to the tape output was absolute phase/polarity-correct. Channel balance was fine at 0.5dB or less error, while the channel separation was typically 55dB over the whole frequency range. The RIAA equalization was very uniform with just a hint of treble lift, +0.15dB at around 10kHz, while the low frequencies were well-extended, only 0.1dB down by 20Hz and within 0.4dB at 10Hz (fig.3). The ultrasonic rolloff above 30kHz was welcome, while it should be noted that there was no infrasonic filtering; thus the matching turntable should have low rumble and a correctly matched/damped tonearm-cartridge combination.
Fig.3 Conrad-Johnson PF-1, phono input, RIAA error (1dB/vertical div.)
A check on the line input showed a very extended low-frequency response (fig.4), perfectly flat to 19kHz 0.5dB, 3dB at 29kHz. Even for a tight 0.5dB tolerance, the low-frequency response extended below 1Hz. It was confirmed that the line stage was phase-inverting, however.
Fig.4 Conrad-Johnson PF-1, line input, frequency response (5dB/vertical div.)
A maximum output of 8.2V (100k load) was available for a low source impedance of 160 ohms, ample for any application. A gain of 20dB was delivered, rather high for CD, which needs typically 10dB. Conversely, some tuner and tape sources required the extra boost. (A series resistor of 100k ohms could be used for CD.) The channel matching for the volume control was excellent, never exceeding 0.06dB error over a control range of 60dB. Interestingly, the line input impedance was a bit lower than expected, measuring 31k ohms with a moderate 100pF of parallel capacitance.
No DC offsets were present at the outputs.Martin Colloms