Overload matters
There are no gain controls on RIAA inputs, so the overload margin, or headroom, is of considerable importance. The issue can get a bit involved, as the frequency-dependant gain is further complicated by a heavy frequency-dependant load in the shape of the feedback network. This heavy loading was a major cause of distortion and headroom-limitation in conventional RIAA stages that had emitter-follower outputs with highly asymmetrical drive capabilities, and for some reason it took the industry a very long time to wake up to this. Once more the 5532 op-amp solved that problem.
There are also interesting limitations to the levels which stylusin-vinyl technology can generate. This chapter tries to sort out the issues involved in maximising disc input headroom.
There was no room in my Preamp ’96 article for a proper discussion of the overload behaviour of RIAA preamp stages.1 Like noise performance, the issue is considerably complicated by both cartridge characteristics and the RIAA equalisation.
There are some inflexible limits to the signal level possible on vinyl disc, and they impose maxima on the signal that a cartridge can reproduce. The absolute value of these limits may not be precisely defined, but they set the way in which maximum levels vary with frequency, and this is perhaps of even greater importance.
Figure 1(a) shows the physical groove amplitudes that can be put onto a disc. From subsonic up to about 1 kHz, groove amplitude is the constraint. If the sideways excursion is too great, the spacing will need to be increased to prevent one groove breaking into another, and playing time will be reduced. From about 1 kHz to ultrasonic, the limit is groove velocity rather than amplitude. If the cutter head tries to move sideways too quickly compared with its forward motion, the back facets of the cutter destroy the groove that has just been cut by the forward edges.
At replay time, there is a third restriction – that of stylus acceleration or, to put it another way, groove curvature. This sets a limit on how well a stylus of a given size can track the groove. Allowing for this at cutting time puts an extra limit on signal level, shown by the dotted line in Figure 1(a).
The severity of this restriction depends on the stylus shape. An old-fashioned spherical type with a tip diameter of 0.0007 in requires a roll-off of maximum levels from 2 kHz, while a relatively modern elliptical type with 0.0002 in effective diameter postpones the problem to about 8 kHz.2
Thus there are at least three limits on the signal level. The distribution of amplitude with frequency for the original signal is unlikely to mimic this, because there is almost always more energy at 1.f. than h.f. Therefore the h.f. can be boosted to overcome surface noise without overload problems, and this is done by applying the inverse of the familiar RIAA replay equalisation.
Moving-magnet and moving-coil cartridges both operate by the relative motion of conductors and magnetic field, so the voltage produced is proportional to rate of change of flux. The cartridge is sensitive to velocity rather than to amplitude (and so sensitivity is always expressed in millivolts per cm/s) and this gives a frequency response rising steadily at 6 dB/octave across the whole audio band. Therefore, a maximal signal from disc (Figure 1(a) would give a cartridge output like Figure 1(b) – i.e., 1(a) tilted upwards.
Figure 1(c) shows the RIAA replay equalisation curve. The shelf in the middle corresponds with 1(a), while an extra time constant at 50 Hz limits the amount of if boost applied to warps and rumbles. The ‘IEC amendment’ is an extra roll-off at 20 Hz, (shown dotted) to further reduce subsonics. When RIAA equalisation 1(c) is applied to cartridge output 1(b), the result will look like Figure 1(d), with the maximum amplitudes occurring around 1–2 kHz.
Clearly, the overload performance of an RIAA input can only be assessed by driving it with an inverse-RIAA equalised signal, rising at 6 dB/octave except around the middle shelf. My Precision preamp ’96 has an input overload margin referred to 5 mV r.m.s. of 36 dB across most of the audio band, i.e., 315 mV r.m.s. at 1 kHz. The margin is still 36 dB at 100 Hz, but due to the RIAA low-frequency boost this is only 30 mV r.m.s. in absolute terms.
The final complications is that preamplifier output capability almost always varies with frequency. In Preamp ’96, the effects have been kept small. The output overload margin voltage – and hence input margin – falls to + 33 dB at 20 kHz. This is due to the heavy capacitive loading of both the main RIAA feedback path and the pole-correcting RC network (R24,25 and C20). This could be eliminated by using an op-amp with greater load-driving capabilities, if you can find one with the low noise of a 5534.
The overload capability of Preamp 96 is also reduced to 31 dB in the bottom octave 10–20 Hz, because the IEC amendment is implemented in the second stage. The 1.f. signal is fully amplified by the first stage, then attenuated by the deliberately slow initial roll-off of the subsonic filter.
Such audio impropriety always carries a penalty in headroom as the signal will clip before it is attenuated. This is the price paid for an accurate IEC amendment set by polyester caps in the second stage, as opposed to the usual method of putting a small electrolytic in the first-stage feedback path, rather than the 220 μF used. Alternative input architectures that put flat amplification before an RIAA stage suffer much more severely from this kind of headroom restriction.3
These extra preamp limitations on output level are shown at Figure 1(e), and, comparing 1(d), it appears they are almost irrelevant because of the falloff in possible input levels at each end of the audio band.