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Phono Stage Mono Switching

Introduction

Since 1958, stereo recordings have been released on vinyl records, to be played either through mono-only setups, which are now quite obsolete, or through stereo cartridges connected to stereo amplification and loudspeaker systems. As a result of the clever way in which stereo is recorded into the groove walls of vinyl releases, there is no reason why a stereo recording cannot be played through a mono set up, or a mono recording played through a stereo one. There are, however, great benefits in terms of sound quality, especially with mono recordings of an older vintage, to be had by playing mono vinyl releases through a pre-amplifier system that sums the two stereo channels together to produce a purely monaural output. As a consequence of this, many systems, particularly more serious ones, from the first age of vinyl (from roughly 1950 to 1985) were equipped with a panel switch that the user could operate when playing mono releases so as to reduce surface noise, distortion, and rumble across the two stereo channels. The mono switch could also be operated to bring the same improvements to heavily worn or damaged stereo recordings, trading the distressing effects of high surface noise and distortion against the illusion of stereo for what would hopefully be a better listening experience.

At the time of writing, the second age of vinyl (from about 2010 onwards) is very well under way and sales of vinyl records have actually exceeded those of CDs for the past two years. Many new phono-stages have been put onto the market, with a great many features that were most often absent from those of the first age of vinyl, the two most often seen being variable gain and variable load impedances. The latter feature is a little more questionable than the former, bearing in mind that all MM cartridges are designed to work with a load impedance of 47kΩ (although using 50kΩ with shorter leads of lower capacitance does bring about some improvement to frequency response). One feature that is conspicuously absent in this new age is the faithful old mono switch, which appeared on all but the very cheapest of stereo equipment from the late 1960s to the demise of vinyl as the predominant medium of high quality music consumption with the advent of the CD and later digital audio formats.


Mono FFRR disc

Figure 1. Cover of a 10" mono LP

Although there are not many mono releases being issued today, there are a plethora of earlier mono releases such as the 10 inch LP of Figure 1 that will give a much more satisfying result when played in true mono mode today. The very poor vertical compliance of early pick-ups designed to play mono pressings would also create a vertical wear pattern that in can generate some very distracting cross channel distortion artefacts when played back using a stereo set-up. It is the opinion of the author that some control that allows summing to mono is absolutely indispensable when playing back these earlier discs. At the time of their cutting, not very much thought was given to their compatibility with the stereo system that eventually superseded them some years later, and although the stereo system for records, like the stereo system for FM broadcasting, was designed for backward compatibility (unfortunately there is no 'pilot tone' on stereo records to allow for automatic summing), there is no doubt that there are significant disadvantages to this simplified approach.


Stereo groove geometry

To understand the improvements that channel summing can bring about on mono or damaged pressings, it is first a good idea to take a brief look at how exactly a stereo signal is recorded into a single groove. Most readers will already be aware of how this is done, but there are some important points to consider, particularly that not all planes of motion are created equally in terms of their usefulness to high quality audio reproduction.


Stereo record groove playback

Figure 2. The groove geometry of stereo encoding


More text.


Surface noise

Section 2


Sample 1A. Left channel of lead in noise

Sample 1B. Mono sum of lead in noise

Sample 1C. Full stereo of lead in noise

Sample 1D. Vertical difference of lead in noise

Some text to discuss the effects of surface noise.


Mono discs

Section 3 text.


Sample 2A. Left channel of mono LP

Sample 2B. Mono sum of mono LP

Sample 2C. Full stereo of mono LP

Sample 2D. Vertical difference of mono LP

A discussion of these effects.


Wear and tear

Heavy distortion and very worn discs.


Sample 3A. Left channel of rock single

Sample 3B. Mono sum of rock single

Sample 3C. Full stereo of rock single

Sample 3D. Vertical difference of rock single

Text to describe the unpleasant effects of excessive distortion.

Sample 4A. Mono sum of scuffed LP

Sample 4B. Full stereo of scuffed LP

Some more text to further discuss the unpleasant effects aforementioned


78RPM discs

Section 5 text.


Sample 5A. Left channel of 78RPM disc

Sample 5B. Mono sum of 78RPM disc

Sample 5C. Full stereo of 78RPM disc

Sample 5D. Vertical difference of 78RPM disc

A short description of 78RPM playback.


Topological implementation

Some text about the correct placement of the mono switch.


Phono pre-amplifier with mono switch and rumble filter

Figure X. A complete pre-amplifier with mono switch

Descriptive text about the phono pre-amplifier.


Conclusion

If you have any questions or comments relating to this article, then feel free to contact the author .