Hot Wiring Your Guitar
Stereo Part 2 - Hexaphony and the Split Precision
In May's gripping instalment of Hot Wiring Your Guitar we described two channel wiring of guitars, popularly known as stereo, and explained why this arrangement doesn't give much of a stereo image. In order to cause the sound of each string to come from a different point would require a very special pickup, and the simplest form of this — although simple is hardly the correct word — is the hexaphonic pickup. This has a separate coil for each string, and is effectively six completely independent pickups in one case. Each string's output can thus be panned to the required stereo position.
The hexaphonic pickup has more advantages than just stereo, however. Once you have the ability to deal with each string absolutely separately, there are virtually no limits to what can be achieved. One spectacular effect is 'hex fuzz', which is simply six separate fuzz or distortion units, one for each string.
Whenever any form of distortion is used to alter an ordinary guitar's sound, two or more notes played at once interact with each other due to a phenomenon known as intermodulation. The result is that when two tones try to elbow their way through a fuzz box, more than two tones come out, and the extra ones needn't necessarily be harmonically related to the original two. With hex fuzz, however, none of this unpleasant rivalry occurs, and the end product sounds more like several guitars playing single notes in harmony.
There are many, many more things you can do once you have a hexaphonic pickup, but before I build up your expectations any more, I'd better deliver the bad news. The only commercially available hex pickup that I know of is the one that ARP used to sell to go with their Avatar guitar synthesiser. ARP are now out of the running, of course, and their pickup cost over £100 back in 1978, so it's obviously not for everyone. There may be other alternatives. Making your own is liable to be tricky, however, since there isn't much room for a coil and magnet in a normal guitar's string spacing. On a brighter note, I see that RS Components, who distribute a myriad of electronic bits to the trade, now stock a magnetic pickup meant for industrial motion sensing. (Equipment from RS Components is only available through established retailers.) It's in the form of a cylinder about 6mm in diameter, and might be usable as one channel of a six-way pickup.
There is one very common instrument which goes part way to true stereo — the Fender Precision bass. This has a split pickup, with each half picking up two strings, and it's fairly easy to feed each 'semipickup' to its own channel. Since the advent of DiMarzio et al quite a few instruments have this arrangement, and they should all be convertible.
Using the split wiring, you can have a simple stereo effect where, each pair of strings has its own amplifier; this benefits from careful playing to keep the notes leaping about between the speakers. Alternatively, you can get really fancy and put separate processing on each channel. For example, with distortion and/or treble boost on the top two strings you can swap instantly between 'solo' playing and straightforward bass sounds from the lower strings.
Unfortunately, hardly any guitars have this split pickup arrangement; the only ones to spring to mind are the Fender Electric XII and the same company's very rare Custom.
Figure 1 shows how to connect a Precision style bass for split stereo, using a DiMarzio Model P pickup pair. Users of other pickups who don't want to use the DiMarzio will, I regret, have to figure out the wire colours appropriate to their own unit. In some cases, extra wires will need to be added to extend all four connections to the control cavity. Ganged controls are called for on volume and tone — there seems to be little point in having separate volume controls, although a case could be made for having separate tone controls. On my own bass the tone control is a single ganged component which works on the lower strings only in stereo, and on all four in mono.
As in the last article, there is a choice of output arrangements, three of which are illustrated in Figure 2. The simplest is a single stereo jack as shown in Figure 2a, with an ordinary DPDT mini-toggle used for the mono/stereo switch on the main circuit. In mono, the wiring of the bass is perfectly conventional and a mono lead may be used. With a stereo set-up, all four strings are heard from the channel connected to the tip of the jack plug. When switched to stereo, the E and A strings remain on this channel, but the D and G strings are sent instead to the second amplifier which is fed from the ring of the stereo jack plug. Alternatively, two mono jack sockets may be used if there is room, connected as shown in Figure 2b. The effect of the switching remains the same.
In Figure 2c, an extra pole is needed on the mono/stereo switch, which will thus be a 3 pole component, or more likely 4 pole with one pole ignored. This ensures that both amplifiers are fed with the same signal when the bass is in mono, instead of one being switched off. If it is required to use a mono lead with this set-up at any time, a separate mono jack will have to be provided (as shown dotted) to avoid shorting everything out. The alternative is to make up a special lead using stereo jacks, but leaving the rings unconnected.
Players who want to put extra processing on the top two strings for 'solo' playing would use arrangement a) or b), whilst those who want the stereo effect would probably use arrangement c). Just to end on a note of caution; in stereo, the two halves of the pickup are no longer in series, and so there should theoretically be a slight volume drop. More seriously, perhaps, the combination will no longer be humbucking. Although I haven't noticed either of these ill effects, they should be borne in mind when converting.
Feature by Peter Maydew
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