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This was an eye-opening exercise.
A couple of months back I decided to try a new source of 4 conductor shielded hook-up cable for our Silent SplitTM models. It was from a reputable dealer and it was a little less expensive, so why not? The first pickup I wired up with it, I noted a possibly significant difference in the final electrical QC specs. I went through the data, and the pickup response appeared to be right on the edge of the distribution, but it was different enough that it was bothering me. I couldn't find anything obviously wrong with the pickup, though. The next one I wired up looked exactly like the first one, and especially since the change was in the muddy direction, I decided I had to dig in deeper. Since I had just switched cable sources, I investigated the cable first.
What I found was that the effect of a 1 foot length of hook-up cable on pickup performance was much more significant than I would have imagined. There are dramatic differences in both the cable capacitance and the frequency response of the pickup as a function of cable type, source and configuration.
But first, a little bit about the specifics of the 4 conductor wiring used in this study. Figure 1 illustrates the Zexcoil(R) Silent SplitTM wiring convention. Zexcoil pickups consist of 6 individual coils, one for each string, and they are arranged to be humcancelling in two pairs of three such that the top three coils and the bottom three coils are RWRP with respect to each other. In normal (series) mode, the pickup is wired in series through all six coils. In Silent Split (parallel) mode, the pickup is wired in parallel through the two sets of three. Note that there is nothing additional that needs to be grounded on the Zexcoil, such that the shield on a multiconductor cable is not required as a ground line.
Figure 1. Zexcoil(R) Silent SplitTM wiring convention.
Figure 2. Frequency response of a Throaty BuckerTM in series mode with various hook-up cable configurations.
Figure 2 illustrates the frequency response of a Zexcoil Throaty Bucker in series mode with various cable configurations (note that the same pickup was used to generate the data in Figures 1-4 and Table 1). Frequency response was measured with a Syscomp CGR101 digital oscilloscope in network analysis mode (see notes on measurement technique in the Appendix). There are two different sources (Vendor A and Vendor B) of 4 x 28 awg conductor shielded cable represented here. Both utilize a foil shield and a fifth bare stranded drain wire as well as stranded conductors. Also illustrated are two examples of a 2 conductor hook-up, using 22 awg stranded pvc jacketed wire. For the two conductor wiring the red and green solder points of the pickup were jumped directly together with a minimum (~1/4") of bare wire and two wires were connected to the white and black solder points respectively. The aligned example places the two conductors linearly adjacent to each other, as they might be in a guitar installation, and the twisted pair consists of the two 22 awg conductors twisted together. Each example utilizes 1 foot of nominal cable length.
The effect of the hook-up cable on pickup frequency response is significant. The difference in resonant frequency between no lead wire (measurement leads attached directly to the pickup solder points) and 1 foot of Vendor B cable with the shield grounded is almost 3500 Hz. The difference between the two cable sources when the shields are grounded in both cases is nearly 1000 Hz. Note that the Vendor A cable with the shield floating (not grounded) is almost identical in response to the 22 awg twisted pair.
Figure 3. Frequency response of a Throaty Bucker in parallel mode with various hook-up cable configurations.
Figure 3 illustrates the response of the same pickup in parallel mode. While the effects in this mode are not quite as significant as the series mode illustrated in Figure 2, they are measurable and follow a trend similar to the series data, with Vendor B exhibiting a depressed resonance compared to Vendor A under like conditions, and the grounded shield exhibiting a significantly lower resonance than the floating shield.
Table 1. Cable capacitance and pickup resonant frequency data from this study.
Table 1 gives the pickup resonant frequency data as well as the appropriate cable capacitance data for each case. Capacitance was measured with an Extech model 380193 LCR meter @ 1000 Hz using the parallel model. For the 4 conductor cable, capacitance was measured between two unconnected wires, in one case when one of the wires was connected to the cable shield (drain wire) and also in the case where neither wire was connected to the cable shield. In the two wire case the capacitance was measured simply between the two unconnected wires.
Figure 4. Pickup resonant frequency as a function of measured cable capacitance. The dotted lines illustrate power law fits with exponents of -0.21 and -0.33 for the parallel and series modes respectively.
Figure 4 illustrates some of the data from Table 1, with pickup resonant frequency plotted as a function of the measured cable capacitance. While this comparison does represent a gross oversimplification of the physics of the situation [we would expect the relationship between resonance and capacitance to follow frequency is proportional to C -1/2, and the actual capacitance is much more complicated than the measured capacitance, especially with the shielded cables], the correlation between resonance and capacitance is clear, as would be expected.
So what does all of this mean? First of all, the pickup is being significantly loaded by the capacitance of the hook up cable, especially when the shield is connected. From our perspective, we want to deliver as much of the tonal spectrum as the pickup is capable of producing, so we want to minimize this loading as much as possible. One could also think of the cable capacitance as another knob in the tonal tuning equation (and this is perfectly valid, Jimi Hendrix famously used a high capacitance cable as part of his tone), but since there are already so many ways to dial out frequencies downstream, we take the approach of trying to deliver as much of the signal as possible to the controls.
So clearly, shielded cable is a significant "tone suck", but can we get rid of it without worrying about adding noise back in to the system? First, realize that there are multiple sources of noise. What we'll call "hum" is magnetic interference, the magnetic component of 60 cycle AC power, that guitar pickups sense the same way they sense the magnetic fluctuations caused by string vibrations. Typical shielding does absolutely nothing to combat this type of interference. Shielding does act to reduce EMI, which we will call "buzz". The best way to reduce noise is to have very quiet pickups. To combat hum, a hum-cancelling pickup (or some other mechanism to generate opposite polarity hum signal such as a dummy coil or hum-cancelling backplate) must be employed. Zexcoils are extremely efficient at hum-cancelling and we find that there is generally no significant noise penalty when using Zexcoils with an ugrounded, shielded cable, and in fact we recommend not grounding the hook-up cable shield. But even the best hum cancelling pickups can be susceptible to buzz in a very noisy environment.
If shielding is necessary to combat buzz, a better route than shielded cable is to shield the guitar body cavities. To a first approximation, capacitance is proportional to the dielectric constant of the insulating medium between the conductor and the shield and inversely proportional to the distance between the conductor and the shield. So capacitance will decrease as the distance between the conductor and the shield is increased, and will also decrease as the dielectric constant of the insulating medium between the conductor and shield is decreased. Since the distance between the conductor and the shield is much larger in the case of a body cavity compared to a shielded cable, and the dielectric constant of the insulator (air) is also much lower than any of the jacketing or insulating materials typically employed in cables (air has the lowest dielectric constant of any material except a vacuum), the added capacitance of a body shield is significantly lower than a shielded cable.
Figure 5 shows the measurement of the capacitance of a pair of unconnected, aligned 22 awg stranded wires, where the black wire is connected to the body cavity shield. With the shielded pickguard in place (as shown at right in Figure 5), the capacitance of a 1 foot length of wire (in roughly the position of a neck pickup lead wire) is only 27 pF/foot, which compares favorably to the 38.5 and 62.8 pF/foot measured for Vendor A and B cables respectively (see Table 1).
Figure 5. Measurement of wire to cavity shield capacitance.
Figure 6. Frequency response of a prototype pickup and a conventional covered humbucker illustrating the effects of grounding and cable capacitance.
How else could this information be utilized to reduce high end loss in pickups? One way is in conventional humbuckers utilizing 4 conductor wiring. Almost every 4 conductor humbucker pickup we have ever encountered utilizes a shielded cable, with the metal parts of the pickup (pickup cover, baseplate, screws, pole pieces, etc.) connected to the cable drain wire and shield. This practice sets up a situation where significant shifts in the resonance will be observed. Figure 6 illustrates the effect on two different pickups. On the right is a prototype Zexcoil pickup with a nickel silver cover and a metal baseplate (basically a construction analogous to a conventional humbucker) but with Zexcoil innards, and on the left a conventional covered humbucker. The Zexcoil pickup utilized nominally 14 inches of Vendor B shielded cable while the conventional humbucker utilized only 7 inches of shielded cable of unknown origin. With the Zexcoil pickup, we were able to analyze three cases, first where all of the metal parts were grounded to the shield, second where a separate wire was used to ground the metal parts and the cable shield was allowed to float, and third where the metal parts were not grounded at all. The most significant effect is between the shield ground and the separate ground wire, with the difference between the separate ground and no ground being much less significant. The conventional humbucker exhibits a similar response between a shield ground and no ground, and while we did not take the humbucker apart to explicitly analyze the case of a separate ground wire, the data at right implies that the same gains in frequency response would be realized with a separate ground wire.
We also see evidence in these data that the pronounced effect of grounding and shielding on frequency response may not be simply due to capacitive losses but may also be due to enhanced coupling between the coils of the pickup. Referring to Figure 6, note that there is a secondary resonance occurring at around 10,000 Hz in both pickups in the case of a shield ground, the resonance is more pronounced on the Zexcoil pickup where there are a higher number of coils to potentially interact. In other unpublished work, we have observed this secondary resonance to be enhanced in pickup design situations where we would expect coupling between the coils to be more significant. Especially in the case of a humbucker with a 4 conductor cable where the leads from adjacent coils are in close contact and also capacitively coupled through the ground shield, it is straightforward to envision how the coupling between coils might also be enhanced.
In conclusion, we have shown how a 4 conductor cable of realistic length can have a significant impact on the observed pickup frequency response. The inherent properties of the cable have a significant effect, as shown by the differences between equivalent lengths of Vendor A and B cable, as well as the grounding and shielding schemes. In particular, cable shielding results in significant downward shifts in the observed pickup resonant frequency. These frequency shifts are highly correlated to the measured capacitance of the hook-up cable and may also be related to enhanced coupling between the pickup coils. These frequency shifts can be minimized by using unshielded conductors, and by utilizing a separate ground wire, not connected to a cable shield, when necessary. Shielding the guitar pickup cavities and utilizing unshielded conductors results in significantly lower capacitance compared to a shielded cable, and should result in less significant resonant frequency shifts.
A. Scott Lawing
The Eye of the Storm
October 29, 2012
A couple of months back I decided to try a new source of 4 conductor shielded hook-up cable for our Silent SplitTM models. It was from a reputable dealer and it was a little less expensive, so why not? The first pickup I wired up with it, I noted a possibly significant difference in the final electrical QC specs. I went through the data, and the pickup response appeared to be right on the edge of the distribution, but it was different enough that it was bothering me. I couldn't find anything obviously wrong with the pickup, though. The next one I wired up looked exactly like the first one, and especially since the change was in the muddy direction, I decided I had to dig in deeper. Since I had just switched cable sources, I investigated the cable first.
What I found was that the effect of a 1 foot length of hook-up cable on pickup performance was much more significant than I would have imagined. There are dramatic differences in both the cable capacitance and the frequency response of the pickup as a function of cable type, source and configuration.
But first, a little bit about the specifics of the 4 conductor wiring used in this study. Figure 1 illustrates the Zexcoil(R) Silent SplitTM wiring convention. Zexcoil pickups consist of 6 individual coils, one for each string, and they are arranged to be humcancelling in two pairs of three such that the top three coils and the bottom three coils are RWRP with respect to each other. In normal (series) mode, the pickup is wired in series through all six coils. In Silent Split (parallel) mode, the pickup is wired in parallel through the two sets of three. Note that there is nothing additional that needs to be grounded on the Zexcoil, such that the shield on a multiconductor cable is not required as a ground line.
Figure 1. Zexcoil(R) Silent SplitTM wiring convention.
Figure 2. Frequency response of a Throaty BuckerTM in series mode with various hook-up cable configurations.
Figure 2 illustrates the frequency response of a Zexcoil Throaty Bucker in series mode with various cable configurations (note that the same pickup was used to generate the data in Figures 1-4 and Table 1). Frequency response was measured with a Syscomp CGR101 digital oscilloscope in network analysis mode (see notes on measurement technique in the Appendix). There are two different sources (Vendor A and Vendor B) of 4 x 28 awg conductor shielded cable represented here. Both utilize a foil shield and a fifth bare stranded drain wire as well as stranded conductors. Also illustrated are two examples of a 2 conductor hook-up, using 22 awg stranded pvc jacketed wire. For the two conductor wiring the red and green solder points of the pickup were jumped directly together with a minimum (~1/4") of bare wire and two wires were connected to the white and black solder points respectively. The aligned example places the two conductors linearly adjacent to each other, as they might be in a guitar installation, and the twisted pair consists of the two 22 awg conductors twisted together. Each example utilizes 1 foot of nominal cable length.
The effect of the hook-up cable on pickup frequency response is significant. The difference in resonant frequency between no lead wire (measurement leads attached directly to the pickup solder points) and 1 foot of Vendor B cable with the shield grounded is almost 3500 Hz. The difference between the two cable sources when the shields are grounded in both cases is nearly 1000 Hz. Note that the Vendor A cable with the shield floating (not grounded) is almost identical in response to the 22 awg twisted pair.
Figure 3. Frequency response of a Throaty Bucker in parallel mode with various hook-up cable configurations.
Figure 3 illustrates the response of the same pickup in parallel mode. While the effects in this mode are not quite as significant as the series mode illustrated in Figure 2, they are measurable and follow a trend similar to the series data, with Vendor B exhibiting a depressed resonance compared to Vendor A under like conditions, and the grounded shield exhibiting a significantly lower resonance than the floating shield.
Table 1. Cable capacitance and pickup resonant frequency data from this study.
Table 1 gives the pickup resonant frequency data as well as the appropriate cable capacitance data for each case. Capacitance was measured with an Extech model 380193 LCR meter @ 1000 Hz using the parallel model. For the 4 conductor cable, capacitance was measured between two unconnected wires, in one case when one of the wires was connected to the cable shield (drain wire) and also in the case where neither wire was connected to the cable shield. In the two wire case the capacitance was measured simply between the two unconnected wires.
Figure 4. Pickup resonant frequency as a function of measured cable capacitance. The dotted lines illustrate power law fits with exponents of -0.21 and -0.33 for the parallel and series modes respectively.
Figure 4 illustrates some of the data from Table 1, with pickup resonant frequency plotted as a function of the measured cable capacitance. While this comparison does represent a gross oversimplification of the physics of the situation [we would expect the relationship between resonance and capacitance to follow frequency is proportional to C -1/2, and the actual capacitance is much more complicated than the measured capacitance, especially with the shielded cables], the correlation between resonance and capacitance is clear, as would be expected.
So what does all of this mean? First of all, the pickup is being significantly loaded by the capacitance of the hook up cable, especially when the shield is connected. From our perspective, we want to deliver as much of the tonal spectrum as the pickup is capable of producing, so we want to minimize this loading as much as possible. One could also think of the cable capacitance as another knob in the tonal tuning equation (and this is perfectly valid, Jimi Hendrix famously used a high capacitance cable as part of his tone), but since there are already so many ways to dial out frequencies downstream, we take the approach of trying to deliver as much of the signal as possible to the controls.
So clearly, shielded cable is a significant "tone suck", but can we get rid of it without worrying about adding noise back in to the system? First, realize that there are multiple sources of noise. What we'll call "hum" is magnetic interference, the magnetic component of 60 cycle AC power, that guitar pickups sense the same way they sense the magnetic fluctuations caused by string vibrations. Typical shielding does absolutely nothing to combat this type of interference. Shielding does act to reduce EMI, which we will call "buzz". The best way to reduce noise is to have very quiet pickups. To combat hum, a hum-cancelling pickup (or some other mechanism to generate opposite polarity hum signal such as a dummy coil or hum-cancelling backplate) must be employed. Zexcoils are extremely efficient at hum-cancelling and we find that there is generally no significant noise penalty when using Zexcoils with an ugrounded, shielded cable, and in fact we recommend not grounding the hook-up cable shield. But even the best hum cancelling pickups can be susceptible to buzz in a very noisy environment.
If shielding is necessary to combat buzz, a better route than shielded cable is to shield the guitar body cavities. To a first approximation, capacitance is proportional to the dielectric constant of the insulating medium between the conductor and the shield and inversely proportional to the distance between the conductor and the shield. So capacitance will decrease as the distance between the conductor and the shield is increased, and will also decrease as the dielectric constant of the insulating medium between the conductor and shield is decreased. Since the distance between the conductor and the shield is much larger in the case of a body cavity compared to a shielded cable, and the dielectric constant of the insulator (air) is also much lower than any of the jacketing or insulating materials typically employed in cables (air has the lowest dielectric constant of any material except a vacuum), the added capacitance of a body shield is significantly lower than a shielded cable.
Figure 5 shows the measurement of the capacitance of a pair of unconnected, aligned 22 awg stranded wires, where the black wire is connected to the body cavity shield. With the shielded pickguard in place (as shown at right in Figure 5), the capacitance of a 1 foot length of wire (in roughly the position of a neck pickup lead wire) is only 27 pF/foot, which compares favorably to the 38.5 and 62.8 pF/foot measured for Vendor A and B cables respectively (see Table 1).
Figure 5. Measurement of wire to cavity shield capacitance.
Figure 6. Frequency response of a prototype pickup and a conventional covered humbucker illustrating the effects of grounding and cable capacitance.
How else could this information be utilized to reduce high end loss in pickups? One way is in conventional humbuckers utilizing 4 conductor wiring. Almost every 4 conductor humbucker pickup we have ever encountered utilizes a shielded cable, with the metal parts of the pickup (pickup cover, baseplate, screws, pole pieces, etc.) connected to the cable drain wire and shield. This practice sets up a situation where significant shifts in the resonance will be observed. Figure 6 illustrates the effect on two different pickups. On the right is a prototype Zexcoil pickup with a nickel silver cover and a metal baseplate (basically a construction analogous to a conventional humbucker) but with Zexcoil innards, and on the left a conventional covered humbucker. The Zexcoil pickup utilized nominally 14 inches of Vendor B shielded cable while the conventional humbucker utilized only 7 inches of shielded cable of unknown origin. With the Zexcoil pickup, we were able to analyze three cases, first where all of the metal parts were grounded to the shield, second where a separate wire was used to ground the metal parts and the cable shield was allowed to float, and third where the metal parts were not grounded at all. The most significant effect is between the shield ground and the separate ground wire, with the difference between the separate ground and no ground being much less significant. The conventional humbucker exhibits a similar response between a shield ground and no ground, and while we did not take the humbucker apart to explicitly analyze the case of a separate ground wire, the data at right implies that the same gains in frequency response would be realized with a separate ground wire.
We also see evidence in these data that the pronounced effect of grounding and shielding on frequency response may not be simply due to capacitive losses but may also be due to enhanced coupling between the coils of the pickup. Referring to Figure 6, note that there is a secondary resonance occurring at around 10,000 Hz in both pickups in the case of a shield ground, the resonance is more pronounced on the Zexcoil pickup where there are a higher number of coils to potentially interact. In other unpublished work, we have observed this secondary resonance to be enhanced in pickup design situations where we would expect coupling between the coils to be more significant. Especially in the case of a humbucker with a 4 conductor cable where the leads from adjacent coils are in close contact and also capacitively coupled through the ground shield, it is straightforward to envision how the coupling between coils might also be enhanced.
In conclusion, we have shown how a 4 conductor cable of realistic length can have a significant impact on the observed pickup frequency response. The inherent properties of the cable have a significant effect, as shown by the differences between equivalent lengths of Vendor A and B cable, as well as the grounding and shielding schemes. In particular, cable shielding results in significant downward shifts in the observed pickup resonant frequency. These frequency shifts are highly correlated to the measured capacitance of the hook-up cable and may also be related to enhanced coupling between the pickup coils. These frequency shifts can be minimized by using unshielded conductors, and by utilizing a separate ground wire, not connected to a cable shield, when necessary. Shielding the guitar pickup cavities and utilizing unshielded conductors results in significantly lower capacitance compared to a shielded cable, and should result in less significant resonant frequency shifts.
A. Scott Lawing
The Eye of the Storm
October 29, 2012
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