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As an electronics design engineer I am used to picking at deeper and deeper levels of understanding in circuits. This issue of the H&K amps, (in my case the GM36), and their upper end is like that. There are the big details which make a lot of difference, like the choice of speaker you make and the actual voicing of the amps, then there are finer details which may not have so much of an effect but which surface when you sort out the coarser ones, like the idea of "blanketing" the speaker with materials to slightly remove some of that top end. Then there are general ideas which may or may not be relevant to the problem at hand as you see it. Here is one of those for you to consider.
Most guitarists level of understanding of electronics is very primitive and is mainly at the level of "repeat what my online guru says". Sorry if that ruffles feathers guys, but in the real world of electronics that is the case, and it is mixed with a variable level of "...and I can't be questioned on that because it is MY opinion!" No amount of verifiable factual engineering, or sometimes even "Laws of Physics", info will change a guitarist from his belief that something is so. Capacitors and their working within circuits are sometimes like that. The belief is simply "capacitors bleed off treble". No, they don't! People with part knowledge often suggest they do online and people with no knowledge repeat that incessantly. Capacitors do much more than simply allow high frequencies to be affected and the H&K amps may have a wrinkle, a detail which has been left in what looks to me to be perhaps a "trimmable state", which we can exploit.
Your guitar pickups are complex things. They are a complex mixture of inductance (most important and least understood), capacitance and resistance on a few levels. Firstly let me start by dispelling a myth which will ruffle some feathers but it needs to be done.
There is no real link between the resistance of the windings and the sound of the pickup unless you have a few other factors at hand to add into the equations. The main factor is its inductance as that dominates the way it shapes the frequency range it works within. Then there are a number of capacitances to consider. Then there is the coil resistance which a number of people think tells you how hot it is wound. In truth it can, but only if you also know other details such as the inner dimensions of the coil(s) and the gauge of the wire. All that resistance is actually telling you on its own is the resistance of the straight length of wire at DC whether it is in the coil or not. If the wire is thick gauge it will mean it has a longer length, if it is thin gauge it will mean a shorter length. And that length will in turn mean more or less turns around the coil former whose size will also affect that number of turns. The number of turns will affect both the inductance and capacitances of the coil greatly. And THEY are the real factors which affect frequency response.
Most pickups are wound with 42, 43 or 44 gauge wire. There you see the resistance per 1000ft of each. The resistance is very different for each so one other factor will also change radically. If you were to use the same length of each wire on the same bobbin you will end up with different resistances and also different numbers of turns on the coil. If you were to use the same resistance of each wire you would have very significantly different lengths with very different numbers of turns and hence very different inductance and capacitance. If you use the same number of turns you will end up with very different lengths which means in turn very different resistances. Other effects like, finer wire means turns closer to the core which means different magnetic properties, also come into play too. If we consider say 6000ohms of each wire, the difference between 42-44AWG actually means that the 43 will only have 77% of the length (and turns) of the 42, and the 44 will have only 64%. So much for resistance of our coil being useful! Yes, if we are comparing pickups with the same gauge of wire and the same sized coil former, meaningless unless we know that to be the case.
Your pickup actually works based mainly on its inductance value with capacitance tagged on too. A generalisation - Inductance does not like passing high frequencies. Effectively the micro-tiny currents the magnets and strings induce into the coil are resisted as their frequency increases. The impedance of the coil, (resistance to the flow of AC currents), increases as frequency increases. The coil capacitances, (there are more than one), do the opposite and resist the passing of low frequency currents. We have a system where, in the first approximation, your pickup looks like a resistor and inductor in series with a capacitor in parallel with them. As frequency increases, the resistance of the coil stays constant, the impedance of its inductance increases, and the impedance of its capacitance decreases.
At low frequencies the inductance is lower in impedance and the capacitance is very high so the resistance of the coil is mainly set by the coil's resistance, (remember it is in series with the inductance). This is the state for the normal range of playing frequencies. At high frequencies the inductance is very high in impedance and the capacitance is very low so the capacitance rules and pulls down the output. This gives the high frequency roll off of our pickups. However, at a magical frequency, the impedances of the rising inductance and the dropping capacitance match up and we have what is known as a "resonance". This is a peak (or a dip) in the value and the interesting thing for us is that it is also pretty much matched by the output of the pickup in voltage terms.
So we inevitably have a peak in the pickup output at the top end of its range. Well, how high a peak, and at what frequency? By changing the values of inductance and capacitance we can move that frequency up or down the range without altering its height too much. By changing the resistance in the setup we can alter the height of the peak without shifting its frequency. Pretty clever if you can do that. In practical terms adding capacitance to the system will allow us to reduce the frequency of resonance moving it down to perhaps a less strident point. It is worth noting that anything attached to the output terminals of a pickup is being added into this resonant system. The Volume and Tone controls are a part of it as is the cable you use and the input components of the amplifier or pedal on the end of it. We cannot easily change the inductance in the system but as I said, the capacitance in the resonant circuit is easy to alter. You just add one little 5p component and you have tuned where your resonance occurs. Add another 2p resistor and you can even tweak how high that resonance is. (Remember how some people swear by 500k pots and some prefer 1Meg?
)
Most guitar amps have a well defined input circuit. They usually have a resistor somewhere in place across the input to set the resistance to ground (which will drop our peak level). This resistor is usually around 1Meg. Many have a capacitor there too to reduce RF interference but, if it is big enough, it will also lower the frequency of resonance. This will usually be only a few 100pF at most. Looking at the input stage of my GM36 I see there is a 1Meg resistor as we would expect, there are two protection diodes to prevent damage from high voltages on the input and these will have a small capacitance, but there is no specific capacitor in there with them. The GM40D will most likely be the same being in the same family of models. H&K will have checked the need and prefers to leave any tuning of this to ourselves, (a good move in my opinion not an error in any way). This leaves us with a question. Is it possible some of us are hearing a too high frequency or maybe even overly large resonance in our guitar's output?
If you have followed any or all of the above, this is easy to implement even for the non-techs. You will simply need a handful of capacitors in the range 470pF to about 10nF, a few resistors of maybe 2.2Meg, 1.5Meg, 1Meg, and the ability to connect them across your guitar terminals at some point. I like to do this inside the cable jack plug at the amp end which is easy if you can solder but I admit I make all of my own cables as I do understand the myths involved in that side too. (No, I'm not starting that here.
) Turn your guitar volume up full to where you will get maximum interaction with your test values, stick a capacitor across those terminals and just play. You won't hear a "night and day" change but, when the capacitor value is high enough the tone should seem to maybe calm down and become a little thicker. you may not like that of course and that is a case by case thing. Play with the values until you get to your best point. Then start with the highest value of resistor and add that across the terminals too, dropping the value until you hear any change and selecting the best one to your ears.
Without oscilloscopes and other test equipment there is no way to check what is actually going on, this is one experiment that you will have to do with your ears. If you can hear no difference then all you have lost is a few pence (cents) for a handful of throw away components. But you just might find that this changes the balance of your tone fundamentally for the better. It relates to any amp and any guitar as each will be different.
I have tried hard to keep this description accurate but simple for non-tech players to understand. I'm sure I have not done so well with that in places and my apologies, I tried. For people who want to know more, and there is a lot more to this topic, I can recommend this site where these points are covered with useful diagrams. Make sure to scan right down both pages as some of the best info is towards the bottom. (For example, look for Fig8, Fig14 and Fig15.) The Secrets Of Guitar Pickups (You can pick up this article as a PDF here: PDF Version.)
Good luck with your tweaking!
Most guitarists level of understanding of electronics is very primitive and is mainly at the level of "repeat what my online guru says". Sorry if that ruffles feathers guys, but in the real world of electronics that is the case, and it is mixed with a variable level of "...and I can't be questioned on that because it is MY opinion!" No amount of verifiable factual engineering, or sometimes even "Laws of Physics", info will change a guitarist from his belief that something is so. Capacitors and their working within circuits are sometimes like that. The belief is simply "capacitors bleed off treble". No, they don't! People with part knowledge often suggest they do online and people with no knowledge repeat that incessantly. Capacitors do much more than simply allow high frequencies to be affected and the H&K amps may have a wrinkle, a detail which has been left in what looks to me to be perhaps a "trimmable state", which we can exploit.
Your guitar pickups are complex things. They are a complex mixture of inductance (most important and least understood), capacitance and resistance on a few levels. Firstly let me start by dispelling a myth which will ruffle some feathers but it needs to be done.
Bordonbert wrote:THE RESISTANCE OF A PICKUP IS IRRELEVANT IN PREDICTING WHAT IT WILL SOUND LIKE.
There is no real link between the resistance of the windings and the sound of the pickup unless you have a few other factors at hand to add into the equations. The main factor is its inductance as that dominates the way it shapes the frequency range it works within. Then there are a number of capacitances to consider. Then there is the coil resistance which a number of people think tells you how hot it is wound. In truth it can, but only if you also know other details such as the inner dimensions of the coil(s) and the gauge of the wire. All that resistance is actually telling you on its own is the resistance of the straight length of wire at DC whether it is in the coil or not. If the wire is thick gauge it will mean it has a longer length, if it is thin gauge it will mean a shorter length. And that length will in turn mean more or less turns around the coil former whose size will also affect that number of turns. The number of turns will affect both the inductance and capacitances of the coil greatly. And THEY are the real factors which affect frequency response.
| AWG | Ohms/1000 ft |
| 42 | 1659 |
| 43 | 2143 |
| 44 | 2593 |
Most pickups are wound with 42, 43 or 44 gauge wire. There you see the resistance per 1000ft of each. The resistance is very different for each so one other factor will also change radically. If you were to use the same length of each wire on the same bobbin you will end up with different resistances and also different numbers of turns on the coil. If you were to use the same resistance of each wire you would have very significantly different lengths with very different numbers of turns and hence very different inductance and capacitance. If you use the same number of turns you will end up with very different lengths which means in turn very different resistances. Other effects like, finer wire means turns closer to the core which means different magnetic properties, also come into play too. If we consider say 6000ohms of each wire, the difference between 42-44AWG actually means that the 43 will only have 77% of the length (and turns) of the 42, and the 44 will have only 64%. So much for resistance of our coil being useful! Yes, if we are comparing pickups with the same gauge of wire and the same sized coil former, meaningless unless we know that to be the case.
Your pickup actually works based mainly on its inductance value with capacitance tagged on too. A generalisation - Inductance does not like passing high frequencies. Effectively the micro-tiny currents the magnets and strings induce into the coil are resisted as their frequency increases. The impedance of the coil, (resistance to the flow of AC currents), increases as frequency increases. The coil capacitances, (there are more than one), do the opposite and resist the passing of low frequency currents. We have a system where, in the first approximation, your pickup looks like a resistor and inductor in series with a capacitor in parallel with them. As frequency increases, the resistance of the coil stays constant, the impedance of its inductance increases, and the impedance of its capacitance decreases.
At low frequencies the inductance is lower in impedance and the capacitance is very high so the resistance of the coil is mainly set by the coil's resistance, (remember it is in series with the inductance). This is the state for the normal range of playing frequencies. At high frequencies the inductance is very high in impedance and the capacitance is very low so the capacitance rules and pulls down the output. This gives the high frequency roll off of our pickups. However, at a magical frequency, the impedances of the rising inductance and the dropping capacitance match up and we have what is known as a "resonance". This is a peak (or a dip) in the value and the interesting thing for us is that it is also pretty much matched by the output of the pickup in voltage terms.
So we inevitably have a peak in the pickup output at the top end of its range. Well, how high a peak, and at what frequency? By changing the values of inductance and capacitance we can move that frequency up or down the range without altering its height too much. By changing the resistance in the setup we can alter the height of the peak without shifting its frequency. Pretty clever if you can do that. In practical terms adding capacitance to the system will allow us to reduce the frequency of resonance moving it down to perhaps a less strident point. It is worth noting that anything attached to the output terminals of a pickup is being added into this resonant system. The Volume and Tone controls are a part of it as is the cable you use and the input components of the amplifier or pedal on the end of it. We cannot easily change the inductance in the system but as I said, the capacitance in the resonant circuit is easy to alter. You just add one little 5p component and you have tuned where your resonance occurs. Add another 2p resistor and you can even tweak how high that resonance is. (Remember how some people swear by 500k pots and some prefer 1Meg?
)Most guitar amps have a well defined input circuit. They usually have a resistor somewhere in place across the input to set the resistance to ground (which will drop our peak level). This resistor is usually around 1Meg. Many have a capacitor there too to reduce RF interference but, if it is big enough, it will also lower the frequency of resonance. This will usually be only a few 100pF at most. Looking at the input stage of my GM36 I see there is a 1Meg resistor as we would expect, there are two protection diodes to prevent damage from high voltages on the input and these will have a small capacitance, but there is no specific capacitor in there with them. The GM40D will most likely be the same being in the same family of models. H&K will have checked the need and prefers to leave any tuning of this to ourselves, (a good move in my opinion not an error in any way). This leaves us with a question. Is it possible some of us are hearing a too high frequency or maybe even overly large resonance in our guitar's output?
If you have followed any or all of the above, this is easy to implement even for the non-techs. You will simply need a handful of capacitors in the range 470pF to about 10nF, a few resistors of maybe 2.2Meg, 1.5Meg, 1Meg, and the ability to connect them across your guitar terminals at some point. I like to do this inside the cable jack plug at the amp end which is easy if you can solder but I admit I make all of my own cables as I do understand the myths involved in that side too. (No, I'm not starting that here.
) Turn your guitar volume up full to where you will get maximum interaction with your test values, stick a capacitor across those terminals and just play. You won't hear a "night and day" change but, when the capacitor value is high enough the tone should seem to maybe calm down and become a little thicker. you may not like that of course and that is a case by case thing. Play with the values until you get to your best point. Then start with the highest value of resistor and add that across the terminals too, dropping the value until you hear any change and selecting the best one to your ears.Without oscilloscopes and other test equipment there is no way to check what is actually going on, this is one experiment that you will have to do with your ears. If you can hear no difference then all you have lost is a few pence (cents) for a handful of throw away components. But you just might find that this changes the balance of your tone fundamentally for the better. It relates to any amp and any guitar as each will be different.
I have tried hard to keep this description accurate but simple for non-tech players to understand. I'm sure I have not done so well with that in places and my apologies, I tried. For people who want to know more, and there is a lot more to this topic, I can recommend this site where these points are covered with useful diagrams. Make sure to scan right down both pages as some of the best info is towards the bottom. (For example, look for Fig8, Fig14 and Fig15.) The Secrets Of Guitar Pickups (You can pick up this article as a PDF here: PDF Version.)
Good luck with your tweaking!
