High Z-Mode

The sound you get when the pickup(s) are directly connected to the input of a tube amp, i.e. no tone or volume controls, is considered a "Gold" standard for many players. What you get from this set-up is very high impedance pickup loading and a sound that is often described as lively or bright. Of course many players either don't have a tube amp or don't want to live without volume/tone controls but we have a solution for that.

High Z-Mode - a high impedance setting that is equivalent to loading the pickups with a tube amp.

The Details

To understand how this tone is generated, we'll look at three major components of the setup: vacuum tubes, pickups and the cable.

The Vacuum Tube is a simple device and by construction has a high input resistance as there is no internal connection from the plate to the anode or cathode. Furthermore the tube contains a vacuum which is a great insulator. The plate connection of the tube it's self does not add much resistance (or capacitance) to load the pickup. Most tube amplifiers do however load the input with an approximately 1 Meg Ohm resistor. This resistor was inserted at the input of the tube amplifier so that if the cable is disconnected at the driving end (i.e. pulled out of the instrument) then the amplifier would not have an unloaded input - which could cause speaker destructive feedback. This 1 Meg Ohm resistor is the main source of resistance loading the pickup in our scenario. Hence we picked a 1 Meg Ohm input resistance for the High Z-Mode. This high input resistance combined with the inductance of the pickup coil and the capacitance within the pickup (mainly due to the details of how the coil is wound) will have a peaked response at some frequency - see the graph below which is an amplitude versus frequency scan of an actual pickup with a 1 Meg Ohm load.

The frequency of the peak is at 3.5 KHz and is plainly visible in the graph.

Pickups - the actual frequency peak for any particular pickup will depend on the internal inductance of the coil combined with the internal and external capacitance which loads the pickup. If more capacitance is added to the pickup (which would happen, for example, when we insert a guitar cable in between the pickup and the tube amp input) then the frequency of the peak will occur at a lower frequency. If the inductance of the coil is changed, for example switching a pickup from a series to a parallel coil connection, then the peak will occur at a higher frequency. If the pickups are wound differently, for example, by a boutique pickup winder which uses a "scatter winding" technique, then the frequency peaks location will also vary. So resonate peak location varies by pickup, as do player preferences.

Cable length and construction affect the capacitance of the cable. By changing the cable capacitance you can shift the location of the resonant peak. Longer cables lower the frequency, shorter cables raise it. Cable construction gets more complex and relates to the distance separating the inner and outer conductors and the material that insulates them; both affect the cable's capacitance per foot. Few guitar cable manufacturers specify the capacitance per foot of their cables although it is a common spec for cables use in radio frequency applications.

Adjusting the location of the resonant peak: you could, and many people do, try different methods to change the resonant frequency peak location. In the past, the only good option was to trade-out equipment. While this is generally an ad-hoc process, it can be effective.

To give you more control of the resonant peak location we've given you a way to make a capacitance adjustment ; the 3ZB module has turn adjustments and the JZ3 has socketed capacitors that can be changed out.

3ZB: The two adjusters (one per pickup) are located at the top of the module near wires. They are 10 turn adjustments which allow very fine control over the resonant peak location. Dial the screw adjustment counter clockwise to reduce the capacitance added to the pickup and the resonant peak frequency will increase. Dial the adjuster clockwise to increase the capacitance and the frequency of the resonant peak lowers. Adjust by ear to your taste and setup.

JZ3: Each pickup has a capacitor installed in sockets; 330pF for the Neck and 680pF for the Bridge. These capacitors can be replaced with other values which will move the frequency peak location. A set of additional capacitor values is available as an option.

Note: In some setups, it may seem that the system gets brighter at first as more capacitance is added to the pickup load. This appears to contradict what I stated above - this apparent backward operation is because most bass and guitar speaker cabinets roll-off the high frequency and as you are making the adjustment you can move the peak back into the speaker's reproducible range.

The effect of the capacitance adjustment is added to the original capacitance level established by the pickup's construction, i.e. if you have a pickup with a low frequency peak already; increasing the capacitance further will only reduce the peak's frequency location. Since most pickups where designed by experimenting with a cable attached to an amplifier you will often want to add some extra capacitance if you desire the same sound as the designer.

In summary: With Audere's preamp you can have a high impedance sound using the High Z-Mode setting. Plus you can easily adjust the sound by adding capacitance - and you get to keep your volume/tone controls.

Nordstrand NJ5 is a trademark of Carey Nordstrand