The idea of putting a preamplifier in a guitar is hardly new. There are many advantages, but a big disadvantage is the requirement for power, which usually means that a battery has to go into the guitar. That means that a space must be found for the battery, it must be replaced when it expires, and has the potential to leak corrosive liquids inside the guitar if left for too long.This circuit avoids those problems by using a common idea from microphone technology, phantom powering. Many microphones obtain power for internal circuits from an ingenious circuit that allows power to be transmitted over audio cables. This system has become a standard, and is provided by most microphone preamplifiers and mixing consoles.
Originally, the power was used to bias condenser microphones, and so the voltage is quite high, 48 volts. Originally, a transformer was used to separate the audio from the supply voltage, but in recent years transistors became available that could do that, and so the “Schoeps circuit” was developed by the engineers at Schoeps Mikrofone to replace it.
The Schoeps circuit has been adapted and modified endlessly since its introduction, and does a fine job of supplying the power provided on a phantom cable, to on board preamplifiers of various kinds. It is a differential amplifier that provides a very low impedance output to drive the output cable, combined with a balanced power extraction circuit. It works well in different configurations, some having Zener diodes for regulation and some not. The driver stage can be configured as an additional regulation stage (which I have done), however this is optional and some designers omit it. It is the kind of circuit that will almost always work, but needs some thought and understanding to achieve the best results.
In my case, I adapted a JFET input stage from some microphones to work as a guitar preamp. The JFET is configured in differential output mode, in order to easily drive the output transistors and audio cable, which are also differential. It has an extremely high input impedance which means that it presents almost no load on the guitar circuit. Thus the guitar tone and volume controls are the only electrical load that the pickup “sees”. This avoids tonal variances that normally happen when a cable is plugged into a guitar. The lower impedance and differential (balanced) signals in the XLR cable are more immune to hum and interference, and can run up to 100 meters (several hundred feet) with absolutely no loss of tone.
The wiring in my guitar is completely conventional, with a three way switch and tone and volume control. The output goes to the preamp, which is connected to an XLR connector that replaces the usual output jack. In fact, it is possible to keep the original jack and have both powered and unpowered outputs from the guitar, but I didn’t want to drill a new hole. In future, I would because the XLR is not a perfect fit in the existing hole (to be honest, a bit of an understatement!).
Here is the installation. The prototype board fits nicely inside with a safety wrapping of insulating Kapton tape.
I wrestled with the design for a long time. There are a lot of variations on it online but no complete explanations that would allow me to optimize the components.
I thought about Zener regulation. The entire circuit is differential except for the JFET current. So I reasoned that I could omit it and depend on a pair of filter capacitors to do the job. What is strange about the Schoeps circuit, is that the pair of driver transistors function as a first stage voltage regulator as well as signal amplifiers. The collectors are common, and go to the first stage filter capacitor. Then a dropping resistor feeds the second stage supply, which depends on the second stage filter capacitor for audio frequency suppression. In my Spice simulations and also in testing, I found that this was more than adequate. There is no noise or instability from omitting the Zener, in fact it is probably quieter because Zeners are notorious noise sources. The only disadvantage that I can find in this arrangement is that all the component values are interdependent, and so you can’t really change one value or component without changing them all. The voltages have to be checked after construction to verify that they are sane. You should measure approximately 10 volts on C6. However, this circuit has a good chance of working the first time if it is built exactly as shown.
Note: R1 in the schematic above is just a simplification for the sake of Spice testing. It really represents the entire usual volume and tone control circuit of the guitar.
I’ve been playing my Godin Redline 2 with the circuit installed and I like it. I have it plugged into an Art Accessories Phantom II Pro, which then goes into my guitar amp. I can just as easily plug it directly into any phantom powered microphone preamp or mixing board, as I mentioned before.