Phantom Powered Guitar Preamp

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.

Undercover Pickups

For quite a long time, I have been wondering about a phenomena called “eddy current losses” in guitar pickups. Essentially, these are signal losses due to various electromagnetic aspects of the pickup’s construction, that cause the tone to become dulled to some degree. All pickups have them, and some depend on a calculated amount of losses to produce a certain tonal balance. However, because the high audio frequencies are conducive to a sensation of “clarity” or “brilliance” in the sound, it is generally good to reduce the losses as much as possible.

While analyzing pickups and examining the analyses of other testers, I began to realize that the metallic covers that contain the coils and internal parts of a pickup, are a prime source of loss. This was actually known to the early designers of the 1950’s era. They responded by finding metals that have low losses, and used those as the base material for pickup covers. These would then be electroplated to any desired appearance.

J.R. Butts, a designer for the Gretch guitar company, chose a different way. He considered the electromagnetic problem more carefully, and designed a metal cover shape that was almost completely immune to the losses – the “Filtertron”. Subsequently, the Fender guitar company adopted the design for a specialty guitar – the “Cabronita”.

After 1960, nobody thought much of the whole thing. High quality pickups always used an alloy called nickel-silver, while the Filtertrons and some other covers remained the more inexpensive brass. But when brass is used in a non-Filtertron design, the sound is usually very dull due to the eddy current losses.

I wondered, why does the J.R. Butts design work so well? The patent mentions it, but offers little explanation. So I began some experiments to determine the exact nature of the eddy currents in a guitar pickup cover. These were extremely revealing. Soon, I realized that the Butts design barely scratches the surface of the techniques that could be leveraged to improve a pickup cover.

I designed and built several prototype alternative designs made from brass to test my theories. These were wildly successful. However, as I considered what I would do with my invention, I realized that I lack the funds and resources to obtain patents, trademarks, set up inventory, place manufacturing orders and such things that are necessary to make and sell a product.

guitar pickup

Prototype Humbucker Cover

So after almost a year of development, I feel that the best course of action is to simply release the information into the public domain. I hope that if it has some small success as a product, that I can at least boast that it was my idea. After all, it probably won’t be the last one coming from me.

The technical article is long, so I should gPrototype Telecaster Neck Pickupive you a summary. The idea is that by cutting small slots in strategic locations on the cover, the tone-sucking eddy currents can be mostly eliminated. This has two applications. One is that cheap brass covers can be used where a nickel-silver one would normally be used. That is a cost advantage. Another is that when a nickel-silver cover is slotted, the losses are so small as to be both non-measurable and inaudible. This means that a protective cover can be added to pickups that have previously shunned covers for reasons of tonal purity (this habit began with the heavy metal players of the late 1970’s).

Here is the full story: pickup_cover_geometry

Rendering Again!

nine_lives_1 These are historical batteries that I have subjected to a cylindrical scan and then rebuilt, using computer rendering techniques. The floor is the actual floor outside my apartment in Hubei, photographed and used as an surface colouring. There are few of these batteries in existence now, because they leak and corrode with age.