One recurring question often seen is how to add a buffer to a circuit to prevent loading and loss of definition of the guitar sound. Buffers present a high impedance to the guitar pickup and have a low impedance output drive with a gain close to unity (unity gain = 1). This is an excellent addition in front of a vintage wah-wah or other circuit that can rob the signal of high frequency response. Buffers are simple, easy and cheap to construct. (Note: Any of these buffers could also be used on the output of an effect circuit.)
Before we see the circuits let us look at a circuit fragment that may be required for some of the buffer variations. As shown to the left, the resistor/capacitor network provides a reference voltage that may be used to bias the transistor or opamp into the best operating range. The point marked "Vr" is connected to the point also notated as "Vr" on the buffer schematic. If there is a reference voltage already established in a circuit to which you are planning to add a buffered input, the existing Vr can be tapped and used for the buffer's reference.
Probably the easiest buffer is the basic jfet common drain amplifier. The input impedance is determined by the value of R1 and is 1M as shown in this example. The value of R2 is not too critical and may be any value from 3.3k to 10k without much change in the sound. I prefer to use lower values since this allows more drive on the negative portion of the audio cycle where the only pull-down is the source resistor. This configuration has the least number of parts but is limited in that if the input voltage exceeds the gate-source forward voltage plus the bias voltage at the source, the signal will be clipped. This configuration is not normally useful with bipolar or mosfet transistors, which require a positive bias voltage (for N-type devices). Input impedance is approximately the value of R1. The output impedance will depend on the jfet but is on the order of a few hundred ohms.
The basic jfet buffer shown in the last paragraph may be improved upon by connecting the gate resistor to a bias voltage instead of to ground. This allows the gate voltage to set the bias at the source to a higher vlaue which increases the headroom and allows a large signal input before clipping. The Vr point on this circuit connects to a bias voltage source as shown in the first paragraph. Input impedance is again approximately the value of R1. It could easily be increased to 10M or more for a cleaner sound with high impedance signal sources such as high-output humbuckers or piezo sensors with only a slight incerase in the thermal noise contributed by the higher value of R1.
If you do not have a bias source available for Vr and you want to keep down the parts count, the gate bias can be set by a pair of resistors as shown in this example. The input impedance is the value of R1 paralleled by R3, or 500k ohms in this example, but you could easily increase their values to 2M to maintain the 1M input Z. This is the buffer that I used on the front end of the Dr. Quack Envelope Filter.
A bipolar transistor may also be pressed into service if the input impedance does not have to be as high as that available from jfets. The advantage is that the bipolars usually have a lower output impedance and are generally easier to find. The disadvantage is the lower input impedance available as compared to fets.
An alternate configuration is shown here that uses the voltage bias on the input in the same manner as the second jfet example above. This is the buffer used in the TS-series distortion boxes.
If you substitute a mosfet transistor into the circuit of the last example and tweak the source and gate resistor values, it is essentially the AMZ Mosfet Booster in buffer mode. (See how these building blocks are useful?) A 9v zener diode (D1) is used for static protection on the mosfet gate. Mosfets have high capacitances between its electrodes and though there is no Miller Effect to multiply those capacitances, their value can nonetheless be high enough to cause high frequency rolloff depending on the characteristics of the individual mosfet transistors.
It is also possible to use a pair of 10M resistors to provide the bias voltage to the gate of the mosfet similar to the earlier Dr. Quack buffer.
An opamp is an even better buffer amplifier, though many believe they are somewhat colder sounding and more sterile than the transistor versions. The opamp gain is exactly unity and the output impedance is quite low; typically measured in tens of ohms instead of hundreds as with the transistors. It also has the lowest parts count of any of the simple buffers presented here.
Voltage divider biasing is also possible with the opamp and the input impedance is calculated the same way as with the transistor versions similarly biased.
This opamp buffer inverts the signal, which is useful when used in conjunction with following gain stages that also invert the signal and therefore would make the output non-inverted when compared to the input. This is important if the signal is mixed with other signals from the same source since cancellation could occur otherwise. The gain is unity and is set by R2/R1. The small 5pF capacitor is optional and gradually rolls off the high frequencies above the audible range. The input impedance of this circuit is the value of R1.
Miscellaneous: Selection of the jfet or bipolar type is not critical, almost any transistor will work fine without problems. The gain of the transistor versions is slightly less than 1, probably 0.9 to 0.96. Transistors with higher hfe will be slightly closer to unity gain.
The transistor circuits have very poor power supply noise rejection. Battery power works well with them but if used with an AC adapter, it must be well filtered and hum-free or the noise will be combined with the signal.
For the jfet or mosfet circuits that use a single resistor on the gate for bias, you can increase the value of R1 to provide a higher input impedance. I typically use jfets on inputs and bipolars for output buffers where their better drive characteristics are needed.
While this article just scratches the surface of buffer amplifiers, it presents enough basic circuits to handle 99% of effects requirements. Any of the building blocks above may be dropped into a circuit exactly as presented.
©2002 Jack Orman This page last modified on Saturday, 12-Apr-2003 08:39:07 PDT
00994 hits since April 16, 2003
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©2002 Jack Orman
This page last modified on Saturday, 12-Apr-2003 08:39:07 PDT
00994 hits since April 16, 2003