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Ken Willmott edited this page Jul 27, 2019 · 24 revisions

Welcome to the integrator wiki!

Overview

This device acts as a front end signal conditioner for guitar pickup measurements. It features an adjustable high impedance input circuit, preamplifier, an integration function, and a line driver amplifier. These are implemented with a single quad JFET operational amplifier. It is designed to be used with a PC sound card or PC oscilloscope and measurement software.

Electrically, a pickup has an LCR circuit response, which can be determined either by directly measuring the two terminal impedance, or by measuring the output response when an external magnetic excitation signal is applied. This system uses an excitation coil. I use about 50 turns of AWG#26 magnet wire on a humbucker bobbin, but any similar coil should work. The coil is placed in proximity to the pickup under test, and the pickup is connected to the integrator board input.

Here is a typical test system: test bench view System diagram:

A Bode plot (frequency vs. amplitude) is produced for the three positions of the input load switch:

  1. no load (actually 11M ohm and about 10pF)
  2. loaded (200k ohm, 470pF)
  3. inductance test load (11M ohm, 4700pF)

The amplitude response of the tester diminishes gradually below 100Hz, and details over 10kHz rarely matter, so the results are usually presented for 100Hz-10kHz:

The peak frequency of the inductance test (here 900Hz) can be used to approximate the inductance value, as the pickup internal capacitance is swamped by the 4700pF test load. The unloaded plot can be examined to determine resonant frequency, inherent Q and eddy current losses. The loaded plot (light blue) approximates how the pickup will respond in an actual guitar circuit, because the 200k/470pF test load simulates a volume/tone control and cable combination.

When values from the plot are entered into an analysis spreadsheet, along with the DC resistance, the pickup capacitance, inductance, and estimates of the eddy current losses can be calculated:

Device Electronics

Here is the integrator device schematic:

The V5 integrator for electric guitar pickup measurements can be divided into four functional blocks. Each one is actually performed by one op amp. One, IC1D, supplies the approximately half supply voltage to provide a reference voltage for bipolar operation from a single voltage supply. Zener D1 reverse voltage of 6.2V is filtered by a capacitor and supplied to the input of the amp, which is configured as a voltage follower.

IC1C is a preamplifier for the signal from the pickup under test. A switch allows a choice of 0dB or -20dB gain. The test input feeds it through a high impedance network that provides isolation from the preamplifier input circuit. It is almost identical to the circuit in a typical oscilloscope probe. However, it can have lower input capacitance because a long cable is not required and so does not need to be compensated as heavily. Like a scope probe, there is a screwdriver adjustable variable capacitor that can be used to calibrate the network. The network input has an approximately 11M ohm resistance and 10pF capacitance. A three-way switch allows the input to be loaded by a standard RC test load of 200k/470pF, an inductance test load of 4700pF, or else no load.

IC1B is an analog integrator with a very low frequency cutoff to provide op amp DC stability. The blocking capacitor C5 is needed to eliminate the DC input current sensitivity caused by the large value of R6.

IC1A is a post amplifier and output line driver.

The gain through the circuit stages was determined by direct experimentation with pickup readings. The signal level after the -20.8dB loss through the high impedance network is either passed to the integrator or amplified by 20 dB, in which case the gain is -0.8dB. The integrator gain varies with frequency, but has unity gain at 884 Hz. The post amplifier has a gain of 33.5dB to restore normal audio levels because the integrator gain is deliberately kept low to increase dynamic range and minimize signal clipping. The preamplifier gain switch is provided to accommodate pickups with different output levels.

The Linear Technologies LT1058 IC was found to perform well, and is the preferred part. However, other FET op amps such as the more common TL084 can be substituted with a slight degradation in accuracy at high frequencies.

The integrator bypass switch introduces some instability in the preamplifier section, because of the effect that the extremely high impedance has on the op amp internal feedback loop compensation, when the large capacitive load of the output cable is connected to directly to the IC1C op amp output. Consequently, the bypass setting is only useful for performing calibration, which can be performed reliably in spite of this problem, by using the high gain setting which is more stable. The output driver amp IC1A can not be used as a buffer for this because the gain is too high. Thus, if the device must be used without the integration function, board modifications or re-design is necessary.

Specifications

Supply voltage: 12V, 2.1mm DC jack

Input impedance: 11M ohm, 10pF

Output impedance: 10k nominal (line audio)

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