Signal Analysis of the FET eTap2HW Part 1

Using a recently acquired National Instruments myDAQ and the Bode Plotter, Dynamic Signal Analyzer and Function Generator Virtual Instruments (VIs) included with it, I’ve been making some detailed measurements of my FET eTap2HW unit.

This is the circuit designed by Piet Verbruggen with the optimizations suggested by Steve Mitchell. It is also fitted with a variable presence control (another Steve M suggestion) and a Cutting Edge Filter (CEF) from the design by Charlie Hall.

Test run #1

Input Signal: 1 kHz sine wave, 400 mV pk-pk

Presence = minimum; CEF = OFF, Echo Model 7 (Reverb + Trem), all Controls = 0, Level = 4

Note there are few harmonics and THD is low

Note that there are few harmonics and THD is low (0.25%) with this low input Level setting, as expected.

Test run #2

Input Signal: 1 kHz sine wave, 400 mV pk-pk

Presence = minimum; CEF = OFF, Echo Model 7 (Reverb + Trem), all Controls = 0, Level = 8

A higher input level is designed to hit the modified ‘Fetzer valve’ circuit’s ‘sweet spot’ and produce desirable valve-like harmonics. This plot shows the harmonics are indeed present now.

The higher input level has 'turned on' the harmonics. THD now about 1.3%

The higher input level has ‘turned on’ the harmonics. THD now about 1.3%

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