Frequency Response Tests Part 2

These tests examine the effect of the Cutting Edge Filter (CEF) which is a steep high pass filter originally designed by Charlie Hall to remove any muddiness in the guitar tone by filtering out the unwanted low end frequencies.

Test 1

CEF: OFF, Presence: MIN.


Test 2

CEF: ON, Presence: MIN


The gain now drops off steeply below about 400 Hz; for example at 200 Hz the gain goes from about -7.5 dB with the filter off to about -22 dB when it’s turned on.

But what’s with the rise again around 80 Hz? It’s probably not significant as the gain there is still about 8 dB down on the response from 1000 Hz up.

Please feel free to leave comments.

Frequency Response Tests Part 1

Bode plots were performed on the FET eTap2HW unit to study the effect of the Presence control and the Cutting Edge Filter (CEF).

The input signal was a 250mV peak swept frequency sine wave. Sweep range 50 – 5000Hz. Echo Model 7, all controls zero, Level = 6

Test 1

CEF: OFF; Presence: Min;

Some crazy phase shift stuff going on!

Some crazy phase shift stuff going on!

Test 2

CEF: OFF; Presence: Max;


Note the presence peak at around 2.24 kHz

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%

Projects Gallery

Automation is beginning to take off in the EchoTapper world and some excellent units are already completed or will be soon. If you send me some photos and details of your project I’d love to feature them on my blog here. Hopefully this will encourage other EchoTappers to ‘take the plunge’ and add automation to their own projects.

Rolf Holmberg kindly sent me some photos of his excellent  Valve EchoTapper which uses my automation code. He has made a superb-looking unit, as I’m sure you’ll agree.

Steve Mitchell was one the very early adopters of the automation and he also made some very useful contributions to the development of the code (which I based on work originally done by Johan Forrer in the United States). Steve has also been instrumental in optimizing the performance of the eTap2hw FET preamp which was designed by Piet Verbruggen, the creator of the eTap2’s DSP code, along with some suggestions from ChrisG.

Johan Forrer’s original automation code forms the basis from which I developed the current system. Johan has himself produced a hybrid unit which uses valves and an FET circuit to produce very authentic-sounding results, as you can hear if you check out his examples on SoundCloud.