Sunday, May 3, 2015

The Nachbaur Diode Limiter, Part 3: Other variations

This post is part three in a series of three.
Part one discussed the basics.
Part two discussed transfer function shapes and resistor selection.

Listening Test

I ran a test of the Nachbaur Diode Limiter as a passive outboard effect.  The aux sends on my audio interface can drive about 8.18 V peak.  I breadboarded a 4-stage version like this:

Resistors were selected using my spreadsheet to have a final knee voltage of 8 V, and a final attenuation 10 (gain of 0.1).

Below are the audible results.  The program consists of a test tone and a DI-recording of my Danelectro 63 baritone guitar:
  1. 5 seconds of 1 kHz sine wave, increasing linearly in amplitude from 0 to full scale.
  2. A simple chord pattern.
  3. A simple single-note lead.
Each guitar section is normalized to full scale in order to just hit the last knee of the limiter.  The program is repeated for each of the following:
  1. Dry sound
  2. Through the limiter.
  3. Through a 160 Hz 1st-order highpass, normalized, and then through the limiter. (This was skipped for the 1 kHz test tone).
  4. Through the limiter, re-normalized, and then through the limiter again.
  5. Through a 160Hz 1st-order highpass, normalized, through the limiter, re-normalized, and then through the limiter again.  (This was skipped for the 1 kHz test tone).
Since the output impedance of the limiter is fairly high, I didn't use my interface's aux return, but rather the DI input. Here is the sound:

As you can see, it really doesn't sound like anything special in this setting.  I think it really needs a much hotter input than I could get out of my interface.

Anyway, here are a few thoughts on some changes you could make to the circuit without changing its topology completely, and while keeping it a passive attenuator:

Lower voltages with Schottky diodes

I might have tried this if I had any small signal Schottky diodes on hand.  Common ones like Vishay's BAT81S will have a forward voltage drop of about 300 mV under the conditions of this circuit.

Add a hard clipper to the last step

By eliminating the top resistor in the ladder, the last step becomes a "hard" clipper:

The transfer function looks like this (additional traces are the voltages at the lower diodes).  Note that the top diodes turn on gradually, so the last step (at about Vin=10 V) is only a "hard" knee in comparison to the earlier steps:

Split the diode ladder into two chains

With one string of diodes conducting on the positive signal voltages and one conducting on the negative, and different resistor selections in each, you can get an asymmetrical transfer function, which might be designed to mimic single-ended tube transfer function:

Break it up

You can save a little on headroom by putting the signal through a hard clipper, followed by enough make-up gain to push the hard clip limit beyond the last knee of the "Nachbaur".  The difference between this and simply including the hard clip in the diode ladder is that the hard clip input voltage is not constrained by the rest of the circuit, and can be selected independently by the gain structure.

You could also do more complicated multi-stage versions, with the extreme case being a single diode pair at each stage followed by makeup gain.


In the end I decided to save the Nachbaur and its variants for a future tube project.  There are other ways of making a soft clipper with a diode ladder that don't need the voltage swing.  More on that in another post. (Hint: feedback).

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