Hi all,
I have just completed my first stomp box, the Dr Quack with the mods suggested by Mark Hammer (and others) in order to use it with bass and I really like it. Now I am thinking about to build a Neutron.
And here is my question: If I got it right the Quack as well as the Neutron have inverting filters. Therefore blending them with the dry signal in some settings may result in both signals cancelling each other. Since I would like to put the Quack or Neutron into the effect loop of my amp to mix it with the dry sinal I was thinking whether there is any possibility to build some kind of signal inverter (preferable on a small PCB) and to add it to the output of each of them.
Thanks in advance for your suggestions.
Markus
The Quack is definitively an inverting filter and hence an inverted output, because there is just the one bandpass filter section there. In the case of the state-variable filter that forms the Mu/Q/Neutron, the polarity of the output would depend on which filter output you select.
There are essentially 4 op-amps in the audio path of the Mutron. Look at the schematic from input to output and you will see the following:
- input stage is inverting
- first filter stage (Highpass output) is non-inverting
- second filter stage (Bandpass output) is inverting
- third filter stage (Lowpass output) is inverting
Taking into account the "double inversions", the highpass and lowpass outputs are out of phase with the input signal, but the band pass is double-inverted, hence in phase.
Probably the smart thing to do for any sort of blending purposes is to have a small inverting buffer that you can bypass.
Alternatively one of the things you sometimes see is a single control that pans between inverted and non-inverted versions of a signal so that the single pot controls both intensity and polarity. A perfect example of this is the Electro-Harmonix Y-Triggered Filter (posted around but not especially legible). This provide an uninverted and inverted version of the envelope signal (the envelope itself, not the audio), each feeding into one of the outside lugs of the pot. The wiper of the pot provides the envelope output. As you move further from the midpoint of rotation, you get less cancellation of the one version by the other. In the case of the YTF, the single control goes from strong downward sweep through mild downward sweep to no sweep, mild upward sweep and strong upward sweep.
Mapping this onto the audio path, you can have separate inverted and non-inverted versions of the Neutron output (stick two consecutive unity-gain inverting stages at the output and run your panpot to the output of each) feeding the panpot, OR separate inverted/noninverted versions of the dry signal, and use the panpot to determine how much of the inverted vs noninverted version of the signal in question gets fed to the subsequent DRY/WET mixing stage.
Make sense?
Hi Mark,
thanks for your great reply. I will work through it and see if I can manage to understand it.
Makes perfect sense (after some hours of reading schematics... I think it would help to work through The Art of Electronics or something similar).
For the Quack I will go with just one inverting buffer added to the output (with a bypass switch) and do the dry/wet mixing in my bass amp (because the Quack is already completed).
But for the Neutron I would like to do the definitely more interesting variant Mark has suggested (sticking the two consecutive inverting stages to the output and including a blending pot that feeds the dry/wet mixing stage).
The only problem is I do not have the faintest idea how to build those unity-gain inverting buffers. I suppose it should be possible to use a low noise OP amp with some caps and restistors. PLEASE HELP.