Re: Tone Stack Question

Posted by induction on Nov 14, 2016; 5:21pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Tone-Stack-Question-tp34153p34191.html

Bear in mind that one of the most important factors when it comes to passive tone stack performance is the input impedance of the following stage, which has a major impact on both output volume and frequency response. (The output impedance of the previous stage is also important, but often not quite as critical.) Very often the boost you get from a make-up gain stage has less to do with the actual gain of that stage than with the input impedance of that stage.

As a make-up gain stage, the LPB-1 is a fairly limiting choice, as its input impedance is usually somewhere around 47k. The MXR Micro Amp, or ZVex SHO, on the other hand, have input impedances north of 1M. Neither is an especially difficult circuit, but the SHO is nearly as simple a circuit as the LPB1, and it doesn't add noticeable distortion of its own when you keep the gain set below 75% or so.

To see the difference for yourself, open up your Duncan Tonestack Calculator, select the Marshall tonestack, change C1 to 680p and you'll have the tonestack shown above. Now change the load resistor (R5) from 47k to 1M and see the difference in output. Play with the TMB controls and switch R5 back and forth. In addition to the obvious differences in output volume, you should notice a massive difference in frequency response for different loads. One setting where the difference is striking is Treble and Bass at max, Mid at min. A good rule of thumb is this: a high load impedance will give you everything you can get from a low load impedance, plus a lot more, especially when it comes to bass response. In other words, your controls will be much more responsive and wide-ranging with a higher load impedance than with a lower one. This is of course in addition to the increased output.

Of course, if you like the LPB-1 as a makeup stage, then by all means use it. With a lossy tonestack, it will almost certainly be better than no makeup stage at all. At minimum, it will prevent your circuit from behaving differently when fed into different amps or other pedals. But if you want more tone options and more output, at least consider a SHO or a Micro Amp (or any other clean boost with a high input Z).

In the end, let your ears and your breadboard be your guide.