Hi folks,
After wearing out 3 bread boards,, here's what I v'e found.
Consider the impedance through Tonemender circuit? Opamp 1 is wired as a buffer (high input imp, Low output imp).
Simple enough but hang on? The output end of the tone stack has just converted your Simple buffer trick right back up to high output imp so the second opamp also sees a high imp, In effect the first buffer is rendered useless.
What is not obvious to the novice is this: High imp input and High gain on the same chip equals *NOISE*
In Tonemender the second opamp also has a high imp input (has to be higher than the tone circuit other wise it suffers massive loss). But it also has a 500k pot for gain!! ouch!!
Even if you use metal film R's and the lowest noise opamps it will make little difference to the noise from such a circuit.
How to fix it, See Schematic.
By utilizing the buffer *After tone controls* you turn the high imp into low.
(The 2 x 10k r's (R5,R6) after the buffer force low imp on the second opamp.
*NOW you CAN introduce substatial gain but without the dreaded noise problem*.
Even then this circuit will be quite hissy if you try to grab to much gain as in all audio setups you are better off to spread your gain over several stages. I use my PhAbbTone circuit as a front end into simple OD pedal type circuits, they do the gain tricks. A high impedance tone stage is not the place to do massive gain.
My PhAbbTone circuit is wired true by pass and I use a 50k pot to trim gain. Only really there to maintain volume balance between the bypass signal. (you can try 100k pot but watch out for the dreaded hiss.)
Mess with the 10k x 10k Voltage divider after opamp 1 at your own peril,, the noise will likely come back.
Note;
Contrary to common practise the buffer in front of a high imp tone stack like this does little. You will only get a tiny bit more bandwidth (most of which is useless for Hi Gain guitar anyway).
You CAN (believe it or not) plug your axe directly into the *Passive front end* of most of the famous tone circuits.
The only time you will encounter a problem is if you use an acoustic with passive peizo PU.
They tend to need a buffer no matter what you plug into anyway so unlikely you will ever have issue.
If the idea erks you then add a buffer,, hey opamps are cheap.
I use my tricked up HiWatt tone stack (shown in schematic) because it has the best tone shaping effect.
Bass and treble response is much like fender circuits but the mid cut ability of the HiWatt is just stunning. (See pic below)
I've built most of the famous tone stacks (both in Valve and SS designs)
IME, Switching tricks like Tonemender and Dumble circuits are just a patchup for a lacking midrange circuit.
Hi watt mid cut delivers a very steep mid notch at around 400hZ.
I've found the common tone circuits just deliver a shallow scoop and by the time you cut enough mid you have also pulled down most of the energy and the sound is very unconvincing.
One note about deriving a bias voltage.
The Tonemender bias is a bit silly as it uses 2x 100k resistors for division. I find the old 1k per volt rule of thumb is good to remember here. i.e. When working with 10 volts supply use a 10k resistor, 100k is miles to big and will likely do nothing except add more noise.
It should be obvious that you can of course use most of the famous tone circuits in place of mine.
As long as you use the setup shown you will have much better success than Tonemender.
No disrespect to *ROG*, They have some good circuits but sadly this one is not one of their better ideas.
Have fun with it all, Phil.