This topology means that the tone control is not suitable for instrument level signals, but only for line level signals ........ Surely enough, the buffer creates a high input impedance, isolating the input from the tone controls, ensuring that the tone controls won't load down your guitar pickup. That's very good.
But because the buffer has unity gain, it means that if the input is some weak signal like 200 mV, the signal going through the tone control is at the same level. This means that your tone control has to be completely shielded, just like the conductor in your coax cable and your guitar's cavities.What you want to do is reverse these two elements. The first thing the device should do is boost the signal through an amplifying op-amp to get it from instrument level to line level as early in the chain as possible. This is where you can have your variable gain also. The buffer can be placed after the tone control, to isolate the tone control's impedance from the next device in the chain.
Secondly, a number of things stand out. Firstly, most of the pots used are 500K. Also, on the gain control, there is a suspicious 47 pF capacitor. Basically, this device looks like it was designed by a guitar player. (``500K knobs are nice and bright in my guitar, so they must be great everywhere. A tone bleed on my strat's volume knob ensures bright highs when I turn down the volume, so I can just transplant into this op-amp circuit and it will do exactly the same thing!'')
But the 47 pF cap in the gain circuit will cause some loss of high frequencies when you crank up the gain! The capacitive reactance of 47 pF with respect to a 10 Khz signal is about 339K. That is significant in regard to the 500K impedance of the pot. The cap provides somewhat of an easier path for high frequencies around the pot, thereby increasing the amount of negative feedback for those frequencies, thereby quieting them down.
Anyway, you could use a 10K pot here (with a compensating for the 4.7K resistor going to ground).
Bigger resistances invite noise because less current flows through them. One source of noise are resistors themselves: they create what is called "thermal noise". It comes from the random movement of electrons in the material itself, and when it is amplified, it sounds like hiss. The way you fight thermal noise is to have a decent signal current flowing, so that the thermal noise is small in comparison to the current. Now in an op-amp feedback circuit, you could achieve the same amount of gain using a 1K resistor over a 2K resistor, or using a 100K resistor over a 200K resistor, or using a 1M resistor over a 2M resistor. It's the voltage divider ratio that matters. However, the absolute resistance also matters: more resistance means lower quality feedback. The op-amp's input does not have infinite impedance; some current flows through the feedback into it. Bottom line: smaller pot.
Further reading: http://en.wikipedia.org/wiki/Resistor#Electrical_and_thermal_noise
The coupling capacitors have wacky values. The 22nF input cap is very low. The purpose of the big resistors around it is to mitigate the loss of bass frequency response that this causes, but it won't work very well.
The 10M resistor is in parallel with the output impedance of the previous device (e.g. guitar pickup and volume knob). Your 500K guitar volume knob provides an easier path for bass than the 22 nF cap, and the 10 M resistor might as well not even be there.
You should remove these silly resistors and put in a 10 uF capacitor.
The 470 nF coupling capacitor at the output is equally silly
, as is the 470 nF capacitor at the bottom of the feedback voltage divider. These both suck bass response. Make them 10 uF.
The 100K output resistor is pointless. It basically means that your device's output faces an impedance that is no higher than 100K, which is a waste if the next device in the chain has a nice and high impedance in the megohm range. Furthermore, your op-amp's output impedance should already be pretty low, so the 100K won't do anything to lower it. You want our output impedance to be as low as possible (ideally zero), and the input impedance to be as high as possible (ideally infinite), for the best possible voltage bridging. Any silly resistors going to the ground at the input or output are just sucking away tone.
Coupling caps should always be at least 10 microfarads.
But 20Hz and up should nicely pass through.
Whatever you do, do not build it using the plain perfboard method that is illustrated. This is completely silly. Nobody in their right mind uses plain perfboard, because there exist PC perfboards which not only have copper pads, but also traces connecting those pads in a kind of breadboard-compatible" layout. You can lay out your components on a breadboard and test the circuit, and then transfer the same layout to this type of board, where you neatly solder everything to the board. The existing traces minimize the jumpers that you have to use, and when you do need a jumper, it is nicely installed through the board like a resistor.
On the other hand, I can't believe someone produced a PC board layout for this device, without correcting any of its obvious flaws.
Let's see it step by step The 47pF capacitor is not "suspicious" but good engineering practice, lowering gain outside the guitar range and preventing instabilities.
Thank you so much for all of your advice. This is what I call the power of network. We are friends. This is the original unit with noisy problem . Sound like "SSSS....." .I have reversed the circuit as your comment. Sound is tighter and less noise. . But it is not enough for me so I have to do something further.
Oh well, the software guys, specially in the Windows world, face much harder incompatibilities than us.
Please take another somewhat larger and better illuminated picture of everything you have in that chassis and explain what you have there.What I don't understand is why you have so many heatsinks there.What are you using as a power amp and speakers?