I built a guitar amp. It consists of various circuit boards (pre amps, equalizer, second tube preamp, tube power amp, various power supply modules etc.) mounted in an alu chassis. I also have a very sensitive AC level meter, down to -120dB, to measure noise etc.
All the individual components work and now I wired the whole thing up and I am disappointed about the noise level.
The chassis is grounded via a grounded outlet, all the other components are connected via coax with each other-so they have a common ground but I have not connected that signal ground to the chassis ground.
In addition to the signal coax cables there are other control cables for power for the op amps, dc for tube heaters, switches etc. which connect the modules.
The overall noise and hum level increases as I connect more and more modules and I think I am picking up noise via the power and control cables and would love to hear a general outline as to how to best wire up a project like this to keep noise out as much as possible.
Uwe
This is an all too common occurrence in multi circuit systems, where you feed the signal from one subsystem to another. In addition, you have a power supply, or multiple power supplies for different voltage levels in your systems.
You may have done some of this stuff, but since this is a forum, here are some ideas:
Signal integrity at the source: The mV signal from the pickups is very susceptible to noise. The front end is critical and needs special treatment. Use shielded wire from the entry point of the signal in the chassis to the first circuits. If the signal goes directly to the 1st circuit w/o any wires (desirable), then is the first stage properly bypassed, with bypassing caps as close to the 1st stage as possible? If the 1st stage is a dual supply, are both rails bypassed?
From one system to another: When you jumper from the pre-amp to the main amp, use shielded wire, as short as possible.
Power supply: Use a toroidal transformer if possible. If not, use a shielded transformer. Be sure that you have enough filtering in the supply. Keep the wire lengths short from the supply to the circuits that require them. Bypass the power supply feeds at the circuit boards that the supply wires feed.
Grounds: Ground the supply with a large gauge wire directly to the chassis if possible. If you are using a 3-prong line cord, the ground needs to also be bonded to the chassis. A "star" ground topology may be desirable if you suspect a ground loop. This means that you bring all subsystem grounds to a single point on the chassis, i.e. don't daisy-chain the grounds.
Digital circuits: If you incorporate any digital circuits that have clocks, use a separate ground wire and segregate from the analog circuit ground. This is especially important in mixed signal A/D & D/A circuits. A star ground topology is paramount. When you finally tie the grounds together at the star ground point, use an RF bead or toroid choke on the digital ground wire at that point.
Ground planes: If you created your own boards, ground and power planes really help in providing shielding in the circuit, as well as provide a low impedance return to the supply. Some miss this 2nd important point.
Shielding: It may be a good idea to physically shield the front end from the rest of the circuit, with the use of a small aluminum box inside the chassis.
Feedback: If the feedback network is not properly handled, it can cause hum or in worst cases an oscillation. Pay careful attention to where the feedback is physically routed. A Zobel network also helps on the output stage in this regard.
Some basic testing on your part will also help. Does the hum show up on an oscilloscope? Does it appear regardless of the load? Does it appear with no load? Does it appear with no guitar plugged in? Does it go away when you turn off the fluorescent lights in the room? You may want to selectively eliminate subsystems until the hum goes away or appears (divide and conquer).
Anyway, all that I can think of before I had my first cup of java this morning. Feel free to add more folks! These are some things that I've observed over the years and have helped me.