Connect the signal ground to the chassis ground.
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
use a shielded transformer
What about all the unshielded spaces, like the "vast expanse' of open circuit boards? I mean I use coax whenever there is a substantial space to be crossed like 3" or such, and then I have an 8" circuit board which receives no shielding??
pre amps, equalizer, second tube preamp, tube power amp
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.
This got me one time. I was pulling my hair out trying to trace a noise source, and it turned out to be MAGNETIC interference. Little coupling transformer, wrapped a piece of copper foil around it and it went away. Look for this in your design, Othello.
How many preamps have you got in this thing? What's your total gain like? Could you post a schematic, pics?
200 ohms sounds pretty low, I seem to remember a guitar pickup is more like 10k.
This is why I asked about the total gain. You may be swamping the tiny signal from the pickup with that 200Ω resistor, then compensating with a lot of gain, which will add noise.
I built a very small op amp preamp/line driver in the guitar which has very low output impedance.
It's not for the gain that I use these amps, its for proper isolation between stages. Most of these stages have an amplification of 1.
That's a good way of doing it, get that output impedance down before it even gets to the cable.
You might try using a 1:1 signal transformer for isolation, or just cap-block the inputs. Each active stage of amplification, even at unity gain, is going to add some noise. You've got, what - three or four dual-channel inline amps before the main stage? Eliminate some of those unity-gain amps and see if that will reduce the noise.
Could be a ground loop - you know about those? Search on "ground loop" if you don't. This is why mikebits suggested a star grounding pattern, also called a single point ground. Ground loops are a common source of 60hz hum.
I made a distortion analyser with oscillators that has many circuit boards. I connected the boards together with shielded audio cable and connected the shield to ground only at the receiving end since the circuits all had the same ground. Then there was no ground loop and the hum, noise and distortion was -100dB.
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