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Started by dazz, January 04, 2018, 08:53:12 AM
Quote from: blackcorvo on January 29, 2018, 11:11:35 AMQuote from: dazz on January 29, 2018, 09:43:13 AMI have loads of hum and noise from the power supply though, a 12V 1.5A unit that came with an external hard driveTime to check this out I guess: Topic: Grounding techniquesPerhaps if we see some pics of your build so far, we might see what's wrong. Also, measure the voltage of the power supply when you have the amp running, maybe the amp is loading the supply down.
Quote from: dazz on January 29, 2018, 09:43:13 AMI have loads of hum and noise from the power supply though, a 12V 1.5A unit that came with an external hard driveTime to check this out I guess: Topic: Grounding techniques
Quote from: Katoda on January 30, 2018, 04:06:39 AMYes, definitely use shielded wire. If you're a cheap scavenger like me you can reuse some old usb cables, they have good shielding and 4 cores.Since you are using a hard drive (probably switching) power supply, you need to filter the hell out of it, using big electrolytics paralleled to small ceramics, maybe even throw a choke in ( you have some of these if you raided the computer PSU ). Otherwise there will be noise in the circuit. Perhaps you could even add in a small LDO voltage regulator for the preamp circuit to minimize the noise.
Quote from: Katoda on January 30, 2018, 06:50:16 AMI didn't mean that it is better (or more usable for that matter) to have 4 conductor shielded cable, I just wanted to say what to expect. You can, however, use two or more of the conductors for the pots in the same signal path, like on a volume/gain pot - one pin is ground, and the other two can be soldered to different wires inside the cable. It just saves some space and results in a cleaner looking build, but the downside is that there is a lot of tension on the individual cores (if the plastic insulation is too thick), which might result in breaking wires.Your cable wiring is correct, the shield is ground and the centre is the signal. You must watch for ground loops here, make sure that there aren't multiple connections to different grounds from any single point.A choke is an inductor, a coil of wire wrapped around a magnetic material (ferrite, in this case). If the switching noise is really the problem, then try googling LC low pass filter. Because you dont know the coils inductance, it might be a hit or miss, but taking that toroidal psu coil and soldering some leads together, so it has all the wraps around the core in series and in the same direction, and then placing it in front of your amps power supply (so that it is followed by paralleled 1000uF and 100nF caps) might help a bit.Keep the input of the power supply short and away from the preamp, as the switching from the external unit might be creating some quite strong EMI.
Quote from: Katoda on January 30, 2018, 06:50:16 AMEDIT: The -3dB cutoff frequency of an LC filter is given asf=1/(2*pi*sqrt(L*C))So if the switching frequency is the same as the LC cutoff frequency, the power supply ripple will be 3dB smaller. Now, electrolytics have higher ESR and ESR, so for effective high frequency decoupling you want ceramics. A lot of them in parallel. Measure the coils inductance if you can, and then try to make the cutoff frequency a couple of times lower than the switching frequency.
Quote from: Katoda on January 30, 2018, 09:01:09 AMNo problemat all, we're all here to learn If the hum is mainly 100Hz-ish, the smps switching noise might not be the problem after all, since they operate at higher frequencies. If you get the power from a switcher, the ground is probably not connected to the "earth" connector of the mains. That would mean that you could be picking up mains noise from everywhere around you. Does the amount of noise reduce if you touch the ground with your finger? If it does, there's your problem. If not, the input might still be picking something from the mains, if it is not completely encased in grounded metal.
Quote from: Katoda on January 30, 2018, 09:37:59 AMAwesome, now that you know the source of the hum, you can take measures to eliminate it. It might be as simple as opening the hard drive PSU and connecting the chassis to the output ground, or as annoying as having to buy/build another PSU, if the current doesn't even have a 3-pronged mains cable.If you are feeling adventurous, you could open the hard drive PSU, desolder the current 2-prong cable and replace it with a 3-pronged one, connecting the mains earth to the output ground (be careful if you do that, small mistake could mean a large fire and/or electrocution!!!)
Quote from: Katoda on January 30, 2018, 10:55:15 AMCrap, i was hoping there would be a cable... I wouldn't recommend mounting it inside the chassis, the coils would probably radiate too much for such a small enclosure. If the smps circuit has a transformer (as in - not just an inductor), you could fit in in another enclosure, one that would provide a connection to earth, but that's quite an advanced challenge, because you're dealing with potentially lethal voltages there. If it's just a buck converter, there's nothing you can do but to buy another PSU. No mains isolation is a no-no.Yes, the earth should be connected to the negative terminal, the ground.