Question about noise from kits.

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Hello again fellow DIY's.

I'm pretty new at electronics and I've got a minor problem (I hope) with a few kits I've purchased. I wanted to "piece together" a preamp kit, a power amp kit, and a power supply kit in 1 complete unit. Problem is, when I put them all together, I get this annoying low frequency hum if I try to turn the volume up past a very minimal point, especially if I try to pump the bass up. And it sounds pretty distorted at any level.

If I hook them up separately, they all work flawlessly at most any level. If I hook them up together with separate power supplies, that seems to work pretty good as well (could my power supply be the problem??) It has a 24 VAC 700 mA transformer. Should I try to up the amps?

I've heard about de-coupling and coupling caps, but I have no idea what it's function is (and I may be way off track with that subject, anyway). Any idea where I can get information on that topic?? Anyway, here are the schematics for the kits.

Power amp:


Power supply:

Any help here is much appreciated, and will point me less towards being the guy asking the questions on this forum and more toward being the guy answering them!!

:confused: :confused:
The hum is almost certainly from the AC power. It is probably getting into your signal via the ground. Look at all the ground symbols on the schematics. Notice there is the same symbol regardles of whether it is associated with signal (at RCA jacks, the speakwe, Volume control, balance, etc.), ot if it is associated with the power supply.

When you do your wiring, the signal grounds should all come together at one place and the power grounds at another. However, they need to be referenced to (connected to) each other. Chances are you have the two ground networks tied to each other in two places - one related to the preamp and one two the amp. If you break this connection in one place but leave it in the other the noise MIGHT go away. This is called a "ground lift".

It is also sometines helpful to put a low value resistor, 10-50 ohms, between the two ground networrks (signal vs. power). This leave them at virtually the same 0V reference but tends to isolate them as well.

If you have the means to draw out you wiring scheme (how you have connected the three modules together and post it here, that might help someone diagnose the situation better as the above comments relate just to "common" situations and your could be different.
I basically have the three kits separated right now, just like in the schematics:

I am running the power supply to both the amp and the preamp at the same time, but they are all 3 on separate boards.

It's basically as if I made the three kits according to the directions, ran a pair of wires for power from my power supply to my preamp, then ran a pair of wires from my power supply to my amplifier. I then plugged my MP3 player into the left input of the preamp by means of an 1/8"male to Stereo RCA male cable. My left preamp output is then ran from another RCA cable into the input of the (mono)amplifier. Then I ran the output of the amp to my speaker.

I'm assuming your saying to try only putting the ground on one one unit (i.e the preamp or the amplifier), and if that doesen't work then put the ground back on both units and put a 10-50ohm resistor between the two units' grounds??
Hi Cax,
1. the resistor is inside the poweramp between the signal ground and the power ground. Remember also to connect the safety ground to chassis and to power ground. This scheme is Leach and others (look up his schematic).

2. An alternative is to omit the resistor and connect all three grounds together (I think this is what you have). It often gives rise to hum problems after you connect in the ancilliaries. Double insulated ancilliaries with no safety ground can usually be connected this way.

3. A third alternative is to connect the power and signal grounds together and resistor between this ground and safety ground.

4. An option on 1. or 3. is to parallel connect a pair of side by side diodes (to limit the voltage) and / or a 100nF cap across the resistor (shorts out RF to earth). In fact some show just the diodes and cap with no resistor (this is equivalent to ground lift).

5. Finally take care with the PSU common. Extremely large peak currents flow between the caps and to/from the rectifiers. Connect all these together but do not connect any of your grounds to this point. Instead take a short wire from your common to your main power ground.

Now, for a separate preamp repeat all the ground philosophy inside the preamp using options 1 or 3.
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