I must be too stupid for DIY...
I made a small passive preamp (just an alps in a box), but when I connect it to a small digital amp for testing, an audible hum is introduced to the otherwise fine signal. The hum does not appear when the volume is either at zero or at max volume, so only when the wiper is on the track.
Wiring is as follows:
The alps takes input from two RCA's, and sends the signals out to two RCA's as well.
The returns of the RCA's are wired to the ground pin of the Alps, the IN signal wires to the IN pin and the wipers are wired to the signal of the OUT RCA's.
IMG_9617 by ctjr, on Flickr
I made a small passive preamp (just an alps in a box), but when I connect it to a small digital amp for testing, an audible hum is introduced to the otherwise fine signal. The hum does not appear when the volume is either at zero or at max volume, so only when the wiper is on the track.
Wiring is as follows:
The alps takes input from two RCA's, and sends the signals out to two RCA's as well.
The returns of the RCA's are wired to the ground pin of the Alps, the IN signal wires to the IN pin and the wipers are wired to the signal of the OUT RCA's.
IMG_9617 by ctjr, on FlickrEach input is a two wire pair.
You must take two wires from the input socket to the vol pot for each input
Each output is a two wire pair.
You must take two wires from the vol pot to the output socket.
Those two wire pairs need to be close coupled along their whole route from source to receiver. Use either coaxial cable, or utp (Unshielded twisted pair), or stp (shielded twisted pair). The screen of the coaxial is the Signal It must not be connected to the metal case/chassis. The screen of the STP must be connected to the case/chassis at both ends. The metal parts (that are not the signal) of the vol pot should also be connected to the case/chassis
At the RCA sockets I can see a BIG gap between the Signal Flow (white, or red) and the Signal Return (black). Those BIG gaps are susceptable to picking up interference.
You need to keep the Return VERY close to the Flow to minimise the gap and thus minimise the interference.
The Alps does not have a "ground" pin.
It has a pin that is common to both the input and to the output. That common sees two wires, one coming from the input socket and one going to the output socket. It is not ground. It does not get connected to ground, nor to the case/chassis.
You must take two wires from the input socket to the vol pot for each input
Each output is a two wire pair.
You must take two wires from the vol pot to the output socket.
Those two wire pairs need to be close coupled along their whole route from source to receiver. Use either coaxial cable, or utp (Unshielded twisted pair), or stp (shielded twisted pair). The screen of the coaxial is the Signal It must not be connected to the metal case/chassis. The screen of the STP must be connected to the case/chassis at both ends. The metal parts (that are not the signal) of the vol pot should also be connected to the case/chassis
At the RCA sockets I can see a BIG gap between the Signal Flow (white, or red) and the Signal Return (black). Those BIG gaps are susceptable to picking up interference.
You need to keep the Return VERY close to the Flow to minimise the gap and thus minimise the interference.
The Alps does not have a "ground" pin.
It has a pin that is common to both the input and to the output. That common sees two wires, one coming from the input socket and one going to the output socket. It is not ground. It does not get connected to ground, nor to the case/chassis.
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Thanks, Andrew. So, bending the tips of the tabs further in and twisting the wires should solve the problem?
Also, I used this pic from hifi collective (and others basically saying the same thing), would you say that this diagram is wrong?
Also, I used this pic from hifi collective (and others basically saying the same thing), would you say that this diagram is wrong?
An externally hosted image should be here but it was not working when we last tested it.
There'll be common-mode currents at mains frequency running along the interconnects. This renders the local 0V reference (at the pot) noisy with hum compared to the 0V at the amp input. When the volume's not at max or min, the quality of this local 0V matters.
There's more than one remedy most likely. The remedy I'd choose is a pair of isolation transformers with various volume taps. Also known as a transformer volume control.
Well... that's outrageously expensive. I bought this Alps because the maker of my amp (audiosector Gainclone) recommend just a pot as volume control. I had to build it outside the case of my amp for convenience and so I could use the amp as a separate in a MiniDSP system.
Lohengrimas, even your wires are short, they are not shielded. Do it properly again with true shielded wires and it will be quiet. I made many passive preamps, nothing to it.
Your creation in the picture is just lousy, you do not even bother to twist the wires! Why?
Your creation in the picture is just lousy, you do not even bother to twist the wires! Why?
I'd build it into the case, keep wiring screened and as short as possible, It could always be bypassed though I don't see the point, why not just turn it right up amounts to the same thing?
Because in the original manual of the Audiosector chipamp, the wires connecting pot to input/board aren't shielded nor twisted (matter of fact, they are just bare copper wires!). I put too much trust in that, apparently. Back to the drawing board.
When you use shielded wires inside the box - do these need to be twisted?
And when using a separate shielded wire for signal and return (again, inside the box, presumably having no "case earth" from signal source box connected) then where do you connect the shields?
If you buy off-the-shelf, then yes. But this is DIYaudio - a TVC can be created relatively inexpensively in parts even though its undoubtedly fairly heavy on the time investment.
I wouldn't base your design on that Audiosector chipamp, it looks pretty but that's about it. You've already discovered it's a poor design
true, true. And they look just awesome. However, I'd like to get this dinky little box working in the meantime 🙂
Connect the ground to the metal of the pot, link the two grounds on the pot left and right, that is it.
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The case is plastic, so the case can't function as a ground.
I tried twisting the wires and soldering them directly to the RCA instead of connecting them to the tab. This made no difference at all.
What still puzzles me, is that there is only hum when the pot is between either ends; at maximum "volume" (i.e. turned fully open) the hum disappears.
How long are your interconnects? Are they thin? Sounds like you need to lower 0V impedances. Try using screened cable in the box. Also pot is plastic case so is offering no screening' Have you a metal case you could use instead?
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