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Passive preamp introduces hum
Passive preamp introduces hum
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Old 18th March 2017, 10:33 AM   #1
Lohengrimas is offline Lohengrimas  Netherlands
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Default Passive preamp introduces hum

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.

Click the image to open in full size.IMG_9617 by ctjr, on Flickr
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Old 18th March 2017, 10:49 AM   #2
AndrewT is offline AndrewT  Scotland
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Each 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.
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Last edited by AndrewT; 18th March 2017 at 10:55 AM.
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Old 18th March 2017, 11:09 AM   #3
Lohengrimas is offline Lohengrimas  Netherlands
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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?
Click the image to open in full size.
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Old 18th March 2017, 11:15 AM   #4
abraxalito is offline abraxalito  United Kingdom
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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.
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Old 18th March 2017, 11:38 AM   #5
scottjoplin is offline scottjoplin  Wales
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@abraxalito. So the remedy is a very low impedance 0V to transmitter AND receiver?
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Last edited by scottjoplin; 18th March 2017 at 11:45 AM.
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Old 18th March 2017, 12:00 PM   #6
abraxalito is offline abraxalito  United Kingdom
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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.
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Old 18th March 2017, 12:07 PM   #7
Lohengrimas is offline Lohengrimas  Netherlands
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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.
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Old 18th March 2017, 12:13 PM   #8
adason is offline adason  United States
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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?
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Old 18th March 2017, 12:15 PM   #9
scottjoplin is offline scottjoplin  Wales
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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?
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Old 18th March 2017, 12:18 PM   #10
Lohengrimas is offline Lohengrimas  Netherlands
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Quote:
Originally Posted by adason View Post

Your creation in the picture is just lousy, you do not even bother to twist the wires! Why?
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.
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