[PROBLEM/SOLUTION] Noise on ICM line with only 1 phone when using CHAN/ZON 12V/2A power supplies

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Sep 19, 2019, 5:04:09 PM9/19/19
to seriss-1a2-ksu
I noticed the following during a recent QC test. I've only seen this with the CHAN/ZON brand power supplies.

When you have one phone plugged into the 1A2 KSU board and
pick up the intercom line to listen, you may hear a weird
repetitive "squeaking sound", like someone rubbing on a balloon.
The pattern of sound may follow the blinking of the KSU's

The sound is typically low volume, but loud enough to be annoying.
This does not happen if there's 2 or more extensions plugged in.
The noise only happens on the ICM line, not the other CO lines.

Apparently under the very low load of a single phone plugged in,
the CHAN/ZON switching power supplies have a small voltage ripple
that the ICM VOICE BATTERY passes through, and the phone's hybrid
picks up this small ripple as audible noise in the voice frequency range.

The tiny load of the CPU status LEDs affects the ripple,
which is why the noise follows their flashing.

It's not unusual for switching power supplies to exhibit noise
under low load. Some larger switching power supplies even
have a minimum load requirement.

The voltage ripple is quite small; less than 1/10th of a volt;
it can barely be perceived on an analog scope at full scale.

The sound goes away completely if you simply add more phones.
Just one phone is enough.

Apparently the 40mA load of just one extra ICM lamp in a 2nd phone
is enough load on the supply to make the problem go away completely.

Since it's not practical to use the intercom line with just one phone,
I'm calling this a non-issue. With two or more phones present,
the problem doesn't exist.

But the issue is at least note-worthy.

I found with one phone attached, just adding a 1W / 480 ohm resistor
across the supply was just enough to make the problem go away, and the
issue doesn't reappear as more load is applied. This issue only happens
under very low load. So in practice, one shouldn't add a resistor to
solve this problem; simply having 2 or more phones solves it.

This issue doesn't show up on the other lines, since the KSU doesn't
introduce power into those lines; their voice battery is provided
by the CO. Only the intercom's voice system is powered by the KSU.
I'm treating this as a non-issue, since it's impractical to make use of the intercom line with only one phone;
when there's two or more phones, the problem goes away.

I didn't notice this in testing, since I do the QC tests with 4 phones plugged into the board, which hides
the problem.


Jun 7, 2022, 6:33:58 PM6/7/22
to seriss-1a2-ksu
I've since switched to using COOLM brand 12V / 2.5A supplies which don't have this problem. These are the supplies I used when people purchased two boards (interlinked).. turns out they have the best performance even under low load.

They cost a little more because they have a higher amperage output, allowing more phone sets to be managed by a single board when CPC connection blocks are used to bridge in more phones.

    US plug versions of the COOLM 12V/2.5A supplies are here:

    There also a UK (England) style of this same manufacturer available here:


If you purchase these adapters for use with the Seriss KSU, then you can just cut off the DC plug, strip and tin the two wires within, and wire them up to the KSU's terminal screws yourself. If you do that, be sure to:
  1. Get the polarity right since there's no polarity protection on board: Red is positive (+), White is ground (-)
  2. When cutting off the DC plug, keep the ferrite bead in-circuit (don't cut it off)

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