CM500 Powering from K2

[Jim Brown-10]

A week or so ago, KS4L asked for info on powering the CM500 from
his K2. He tried the instructions I gave, but it didn't work. Bear
in mind here that several variations of output stages are used for
the FET impedance converter in electret mics, and not all mfrs wire
them the same way to their plugs. For this reason, a single
powering method doesn't work for all electret mics. We learned this
two decades ago in the pro audio world when we started using
lavalier (clip-on) mics with wireless mic transmitters, and every
mfr of both mics and wireless systems published lots of info about
how to hook their stuff up.

The CM500 works directly with the K3, so I looked at what the K3
does for power. It's 5.6K to the tip only. The K3 makes no
connection to the ring. Randy tried that and it works fine.

So -- revised advice. From the CM500 mic plug, wire the tip to the
mic input, add a 5.6K resistor between +DC on the K2 mic jack and
the mic input. The resistor value is not critical, 4.7K to 8.2K
should work fine. There is very little current, so very small
resistors are fine. The sleeve (common) of the plug goes to common
at the mic jack.

The same wiring should work with this mic for nearly all other ham
rigs.

I would expect the CM500 to work really well with the K2. One of
the few shortcomings of the K2 is that since it was originally
designed as a QRP CW rig, the support for SSB is not a strong
point. One way this shows up is in relatively low audio gain and
not enough LF rolloff. The CM500 helps this situation, first
because it's a pretty "hot" mic (that is, higher than average
output level) and because the low end is rolled off internally.

There are several published mods to improve the gain of the SSB
signal chain, some of which go all the way to the RF end of the
chain. I took the simpler approach, modifying my K2s by changing a
few resistors and capacitors in the mic stages to provide rolloff
around 500 Hz and increase the gain by about 7dB. This allows the
mic to hit the compressor harder (and the rolloff prevents low
frequency sounds from hitting the compressor), making the audio
more competitive. The CM500 should provide the same improvements
but without the mods.

73,

Jim Brown K9YC



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[w7aqk]

Jim and All,

The K3 provides the necessary bias through a menu setting.  The K2 requires
an internal adjustment for whatever type microphone you are using, and how
the 8 pin connector is wired.  I think all this is covered in the manual,
but the bottom line is that for electrets, a resistor is added on the
internal microphone wiring terminal which goes from the bias voltage supply
connector to whatever pin the microphone uses for the microphone lead.
Kenwood and Elecraft use pin 1 for the microphone lead, but some other
microphones may have it on a different pin on the 8 pin connector.  My guess
is that internally, KS4L didn't have his K2 wired to match whatever adapter
he is using.  The K2 has only one input for the microphone, and that is the
8 pin connector on the front.  So, the internal microphone terminal has to
be wired accordingly so that the microphone pin selected (and the PTT, GND,
etc.) will properly correlate to however the microphone connector (8 pin) he
is using is wired.  Presumably he has some sort of pigtail wired up to
convert from the 3.5 mm plug on the CM500 to an 8 pin connector which goes
to the K2.

So, it's not necessarily your instructions that were wrong--it's just how
was he going from the CM500 to the K2's front panel connector, and further,
how is that wired internally--at least I think that must have been the
problem.

Dave W7AQK


----- Original Message -----
From: "Jim Brown" <[hidden email]>
To: "Elecraft List" <[hidden email]>
Sent: Wednesday, January 20, 2010 10:22 AM
Subject: [Elecraft] CM500 Powering from K2

>A week or so ago, KS4L asked for info on powering the CM500 from
> his K2. He tried the instructions I gave, but it didn't work. Bear
> in mind here that several variations of output stages are used for
> the FET impedance converter in electret mics, and not all mfrs wire
> them the same way to their plugs. For this reason, a single
> powering method doesn't work for all electret mics. We learned this
> two decades ago in the pro audio world when we started using
> lavalier (clip-on) mics with wireless mic transmitters, and every
> mfr of both mics and wireless systems published lots of info about
> how to hook their stuff up.
>
> The CM500 works directly with the K3, so I looked at what the K3
> does for power. It's 5.6K to the tip only. The K3 makes no
> connection to the ring. Randy tried that and it works fine.
>
> So -- revised advice. From the CM500 mic plug, wire the tip to the
> mic input, add a 5.6K resistor between +DC on the K2 mic jack and
> the mic input. The resistor value is not critical, 4.7K to 8.2K
> should work fine. There is very little current, so very small
> resistors are fine. The sleeve (common) of the plug goes to common
> at the mic jack.
>
> The same wiring should work with this mic for nearly all other ham
> rigs.
>
> I would expect the CM500 to work really well with the K2. One of
> the few shortcomings of the K2 is that since it was originally
> designed as a QRP CW rig, the support for SSB is not a strong
> point. One way this shows up is in relatively low audio gain and
> not enough LF rolloff. The CM500 helps this situation, first
> because it's a pretty "hot" mic (that is, higher than average
> output level) and because the low end is rolled off internally.
>
> There are several published mods to improve the gain of the SSB
> signal chain, some of which go all the way to the RF end of the
> chain. I took the simpler approach, modifying my K2s by changing a
> few resistors and capacitors in the mic stages to provide rolloff
> around 500 Hz and increase the gain by about 7dB. This allows the
> mic to hit the compressor harder (and the rolloff prevents low
> frequency sounds from hitting the compressor), making the audio
> more competitive. The CM500 should provide the same improvements
> but without the mods.
>
> 73,
>
> Jim Brown K9YC
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html 

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