Elecraft › [K2]

K2: connecting a condenser mic

Classic

List

Threaded

6 messages Options

K1LI

K2: connecting a condenser mic

I have a condenser mic that is not on the list of "known" mics in the docs
or on the Elecraft web site. I'm trying to decide whether to simply short
the +5V to the AF when connecting my condenser mic to the KSB2, or to place
a resistor in between. I am more concerned about potential long-term adverse
effects on the mic element by operating it out-of-spec than I am concerned
with damaging the K2.

A DMM shows the DC resistance of the element to be 400 Kohms! Not really
surprising, I guess, since a condenser mic is electrically similar to a
capacitor.

The manufacturer specs the mic element at 4.5Kohms and 1.5V to 9.0V bias.
Applying the KSB2's +5V directly to the mic element's 4.5K ohms should
produce 1mA drain, no sweat for the KSB2, and right in the middle of the
manufacturer's applied DC voltage spec.

The manufacturer's tech sent me a wiring diagram that shows a +12V supply
with a series resistor between 470 and 2.2K ohms to the mic. The 470 ohm
resistor would apply nearly 11V to the mic, if the mic element and the
series act as a pure voltage divider. The 2.2K ohm resistor would produce 8V
at the mic.

The KSB2 schematic shows a 2.2uF electrolytic cap between the MIC AF and the
rest of the KSB2, so I wouldn't think there's any risk of a short.

Help from those in the know, please?

--
73 -- Brian -- K1LI
______________________________________________________________
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

Don Wilhelm-4

Re: K2: connecting a condenser mic

Brian,

Is that a 3 terminal mic element? If so, then you can follow the
manufacturer's instructions and apply the 5 volts to the voltage
terminal on the element.
OTOH, most of these elements are 2 terminal, and the voltage should be
applied to the AF pin through a resistor - try 5.6k like the Elecraft
MH2 uses as a first trial, it will likely work because the resistance is
not critical.
If you try to connect the 5 volts directly to the AF terminal, you will
effectively bypass all the audio to ground. For AC signals, a power
rail is jst the same as ground because there are large bypass capacitors
on the voltage rail.

73,
Don W3FPR

Brian Machesney wrote:

> I have a condenser mic that is not on the list of "known" mics in the docs
> or on the Elecraft web site. I'm trying to decide whether to simply short
> the +5V to the AF when connecting my condenser mic to the KSB2, or to place
> a resistor in between. I am more concerned about potential long-term adverse
> effects on the mic element by operating it out-of-spec than I am concerned
> with damaging the K2.
>
> A DMM shows the DC resistance of the element to be 400 Kohms! Not really
> surprising, I guess, since a condenser mic is electrically similar to a
> capacitor.
>
> The manufacturer specs the mic element at 4.5Kohms and 1.5V to 9.0V bias.
> Applying the KSB2's +5V directly to the mic element's 4.5K ohms should
> produce 1mA drain, no sweat for the KSB2, and right in the middle of the
> manufacturer's applied DC voltage spec.
>
> The manufacturer's tech sent me a wiring diagram that shows a +12V supply
> with a series resistor between 470 and 2.2K ohms to the mic. The 470 ohm
> resistor would apply nearly 11V to the mic, if the mic element and the
> series act as a pure voltage divider. The 2.2K ohm resistor would produce 8V
> at the mic.
>
> The KSB2 schematic shows a 2.2uF electrolytic cap between the MIC AF and the
> rest of the KSB2, so I wouldn't think there's any risk of a short.
>
> Help from those in the know, please?
>
>

______________________________________________________________
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

K1LI

Re: K2: connecting a condenser mic

Thanks, Don.

Not knowing any better, I simply shorted the AF and MICBIAS when I connected
the mic to the K3. From what I can see in the K3 schematics, the only
bypassing on the MICBIAS line is 1uF (rev B, sheet 5 of 7, "Front Panel -
Miscellaneous"). Can I build the same resistor into the mic plug and use it
that way for both the K2 and K3?

Brian K1LI

On Fri, Jan 29, 2010 at 9:34 AM, Don Wilhelm <[hidden email]> wrote:

> Brian,
>
> Is that a 3 terminal mic element? If so, then you can follow the
> manufacturer's instructions and apply the 5 volts to the voltage terminal on
> the element.
> OTOH, most of these elements are 2 terminal, and the voltage should be
> applied to the AF pin through a resistor - try 5.6k like the Elecraft MH2
> uses as a first trial, it will likely work because the resistance is not
> critical.
> If you try to connect the 5 volts directly to the AF terminal, you will
> effectively bypass all the audio to ground. For AC signals, a power rail is
> jst the same as ground because there are large bypass capacitors on the
> voltage rail.
>
> 73,
> Don W3FPR
>
>
> Brian Machesney wrote:
>
>> I have a condenser mic that is not on the list of "known" mics in the docs
>> or on the Elecraft web site. I'm trying to decide whether to simply short
>> the +5V to the AF when connecting my condenser mic to the KSB2, or to
>> place
>> a resistor in between. I am more concerned about potential long-term
>> adverse
>> effects on the mic element by operating it out-of-spec than I am concerned
>> with damaging the K2.
>>
>> A DMM shows the DC resistance of the element to be 400 Kohms! Not really
>> surprising, I guess, since a condenser mic is electrically similar to a
>> capacitor.
>>
>> The manufacturer specs the mic element at 4.5Kohms and 1.5V to 9.0V bias.
>> Applying the KSB2's +5V directly to the mic element's 4.5K ohms should
>> produce 1mA drain, no sweat for the KSB2, and right in the middle of the
>> manufacturer's applied DC voltage spec.
>>
>> The manufacturer's tech sent me a wiring diagram that shows a +12V supply
>> with a series resistor between 470 and 2.2K ohms to the mic. The 470 ohm
>> resistor would apply nearly 11V to the mic, if the mic element and the
>> series act as a pure voltage divider. The 2.2K ohm resistor would produce
>> 8V
>> at the mic.
>>
>> The KSB2 schematic shows a 2.2uF electrolytic cap between the MIC AF and
>> the
>> rest of the KSB2, so I wouldn't think there's any risk of a short.
>>
>> Help from those in the know, please?
>>
>

Don Wilhelm-4

Re: K2: connecting a condenser mic

Brian,

You *could* put the resistor in the mic plug, but I would think it
easier to add the resistor in the K2 to the back of the microphone jack
between pin 1 and pin 6 as is normally done. In the K3, you simply turn
Bias on in the menu.

If you choose to put the resistor in the mic plug, run the K3 without
Bias selected, it will pick up the voltage from pin 6

73,
Don W3FPR

Brian Machesney wrote:

> Thanks, Don.
>
> Not knowing any better, I simply shorted the AF and MICBIAS when I connected
> the mic to the K3. From what I can see in the K3 schematics, the only
> bypassing on the MICBIAS line is 1uF (rev B, sheet 5 of 7, "Front Panel -
> Miscellaneous"). Can I build the same resistor into the mic plug and use it
> that way for both the K2 and K3?
>
> Brian K1LI
>
> On Fri, Jan 29, 2010 at 9:34 AM, Don Wilhelm <[hidden email]> wrote:
>
>
>> Brian,
>>
>> Is that a 3 terminal mic element? If so, then you can follow the
>> manufacturer's instructions and apply the 5 volts to the voltage terminal on
>> the element.
>> OTOH, most of these elements are 2 terminal, and the voltage should be
>> applied to the AF pin through a resistor - try 5.6k like the Elecraft MH2
>> uses as a first trial, it will likely work because the resistance is not
>> critical.
>> If you try to connect the 5 volts directly to the AF terminal, you will
>> effectively bypass all the audio to ground. For AC signals, a power rail is
>> jst the same as ground because there are large bypass capacitors on the
>> voltage rail.
>>
>> 73,
>> Don W3FPR
>>
>>
>> Brian Machesney wrote:
>>
>>
>>> I have a condenser mic that is not on the list of "known" mics in the docs
>>> or on the Elecraft web site. I'm trying to decide whether to simply short
>>> the +5V to the AF when connecting my condenser mic to the KSB2, or to
>>> place
>>> a resistor in between. I am more concerned about potential long-term
>>> adverse
>>> effects on the mic element by operating it out-of-spec than I am concerned
>>> with damaging the K2.
>>>
>>> A DMM shows the DC resistance of the element to be 400 Kohms! Not really
>>> surprising, I guess, since a condenser mic is electrically similar to a
>>> capacitor.
>>>
>>> The manufacturer specs the mic element at 4.5Kohms and 1.5V to 9.0V bias.
>>> Applying the KSB2's +5V directly to the mic element's 4.5K ohms should
>>> produce 1mA drain, no sweat for the KSB2, and right in the middle of the
>>> manufacturer's applied DC voltage spec.
>>>
>>> The manufacturer's tech sent me a wiring diagram that shows a +12V supply
>>> with a series resistor between 470 and 2.2K ohms to the mic. The 470 ohm
>>> resistor would apply nearly 11V to the mic, if the mic element and the
>>> series act as a pure voltage divider. The 2.2K ohm resistor would produce
>>> 8V
>>> at the mic.
>>>
>>> The KSB2 schematic shows a 2.2uF electrolytic cap between the MIC AF and
>>> the
>>> rest of the KSB2, so I wouldn't think there's any risk of a short.
>>>
>>> Help from those in the know, please?
>>>
>>>
> ______________________________________________________________
> 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
> ------------------------------------------------------------------------
>
>
> No virus found in this incoming message.
> Checked by AVG - www.avg.com
> Version: 9.0.733 / Virus Database: 271.1.1/2655 - Release Date: 01/29/10 04:08:00
>
>

Jim Brown-10

Re: K2: connecting a condenser mic

In reply to this post by K1LI

On Fri, 29 Jan 2010 08:49:43 -0500, Brian Machesney wrote:

>I have a condenser mic

If it's like most 2-wire mics, you want a series resistor between V+
and the "hot" terminal of the mic. It serves as the load resistor
for the output stage of the mic, and also limits current to that
output stage to a safe value. If I knew nothing about the mic, a
value on the order of 4.7K - 5.6K would be a good starting point.
The mfr is giving you lower values. If you use NO resistor, the V+
shorts out the audio.

73,

Jim 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

David Woolley (E.L)

Re: K2: connecting a condenser mic

In reply to this post by K1LI

If it really is a simple condenser microphone, it will not work with
only 5, or even 12 volts of bias.

It is almost certainly an electret microphone, which is a composite
device, consisting of an electret transducer and a MOSFET pre-amplifier.
The element is biased by the permanent charge on the electret, and the
power supply is actually for the pre-amplifier.
-----
400k is too low for the element itself and too high for the FET, in a
two wire configuration. I suspect a two wire configuration and that the
reading is being distorted by reverse polarity or too low a measurement
voltage.
-----
Most, if not all, amateur use electret microphones are two wire devices,
even though those for PC sounds use stereo plugs; the ring and tip are
actually connected together, and the plug is just a trick to ensure that
the bias is not applied to a dynamic microphone, which will use a mono plug.

Applying 5 volts DC to one of these without either a resistor or AF
choke, will create an AC short on the output, and, depending on how good
the power supply bypassing is, you may get no output at all!
-------
A two wire electret will not behave like a resistor. It will have the
approximately square law characteristic of an FET drain, so you cannot
use simple potential divider calculations.
-------
The risk of a short would be due to a cable fault

Non-interleaved top posting by list policy, not desire. --------
indicate where to interleave.

Brian Machesney wrote:
> I have a condenser mic that is not on the list of "known" mics in the docs
> or on the Elecraft web site. I'm trying to decide whether to simply short
> the +5V to the AF when connecting my condenser mic to the KSB2, or to place
---------
>
> A DMM shows the DC resistance of the element to be 400 Kohms! Not really
> surprising, I guess, since a condenser mic is electrically similar to a
> capacitor.
---------
> The manufacturer specs the mic element at 4.5Kohms and 1.5V to 9.0V bias.
> Applying the KSB2's +5V directly to the mic element's 4.5K ohms should
> produce 1mA drain, no sweat for the KSB2, and right in the middle of the
> manufacturer's applied DC voltage spec.
---------
> resistor would apply nearly 11V to the mic, if the mic element and the
> series act as a pure voltage divider. The 2.2K ohm resistor would produce 8V
> at the mic.
--------
>
> The KSB2 schematic shows a 2.2uF electrolytic cap between the MIC AF and the
> rest of the KSB2, so I wouldn't think there's any risk of a short.
>

--
David Woolley
"we do not overly restrict the subject matter on the list, and we
encourage postings on a wide range of amateur radio related topics"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
______________________________________________________________
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