100D with Icom SM20

laserbill
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Joined: Thu Apr 05, 2018 8:43 am
Location: West Yorkshire

100D with Icom SM20

Postby laserbill » Thu Jul 30, 2020 1:04 pm

Hello,
I need some help trying to get this combination working please. My radio is set up to supply bias voltage to the existing mic I use.
I have made a 3.5 mm jack to 8 pin round male patch lead up for the SM20 but when I connect It no audio output from the mic .
My Anan front mic socket is configured
TIP Mic In with Bias
RING PTT
SLEEVE Ground
I have not wired the PTT line in the patch cable until I get the audio working
Many thanks
Bill
G6SVN
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w-u-2-o
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Re: 100D with Icom SM20

Postby w-u-2-o » Thu Jul 30, 2020 10:25 pm

Post the schematic of your patch cable.
laserbill
Posts: 14
Joined: Thu Apr 05, 2018 8:43 am
Location: West Yorkshire

Re: 100D with Icom SM20

Postby laserbill » Fri Jul 31, 2020 5:48 am

Good question Scott ;)
Have tries several combinations and this is the last attempt
Got lots of noise and slight audio with this

TIP to pin 2 and connected to 47uf cap, other end of cap connected to pin 1
RING N/C
SLEEVE to pin 7

Thanks
Bill
laurencebarker
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Joined: Mon Nov 11, 2019 7:39 pm

Re: 100D with Icom SM20

Postby laurencebarker » Fri Jul 31, 2020 10:26 am

"Slight audio" is usually caused by inadvertent decoupling of the audio to ground.

If you look at the top right of your microphone circuit the bias input (from pin 2) has a 270 ohm series resistor and 100uF capacitor to ground. Great to get rid of RF, but.... it is also a 5Hz lowpass filter. Your voice is typically 300-2700 Hz, so your audio is ending up in that filter.

I think you will need to remove the 100uF capacitor and the 10uF capacitor at top left, which is having a similar effect.
Laurence Barker G8NJJ
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w-u-2-o
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Re: 100D with Icom SM20

Postby w-u-2-o » Fri Jul 31, 2020 10:58 am

Laurence: if that is the case, how did the microphone work with Icom equipment in the first place?

However, that does beg the question: does the microphone work OK with compatible Icom equipment right now? I.e., is the mic itself broken somehow? Because the proposed adapter should work.
laurencebarker
Posts: 24
Joined: Mon Nov 11, 2019 7:39 pm

Re: 100D with Icom SM20

Postby laurencebarker » Fri Jul 31, 2020 11:44 am

With the original 8 pin microphone connector, the bias supply and the audio signal are separate. So the supply to the bias (pin 2) is derived from +12v and can be well decoupled but it is not electrically connected to the audio signal. With that arrangement, any filtering on the bias supply in the microphone does not affect the audio.

The limitation is the 3.5mm audio connector providing bias supply AND audio through the same connection. With the adapter as described, the tip provides the audio and bias connection. The filter in the microphone bias circuit is in parallel with the audio, and filters it out.

The alternative is to make up a slightly different adapter that also brings in +12v from the radio power feed; regulates the 12v down to 5v-8v (even a series zener diode may do) and provides the bias supply from a 12v source. Then connect the audio to the 3.5mm tip, and it is isolated from the filters in the power feed.
Laurence Barker G8NJJ
laserbill
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Joined: Thu Apr 05, 2018 8:43 am
Location: West Yorkshire

Re: 100D with Icom SM20

Postby laserbill » Fri Jul 31, 2020 12:59 pm

I appreciate the help guys, the microphone is working fine with my Icom 775, always had good reports.
Measured the voltage from the Anan mic socket and its 2.33 volts without anything connected, Is this enough to power the SM20?
There is a seller on Ebay with a made up lead https://tinyurl.com/yxolpgwq
No Bias required according to the description.
Thanks again for chipping in
Bill
laurencebarker
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Joined: Mon Nov 11, 2019 7:39 pm

Re: 100D with Icom SM20

Postby laurencebarker » Fri Jul 31, 2020 1:18 pm

It all hinges on the two transistors. I don't know whether they will work with a lower input voltage.

The other thing the ebay supplier could have done is: put a large series resistor into the bias circuit (ie into pin 2). Say 47K. That would limit the effect of the filters.

I've has this issue with various home made microphone arrangements. It's really easy to put a capacitor in what is supposed to be a different part of the circuit, and not spot that it is joined to the audio part.
Laurence Barker G8NJJ
laserbill
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Joined: Thu Apr 05, 2018 8:43 am
Location: West Yorkshire

Re: 100D with Icom SM20

Postby laserbill » Fri Jul 31, 2020 1:42 pm

Thanks for the suggestion Laurence.
I will try a 47k resistor in series with Pin 2 and tip and report back
Bill
laserbill
Posts: 14
Joined: Thu Apr 05, 2018 8:43 am
Location: West Yorkshire

Re: 100D with Icom SM20

Postby laserbill » Fri Jul 31, 2020 2:07 pm

No joy with the resistor...no audio whatsoever.
Have messaged the Ebay seller for more info on the patch lead.
Thanks
Bill
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w-u-2-o
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Re: 100D with Icom SM20

Postby w-u-2-o » Fri Jul 31, 2020 4:57 pm

laserbill wrote:Measured the voltage from the Anan mic socket and its 2.33 volts without anything connected, Is this enough to power the SM20?

D'oh! I should have thought of that. No, not even close to enough. You need to do what Laurence suggests...

laurencebarker wrote:The alternative is to make up a slightly different adapter that also brings in +12v from the radio power feed; regulates the 12v down to 5v-8v (even a series zener diode may do) and provides the bias supply from a 12v source. Then connect the audio to the 3.5mm tip, and it is isolated from the filters in the power feed.

Don't forget to remove the bias jumper--you don't want/need that connection if you are feeding the bias pin from a separate power supply.
laserbill
Posts: 14
Joined: Thu Apr 05, 2018 8:43 am
Location: West Yorkshire

Re: 100D with Icom SM20

Postby laserbill » Sat Aug 01, 2020 3:51 am

Hi Scott and Laurence.
I heard back from the Ebay seller and although he says the patch lead works 100% with all Icom Microphones, he cant guarantee it will work correctly.

"Hi. I do have schematic and in my opinion it will work however since I've never tried with 2-3Vdc I can NOT guarantee full compatibility. Minimum it should be powered by 5Vdc.
Regards,
HAMdaptor"

So.....Will stick with my existing arrangement
Thanks again and have a great weekend
Bill

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