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Author Topic: What is the difference / why is it there, between mic ground and PTT ground?  (Read 555 times)

N1AUP

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I'm wiring an Kenwood MC 60 to a Yaesu FT991a.

I get the connections for PTT +, Up, Down, Mic, and Mic ground.  Why is PTT ground separate from Mic ground?  Don't they both go to the same bus on the radio?  It seems that mic cables don't have shielding these days - at least the ones in the hand mics.

Thanks
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AA4PB

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The mic ground probably goes to a PC board ground run close to the mike input circuit. This minimizes possible noise from the PTT circuits from mixing into the audio.
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Bob  AA4PB
Garrisonville, VA

WA3SKN

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Instead of calling it a "ground", consider calling it a "return path".
By separating the PTT and Mic returns potential noise problems are eliminated.
This was first observed in the RS232/DB25 connection when going from 9600 bps to 19200 bps data speeds.  Bell Labs learned to separate the "signal ground" and "frame ground" to minimize noise issues.
Also note the PTT is usually working with 12 volts while the mic levels are at the -45 dBm to -75 dBm range... a big difference!
73s.

-Mike.
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N1AUP

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I think part of my problem is that Kenwood labels PTT Ground as Pin 8, while Yaesu doesn't mention PTT ground at all.

https://forums.qrz.com/index.php?attachments/screenshot-from-2019-06-27-18-41-14-png.566673/
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N8NK

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WA3-'Straight Key Night' nailed it. Most manufacturers take what he said to heart and separate all analog 'gounds' from digital 'grounds' all the way to the power supply, and are given their own separate 'rails'. Induced noise is a mic line is nothing; induced noise in medical ultrasound and MRI would render the technology useless.
N8NK
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N1AUP

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So in the FT991a, is pin 7 PTT ground?
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WA3SKN

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OK, the ft991a mic connector is an rj45 style (8P8C) connector on its side with pin 1 at the top and pin 8 on the bottom. I noted they recommend that the radio be powered down when connecting/disconnecting the mic.
The mc60 you will want in the 500 ohm setting with the pre-amp off.  It uses an 8 pin DIN style connector.
Pin 1 of the mc60 should connect to pin 4 of the FT991a.
Pin 7 of the mc60 should connect to pin 5 of the FT991a.
Pin 8 of the mc60 should connect to pin 6 of the FT991a.
Pin 2 of the mc60 should connect to pin 7 of the FT991a.
VOX operation might not work with these settings.
73s.

-Mike.
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W9AC

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Why is PTT ground separate from Mic ground?  Don't they both go to the same bus on the radio? 
Thanks


They do.  From the mic, analog audio (shield lead) and the PTT return lead should connect at one point inside the transceiver, typically very close to the mic jack.  In addition to what others have said, the point is to control current back to a common ground point.  To help ensure low audio noise, current between functions is split into two pathways back to the transceiver.  Otherwise, joining the two at the mic end can result in high noise current on the audio shield when the PTT button is activated, especially with older transceivers that use high-current PTT circuits.  Keep in mind that the two returns are at equal ground potential, but currents are appropriately diverted.

The designers of older gear like Heathkit, Drake, Collins, etc. used one common mic ground for audio and PTT return current on the mic cable.  Back in the day, it was very common to hear an annoying click in the mic audio as the op activated the mic. 

Have a look at some of the best test equipment made and you'll see perhaps a half-dozen grounds: audio ground, multiple digital grounds, chassis ground...all of which are on isolated paths to a common ground point. 

Sometimes, it's necessary to bifurcate two or more audio ground paths.  For example, on my QRZ page is a low-noise audio amp that has two separate ground planes, both for audio.  The idea is to ensure that the high-current return path feeding a speaker doesn't modulate the high-gain, low-noise stage's ground plane.  The planes are separate but come together at exactly one point:  the voltage regulator ground pin.

So, in a nutshell, it's all about low-noise return path current management. 

Paul, W9AC   
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KM4AH

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It's been so long that I have forgotten the details, but I connected an old three wire mic to a Kenwood TS-940S using the single mic ground for PTT and Audio. It didn't work. Pretty nasty noise. Not sure whether one would describe it as RF or feedback. Either way.
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N6YWU

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Keep in mind that the two returns are at equal ground potential ...

The key to understanding the issue is realizing that the above statement is false.  Nothing is at ground potential.  Why?  Because every wire is an inductor, and the moment you switch current or send any signal, there will be voltage differences between the ends of the "ground" wire.

For a cleaner signaling, you use a separate signal "ground" connection using a wire that won't carry a lot of current for other non-signal things.  Then the two ends of your "ground" won't be too different, and less noise will couple into your signal (microphone in this case).

Other, large swing digital signals or power returns can go other "ground" wires where the voltages on one end or the other might bounce around a lot more.

Other issues will arise due to the fact that 2 or more parallel "ground" wires are also transformers and capacitors, which is why just grounding signals and stuff in high frequency and high speed digital design is non-trivial.
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K0UA

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It is simple.  Whom ever said "ground is ground the world around" was an idiot, and no knowledge of "ground" nor the world. :)
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73  James K0UA

W9AC

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Keep in mind that the two returns are at equal ground potential ...

The key to understanding the issue is realizing that the above statement is false. 

I'm referring to a return at the mic jack.  In most modern transceivers, the common 8-pin Foster mic jack has an audio signal ground pin and PTT/control ground pin. Those pins are bonded together at the jack and are further bonded to the chassis with a short length of wire to minimize inductance. 

Some transceivers incorrectly bond to a PC board as a signal ground and not the chassis.  And in the case of the early production Elecraft K3, the designers chose to use RF chokes on the signal ground leads -- exactly opposite of what's required. The result was customer RFI complains.  In addition to creating higher inductance, not bonding directly to the chassis creates the "Pin 1 problem":

http://www.audiosystemsgroup.com/Pin_1_Revisited_Part_2.pdf

Paul, W9AC

   
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K7LZR

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I wouldn't worry too much about tube availability except for perhaps a few offbeat high power transmitting types. There are likely millions of receiving tubes still available, NOS & good used. They are like crude oil - eventually the supply will vanish, but not for a long, long time.....
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WB6BYU

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Quote from: KM4AH

... I connected an old three wire mic to a Kenwood TS-940S using the single mic ground for PTT and Audio. It didn't work. Pretty nasty noise. Not sure whether one would describe it as RF or feedback. Either way.



I've run into that problem due to dirty PTT switch contacts.

There is current through the contacts, so any varying resistance
causes varying voltage / current on the ground wire.  If that
wire is in series with your mic circuit, you get a lot of noise
that may be stronger than the signal from the mic itself.

You really don't want that ground wire common to both circuits.
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