Max Microphone Cable Length

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Greetings!

What is the maximum microphone cable length that can be used with a 6700?  I am interested in having the transceiver in one room and talk from another room.  I know there are many ways of using remote operation, but I would like to know what the maximum cable length that can be used before going to a remote setup.

Many Thanks!

Jeremy
W2JG
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Jeremy - W2JG

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Posted 10 months ago

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Doug - W3UB

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Well, if you are using a balanced mic and cable, it can be quite long. Mine is 35 feet to the radio and amp, to keep fan noise down. The comments I get are always positive.
/Doug
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Jeremy - W2JG

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Thanks for the quick reply, under 10 minutes!  Do you think a 50ft cable would work?  Did you purchase the cable and from who or did you make it?

Thanks again Doug!
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David Merchant

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If you use a balanced microphone with the XLR input, it can be thousands of feet. How long are you thinking and what type of Mic are you using?
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Jeremy - W2JG

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David,

Thanks for your quick reply!  The length is easy answer, about 50ft.  The mic is harder because I wanted to run all 8 pins so that I can use PTT and have it available for any future microphones I might want to use.  In the beginning I will be using a Radiosport RS60CF headset with M350 mic.
I have been trying to look for the spec on the M350, but cannot find the impedance of the microphone yet.  I will keep looking, but if anyone else knows, please let me know.

Thanks!

Jeremy
W2JG
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David Merchant

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I wouldn't use the 8-pin connector for this application.  I would use the XLR Mic input on the back of the 6700 to connect to the headset.  Use the following cable to convert the headset to XLR, then use an XLR extension cable:

http://cart.flexradio.com/Radiosport-Headset-cable--3-pin-XLR_p_951.html

Run a separate PTT line to the RCA jack to key the radio.  

The benefit of using the XLR connection is that it uses a balanced or differential signal that is much more immune to noise.  It's why this Mic connection is used for virtually all professional audio applications.
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Paul Christensen, W9AC, Elmer

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The source Z of the RS60F is almost certainly under 1K-ohm.  50 ft. will be fine.

One more point to mention:  The days of strict audio impedance matching have been over for 3+ decades.  Some folks still think that a 10K-om input impedance needs a 10K-ohm microphone when using modern audio equipment. We no longer need to maximize the power transfer function as we did in the days of input and output transformer matching. 

The audio industry has long been using voltage-based audio matching, rather than power-based matching.  That means, even if the rig's input Z is 10K-ohm, the transceiver is simply a bridging termination on the low-Z source. 

A good rule of thumb is to use a low source Z at every opportunity, and a moderately high-Z at the termination in at least a 10:1 ratio.  Excessively high input Z is not always a good thing either, especially when using switched audio systems.   

Paul, W9AC
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Paul Christensen, W9AC, Elmer

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>"I wouldn't use the 8-pin connector for this application.  I would use the XLR Mic input on the back of the 6700 to connect to the headset.  Use the following cable to convert the headset to XLR, then use an XLR extension cable"

Any benefit derived by using an XLR connector to a differential (balanced) audio input means the cable should be balanced (e.g., 2 conductors with or without shield and preferably with a high-twist ratio, star quad, and CAT 5/6 as examples).  One cannot just put an XLR connector on any mic cable and call it "balanced."   

If his RS headset's mic line simply has one conductor with an overall shield, then the only part of the system that's truly balanced is the 6700's XLR mic input, which hopefully uses a 3-stage instrumentation amplifier for best common-mode rejection.

Paul, W9AC 

(Edited)
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Jeremy - W2JG

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David,

THANKS!  That is a GREAT comment about using the back panel XLR connector!  I was not aware that the front connector did not provide a balanced/differential input!

Jeremy
W2JG
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Jeremy - W2JG

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Hi Paul!

I noticed your comment about one of the replies.  I do not want to start a grudge between commenters, but I need to follow up because I am now more confused.  I completely understand your comment about connecting single ended to differential, but I don't know what the M350 uses, I have been searching for specs since starting this post, but have yet to find the info.  I bought the headset with the 8-pin Foster connector, but don't know how many pins the M350 uses.  Do you know?  (My headset is currently in storage, so I cannot look at it)  There was also an implication that the front panel connector was not balanced but the rear XLR connector was.  I went back to the FlexRadio documentation and saw that the front panel uses two pins, +/-, for the microphone.  I would assume that means it is differential, if not what is the difference between the front and rear connectors for the microphone.  I know that the front also has pins for PTT, Up, Down, and Fast.

Thanks so much!

Jeremy
W2JG
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David Merchant

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I sent a quick note to Dave Bottom, the owner of RadioSport Headsets.  Here's what he came back with for the benefit of the group: 

From: Dave Bottom
Date: Friday, December 14, 2018 at 1:32 PM
Subject: Re: M350 Mic with XLR Cable - Is it balanced?

Hi David,

Thank you for your email.

Virtually all desktop HF radios since the introduction of the 8-Pin Foster Mic connector have a quasi-balanced Mic input, in that the Mic+ and Mic- leads are separate leads from the Mic, through the front panel Mic connector, then often via a computer like ribbon cable (every other lead is a ground lead) from the front panel PCB to the TX/IF PCB where the two Mic leads get terminated at the Mic Pre-Amp.  

The radio then determines whether this is single-ended or a balanced differential input.  Few radios in the past actually offered true differential Mic input prior to the Flex 6700, and those being the TenTec Orion I and Orion II, and the Yaesu FTDX-9000.

Since with a true differential Mic input, there is no possibility of a ground return for Mic BIAS to power an Electret-Condenser Mic, so a Dynamic Mic (our M208) must be used.

That said we offer a 5-foot headset extension cable and a 5 foot PTT extension cable when rack mounting the radio, that like our headset cables, the lengths are carefully chosen to prevent any cable from being an electrical quarter wavelength on the HF Ham bands.  These may be used with our HTR cables with Dynamic or Electret-Condenser Mics.  These may also be used with our CS6-XLR or CS6-BAL250 (1/4” Plug) Headset-To-Radio Cables and our M208 Dynamic Mic.

If longer lengths are really intended - running remotely with Win SSDR and the headset connected to the PC, or with a Maestro and the headset connected to the Maestro would seem the prudent move. 

73 Dave WI6R

Happy Holidays to all!


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Jeremy - W2JG

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Thank you Dave for all your efforts to help track this down!  I greatly appreciate it!
I would also like to once again thank everyone else for your extremely prompt responses and good discussion of my question.  I really appreciate eveyone's help!!!

Jeremy
W2JG
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Jim Gilliam

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theoretically there should be no practical limit to the length as the loss in the cable would be negligible. The problem arises with noise: There will be noise generated in the cable itself that will be amplified and there is the high susceptibility of the cable acting as an antenna and picking up large amounts of RF. The amount of RF depends on how "clean" your operating is regarding feed line radiation running to  your antenna and the close signal intensity that could couple into your cable.


I don't think there is a clean cut answer to your question and it would best be answered by giving it a try to see what happens. Adding plenty of iron core clips to the cable would go a long way in suppressing the RF but other noise factors might enter into the equation.


Jim, K6QE
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Jeremy - W2JG

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Thanks Jim!

Your point about the noise is why I asked the question.  Thanks for your suggestion for iron core clips!
This now raises a second question, are you aware of any double shield cables?  If available, I would think that might help.
Thanks again for your quick reply!

Jeremy
W2JG
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Paul Christensen, W9AC, Elmer

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Depends on the source impedance of the mic.  If Hi-Z from, for example, an Astatic D104 crystal element, then cable length can become critical.  If the Hi-Z source is mostly resistive, then the effect of a long cable is attenuated high frequency response.  If the Hi-Z source is mostly a capacitive reactance (like the D104), then the overall output is reduced with cable length, but little effect on frequency response.  Most of us use under 4 kHz audio for SSB, and under 10 kHz for AM.  The wider AM bandwidth would be noticeably affected first.

Hi-Z mics when used with long cable runs are also subject to cable noise.  Just kick the cable and you'll see what I mean.  Keeping the source Z low is always a good idea.

For more common 150/600 ohm mics, you can generally run much longer mic cable lengths.  I would use a Lo-Z 150 or 600 ohm mic -- or really anything under 1K if you're running a long mic cable like from a home basement up to a second story room. 

Paul, W9AC
(Edited)
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Jeremy - W2JG

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Thanks Paul for your quick reply!

To start with, I will be using a Radiosport RS60CF headset with M350 mic.  Unfortunately,, I do not know the impedance of the M350 mic nor have I been able to find it.  I will keep looking.  Off hand, are aware of its impedance?


Thanks Again!

Jeremy
W2JG
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Doug - W3UB

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I would think 50 feet would be no problem. I use a PR781 mic. Lots of places to get the cables, I got mine from (where else) Amazon ...

https://www.amazon.com/gp/product/B01JY2BDAU

On a side note, I do everything I can to avoid transporting the TX and RX audio over to the client, as the quality gets degraded in my experience.

/Doug
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Jeremy - W2JG

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Thanks Doug!

A couple of questions comments...  First, excuse my ignorance, may be lack of sleep, but what do you mean by transporting TX and RX audio to client, isn't that the point of cable?  I also saw from the link that you are only using a 3-pin cable.  I was hoping to use all 8 pins so that I could also use the PTT.  How did you do the PTT with your 35ft cable?

Thanks Again!

Jeremy
W2JG
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Doug - W3UB

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For me, I use VOX instead of PTT.

You can connect the TX and RX audio either to the radio itself, or to the client (PC/Maestro/IPad). You can mix and match if you want as well. I find the best audio by far is tp/from the radio.

The headset mic is not really balanced. You could make up an adapter to drive a balanced cable, it might work. I have the same headset and picked up 60Hz hum on the long run. In the end when I use it (not often) I plug it into the Maestro. You can probably make it work, but it may involve some tinkering.

Your best bet by far is to use a balanced mic with a balanced cable to the balanced input.

/Doug
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Jeremy - W2JG

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Thanks Doug!

I didn't follow that your comment about the client was referring to the PC.  Now I understand!  I also appreciate your comments about the headset.  My headset is in storage right now, so I could not look at it.  I want to wire this so that it is microphone independent, meaning extending exactly what is at the connector, so that I would add any adapters at the extended side, just like I would have to right at the unit.

Thanks Again!

Jeremy
W2JG
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Mark WS7M

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I am running balanced to my radio which is in a shed next to my op position.  I have RFI chokes on both ends but the distance is roughly 30 feet and also no issues.