What is the best microphone wire to use on a D-104?

[N2IDU]

I have to replace the old microphone wire to my unamplified d-104 mic. Somewhere I remember that certian types of replacements cord will attenuate the highs. I don't want to do that if at all possible. The d-104 is loading into about a 5 or 6 megaohm resistor in the first audio. Would appreciate info on suitable replacement for same. The present mic cord and plug fits my Ranger and 32V3.

Thanks,

Peter N2IDU
"OLD SHINCRACKER, VT"

[N2DTS]

Good question.
I once tried to replace the to short cord on my D104, I used balanced microphone wire, and it sounded really nasty, no high end at all.
 
I suspect you need something with a really low capacitance per foot.

I wonder if small coax would do, with the ptt outside and all run inside some heat shrink or some sort of outside cover....

Brett

[Pete, WA2CWA]

Several years ago I needed to replace the cable on one my D-104's. Went to a flea market, bought a CB mobile mike in someone's junk box for 50 cents. Chopped the connector end off since it was the wrong type, unsoldered the wire connections in the mike head, and I had a replacement cable. Been using it ever since. No bad mike reports when connected to the Ranger, Valiant, or Viking II and several other rigs.

[k4kyv]

If you use one of those PTT stands, you don't want to use two conductor mic cord.  Use shielded cable and a separate external line for the PTT.  Otherwise, if the audio line and the control line are inside the same shield, the PTT line current will induce a spike in the audio whenever the switch makes contact, and that could cause the final to flash over or even crap out the mod xfmr.

There is one case for using a two-conductor mic cord; running the D-104 with balanced output.  The stock Astatic xtal element is not polarised with specific terminals for hot and ground.  You can connect each of the two terminals on the xtal element to one of the conductors of a two-conductor shielded mic cord, and ground the shield to the mic case. Then, you can use a push-pull mic input stage, with each of the two conductors connected to the grid of one of the tubes.  Each tube has its own grid leak; the two grid leaks in series with a common ground is what establishes the mid-tap of the balanced output.  Try to get the grid leaks on each tube as close to the same resistance as possible.

Besides reducing hum, this configuration allows you to double the load resistance on the crystal element while still maintaining stable grid bias on the mic preamp tubes.  I run 10 megohms of grid leak in my present single-ended mic preamp.  With the pushpull input, I should be able to run 20 megohms of load on the xtal element, which should give a superior low end response to the mic.  The pushpull mic preamp is one of my present works in progress.

I don't use a PTT mic stand.  I hate desk stands.  They always get in the way, and inevitably, no matter how careful I try to be, I manage sooner or later to knock the mic off the desk onto the floor.  I prefer a stationary boom for the mic, and a separate hand-held PTT switch connected to the transmitter via an old spiral mike cord.

[KE6DF]

I think you need to use Oxygen free cable using copper mined in only the Sugindo mine in Chile and refined by the waterless process in the presence of Polonium.

I have some I can sell you for $1000 per foot.

 

[Steve - WB3HUZ]

Whatever you replace it with, keep it short, 2 feet or so and you won't lose those highs.

[ke7trp]

The thing that really matters in my opinion, Is that the audio wire, has its own shield.  I have 500Ft of this.. If you need some, Just let email me your address. I will send you 10Ft.  Just help someone on the forum down the road.


Clark

[N2DTS]

I really like Don's idea!
I only use the D104 in the 32V3, and that rarely, but love the balanced idea.
Not sure if I could work it in the small space, I have VR regulator tubes mounted in there to regulate the KT90 tube screens, but will have to look and see if I could do something like that.
Need to replace the connector also to give 3 wires plus ground....

Don, your picture sure looks scary, like Mosses with homebrew...or some crazy doctor,
Fire and brimstone kind of thing.....

Brett

[WBear2GCR]

The D-104 element is a two terminal device, no ground.

Connecting the terminals to a balanced in will not buy you a balanced input, so there will not be any hum or RFI cancellation that I can see. The mic body is at ground, but that ground knows nothing about the mic element, unless one side of the mic element is connected to it (ground). Most balanced inputs are not high-Z, and are low Z, so that is a bad match to a D-104. The D-104 wants an ultra super maxi high-Z load (in the megohms) so that isn't even a standard high-Z input.

When you get down to it, you need a special input set up in the rig for the D-104, or in each rig that you want to use the D-104. OR ELSE, you can just put the single FET follower circuit which takes care of the problem 100% by changing the output impedance to a low Z and providing a fixed ultra super maxi high-Z load to the element (which if it is in the base is only 8" away) and also makes the choice of mic cable rather unimportant, and you can use it with any rig's stock mic input too.

Go for the JFET source follower in the base.

                             _-_-bear

[KC4VWU]

     Now I would have to double check myself, but if I recall correctly, when the D-104 is set up to be used on a G stand one side of the element is connected to ground. The 3 pin plug in the neck of the D-104 has both the wires from the element going into pins 1&2. Pin 2 is connected to pin 3 in the stand, pin 3 is grounded through the screw which holds the plug in the bottom of the D-104. Check pin 3 to case, I believe it will show continuity.
-- Phil

[N3DRB The Derb]

Don, your picture sure looks scary, like Mosses with homebrew...or some crazy doctor,
Fire and brimstone kind of thing.....

Dude! 

Why you gotta be so rude and nasty mean? Lets see your pitcher so we can discuss yoo.

Don is the AM Phone MC Hammer of the Mid South. Dont diss on my buddy. He's got moves you cant touch.

....besides, I took that picture. 

[Pete, WA2CWA]

Why you gotta be so rude and nasty mean? Lets see your pitcher so we can discuss yoo.

Don is the AM Phone MC Hammer of the Mid South. Dont diss on my buddy. He's got moves you cant touch.

....besides, I took that picture. 

But, does he have the pants!   

[W7TFO]

I think Don's photo personifies perfectly the feeling a lot of older hams have today:  Disgruntled disbelief over stupid technology like BPL, the ARRL's stance on AM, bandplan non-compliance, eTc.

Like Johnny Cash wearing black, maybe Don will put up a happy photo when things get better for us.  Derb can take it for us, too.

Godspeed, Donald Chester!  

Now back to our regularly scheduled program.....

[W7TFO]

Bear sez:

"Connecting the terminals to a balanced in will not buy you a balanced input, so there will not be any hum or RFI cancellation that I can see. "

IMHO, balanced doesn't necessarily mean a ground referenced 'center tap'.  Merely the pair is used rather than a single-ended power transfer.  To wit, the phone loops in use are not ground referenced for voice service.  Most old-school audio mixer mic inputs are floating with a grounded shield. 

This all goes back to the technique used back then calling for impedance matching for proper power transfer, rather than todays' style of 'constant voltage', low impedance symmetrical dual-rail powered output and bridging impedance inputs.

Nobody says you can't have a center-tapped reference, rather it is just not necessary.  Most older tube pro & broadcast gear had no ground in the output windings. 

I have seen old gear with an input scheme similar to Don's, to get away from a costly input transformer.  It is, in fact, a harbinger of what we now call 'active balanced input' found everywhere in pro audio.

The plan of using a split grid leak resistor deal does give it a point of phase inversion to use twin triodes in push-pull, and also gives it the ability to reject hum and noise even with very high impedance levels.

As far as cable goes, I'd use a star-quad type from Mogami or Canare.  Absolutely immune from external fields when properly used, and works well for hi-impedance or low.  This does NOT have extra conductors for switching, BTW.

[N2DTS]

Over many years, I have worked Don many times, and know he is a nice guy, nothing like the idea you get from that picture!

Brett

Don, your picture sure looks scary, like Mosses with homebrew...or some crazy doctor,
Fire and brimstone kind of thing.....

Dude! 

Why you gotta be so rude and nasty mean? Lets see your pitcher so we can discuss yoo.

Don is the AM Phone MC Hammer of the Mid South. Dont diss on my buddy. He's got moves you cant touch.

....besides, I took that picture. 

[N2DTS]

Oh, no wonder he looks so GRIM...

Brett
 

....besides, I took that picture. 
[/quote]

[N2IDU]

Ok FINE!!

Thanks for all your input. It was very helpful. This is always a great board.

Peter N2IDU
Old Shincracker, Vt.

[WD8BIL]

At least 3 conductor shielded. 4 conductor is better yet.

[k4kyv]

The D-104 element is a two terminal device, no ground.

Connecting the terminals to a balanced in will not buy you a balanced input, so there will not be any hum or RFI cancellation that I can see.

This would be just like a balanced 500Ω line that connects one piece of audio equipment to another.  Professional grade audio equipment doesn't necessarily have a ground reference at the input and output of each component, such as a midtap on the input/output transformer.  The balanced line can float in reference to ground, while the shield is grounded at one end only, to avoid ground loops. Transformerless solid state units often use a pair of resistors to establish balance to ground.

With the D-104 element, the two grid leak resistors in series at the pushpull mic input stage form the load on the xtal element, and if they are carefully chosen so that the resistance is equal (within a few %), each grid will see the same voltage in reference to ground, but 180° out of phase.  Common mode hum and rf theoretically should cancel, leaving only the differential mode, the audio voltage from the xtal.  This should work equally well with FETs.

The mic body is at ground, but that ground knows nothing about the mic element, unless one side of the mic element is connected to it (ground).

In that case you have an unbalanced high-Z output, which it typical for crystal mics.  Unlike the D-104 element, most xtal elements have a metal case and one terminal for the output.  The other terminal of the crystal element is internally grounded, so you have no choice but to use unbalanced output.  OTOH, both terminals of the xtal in the D-104 element are isolated from ground, so you can feed the output of the element to the grids of a pushpull stage, independent of ground, through a two-conductor shielded audio cable.  The balanced reference to ground is established via the grid leak (or gate) resistors at the input of the push-pull mic preamp.

Most balanced inputs are not high-Z, and are low Z, so that is a bad match to a D-104. The D-104 wants an ultra super maxi high-Z load (in the megohms) so that isn't even a standard high-Z input.

This doesn't have anything to do with a "standard" balanced line.  The standard balanced line is usually somewhere between 50Ω and 500/600Ω, but there is nothing that says it has to be limited it to those low impedances.  The D-104 xtal element can work into a balanced high-Z load just as easily as it can work into an unbalanced high-Z load.

With the older D-104's (before the advent of the CB "power mic" pre-amp in the base of the desk stand), Astatic recommended the high-Z balanced line when the mic cord was to be more than a few feet in length.  They claimed that the xtal element could work into a balanced shielded cable up to 100' to 200' in length, without deterioration of the frequency response and with good rejection of hum and rf.

[W1VD]

Might be interesting to try a balanced high-input Z / balanced 600-ohm output FET preamplifier (in the base of the microphone) feeding a high performance audio chain. Can't recall having heard a D-104 run through processing...usually they're plugged directly into BA transmitters. 

[K1JJ]

Probably a good W1VD project and write-up:


Terminate with a pot (try various resistive loads) and run an audio sweep on various types of mike cable.  Also check its capacitance per foot. This would shed some light on what's what with attenuating highs.


Yes, as suggested, coaxial cable would probably do very well being only 10pf-30pf per foot or so.

T 

[KD6VXI]

Might be interesting to try a balanced high-input Z / balanced 600-ohm output FET preamplifier (in the base of the microphone) feeding a high performance audio chain. Can't recall having heard a D-104 run through processing...usually they're plugged directly into BA transmitters. 

That's what I do, run my D104 into the signal processors.

People can't tell the difference, really... And it sounds SSMMOOOTTHHHH.

Like Kool... Smoke KOOOL SMOKE KOOL!

--Shane

[N2DTS]

I happened to look at an old electric radio and notice that heil makes wire, low capacitance, balanced, with seperate (2) ptt wires.
The shield is 100% and silver.

Might work well on a D104!

Brett

[k4kyv]

I'd have to go back an re-study the  details, but I recall that the capacitance between conductors and shield, and presumably between two conductors, of a shielded mic cable are not supposed to affect frequency response, but instead attenuate the signal across the frequency range.  The xtal element is equivalent to an ideal a.c. generator with a series capacitance of about 500-1000 pf.  The capacitance between conductors is equivalent to a second capacitor shunted across the load.  The series capacitor and load shunt capacitors act as a capacitive voltage divider, attenuating everything equally across the range.  The series capacitance working into the load resistance attenuates the low frequency response, so the higher the load resistance the better the low frequency response.

There is a limit to how high the load resistance may be before the grid (or gate) voltage becomes unstable, so you can't just let the input float without a resistance to ground.

The advantage of the push-pull circuit, besides common-mode attenuation, is that it allows you to double the load resistance on the mic, since the two grid leak resistors are in series across the output of the mic element.  This should shift the 3 dB point of the low frequency roll-off one octave lower.