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Author Topic: Best microphone for AM double sideband xmission?  (Read 649 times)
KC9KEP
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Posts: 208


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« on: November 19, 2007, 07:04:06 AM »

Hello Elmers!

I'm planning on building a 75 to 120 watt AM phone
vacuum tube transmitter (double sideband) :-)

I've been listening to the 80 meter, AM double
sideband phones guys lately.

One operator was commenting on someone’s transmission
sounding "bassy". He speculated that they may have
been using a dynamic microphone.

Another operator was commenting about how important
that (audio) pre-emphasis is for phone quality.

Well, I'm quite versed in various microphones, having
owned & operated my own audio recording studio.

And, I realize, of course, that the bandwidth and
fidelity requirements for ham transmissions are
quite different than that of hi-fi requirements.

Bottom line, should I pick up a crystal (or ceramic)
mic to use with my home brew transmitter for better
legibility?  Or can I get away with a dynamic
low-Z (& matching transformer) mic such as a Shure
SM-57 (which I have many of!).

Thanks again .. I appreciate your replies!

--Tom Nickel KC9KEP


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AC5E
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Posts: 3585




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« Reply #1 on: November 19, 2007, 08:01:09 AM »

There are no "best" microphones for AM - although many hams think there are. Some of the best AM audio I ever heard came from Johnson transmitters.

One in particular, a Ranger with an Astatic JT30, was probably the best sounding AM rig I have ever heard on the air, but the Valiants with a variety of mikes also did very well. (Of course, the fact I built a bunch of Rangers and Valiants for the local dealer may have colored my opinion!)

The real secret for a homebrew AM transmitter? Nothing more complicated than low distortion audio stages with a reasonable high frequency rolloff, plenty of headroom in the PA, and plenty of iron in the modulation transformer.

If you do it the old fashioned way, with an LC low pass filter beginning at 2700 to 3300 cycles, excuse me, Hertz; preceded and followed by low intermod and odd harmonic distortion amplifiers will sound great regardless of the mike.

Back in the days I "rode gain" for a variety of announcers we also had a variety of mikes. Dynamic, ribbon, and condenser - and every announcer had a mike they thought flattered their voice. What the engineering staff (me) thought was another matter.

My opinion? If you have a "commercial quality voice" you should get commercial quality results from almost any decent mike. And if you do not, all the massaging in the world is not going to help much.

73  Pete Allen  AC5E
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WA3SKN
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« Reply #2 on: November 19, 2007, 09:50:25 AM »

There are a lot of "pseudo-audio experts" out there, especially on 80 meter AM.
The truth is that you are limited to 0-3000 Hz audio (6 Khz AM bandwidth), and thats with a 25 dB rolloff bandwidth, at least on the HF bands.  The lowest frequency passed is about 80 Hz due to the carrier. Just about any microphone can supply this.  The crystal mics hilighted the upper frequencies, peaking about 12 Khz.  You can get equalizers to adjust the frequecy response to your liking, but I would not invest too heavily in this.
Good old class A amplifiers, a contolled sound booth, and a natural sounding voice are the main assets for good audio.

-Mike.
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K6AER
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« Reply #3 on: November 19, 2007, 10:03:27 AM »

As Pete said build your transmitter with well defined low distortion audio drivers and modulator.

Where I would divert form the way it used to be done is I would use wide band audio circuits in the AM transmitter and then feed the transmitter modulator from an Dynamic EQ box such as the Behringer DEQ2496 Ultra-Curve Pro to set up the equalization, bandpass both high and low as well as and dynamic processing. You can buy this unit for under $300 which includes an audio spectrum analyzer, level meters and DEQ (noise gate).  If you need more than 30 dB of bandpass you can loop the left side back through the right side for over 60 dB of bandpass equalization.

I use this unit on my Pro-III and it really cleans up the spectrum profile of your signal. At 1000 HZ above my cutoff frequency of 2900 HZ Low Pass, my signal sidebands are 35 dB down. At 2000 Hz above the top bandpass the sidebands are down 47 dB.

As for a microphone (under $150) almost any quality microphone will reproduce 18-18,000 Hz with +/- 2 dB for flatness. I use an M-Audio unit I picked up at the Guitar Center for $79 and it work just fine.
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VK1OD
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« Reply #4 on: November 19, 2007, 11:14:40 AM »

WA3SKN said "The lowest frequency passed is about 80 Hz due to the carrier."

How so?

Owen
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KG6OMK
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Posts: 107




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« Reply #5 on: November 19, 2007, 03:14:40 PM »

People seem to be thinking only about the frequency response of the mic but I think it's pattern may mater more.  Is it directional or omni?

Why I think ity matters is that your voice may be picked up by the mic but then continue on and reflect off the wall and be picked up again.  This will mix slightly out of phase.  The effect is a bit like a comb filter.  The mic pattern, how close the mic is to the operator and room acoustics all combine.

Now about the fact the the mic has frequency range well outside of the 3KHz that can be transmitted.  true.  But think about reflections,  Some higher pitched component of the voice and a reflection of the dame might combine and alias to a frequency below 3khz.  In radio we call this "intermod" it's where two stations both outside your passband are spaced such that the sum or difference of their frequencies fall inside your passband.  Room acoustics, reflections and mic patterns all work together and I'm sure some of the beat frequencies are in the 3Khz passband.  

That said, I'm sure the effect is subtle at best but may explain why different mics have a different "sound" even when bandpass filtered.
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ONAIR
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« Reply #6 on: November 19, 2007, 11:19:44 PM »

  The good ole D-104 and the Turner +2 mikes were used on AM by many hams and CBers way back in the '60s with really good results!
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W8JI
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« Reply #7 on: November 20, 2007, 04:46:01 AM »

In order to generate NEW frequenies there has to be something very non-linear in the system. Reflections won't "mix" and make new frequencies that distract from the primary frequencies unless something is terribly wrong somewhere. I doubt any such system would sound even remotely decent no matter what the room accoustics were like.

As for audio amplifiers, the PA stage in the transmitter has to modulate with a square law response to voltage changes. If the voltage on the PA doubles the PA has to have four times the envelope power. It's almost silly to run a class A amplifier or do all these low distortion mods to a rig like a Valiant or any other tetrode PA since virtually none of the tetrode radios have low distortion envelope response. Even a class B modulator will be cleaner than the modulated stage unless the PA stage is a really hard switched low mu triode.

Most of the "sound" people rant about is really just frequency response and nothing to do with distortion. The personal tonal likes of the listener mean more than anything else in most cases, unless you have terrible audio distortion. People like to delude themselves into thinking they can hear a fraction of a percent distortion when they would be lucky to sort out 5% if it was a blind A B test.

As for the microphone, the impedance match means quite a bit. If the radio has a high impedance input a dynamic mic can sound extra bassy because the mic is running into an open load and can have sharp resonant peaks in frequency response that would not be there if loaded properly.

A crystal or ceramic mic can sound pinched or tiny if run into a low impedance input.

So the mic has to match the radio input impedance.

In general something like a D104 sounds great on AM or SSB if the input impedance of the radio matches the mic. Other mics can sound just as good or better, IF they are a good fit for the mic input port on the transmitter.

I wouldn't waste a lot of money. I'd just get a D104 and start there until I decided if I liked AM enough to play like a pretend radio announcer in a big studio. :-)

73 Tom

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KE3WD
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Posts: 5689




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« Reply #8 on: November 20, 2007, 07:58:13 AM »

You are planning on building an AM transmitter from the ground up and are fixated on the microphone?

That will be the least of your possible problems and possibly the easiest to deal with, considering that the mic attaches with a plug...


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W5RKL
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Posts: 894




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« Reply #9 on: November 20, 2007, 08:29:58 AM »

A D-104 or some other crystal or dynamic microphone
should work fine. I do not recommend using a
microphone with a built-in amplifier.

A plate modulated AM transmitter speech amplifier is
usually a dual triode such as a 12AU7 in a Class A
amplifier configuration. Other dual triodes can be
use as well. However, a Class A amplifier cannot
drive a modulator tube directly without creating
distortion.  A power amplifier is required to drive
modulator tube(s). Class A amplifiers are voltage
amplifiers not power amplifiers. Therefore a power
amplifier such as a 6AQ5, 12BY7, or some other
power tube will be needed between the speech amplifier
and the modulator.

Whether you use parallel or push pull modulator is up
to you. Push Pull would be my choice. Modulating a
pentode such as a 6146 will also require the screens
to be modulated along with the plates. This is quite
easily done by supplying the final tube's screen
voltage from the tubes plate voltage through a
dropping resistor.

The power supply should be designed to provide all
the necessary voltage and current requirements without
over loading the supply.

The PI network coil requires a thicker wire than
used in a sideband transmitter. An AM transmitter
is 100% duty cycle which means RF current is flowing
through the PI network 100% of the time while
transmitting. Therefore, the wire must be thicker
in order to withstand the high constant current flow
through the coil. A sideband PI network will be
destroyed rather quickly in a 100 watt plus
AM transmitter. Sideband PI networks are designed
for 50% duty cycle, not 100% duty cycle.

If you change your mind and decide to use controlled
carrier screen modulation, the rules change and so
does the circuitry. Many commercial made transmitter's
have successful designs using this method of AM
modulation such as Drake, for example.

Also recommend including a final amplifier clamp
tube. The clamp tube controls the final amplifier's
plate current in the event the RF drive to the
final amplifier is lost. A simple 6AQ5 clamp tube
can be used. The final amplifier will most likely
be Class C and a clamp tube works very well here.

I recommend reading some of the older ARRL handbooks,
mid 60's and earlier, or W6SAI's Radio Handbook for
further details on plate modulation, speech amplifier,
and modulator design. The W6SAI's Radio Handbook is no
longer in production but I believe it is available for
download on the Internet.

73's
Mike
W5RKL


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W8JJI
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Posts: 291




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« Reply #10 on: November 20, 2007, 12:14:48 PM »

Best mic for AM ?

 I say use any mic that you like but put a 31 band mono  EQ after it. This keeps it simple and is vastly adjustable.
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WA6HDZ
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Posts: 52




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« Reply #11 on: November 20, 2007, 02:48:49 PM »

If you're copying a schematic from back in the day, remember that most home station transmitters had the mic feed a tube grid directly. This necessitated a high (50k-500k) impedance microphone (this question was on my General test!) Common ones used then were crystal, ceramic or dynamic (with a built-in transformer) types. Used Astatic and Shure mics get pricey on "the bay" but lots of others sound just fine and are cheaper.

You'll need to either use an input transformer or redesign the input stage for a lower impedance to properly use a more modern style low (500 ohm-1k) impedance mic. Radio Shack's decent "Optimus" dynamics are often on sale and some come with the transformer built-in and an impedance switch. To use an electret mic you also need a DC bias.

If you're still really hung up on the vintage look, find a CB'er with a D104-clone he can't wire and offer him five bucks! (be sure to disable the roger beep or you'll become a forum topic)  
       
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W5ONV
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Posts: 0




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« Reply #12 on: November 20, 2007, 03:06:06 PM »

 I use the Heil HC-5 mic cartrige and get good results on AM .People say that it sounds real good.
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W5RKL
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Posts: 894




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« Reply #13 on: November 20, 2007, 05:07:52 PM »

The "bias" voltage is actually the operating voltage
that powers an FET amplifier built into the microphone
itself.

73's
Mike
W5RKL
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KC9KEP
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Posts: 208


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« Reply #14 on: November 21, 2007, 07:18:04 AM »

Thanks everyone for all of your ideas!

Here is a link to the schematic of the transmitter
that I plan to build;

http://www.bignick.net/temp/6146_xmitter.jpg

Also, here are some of my recent homebrew projects;

http://www.bignick.net/Morgan_Radio/Radio.htm
http://www.bignick.net/TubeRadio.htm

--KC9KEP
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