d104, tube amp?
This is my first post here, and I can see several posts dealing with how to get an a-static 104 to work with modern equipment, so I hope I don't annoy anyone with this question. I have a silver eagle mic I just scored from ebay (partly because of its looks) and had no idea about the history of this mic. Having read about it makes me glad I got it. Anyhow, mine has no amplifier circuit in the base. From what I can see, it seems to match the 'UG-8' stand setup as shown here: http://www.barovelli.com/projects/ra...dioastatic.htm
I've seen various simple circuits I could try to make to get this thing to work with modern equipment, but I've read that it really sounds best with the old tube stuff, like the radios it interfaced with. My end goal is that I want to be able to record stuff with this mic, into my computer's soundcard, or to just be able to use it with other line level stuff. I know that the old tube gear often requires an overhaul with the caps and I've done this a few times with old tube AM/FM radios so thats ok. What I want to know is, would I maybe be able to score a cheap tube ham/cb transceiver on ebay, and use that just as an amp, and somehow use outputs from that and go from there to line-level inputs? I honestly don't know anything about ham radios or cb radios, and don't know if any of them have outputs like this. Is this idea do-able or just a silly way of doing it? If it could work, any specific suggestions as far as which transceiver or even just tube amp would be best? Main reason I'm looking at this method instead of the FET circuit is sound quality, as this is for a 'project'. Anyone's suggestions appreciated.
The following information is widely available on the Internet web pages you have been reading -- the theory presented is standard Audio theory -- known to a wide audience of audiophiles and professional audio personnel -- who don't even use microphones for radio !
There are a large number of Audio only web sites (like QRZ) focused on your type of question.
The Astatic D-104 microphone uses a Crystal (e.g. Rochelle crystals) microphone element.
Crystal elements (as well as ceramic) have a High Impedance (Hi-Z) output
Older tube equipment accepted these Hi-Z (typically 50 Kohms) microphones
for their audio input (microphone jack).
As solid-state audio and radio equipment appeared after 1960 --
these designs accepted Lo-Z or Mid-Z (typically about 500 ohms).
YOUR SOUND CARD (Computer) INPUT is also for a Lo-Z to Mid-Z (~ 500 ohms) microphone.
The cheap computer microphones (less than $5 or for FREE) at GoodWill and surplus stores
are actually just as good or better that the D-104 !!
Since you said that you have an Astatic "G" stand .. you have no microphone pre-amp.
This microphone then has a H-Z output.
Your computer sound card needs Lo-Z (500 ohm input) --
so a small audio transformer (impedance matching) can handle that conversion --
available ($$) from Heil Sound, Mouser and other audio/electronics suppliers.
Last edited by W9GB; 09-05-2010 at 12:47 AM.
First, don't expect a great full range audio from this type of mic capsule.
Second, there is more needed than just impedance matching in order to drive the Line Input of the computer soundcard. You will also need a significant amount of preamplification in order to get the AC signal voltage output of that mic capsule up to the Line level.
Perhaps the easiest way to try this out would be to find an old tube driven TAPE RECORDER and wire the mic capsule output to a plug that fits the mic input of the recorder. Then use the Line Out or the Speaker Out of the recorder, connected to the right sized plug to fit in one channel of your soundcard's Line Input. You will need to find the kind of tube tape recorder that can monitor its own recorded signal in order to do that without going inside the recorder and modifying circuits.
Another simple way to obtain both the impedance match and the gain required would be to connect the mic to one of those little solid state or tube practice guitar amps and use the Line Out on that amp to the soundcard. The guitar input is high impedance. The earphone jack can be used to get your Line output, again, just adapt to the type of jack in one channel of the line input of your soundcard.
Again, don't expect great audio from a mic capsule designed for voice range communications, which are typically about 3KHz wide. Expect a rather special sound, which, for the right purposes, may be a good effect.
Trying to modify a CB or ham radio transceiver or transmitter just to drive a soundcard wouldn't be my first choice at all, actually, for the simple reason that the two devices mentioned would make it a lot simpler task. Guitar amp would be likely the most easiest and inexpensive thing to find these days, I've seen so many of those small practice amps in the local thrift shops at rather low prices. Guess the kid wasn't headed for stardom with that Christmas present after all...
However, WD, keep in mind that many soundcards have a "mic boost" software button, that if activated, will bring the gain up to an acceptable level for just about any microphone. YMMV, but before dismissing the possibility of using that mic, he should check out the software driver for that particular soundcard.
However, comments regarding the audio quality should not be dismissed. Many communications microphones do have an altered frequency response ( attenuated bass, boosted high midrange ) to make them more intelligible under less than ideal condtions ( noisy cab of truck, for instance )
Ham Radio, Amateur Astronomy, and Model Airplanes - what better way to spend some time!
No time is ever wasted that is spent LEARNING something !
The Mic Boost is done in software.
Which means that it does nothing at all about the fact that the high impedance capsule will be looking into a low impedance load, the worst of all situations for audio. Expect weird frequency cancellations and the like. In the audio world, just because you can hear it doesn't mean that you are hearing it correctly.
Originally Posted by KE3WD
Mac is totally dead on about this.
But, the D104 will sound great if it is impedance matched. I would not use any of the circuits on the link orignally posted.
I would do something like this:
According to Astatic, the recommended resistive load for the D-104 is 5 megohms. Using any less load resistance will degrade the low frequency response. A rochelle salt crystal microphone can be thought of as an ideal a.c. generator with about a 500 pf capacitor in series. Therefore, to maintain good bass response, the load resistance must be very high.
With tube type amplifiers, the load resistance on the high-Z microphone is achieved with the grid resistor (sometimes incorrectly called the grid leak resistor). One problem with using that much grid resistance is that some tubes may develop unstable plate current because an infinitesimal grid current may flow and be enough to slightly shift the operating bias. It may be necessary to try several tubes to find one that performs satisfactorily. The easiest way to determine the operating plate current is to measure the voltage drop across the cathode resistor and use Ohm's law. The RCA Receiving Tube Manual generally recommends that the grid resistor for most tubes in class A service, whether triodes or pentodes, be 0.5 megohms or less. This is far too low a resistance for the D-104 or other crystal microphone. I have successfully used as high as a 10 megohm grid resistor with good results and stable tube plate current.
I am presently using 20 megohms load on my D-104. Mine is wired for balanced output, using a two conductor shielded microphone cable. The shield is grounded to the microphone case, and each of the two conductors is wired to one of the terminals on the crystal element. The D-104 element is manufactured in a bakelite case with both terminals brought out to ungrounded solder lugs. The microphone pre-amp is a balanced push-pull stage, using a pair of high-mu triodes. I selected a pair of tubes from about a half-dozen, that displayed stable, balanced plate currents each with a 10 megohm grid resistor, and used matched grid resistors. The common grounding point of the two grid resistors establishes the mid-tap for the balanced output to feed the push-pull stage. In the push-pull circuit, the two resistors appear in series as a load across the crystal element. The two microphone cable conductors are wired directly to the grids of the tubes. Blocking capacitors are not necessary with a crystal microphone. With this configuration, the D-104 has excellent bass response.
In a previous configuration, I used a D-104 wired in the conventional manner, with a 10 megohm grid resistor at the input of the single-ended stage. It also has excellent low frequency response.
If the mic uses the desk stand, a FET pre-amp can be easily built in the base, using a circuit that allows an adequately high resistance load on the crystal element. The stock D-104 "power mic" pre-amp that comes with the microphone is a piece of crap; the mic can be made to sound much better by trashing it and building a homebrew one from scratch. There are numerous circuit descriptions for both tube type and FET pre-amps on the net; just do a Google search.
That mic should work fine with the old tube/valve radios, and that is what it was made for. But Computers?? That would take some doing. LINE VOLTAGE? Phantom Voltage? The microphones that need that is the High End microphones that are for broadcast and have a very wide response. Like below 50 HZ and up. If you want Professional Audio to record with, then go to a site that sells professional microphones like the condenser microphones and the Large Dynamic Microphones like the Electro Voice RE-20. That's the mic I'm using now on my Kenwood 950 SD. With those mics, you MUST have an Equalizer and maybe some item that will create Phantom Voltage, a Pre-Amp, a Compressor. Maybe too much stuff than you will want to monkey with.
This guy says he wants to use the D-104 mic to RECORD into the computer soundcard.
As I stated previously in this thread, he has some rather nice and inexpensive options that would allow him to do that easily.
The Art Tube microphone preamp would do this job. To get a rather high impedance input for the D-104 cartridge, he should put a 1/4" mono Tip/Ring plug on the end of the cable, wired only to the mic cartridge output wires, and plug the thing into the "Instrument" (not the MIC, which is low impedance) input on the preamp. Line out of preamp goes directly to either L or R Line input of the soundcard.
Another option would be a small solid-state Guitar amplifier that features high impedance input and a headphone or line (sometimes marked Record) output, either of which can be connected to the soundcard's line input directly and safely. The Guitar amplifier, however, may not be as fullrange or as flat in response as the Art Tube mic preamp.
While the Guitar input won't be 10 megohms, it will likely be around 1 megohm in all but the cheapest examples. Check the specs for Instrument input impedance before purchase. The aRt Tube Instrument Input is specified as being at 840K, not quite 1 meg, but certainly won't load down the audio from that capsule too much. You'd hear it pretty much as it was designed to be heard.
Art Tube preamp is available in single channel basic use design for not much money at all these days.
OR just replace the original mic element with one that matches the modern equipment. Cheap and easy. I like the looks of the D104 too and buying a new element was the simple solution. You can always change it back if you want to.
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