Astatic D104 question

Discussion in 'Microphones, Speakers & Audio Processing' started by KK7BMS, Dec 1, 2021.

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  1. KK7BMS

    KK7BMS Ham Member QRZ Page

    I have an Astatic D104 with the preamp base that I would like to use with a HW101 Boatanchor. I understand the preamp version is ~5000 ohm impedance, and the HW101 would like a high impedance microphone. My question is if 5000 ohms is high enough, or if I need to modify the mike back to the original unamplified design? I am a few days from being able to align and test the rig of my dreams 45 years ago.....
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You'll have no problems running the HW-101 with the D104 with the transistorized base. I've done it many times with my SB-102 which is very similar in terms of mic input impedance requirements.

    Basically the crystal element Astatic used in the D104 should really be loaded with extremely high impedances in the 100k ohm to mega ohm range. When you drive that element directly into a rig with lower mic input impedance (and even 50 k is getting low for that crystal element) the mic loses bass response and starts to get pretty tinny with excessive highs. That can actually be a good thing for working in noisy band conditions or weak signal work like trying to break a DX pileup but for day to day use many don't find it that pleasant.

    The 'preamp' in the D104's main role is to provide impedance buffering for that crystal mic element so lower impedance rigs like we started to see with solid state rigs in the late '60s and '70s don't overly load that crystal mic element such that it starts losing bass response. But the mic with the active impedance buffer has no trouble driving low impedance or higher impedance rigs. You don't strictly need the preamp to maintain decent audio quality and sufficient drive when running into most older tube rigs but it doesn't really hurt to do so and the mic stays compatible with newer solid state rigs.
    KB0MNM, KV4PD, W9WQA and 1 other person like this.
  3. KE0ZU

    KE0ZU Ham Member QRZ Page

    Here are plots of the original D104 element response with different load resistances. I'd recommend you use the amplified output, so you don't have to be concerned about the input resistance of what ever transmitter you are using, unless its designed strictly for a low Z dynamic, or Carbon microphone, or line input. All of which are on the order of 600Ω or less.

    Last edited: Dec 3, 2021
  4. W9WQA

    W9WQA Ham Member QRZ Page

    even if the out z were 10 ohms it would work.!
    there is always confusion about going up...
    K7TRF likes this.
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    The input resistance to the HW101 6EA8 pentode stage is 1 Megaohm. The output resistance of an amplified D-104 is ~ 10k ohms. So you're ok since the input load resistance is high enough so as to not excessively load the D-104's amplifier.

    If one wants to use an unamplified D-104 mic exclusively to drive a HW-10X or and SB-10X, I recommend that R3 be changed to 3.3M and R4 be changed to 680 ohms.

  6. W9MT

    W9MT Ham Member QRZ Page

    Jumper around the pre-amp in the D-104 base and simply drive the input of the 101 mike input directly with the crystal element. I've done that with mine to drive Heath's, Swan's, and Drakes. As long as you have a shielded mike cord this is an easy mod to the mike that's easily reversible. Plus, you don't need to replace batteries from time to time in the mike base.

    I even used my D-104 and Turner 254X crystal mikes to drive the low impedance input of my Ten Tec Jupiter via a JFET (MPF-102) follower to transform back to a low impedance that the Jupiter liked. Ten Tec was nice enough to supply 8VDC on one of the four pins of its mike jack on the Jupiter. (There are representative circuits for this on-line if you ever need to do this. But your problem is having a high enough impedance out of the mike.)

    There are many ways to "skin this cat"....(and please forgive my archaic cliche, feline afficianados).
    KB0MNM and KY4GD like this.
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    It can work just fine.

    Turn the Gain for the preamp down low, To prevent over driving.

    A low will drive a high input just fine, With a lot of power left over. Just reduce Mic gain and all is good.

    Bypassing the amp can work, But your average power out will be lower.

    For Max Meter Swing, Keep the preamp. :D
    Last edited: Dec 5, 2021
  8. W9AC

    W9AC Subscriber QRZ Page

    I agree with others who suggested a simple bypass of Astatic's 2-stage preamp -- provided you keep the mic cable length reasonably short. You can bypass the preamp in such a way to easily return it to the original configuration.

    AC0OB is correct concerning the HW-101's input resistance. A 22K resistor is on series with the mic lead but a 1 meg "grid-leak" resistor is what largely sets the total mic input impedance.

    Concerning the mic cable length: any time a highly reactive source (i.e., a crystal mic element) is in series with paralleled cable capacitance, a capacitive voltage divider is inadvertently created which can result in lower audio level reaching the mic stage. The cable also becomes much more prone to noise and handling microphonics when it's moved while talking. Even worse, when a Hi-Z source is mostly resistive, cable capacitance results in the creation of a low-pass filter. In either case, the problem is completely overcome with W9MT's JFET suggestion. When practical, it's always best to convert a Hi-Z source to Low-Z to offset the effects of cable capacitance. Again, for short mic cable lengths, it's a non-issue.

    Paul, W9AC
    W9GB likes this.
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I wired mine up for balanced output to feed a push-pull first preamplifier stage. The D-104 crystal element is inherently non-polarised; two solder lugs on the back of the plastic case. Not one designated "hot" or "ground". The two terminal connections come out to the close-spaced prongs of the connector; the third prong is shield/ground. All you need is a two-conductor plus shield audio cable (plus any PTT control lines). Feed each of the two conductors to the grid/gate of the tube or FET. I use a pair of high-mu triode tubes (6F5, equivalent to one section of a 12AX7). 10 meg grid resistor at each tube, for total of 20 megohm load on the mic. The high resistance load allows full low-frequency response capability of the mic, which extends well below 100 Hz.

    Balanced-to-pushpull configuration greatly reduces a.c. hum and r.f. pick-up. I didn't invent this circuit; it was suggested by Astatic in early production D-104 data sheets for long mic cords (over 20 ft). My mic cord is only 6 ft long, but this totally eliminated tendency for the pre-amp to pick up r.f., and the balanced push-pull circuit greatly reduced a.c. hum in the audio.

    Mine is on a boom stand. I despise desk-stand mics; they are always in the way, and it's not a matter of if but when the mic will inevitably get knocked off the table to crash onto the floor. The boom holds the mic at an elevated position approximately nose to eye level so that the operator tilts his/her head slightly back to talk into the mic, rather than scrunching down to talk into a desk mic. This allows the chest and larynx to expand, instead of being compressed and squeezed by the facing-down posture, greatly improving transmitted voice quality.

    A small hand-held PTT switch is attached to a spiral cord, a repurposed headphone extension cord. A spiral cord from a discarded hand mic would work as well.
    K5UJ likes this.
  10. KB0MNM

    KB0MNM Ham Member QRZ Page

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