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Author Topic: D-104 amplified mic freq. response?  (Read 8175 times)
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W0BTU
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« on: August 15, 2014, 03:17:36 PM »

One of my D-104 microphones has the Astatic crystal element with the stock bipolar preamp. Does anyone know what the frequency response curve is?

I'd also like to know what that preamp's input impedance is.

I found nothing specific to the above on Google.
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73 Mike 
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Patrick J. / KD5OEI
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« Reply #1 on: August 16, 2014, 12:22:12 AM »

I have one of those as well, bipolar, I think, but no method of measuring the response and no generator that would make a flat acoustic sound or a calibrated one..

I think the schematic was inside the mike base, or drawn somewhere. Wouldn't the Z be whatever the resistance there is?
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W0BTU
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« Reply #2 on: August 16, 2014, 01:08:09 AM »

I found this schematic of the preamp:

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« Reply #3 on: August 17, 2014, 08:05:56 PM »

the un-labled resistor from collector to base.. I guess measuring it is one way to get close. I think the schematic in mine looks like that too.

A way to reasonably find the Z should be the same as with any 'two terminal-one side grounded' thing. - send a signal in through a 500K rheostat, adjust it so the signal at the generator is twice that at the mike preamp base, the rheostat value should be set to the same resistance as the input impedance. You can do it at a few frequencies in the voice band to be sure, and the preamp should be on and should not be driven to clipping -just so things are normal.

Does the attachment make sense? I'm not an expert on that.


* mic-amp-z-meas.png (102.26 KB, 822x359 - viewed 435 times.)
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Pete, WA2CWA
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« Reply #4 on: August 17, 2014, 09:07:54 PM »

You can review the specs:



circuit:



Google is your friend
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wa3dsp
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« Reply #5 on: August 18, 2014, 02:31:05 AM »

Mike,

 Not sure what your application is for the D104 but the idea is to NOT load the element and a FET is a much better choice then a bipolar transistor. I would scrap the stock circuit and use a FET with a 10 meg "gate leak" resistor. I use the attached circuit but there are many variations out there. This circuit gives excellent results with modern rig 600 ohm input microphone circuits but would also work well into high impedance circuits. The element has plenty of voltage output when lightly loaded and the single FET gives ample output. It could be powered in several ways, from a battery if you have extra contacts in the mic, over the microphone lead - modern rigs do this, or on a separate line to the mic from an external source.

If you are using a vacuum tube mic input circuit usually the grid leak resistor could be increased to 10 megs and the element connected directly to the grid or through RFI bypassing to the grid eliminating the need for a FET. The FET circuit would however allow you to change the input to lower impedance keeping the high impedance circuitry within the microphone reducing stray pickup.

Loading the D104 element down will shoot the bottom out of the frequency response curve.

* D104 Fet Source Follower.pdf (18.9 KB - downloaded 212 times.)
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AB2EZ
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« Reply #6 on: August 18, 2014, 10:54:07 AM »

Mike

From the schematic posted by Pete, and the specifications for a 2N2712 that are available on the web...

The input impedance of the 2 transistor amplifier should be somewhere between 475k ohms and 780k ohms, depending upon the actual current gains (hfe) of the two transistors.

The output impedance will be 5000 ohms, or less, depending upon the setting of the audio gain pot.

You can model a typical D-104 microphone head as an ideal audio voltage source in series with a 1000pF capacitor.

http://en.wikipedia.org/wiki/Piezoelectric_sensor

Therefore, with (for example) a resistive load of 500k ohms across the D-104 microphone head, the input to the amplifier will start to roll off, at low frequencies, when the impedance of the capacitor increases toward 500k ohms.

In this example, with a 500k ohm load across the output of the D-104 microphone head, the 3 dB roll off point would be at 318Hz.

As many have pointed out, if the load across the microphone head is increased to 10 megohms, then the 3dB rolloff point moves down to 15.9Hz.

Some of later D-104 microphone heads had a larger value of equivalent series capacitance.

Stu
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WD5JKO
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« Reply #7 on: August 18, 2014, 11:21:19 AM »


With one of my D-104 stands, I added a switch to insert a parallel resistance across the element. I had a 3.3 meg in the rig, and in the base I added a 1 meg, and 470K with a switch to pick either resistor to be in parallel with the rigs 3.3 meg. As mentioned, all this does is move the -3db point for lows around. It does not change the mid band level at all.

Under marginal conditions, chopping out some of the lows can make a 100 watt rig get out like a 375 watt rig that has full response. Then when conditions improve, you can insert the lows again by the flip of a switch.

Jim
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AB2EZ
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« Reply #8 on: August 19, 2014, 10:43:54 AM »

Mike

A number of people responded to your questions.

We're any of the responses of any help to you?

Stu
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W9ZSL
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« Reply #9 on: August 19, 2014, 02:16:00 PM »

The microphone responds with rising characteristics from 500-4,000 Hz.  That's the limit of the crystal element regardless of the frequency response of the amplifier which may be broader banded.  Makes no difference.  The restriction is in the crystal element itself.  Here's an article about converting the mic to work with modern transceivers: QST, August 1999, page 34.
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Steve - K4HX
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« Reply #10 on: August 19, 2014, 08:18:19 PM »

Makes a big difference in the low frequency response.
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W9ZSL
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« Reply #11 on: August 19, 2014, 08:34:03 PM »

Huge.  Classic "telephone" equalization.  Best-suited for speech alone unless you were dumb enough to put it in front of a guitar amp.
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Steve - K4HX
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« Reply #12 on: August 19, 2014, 11:37:04 PM »

It's been done.  Grin

With a load Z of 5 Meg or more , the D-104 has good response to at least 100 Hz. That is better than the traditional telephone quality.
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wa3dsp
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« Reply #13 on: August 20, 2014, 12:13:20 AM »

The D104 is an extremely good microphone. With a good element and loading it extremely lightly it gives very good response. When the mic was originally designed inputs on ham rigs were high impedance. The bad rap came when solid state rigs appeared and everyone thought they could just hook it up. Many adaptions used a 33-47K resistor in series with the element to the mic input. This caused lighter loading but not nearly enough and also dropped the overall voltage. Try running the element into a FET or vacuum tube with 10 Megs of "leak" resistance. I think you will see just how wide the response is. 
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W0BTU
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« Reply #14 on: August 20, 2014, 04:47:46 AM »

Mike

A number of people responded to your questions.

We're any of the responses of any help to you?

Stu

Absolutely they were! I was just now able to look at the many helpful replies.

Many thanks to all who replied. I could not find the answer on Google. This info is exactly what I was looking for, and much more.  Smiley

Thanks again!
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73 Mike 
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VE3AJM
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« Reply #15 on: August 20, 2014, 08:47:31 AM »

Finding a good D104 crystal mike element is important of course. This is becoming becoming more difficult to do as time goes on, as the elements are no longer made by Astatic.

I've found that as the elements age, their output level and frequency response can drop off. i.e. overall output drops and loss/reduction of its low end response. The replacement crystal elements out there that are commonly used i.e. Kobitone etc. don't seem to perform as well.

Al VE3AJM
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