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D104 MPF102 follower static sensitivity




 
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Author Topic: D104 MPF102 follower static sensitivity  (Read 34204 times)
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Steve - WB3HUZ
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« Reply #25 on: January 03, 2009, 09:52:19 PM »

Here ya go. Preamp and a compressor - all-in-one.

http://k7dyy.com/products.htm
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kb3ouk
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« Reply #26 on: January 04, 2009, 10:59:01 AM »

I would seem to think that maybe you could put together another mic plug and tie all the pins, or just the problem ones, then connect it to ground, and stick that in there before you plug the mic in to short the pins to ground, so that it won't discharge through the mic. However that may not be a good idea, depending on how the  mic jacks on the two rigs are wired, you may be shorting some voltage to the ground.
Shelby KB3OUK
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« Reply #27 on: January 04, 2009, 11:01:25 AM »

Howdy lollypop microphiles,

I used to work for ADI and those designers put a lot of work into protecting the chips from ESD. In fact finding clever circuits that protect Hi-Z inputs without destroying frequency response was guarded information. So as radical as it may sound, how about designing up a simple Hi-Z Op-Amp mic preamplifier? The thing can even be fully differential on the element side with your choice of single or balanced Low-Z on the output.

In a D104, which needs little actual gain, I would use the classic 2 op-amp INAMP circuit for simplicity. Vo = (Vin+ - Vin-)( 1+ R1/R2) where R1 = R4 and R2 = R3
Ground the minus rail and set VREF to 1/2 of the 9V rail for single supply operation.

FET Cascode inputs allow input Z's of 10 to the 12 or 13th power with capacitances of less than 2 pf.

There are plenty of dual FET op-amps you can get on Digi-Key or EPay for less than 5 bucks which are granchildren of the TL082 which would run on a single 9V battery for a long time. http://parts.digikey.co.uk/1/1/94083-ic-dual-fet-input-op-amp-8-dip-opa2137p.html

73's Mike - on the road again!


* TwoOp-Amp.GIF (5.56 KB, 339x213 - viewed 680 times.)
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W3RSW
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Rick & "Roosevelt"


« Reply #28 on: January 04, 2009, 11:07:02 AM »

U2d, interesting op amp design.  "lollypop microphiles," - good one.
Back to strays: The 32V is polarity checked for proper ground. I've marked the old two wire AC line with a black mark on the plug.  Yeah, thought of stray 115 field somehow being present at places it shouldn't.  Come to think of it both mike cable wires are inside the shield, both audio and PTT lines.  On the 813 rig the PTT line carries about 6 to 8 volts, on the 32V2 it's 115.  So there's an obvious solution.

I've probably been too lazy to do anything other than change out MPF102's, they're so cheap and the fix lasts up to a year when I forget not to wildly change out mikes without turning off and upplugging everything.
 
Been an interesting discussion. Thanks all for the neato input. A lot to think about...

Steve, I thought about DYY's preamp but have heard some stories about it too. Maybe I don't want to be compressed  Grin
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kb3ouk
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« Reply #29 on: January 04, 2009, 11:31:11 AM »

I think someone already mentioned something similar to this already, but what if you used the spare contacts on the PTT switch to short the output of the amp  to ground, that way the amp is disconnected from the connector when it isn't keyed up. Then the only way the MPF102 would get zapped would be for you to be holding the PTT switch down as you plugged the mic in to either rig. Also, you may want to check to see if there is any stray voltages on that connector that could cause the MPF102 to get zapped.
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Steve - WB3HUZ
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« Reply #30 on: January 04, 2009, 11:36:03 AM »

Yea, the DYY thing is just another option. One could always bypass the compressor.

The circuit below has been used successfully by many. TNX to K1DEU.


http://www.amwindow.org/misc/gif/micamp.gif

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w9ac
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« Reply #31 on: January 13, 2009, 08:58:28 AM »

Joining in on this a bit late, but here goes...

To help with JFET static issues, I generally use a 5-megohm resistor ahead of the FET's gate.  Also, a bifilar wound CM choke at the gate will assist in suppressing transients, while leaving the differential mode audio waveform intact. 

I see far too audio many designs that use brute-force differential-mode filtering (e.g., individual inductors on separate cores).  Even with proper termination, the result almost always manifests with overshoot and ringing, owing to the passing of complex audio waveforms through inductance.  Absent winding leakages, the CM choke completely cancels differential-mode inductance, while raising common-mode inductance by a factor of 2L.  A well-designed CM choke with a very large L value is capable of preserving the integrity of complex non-sinusoidal audio waveforms.

On my D104, the crystal cartridge (+) and (-) leads are twisted from the mic head connection,  then pass through the 5M resistor and CM choke, both components located near the FET's gate.  No other components are used at the gate.

Although R and C values between the FET gate and ground may be used to help with static, I have always been concerned with degrading the input Z of the FET buffer circuit.  If you conduct a web search on "D104 FET" you'll get many FET buffer circuit designs that IMHO, are either deficient, or incorrectly designed. 

For example, on the source-follower designs, I've seen 100K resistors brought from the gate to ground.  Why?  For gate input protection?  For "gate leak" biasing as if the gate is identical to the grid of a vacuum tube were a "grid leak" may be required?  In any event, that's far too extreme and any value below the meg-ohm range will degrade low-end performance from the crystal cartridge.  A "gate leak" resistor is not required in the source-follower.  The FET as a source-follower is 100% self-biased.  The V drop across the Rs value produces and sets the Vgs bias. 

To set mid-point bias on the source-follower, here's a good approximation:  Rs = 1/Gmo; where Rs is the source resistor, and Gmo is the transconductance value from the device's data sheet.  Many FET datasheets will show large sample disparities between Gmo values.  Without manually setting the optimum point, I take the geometric mean between the lower and upper limits from the datasheet.  Moreover, self-bias stabilizes the Q point against changes in FET parameters (e.g., Gmo, Idss, etc.).

Whatever components are used at the gate, it's important to keep the loading of the crystal cartridge at a minimum in order to preserve low-frequency response.
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K3ZS
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« Reply #32 on: January 13, 2009, 10:55:33 AM »

I used that classic twin opamp design 25 years ago using LM308 opamps.    I used it as a substitute for the "instrumentation" balanced input amplifiers for thermocouple amplifiers and other balanced inputs.   The LM108 worked even better.   The common mode noise rejection was pretty good as long as you used 1% resistors.    I would add a 10Meg resistor from each input to ground to provide a current return.   The gain is adjusted by the ratio of the resistors.
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WA1GFZ
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« Reply #33 on: January 13, 2009, 11:08:23 AM »

Avoid bipolar op amps around RF. da suck
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K1DEU
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« Reply #34 on: January 15, 2009, 10:45:56 AM »

    Yes I've use clamps on the output to bullet prof the pre-amp since 1987.

  Here is a link to my shaky drawing

  http://hamelectronics.com/k1deu/pages/ham/audio/mikes/jfet_mike_preamp_equalizer.htm


   JFET Mike Preamp/Equalizer

Some typical Voltages are;

Battery 9 VDC Velcro-ded underneath

Drain  7.1 VDC

Source 1.8 VDC

Gate= near Zero

The 1 Mfd. Output Cap is non-polar.  To RF proof the input I put a miniature 470 micro henry choke in series between the gate input and the 10-20 megohm resistor along with a 18 pf cap from gate  to ground at JFET. All short leads please !  You may build the circuit without pots. Full bass boost would be up in the drain near +9 VDC the .1 across a fixed 10K Resistor, which is good feeding many transmitters. In most cases I would use a pot for treble/sibilance boost control running more boost when my signal was weak at the other end, less when strong. 
   The bass boost magic is from a triode Vacccccum Tube ckt in the 1938 Radiotron Designers Handbook ! Aparently  a triode is a triode sometimes.
Sorry I'm so late to reply.  Regards  John
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k4kyv
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Don
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« Reply #35 on: January 15, 2009, 02:21:53 PM »

    Yes I've use clamps on the output to bullet prof the pre-amp since 1987.

  Here is a link to my shaky drawing

  http://hamelectronics.com/k1deu/pages/ham/audio/mikes/jfet_mike_preamp_equalizer.htm

Interesting circuit.  I have used the  same basic idea for treble boost for many years, in  cascaded 12AX7 stages.  The low value of cathode (or source) bypass capacitor makes the cathode (or source) resistor appear bypassed at higher audio frequencies, while at lower frequencies it appears to be unbypassed, resulting in degenerative feedbark at the lower frequencies.

It had never occurred to me to do the same thing with the plate or drain resistor to boost the bass.

Does the bass boost shift the phase at low frequencies?  I have an Altec tube-type mixer board, which is equipped with a set of consumer-audio-like bass and treble controls.  The bass boost works well, but it reverses the phase of the lower frequencies.  I could phase my audio for upward positive peaks, but with the bass boosted, some of the higher amplitude peaks would go in the negative direction while others went in the positive.  To  maintain my original voice asymmetry, I had to run the controls at flat response.

Actually, that circuit is not a  treble boost.  It is a bass attenuator.  And the bass boost is actually a treble attenuator, since they depend on the degenerative feedback of an unbypassed cathode/source resistor, and the gain reduction of a lowered plate/drain resistance.

My treble boost is not variable.  I use the fixed value that gives me the desired pre-emphasis curve.  Two 12AX7 stages gives about 9 dB of boost (or attenuation, if you prefer).
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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Rick & "Roosevelt"


« Reply #36 on: January 16, 2009, 11:48:31 AM »

Ok,
In the K1DEU/W1ECO circuit I see back to back 5v zeners. Why are they in series and not paralled from signal to ground? They are already on the AC side of the 1uf capacitor.  How can they act at 0.7 volt threshold (+/-) when they counteract each other?  That might have been my problem with the hv silicon diodes.
Somedays the fuzzy logic portion of my head just can't seem to compute stuff so I'm perplexed. 
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RICK  *W3RSW*
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« Reply #37 on: January 16, 2009, 12:33:41 PM »

Ok,
In the K1DEU/W1ECO circuit I see back to back 5v zeners. Why are they in series and not paralled from signal to ground? They are already on the AC side of the 1uf capacitor.  How can they act at 0.7 volt threshold (+/-) when they counteract each other?  That might have been my problem with the hv silicon diodes.

The back to back zeners act to clip both the positive going and negative going peaks to about the zener rating.

Rich

W7SOE
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W3RSW
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Rick & "Roosevelt"


« Reply #38 on: January 16, 2009, 03:42:50 PM »

Yes your saying what it does but how does it work?

Why can't I get the same effect with the diodes paralleling each other but flipped in polarity?

so your saying that the 'top' diode will pass half cycles less than 5 volts to the bottom diode (but block those same phased half cycles over 5 vac).  But then doesn't the bottom diode and pass the other half cycle (inverse) to the bottom diode which will then block ..   uh, guess I better sketch out my thinking.

Also if I inadvertantly put 115 vac across the output from the T/R line of, say, a 32V2 then I'm going to pop the 5 volt zeners unless there is a dropping resistor, say 20k or so between the jct. of the top of the zeners and the output.  20k ought to be fairly impervious to audio at the impedances involved.

....I must be missing something  Grin
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RICK  *W3RSW*
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« Reply #39 on: January 16, 2009, 04:08:14 PM »

The top diode will pass half cycles MORE than 5V to the bottom diode, you then have to add the forward diode drop of the bottom diode.  That way the top diode clips half cycles more than 5V (ish).

On the negative cycle the bottom diode conducts in reverse when the voltage is below -5V (ish) plus the (now forward biased) voltage drop of the top diode.

Rich
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Rick & "Roosevelt"


« Reply #40 on: January 16, 2009, 04:38:45 PM »

ok I got it.
well you know..... brain cells are the second thing to go.  Cool
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RICK  *W3RSW*
K1DEU
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« Reply #41 on: January 19, 2009, 10:39:34 PM »

Don; The Bass boost circuit changing the plate load resistor alters the GM (Transconductance) around 77+- (broad(low q)) Cycles per second. Yes, many more hams in the mid 30's studied and knew vaccccccum tube theory.

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

Regards  John, K1DEU


Please click on http://www.tvfools.com  , once its a bookmark you will use again in our future even if we have cable or satellite TV. Do not be mislead by the url its serious terrain path software !

http://amfone.net/Amforum/index.php?topic=18294.0
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