D-104 Amplified MODS

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WBear2GCR:
Ok, so the stock D-104 amplfiied base is probably best bulldozed to use Tim's term and replaced with a simple FET follower... but I had one, and it was functional, and I really did not want to rip it out, and I was curious if it could be made to sound halfway decent. Keep in mind that the follower is unity gain and the FET follower, and the stock amp has beaucoup gain, like >20dB no problem!

The first image is the stock circuit.
It sounds like hell.
The transistors are biased the crappy way, but that probably keeps the batt draw low.

In a QSO with Stu, AB2EZ he mentioned that he had dropped a cap to ground in parallel with his mic at the input, and that this serves to tailor the response by effectively boosting the bass... so the 470pf in the stock schematic might be doing some of that, although it is in series with a 150k looking to ground...

The modified schematic shown has the response shown in the plot below. The yellow line is ouput B and the blue line is output C. The main diff is that C is lower in level, because of the resistor divider and has the greater roll-off at 1mHz., which might be good in some situations. The cap at C does that extra roll-off out of the passband.

The overall response is down a little at 10kHz. which is generally an advantage, plus the mic's response is NOT modelled here, meaning that there is a presence peak anyhow that goes out to ~5khz. The LF response is where the greatest diff is between this and the stock amplifier. As you can see this is flat to 100Hz. and rolls gently below that.

The key here is still the increased value of the first base resistor and the increased value of the input cap... I adjusted the values of the other two coupling caps, C2 and C3 to achieve the best looking response below 100Hz. C3 in the original is quite large, even though the input end of the circuit doesn't pass any bass! If you scale up C2 and C3 just a little bit, like double the value, the LF rolloff point drops a bunch, and by 1ufd you're looking at flat response down to like 10Hz. A bit much, imho.

C5 has been added to give a HF rolloff just starting at ~10khz or so... then that second cap on the "C" output (not in the original) gets rid of ultrasonic, RF even more.  It doesn't take much reduction in C5 to make the rolloff point 20kHz and not 10kHz.

It looks like C6 should be increased. Maybe more than the listed value. That input cap is in series with the capacitance of the crystal element, so it wants to reduce the capacitance, making the lows roll off more - the bigger that cap the less that effect happens. I did not find a good way to model the capacitance of the source in the SPICE simulation program... so it ain't there. The value I show is 10x the stock value.

C4 is there for possible RF control - it might have an effect in the audio band with the real capacitive crystal element, but I couldn't get the model to reflect that. Without that it has nil effect on the audio.

The pot in the stock unit is shown as fixed resistors. The gain is high in the stock unit, so you'd have to set the levels low anyhow. The output at "C" seems more reasonable in level, being only 8dB hotter than the input level.

I didn't get around to increasing the emitter resistor on the second transistor, but that seems like it might be a good idea - increases the linearity and drops the gain.

Bottom line is that I think you can use the stock amp with these mods and get decent audio.

_-_-WBear2GCR

AB2EZ:
Bear

Hi!

A few suggestions:

1. If you want to model the series capacitance of the microphone element, just change the value of C6. The series capacitance of the microphone element is around 5000 pF on my D104 element. I think that some of the older D104's may have a series capacitance that is significantly lower (around 1000 pF). Since the existing 0.1uF capacitor, C6, in your schematic will be in series with the series capacitance of the microphone, just change the value of C6 to somewhere between 1000 pF and 5000 pf for the purpose of the simulation (or add another capacitor in series with C6).

2. In the original D104 circuit, the current flowing through the 470 pF capacitor bypasses the gain of the first transistor. For frequencies below 1000 Hz, it behaves as if it were going directly to ground... because it's impedance is much larger than 150k ohms. The 150k ohm resistor will boost the frequency response above around 2000 Hz, because it sets a lower bound on the impedance that is effectively connected between the base and ground. I.e., the impedance of the capacitor equals 150k ohms at 2257 Hz.

3. I think you will find that once you change C6 to reflect the 1000pf - 5000pf capacitor that is in series with the voltage source in the equivalent circuit of the microphone element, you will find it desirable to add some capacitance to ground (~ .005 uF) at the input of the amplifier... if you want to get down to very low frequencies in the overall response of the microphone-amplifier combination

4. With 150k ohms in the emitter of the first transistor, and assuming a current gain of 50, the resistive part of the impedance looking into the base of the first transistor is ~ 7.5 megohms (i.e., 50 x 150 k ohms). From that perspective, the large biasing resistor (R2) makes sense. However, I think you will find that if/when you place some capacitance across the input (as described above) it will not be necessary to have such a high impedance first stage design. The design you have now has such a small base bias current, that I wonder if it has good linearity. Reducing both the value of the 150k ohm emitter resistor and the value of the biasing resistor by a factor of 10 (after adding capacitance from base to ground) might improve the linearity

Best regards
Stu

WBear2GCR:
Stu,

Re: 2) no voltage gain, it is a follower. Adding or removing the cap in the simulation did not materially change the response - I think it was for RF in the main, unless it works as you surmise - but that did not show up in the simulation so far (there may be a few more things to try in that regard).

Re: 3) changing the input cap or placing an additional series cap of 2000pf did not materially change the overall response in the simulation.

re: 4) seems surprisingly ok... in the simulation. In practice your idea may or may not be an improvement.

Thanks,

      _-_-bear

w3jn:
Beware the THWONK coming from the coupling cap charging up; the PTT circuit switches the preamp on an off as I recall.

This transient can be a mod xformer killer.  Better to leave the amp powered all the time.

As an aside, I found another transient in my FT1000MP that I was concerned was gonna kill my linear.  I noticed a full power spike on the scope when I keyed the mike; I traced it down to the PTT switch clicking when it was keyed, this sound spike was then sent thru the audio chain and appeared as a full-power spike no matter what the power setting.  Solution was to use a different mike and a footswitch  ;D

WBear2GCR:
Yeah it does! Bwwwwammm!

I think one could use a transistor to switch the power to the circuit without any trouble.
Probably that would reduce the transient... especially if the transient is being caused by switch "bounce" or straight arcing....

Leaving the power on all the time will reduce the batt life...  :-[

   _-_-bear

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