D104/desktop preamp mods pt 1

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31
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D104/desktop preamp mods pt 1

Post by 31 »

In the process of doing projects on my bucket list. The desk top D104 were built with or without preamps in the base. I been wanting to try 3 or 4 different circuits floating around on each. The preamp in the model I have is a 2 stage NPN preamp. The difference between the desktop and the newer handheld mic preamps are the preamps being used. The D104 elements are the same across all models, they like to see a high impedance load across their output. The difference between a D104-m6 and a 575-m6 is the load the element sees. The D-104 has 270k fixes resistance and the 576 has 2.1M of variable restance. Change the gate resistance and voila you got a 575 or D104 , whichever way you go. The more the resistance/impedance the bassier or more you see the element natural tone. Without going in to far thr JFets in the handhelds produce a different type of sound then Solid state NPN/PNP type transistors. You can think of a FET as a "Vacuum tube" in a small package. Also without getting to deep, each type(FET or Bipolar) produces harmonics that our ear picks up. If you've ever listened to a vacuum tube guitar amplifier , you could immediately hear the difference between that of a transistorized amplifier. The difference you hear is the distortion each produces in the form of harmonics. Tube types produce even order and bipolar types produce odd order harmonics. The even order produces round waveforms that also sound warmer and pleasing to the ear. Where as odd order harmonics produce a square waveform that will produce a harsher type of distortion to the ears. Which ever way you ears hear it , they're both distortion. Sorry for the rambling and if not technically accurate , I apologize, it's gathered knowledge after years of reading online. Anyhow, didn't see many pictures of the preamp I was working on so I decided to post pictures here for future reference. Standby for further progress.
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http://www.tungsol.com/tungsol/html/faqs14.html
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Re: D104/desktop preamp mods pt 1

Post by 31 »

D104 desktop 2x2n3904
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575m6
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D104m6 2n5484
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http://www.qsl.net/hcara/D-104.htm
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Re: D104/desktop preamp mods pt 1

Post by Goldfinger »

Looking forward to your updates.
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Re: D104/desktop preamp mods pt 1

Post by 31 »

RB Goldfinger, update #1. The circuit I followed was based on N9WB's Fet Preamp.
http://www.qsl.net/hcara/D-104.htm
In his arrangement, the JFET is configured in a common drain setup. Everything has advantages and disadvantages. Electronic theory says that this arrangement is setup for current gain. The output from the drain, is a faithful reproduction of the input however the gain in signal amplitude is considerably lower then in a common source. An analogy that some maybe familiar with is to think about a Texas Star being class AB, its sounds good and clean, however the watt gain isn't as great as a Davemade class C amplifier, where the signal is 180 degrees out of phase (dirty) but has a significant amplitude increase. Similar thing here, although the ear might not necessarily hear the difference in reproduction, the level of audio increase is hard to ignore.
With that being said, a side be side comparison showed me the difference in gain using a modified 575 and the D104 in test. I have two different game plans to consider here. The first is , reconfigure the setup as is and adding the original second stage bipolar in for another stage of amplification. Or copy the 575/D104M6 JFET preamp and go with that. Either way my goal here in part one is to reuse the existing circuit board as much as possible. Part two will be using a Tug8/9 base that I will be perf boarding a dual FET pre on.
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Re: D104/desktop preamp mods pt 1

Post by 31 »

In part 2 I will be using this circuit by N1HFX ,with the exception of removing the input cap and increasing the gate resistor to 10-15 Meg. The D104 element likes impedance over 10 Meg. May also play with the coupling cap values. What they say, the more the merrier or how about a play on that 80's song "we like the mic's the mic's that go BOOM"
http://www.rason.org/Projects/jfetamp/jfetamp.htm
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Re: D104/desktop preamp mods pt 1

Post by grasshopper »

Very good research 31.
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