D-104 / FET Circuit

[WA2SQQ]

I’m using a FET circuit to match my D104 to my Flex radio. I’m using a 10 meg input resistor - works fine. I recently acquired a few Turner desk mics including the 254C and 454X. Can someone suggest what I should change the input resistor to?

Also, I was given one of the Silver Eagle amplified D-104’s. Were they still using the crystal mic elements used in the late 50’s - early 60’s D-104’s?

[KD6VXI]

Dunno on the turners.

The Silver Eagle is an xral element until the mid to late 70s and they went ceramic, is what I was told.

The silver eagle has so much gain that just putting a 4.7m in series with the mic element lead will still allow so much gain it will overdrive almost anything but the element will be loaded properly for Wie TX.

--Shane
WP2ASS / ex KD6VXI

[WD8BIL]

The Turners were Xtal mics. The 454 replacement cartridges were ceramic beginning in the mid '70s. Either way, the pre-amp setup for the D104 should work with the Turners.

[WA2SQQ]

OK so I opened the D-104 and found the schematic. The preamp's input shows a 10Meg resistor, so I think it is safe to assume that the element is crystal - correct?

So I connected the mic to my Flex 6500 and each time I key up the audio seems to mute, and quickly ramp up to a normal level. I'm looking at the schematic and I see that both power and the preamp out are connected through the PTT contacts. That might be causing the "surge" causing the Flex to react. Thinking about hard wiring power and audio. What do you think?

[ki4nr]

I'm pretty sure that input bias resistor is One meg ohm. D-104's do make a small surge / pulse on key up because the amps emitter ground is being connected thru the leaf switch.  

Dirty switching contacts can make it way worst ( crystal element ground / emitter ground, audio output are all switched) possibly the 5uf coupling cap could be leaky on top of all that.

Your Flex digital limiting circuitry, most likely the compressor is being triggered by the surge and the AGC goes full compression ( Lowers the audio ) and then falls back to full gain. I've seen the same on my external analog compressor with the D-104 spiking the AGC on key up.

[WA2SQQ]

I checked the preamp, and the resistor is 10 M. I'm going to try and bypass the contacts so that its permanently connected and powered on. It does sound very good once you get past the slight ramped up delay

[ki4nr]

It will sound impressively better if you junk the stock bipolar preamp and built a MPF-102 FET buffer/ amp for it and set the input load at 10 megs. Many good circuits on the net. The excellent bass response and over all sound/clarity with the FET amp is vastly superior and a slight polish with an EQ makes it even better.

[n8fvj]

3 meg is ok on the Turner mics.

[MikeKE0ZUinkcmo]

...It will sound impressively better if you junk the stock bipolar preamp...

Not so, both amps have basically a flat response, with little distortion.   The FET amplifier was just considerably cheaper to make.

As noted by the OP the input resistor is in fact a ten meg device.   

The first stage is simply an emitter follower to convert the 10Meg input to a much lower "Z" to drive the output stage device.