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AMfone's Online AM Handbook => Audio => Topic started by: Steve - K4HX on March 29, 2020, 11:38:52 AM

Title: D-104 Preamp by N1PS
Post by: Steve - K4HX on March 29, 2020, 11:38:52 AM
Thanks to Pete for sending this along. The FET-follower circuit presents a 10 MegOhm load to the D-104 for best low frequency response (see the response curves from Astatic below).

Title: Re: D-104 Preamp by N1PS
Post by: DMOD on May 04, 2020, 12:05:19 PM
Does anyone know if he used an NMOSFET or a junction FET and what was the part number?

Phil -AC0OB

Title: Re: D-104 Preamp by N1PS
Post by: ae7db on May 04, 2020, 05:18:04 PM
The schematic symbol shows a MOSFET, but in the notes he lists acceptable FETS and they are all JFETS.

I use a similar circuit with a J310 myself.


Title: Re: D-104 Preamp by N1PS
Post by: n1ps on May 05, 2020, 08:09:31 AM
Yes correct...a JFET. 

Here is a video too:


Title: Re: D-104 Preamp by N1PS
Post by: WBear2GCR on May 09, 2020, 10:57:46 AM
JFET needed for the high input Z.
Also for the self bias aspect.

Almost any JFET will do.

The IDSS (quiescent current) can be selected, IF ur driving something of
a low-ish Z at the mic input. More current being = to lower output Z.

Otherwise, lower IDSS = less battery draw, IF ur powering off a battery.

There are a number of neat circuits out there.
K1DEU (sk) has one up somewhere, maybe here, with high/low tone control capabilities
built in...


PS. the 47ufd cap at the B+ side is probably a bad idea, it will have to charge each time
and the draw off the jfet is very low... that will introduce a time constant in both turn on
and turn off. Skip that. The small cap is ok. The 180ohm resistor too is likely unnecessary
unless there is some turn on transient when keyed... which usually does not happen. No need
for current limiting, as the IDSS of the JFET and the 1kohm source resistor does that automatically.

The other thing is that the cap to ground at the gate, likely not going to do what is expected.
The xtal element looks like a capacitor, afaik - so that looks like a divider...  I'd consider another method
to make sure there is no RF getting into the JFET if there is any. A ferrite bead, or simply a 100-200ohm
gate resistor instead...

Title: Re: D-104 Preamp by N1PS
Post by: n1ps on May 10, 2020, 07:13:51 AM
Good comments Mr. B-E-A-R.  The 47mike cap is a little heavy (could be a 1-2mike with a battery), but who cares if it charges up (its not noticeable)?  I wanted maximum protection against transients with vacuum tube transmitters.  I have used this circuit since 2014 almost every day with no failures on multiple TXs.  I replace the bat once a year.  The Vcc input would work also for a non battery supply.  So besides decent audio, my goal was maximum reliability. 



Title: Re: D-104 Preamp by N1PS
Post by: WBear2GCR on May 11, 2020, 12:03:54 AM
I figured it might turn out to be in the several mS to come up or drop off, keeping the mic hot on the turn off and slow to
come on, and be audible... it also does use up some batt power. I'd throw in a small 3 legged voltage regulator if I was going to
run off external power, that and one of these series diodes to make sure reverse polarity does not happen...

But, if it works, then it is good! :D


Title: Re: D-104 Preamp by N1PS
Post by: ka8gef on May 11, 2020, 11:56:28 AM

Was the original 'stock' Astatic D104 transistor pre-amp designed for high or low impedance input rigs? Just wondering, for HI-Z input rigs, if I should replace it with this circuit, use original with a low to high Z xfm'r or...? Thanks.

Title: Re: D-104 Preamp by N1PS
Post by: n1ps on May 12, 2020, 09:59:16 AM
The FET circuit would be a much better arrangement than that Astatic transistor amp.  I have heard several of them and they sounded like "Uncle Albert".   :)

Bear - yes agree on the regulator for a non battery supply. 


Title: Re: D-104 Preamp by N1PS
Post by: ka8gef on May 13, 2020, 08:36:02 AM
Here is the circuit from K1DEU. It appears to be more of a impedance altering circuit than a pre-amp...


Title: Re: D-104 Preamp by N1PS
Post by: KD6VXI on May 13, 2020, 11:32:30 AM
I've found that the original a static d104 circuit can be made to sound good with a resistor.

Problem is, it loads the mic down.  Throw a 3 to 10 megohm resistor in series with the microphone lead.

Element is now properly loaded.....  Although you'll find total output is down, I haven't had a problem with it driving my Viking or any solid state stuff.

That said, I use j310 or mpf102 fets in the bases of anything that doesn't have an a static preamp in it.


Title: Re: D-104 Preamp by N1PS
Post by: WA2SQQ on May 14, 2020, 08:24:42 AM
So where is the schematic for this?

Title: Re: D-104 Preamp by N1PS
Post by: WBear2GCR on June 07, 2020, 06:12:14 PM

The schematics are all up in the thread.
Also in the "Technical Info" area on the masthead of the site...

The K1DEU version includes a provision for "bass" and "treble" controls.
You can take them out and replace with just a resistor... or just use one, if
you like.

The N1PS version shows a polarity reversing switch.
Others have chosen to take the output off the other side of the JFET to
do this trick...

Some have no resistor on the Drain side (aka "plate") side of the JFET, in which
case you can't do the "phase reversal" on the output side.

These are follower circuits. No gain.
The goal is to provide a very high Z to the mic element, thus getting good bass
response (see the graph in post 1). Then the output is low Z to drive the mic input
of a typical transmitter with ease. That's all.

Originally the D-104 was use to give "communications quality" audio. Note the curve
showing a lower Z impedance loading the mic??

The Astatic mic circuit, aka "power mic" was to either A) overdrive the input of a CB rig
(for "talk power") or B) sell more mics? It's a pretty crap effort overall, if you want to
sound better than "space shuttle audio". So, no, it's not "good" for today's AM use.

In the N1PS circuit, which is mostly fine (dunno why the transorb?) I'd only add a ~100k
resistor at the output end of the coupling cap. Just to make sure it doesn't have a charge
on it if plugged into a live circuit. Minor detail.

You can also just run it off a 9v battery in the base... etc.


Title: Re: D-104 Preamp by N1PS
Post by: n1ps on June 10, 2020, 07:49:30 PM
Hey Bear....good comments.  The transorb would not be needed if the user doesnt switch the Bat + or use external/radio power source.  IIRC I show the battery being switched, so it adds a layer of safety for the FET. 

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