With a TR7 I use the high level input which puts the total load on the cartridge to a around 1 Meg. What the higher load does is bring up the low end response (see attached graph). There's not much more you could get with a JFET. You could go even higher with the resistor value until you get into the low pass filter effect of the cable capacitance or the point of not enough drive to the radio. Just my personal opinion but I don't see the need to complicate things with a JFET. The resistor is simpler and easier for non-techy hams to implement.
The solution to the chassis ground issue of the Foster connector used in the 5/7-line is nicely described by Bob Heil.
See the section on RFI Removal.
Word wrap broke your link. This should fix things.
,-Cable%20shields%20are&text=The%20problem%20is%20that%20any,injected%20into%20the%20audio%20circuitry
73
-Jim
NU0C
On Fri, 23 Feb 2024 16:05:28 -0500
"Paul Christensen" <w9ac@...> wrote:
Jim's 470K resistor solution is a good one where the transceiver limits
audio response to what we're normally accustomed to for SSB. I use a 470K
resistor in the base of a D104 when using my Drake TR5 and Kenwood TS-830S
transceivers. For AM and ESSB modes, the JFET source follow helps to extend
the low-end range by completely unloading the crystal mic element.
Another reason to use a JFET as a source follower isn't always apparent: by
keeping the source Z low, the attenuation effect from mic cable capacitance
is minimized. True, it's really only needed on longer than normal mic cable
runs but it does help with physical microphonics on a cable with a high
source Z. For example, kick a mic cable with a high source and termination
Z and you'll hear it. But sure, we don't kick our mic cables. With a
capacitive source, which the crystal element is, cable capacitance creates a
voltage divider. The level at the mic jack is reduced but the frequency
response is unaffected with a capacitive voltage divider.
The worst case occurs when a mic with a highly resistive source Z feeds a
long cable. In that case, the parallel capacitance of the cable in series
with the high source resistance creates a low-pass filter. The JFET as a
source follower *placed at the source end* solves either problem.
Unrelated but I recently rewired the 4-pin mic jacks on my TR5 and TS-830S
transceivers. In both instances, the mic ground pin isn't bonded to the
chassis at the mic connector. Instead, and despite what the TR5 schematic
shows, the ground pin is attached to the audio shield and the shield is
connected to a 0.1" header on a PC board some considerable distance away
from the mic jack. That's a classic example of the so-called "Pin 1
problem."
roblem,-Cable%20shields%20are&text=The%20problem%20is%20that%20any,injected%
20into%20the%20audio%20circuitry.
Paul, W9AC
Paul, W9AC
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Jim Shorney
Sent: Friday, February 23, 2024 3:18 PM
To: [email protected]
Subject: Re: [DRAKE-RADIO] Drake T4XC and D-104 wiring
That does not change what I wrote.
73
-Jim
NU0C
On Fri, 23 Feb 2024 09:48:33 -0800
"jerry-KF6VB" <jerry@...> wrote:
On 2024-02-23 08:51, Jim Shorney wrote:
My "secret" is to do away with the amplifier and replace it with a
series 470K resistor. This is actually an old-timer's trick to
mellow out the frequency response of the D-104 head. The amplifier
is enhanced uselessness for our radios.
*** My homebrew amp is just a source follower. It has no voltage gain.
Its only purpose is to present a high impedance to the element.
I see that the "silver eagle" had a 26dB amp in the base. I scratch
my head as to why that might ever be needed. Crystal elements put out
plenty of volts as is.
- Jerry, KF6VB
--
73
-Jim
NU0C
Jim Shorney
