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Construction Manual for the K2DI v.III and K2DI-PCI boards © EA3BLQ - Revision June 2005
(*) NOTES ON THE R7 RESISTOR'S VALUE TO CONTROL LM386's GAIN.
The manufacturer's General Description of the LM386 reads: "…The gain is internally set to
20 to keep external part count low, but the addition of an external resistor and a capacitor
between pins 1 and 8 will increase the gain to any value up to 200."
The factory default gain of ±20, attained through the inner 1.35Kohm resistor allocated
between pins 1 and 8 is too low for our needs.
On the other way if we externally bypass that resistor by inserting a 10µF electrolytic
capacitor between those pins (+ lead to pin 1) the gain is increased to 200. Good enough
to drive a 4Ω speaker but too high for the sound card, as it gets overdriven.
On this account and by inserting one external resistor in parallel with the inner resistor and
in series with the electrolytic capacitor, between pin 8 of the IC and the (-) lead of the
capacitor, we may set that gain to the level that meets with our needs, within the range of
20-200.
As a result of some computing, I've got the following options for R7's resistor value:
1) One 1.2Kohm resistor (brown-red-red) will render a gain of about 50 (perhaps a bit on
the low side)
2) One 330Ω resistor (orange-orange-brown) will render a gain of about 75
3) One 220Ω resistor (red-red-brown) will render a gain of about 90.
4) One 180Ω resistor (brown-grey-brown) will render a gain of about 100 and
5) One 100Ω resistor (brown-black-brown) will render a gain of about 125
I have chosen option #3, but the builder may choose the option he/she thinks most
suitable for his/her needs and/or preferences.
CAUTION:
• Please, remember to configure J2 header (the microphone configuration header
extender) in accordance with your mic needs, as per Table 2 on the Elecraft’s
“Assembly and Operating Instructions” for the KSB2 SSB Adapter: “P1 Wiring for
Selected Microphones”, as you would do on the Microphone Configuration Header
at the Front Panel board itself.
• If your intention is to construct the K2DI-PCI board too, then it is most
convenient to set up the mic configuration at this step, before constructing and
assembling the K2DI-PCI daughter board to the K2DI v.III board. If you fail to do
so you may forget this step and be compelled to undo the K2DI v.III / K2DI-PCI
assemblage so that you can reach the J2 header at the K2DI.
(35 stránky)
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