Amen, Larry! The LM386 data sheet shows up to a 50db Power
Supply Rejection Ratio (PSRR) with a 10uf cap from pin 7 to
ground and the gain at 20 with 6V supply and 1 khz operation.
My Ark30 had a squirrley sounding low level noise before I put a
10uf tantalum on pin 7 to ground. It might have been coming from
some of the digital synthesis circuitry and was most noticeable
when using headphones.
One glance at the LM386's internal equivalent schematic shows
why this decoupling cap is so needed. Pin 7 is right on the chip's
bias network.
BTW, on the ARK30, I placed the 10uf tantalum cap on the foil
side of the transceiver board. It just fits between the PCB and
the shield plate.
I have also found on many op amp circuit designs, that the unused
input pin ALWAYS need to be decoupled if it is not grounded or you
will invariably get keying or T/R thumps in the audio chain. Several
radios I have or have looked at omit this decoupling cap on the
resistive divider on the non-inverting input (+ lead). The TAC-1 had
this problem on the op amp following the CMOS bilateral switch chip.
A 0.33 uf monolythic axial cap fixed it completely, and killed the thump.
Bypass that lil sucker!
72,
Gary Surrency AB7MY
S&S TAC-1(40&80m) and ARK30, AT-11 (QRO and QRP)
QRP-L #571 Chandler, AZ (near Phoenix)Grid Square DM43BH
Az ScQRPions
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]