Re: Oscillation with Hycas AGC Amplifier

Larry Acklin

I second that LM386 thought.

cribbed from Stackexchange.com-

• GAIN CONTROL To make the LM386 a more versatile amplifier, two pins (1 and 8) are provided for gain control. With pins 1 and 8 open the 1.35 kΩ resistor sets the gain at 20 (26 dB). If a capacitor is put from pin 1 to 8, bypassing the 1.35 kΩ resistor, the gain will go up to 200 (46 dB). If a resistor is placed in series with the capacitor, the gain can be set to any value from 20 to 200. Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground.

Gain = 15,000 x 2 / (Ra + Rb)
Initially Ra + Rb = 0.150 k + 1.350 k = 1.5k
So gain = 2 x 15,000 / 1.500 = 20.
Shorting the 1.35K FOR AC increases gain to 2 x 15k/.15k = 200

Try reducing gain by pulling out the cap if it is there.

If an lm386 is overdriven, it will oscillate too.  And be careful- I seem to remember that you can't ground either of the outputs- like a headphone jack would ground one side.

73

Larry

KB3CUF

On Thu, Aug 12, 2021 at 1:57 AM Alan Jones <oalanjones@...> wrote:
Hi Rich,
I would take a look at the preamp circuit first and then the gain on the LM386
This is just a guess since I don't have your schematic in front of me.

Kind Regards,
Alan, N8WQ

On Wed, 11 Aug 2021 21:25:37 -0400, Richie Chambless <rchambless7@...> wrote:

Hi,

I've recently built a receiver using the Hycas amplifier and I am have some oscillation issues. First, here is the basic layout of my test receiver:

Antenna > 40 Meter BPF > SBL-1 > TIA Amp (copied from uBitx) > 12 Mhz SSB filter > Hycas amp (L match retuned for 12 Mhz) > SBL-1 > LM386 AF Amp with 2N3904 preamp

When I power up the Hycas amp and AF section (with no TIA, LO, or BFO running), I get a loud motorboating sound. When I jumper across the AGC off pins, the motorboating stops. When I turn the rest of the receiver on, I get a high pitched oscillation with a frequency that varies with turning C2. I can null out the squeal and here some stations, but the noise floor (hiss) is very high. But, when I jumper the Mute pins, the gain is reduced and I am able to hear stations at a comfortable level. Now, I know this module wasn't designed to be used on mute, so I know I have something wrong. Any suggestions on where to start? Is there any particular thing I need to do, such as decoupling caps or shielding? I appreciate any help you can give.

Thank you,

Rich

KM4TLR

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