Oscillation with Hycas AGC Amplifier


Richie Chambless
 

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


Alan Jones
 

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


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


ken WA2MZE
 

The LM386 has a single ended output.  It MUST be coupled to the load through a capacitor (value depends on load impedance, 100-250uf is fine).  One end of speaker or headphone to ground is OK if capacitor is present at '386 output.

On 8/12/21 9:24 AM, Larry Acklin wrote:
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



Richie Chambless
 

I thought it was the LM386, so I tried two other amps but got the same result. I swapped the Hycas with another IF amp (around 20 db gain) and it worked fine. So, I know the problem is in the Hycas. My next step was to terminate the input and output with 50 ohms and test the AGC voltage. Sure enough, it was only 1.5 volts, meaning something was oscillating internally and driving the control voltage low. I then changed R4 and R8 to 100 ohms, no luck. But when I held a 4.7k resistor across R2, the oscillation stopped! While holding this resistor in place, I put the volt meter on the AGC pin. It had around 5 volts the reduced with incoming signals. So, I soldered the 4.7k resistor in place on top of R2, but the oscillation came back! It's something about my hand capacitance that calms the instability. I think I may experiment with higher R4 and R8 values, along with a transformer impedance match in lieu of the L match. Maybe something will work!


Larry Acklin
 

have you looked here?

oscillation was apparently an issue. 

73
Larry
KB3CUF 

On Fri, Aug 13, 2021 at 7:50 PM Richie Chambless <rchambless7@...> wrote:
I thought it was the LM386, so I tried two other amps but got the same result. I swapped the Hycas with another IF amp (around 20 db gain) and it worked fine. So, I know the problem is in the Hycas. My next step was to terminate the input and output with 50 ohms and test the AGC voltage. Sure enough, it was only 1.5 volts, meaning something was oscillating internally and driving the control voltage low. I then changed R4 and R8 to 100 ohms, no luck. But when I held a 4.7k resistor across R2, the oscillation stopped! While holding this resistor in place, I put the volt meter on the AGC pin. It had around 5 volts the reduced with incoming signals. So, I soldered the 4.7k resistor in place on top of R2, but the oscillation came back! It's something about my hand capacitance that calms the instability. I think I may experiment with higher R4 and R8 values, along with a transformer impedance match in lieu of the L match. Maybe something will work!


Larry Acklin
 

Diz is a fan of gimmick capacitors- 1 inch of enameled wires twisted tightly, cut off to suit.  Maybe that across R2?

On Fri, Aug 13, 2021 at 9:42 PM Larry Acklin via groups.io <acklin=gmail.com@groups.io> wrote:
have you looked here?

oscillation was apparently an issue. 

73
Larry
KB3CUF 

On Fri, Aug 13, 2021 at 7:50 PM Richie Chambless <rchambless7@...> wrote:
I thought it was the LM386, so I tried two other amps but got the same result. I swapped the Hycas with another IF amp (around 20 db gain) and it worked fine. So, I know the problem is in the Hycas. My next step was to terminate the input and output with 50 ohms and test the AGC voltage. Sure enough, it was only 1.5 volts, meaning something was oscillating internally and driving the control voltage low. I then changed R4 and R8 to 100 ohms, no luck. But when I held a 4.7k resistor across R2, the oscillation stopped! While holding this resistor in place, I put the volt meter on the AGC pin. It had around 5 volts the reduced with incoming signals. So, I soldered the 4.7k resistor in place on top of R2, but the oscillation came back! It's something about my hand capacitance that calms the instability. I think I may experiment with higher R4 and R8 values, along with a transformer impedance match in lieu of the L match. Maybe something will work!


Richie Chambless
 

Hi All,

I have good news. I took the Hycas module out of my test receiver and powered it directly (with short leads) with a 13 volt battery. With two 40db attenuators (one on input, other on output), it does not oscillate when connected to my nanovna. I measured around 50db of gain, and the agc action was working to reduce the control voltage down around 4.75 volts, so I know everything is working. I'm thinking I had a ground loop in my receiver, so I will need to change the power connection and build an RF shield for it. Thanks for all of your help!

Rich KM4TLR