Topics

Bitx40 low volume oscillation squeal

Graham <planophore@...>
 

Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas? Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc

Ken
 

Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc





Graham <planophore@...>
 

Yes.


On 2017-03-26 20:55, Ken wrote:
Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc






Ken
 

Try a new one. I found that mine had dead zones at both ends of travel. Replace with a log or audio taper.

73 Ken VA3ABN

On Sun, Mar 26, 2017 at 5:04 PM, Graham <planophore@...> wrote:
Yes.


On 2017-03-26 20:55, Ken wrote:
Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc







Graham <planophore@...>
 

I tried a second one and had the same behavior.  I will dig through my junk box and see if I can find another to try.

I am not sure how a pot that has dead zones on one or both ends would cause this type of behavior.

cheers, Graham ve3gtc


On 2017-03-26 21:05, Ken wrote:
Try a new one. I found that mine had dead zones at both ends of travel. Replace with a log or audio taper.

73 Ken VA3ABN

On Sun, Mar 26, 2017 at 5:04 PM, Graham <planophore@...> wrote:
Yes.


On 2017-03-26 20:55, Ken wrote:
Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc








Ozemail
 



Try a 4 ohm speaker - solved the problem for me!
De VK2AHF Syd

On Mar 27, 2017, at 8:29 AM, Graham <planophore@...> wrote:

I tried a second one and had the same behavior.  I will dig through my junk box and see if I can find another to try.

I am not sure how a pot that has dead zones on one or both ends would cause this type of behavior.

cheers, Graham ve3gtc


On 2017-03-26 21:05, Ken wrote:
Try a new one. I found that mine had dead zones at both ends of travel. Replace with a log or audio taper.

73 Ken VA3ABN

On Sun, Mar 26, 2017 at 5:04 PM, Graham <planophore@...> wrote:
Yes.


On 2017-03-26 20:55, Ken wrote:
Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc








Ken
 

That is strange. This was the problem with mine and 2 others had the same problem. I am assuming your smd version is built the same as the through hole version.

73 Ken

On Sun, Mar 26, 2017 at 5:29 PM, Graham <planophore@...> wrote:
I tried a second one and had the same behavior.  I will dig through my junk box and see if I can find another to try.

I am not sure how a pot that has dead zones on one or both ends would cause this type of behavior.

cheers, Graham ve3gtc


On 2017-03-26 21:05, Ken wrote:
Try a new one. I found that mine had dead zones at both ends of travel. Replace with a log or audio taper.

73 Ken VA3ABN

On Sun, Mar 26, 2017 at 5:04 PM, Graham <planophore@...> wrote:
Yes.


On 2017-03-26 20:55, Ken wrote:
Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc









Graham <planophore@...>
 

nothing ventured, nothing gained - I will see if I have one lying around and will give it a try.

I still don't understand what causes this behaviour. I will have to breadboard the circuit and play some with it.

cheers, Graham ve3gtc


On 2017-03-26 21:32, Ozemail wrote:


Try a 4 ohm speaker - solved the problem for me!
De VK2AHF Syd

On Mar 27, 2017, at 8:29 AM, Graham <planophore@...> wrote:

I tried a second one and had the same behavior.  I will dig through my junk box and see if I can find another to try.

I am not sure how a pot that has dead zones on one or both ends would cause this type of behavior.

cheers, Graham ve3gtc


On 2017-03-26 21:05, Ken wrote:
Try a new one. I found that mine had dead zones at both ends of travel. Replace with a log or audio taper.

73 Ken VA3ABN

On Sun, Mar 26, 2017 at 5:04 PM, Graham <planophore@...> wrote:
Yes.


On 2017-03-26 20:55, Ken wrote:
Graham

Are you using the original volume control that came with the Bitx40?


73

Ken VA3ABN

On Sun, Mar 26, 2017 at 4:51 PM, Graham <planophore@...> wrote:
Good day all,

I have been messing about with my Bitx40 SMD on an off for a couple of weeks working towards getting it mounted in a case.

I have removed C113 across the LM386 pins 1 and 8 to reduce the gain, makes a big difference. Problem is that I get a high pitched "squeal" or oscillation at very LOW volume setting; turning the volume up a bit more and it disappears.

I have spent some time searching the archives for similar posts and fixes but just haven't managed to find much that is relevant.

Ideas?  Better bypassing on the power lines to the LM386?

Speaker is smallish 3" diameter generic 8 ohm speaker.

cheers, Graham ve3gtc









Michael Davis <maddmd818@...>
 

I was going to mention speaker impedance. Mine did oscillate with a 4 ohm speaker, but not 8 ohm. I believe you said that there was no oscillation before removing the capacitor. Try adding an 8-16 ohm resistor in series with the speaker, or try a 16 ohm speaker, to see if the circuit change somehow affected what the audio amplifier load will be happy with.

Sent from Mike's iPad WA1MAD

Graham <planophore@...>
 

Still no luck in resolving my "squeal".

So far, I have removed C113 across pins 1 and 8 of the LM386 in order to
reduce it's gain. Big improvement to audio, at least to my ears.

The "squeal" at low audio levels existed before I removed C113 and
persists after removal.

I have now tried three different 10K pots - the two that were included
with the kit and a third new 10K pot (linear taper).

I have now tried three different small speakers, two are 4 ohm and the
third is marked 8 ohm.

And for good measure, combinations of these different 10K pots and
speakers.

All of this to no avail - I still get a "squeal" i.e. oscillation at
very low volume level for about five degrees or so of pot rotation from
full counter clockwise position turning clockwise to a higher volume
setting.

The "squeal" i.e. oscillation, disappears once the control has been
turned past this very low volume setting.

I am using a good quality HP / Agilent lab power supply and have tried
another with the same reuslts.

Still at a loss. There is something I don't quite understand about the
why of this behaviour. I guess it's back to basics and to prototype the
circuit to see if I can replicate the behaviour and if so then what I
need to fix it.

News at 11.....

cheers, Graham ve3gtc

On 3/27/2017, "Michael Davis" <maddmd818@...> wrote:

I was going to mention speaker impedance. Mine did oscillate with a 4 ohm speaker, but not 8 ohm. I believe you said that there was no oscillation before removing the capacitor. Try adding an 8-16 ohm resistor in series with the speaker, or try a 16 ohm speaker, to see if the circuit change somehow affected what the audio amplifier load will be happy with.

Sent from Mike's iPad WA1MAD


T Williams
 

To eliminate the audio squeal, try rearranging wires.  Twist pairs that go together.   Route all wiring away from the Raduino board.  Neatly bundle unused wires and put them away from the wiring that is in use.  This worked for me.


TW

Graham <planophore@...>
 

thank you for the suggestion.

However, doesn't help. Bitx40 is still not mounted in case, The wires
for speaker and volume control are the only ones near each other and
both are twisted i.e. volume control wires are all twisted together, the
two wires for the speaker are also twisted together, makes everything
tidier and is my normal course.

I am not using a raduino but rather feeding the bitx with an external
analog vfo connected through RG174 to the DDS connector on board the
bitx40.

I haven't had time to poke around at this problem so far today. Perhaps
I will find a wee bit of time later this evening.

cheers, Graham ve3gtc
+
+`

On 3/27/2017, "T Williams via Groups.Io" <n41w124=yahoo.com@groups.io>
wrote:

To eliminate the audio squeal, try rearranging wires.  Twist pairs that go together.   Route all wiring away from the Raduino board.  Neatly bundle unused wires and put them away from the wiring that is in use.  This worked for me.

TW

Allard PE1NWL
 

Graham,
you can also try to shield the wires to the volume pot.
In my case this fixed this problem, and it also greatly reduced the tuning
clicks.
73, Allard PE1NWL

Lawrence Galea
 

Hi.

First of all, all audio wiring should use shielded wires.

 

I had a look at the BITX circuit and also at the LM386 data sheet that I am attaching.

 

I noticed that the volume control slider goes to the non-inverting input pin 3 of the LM386 directly without an isolating capacitor.

 

This means that not only will the volume control go scratchy over time as it has a dc voltage between the slider and earth, but also that the internal biasing of the LM386 will be upset depending on the setting of the volume control.

 

I suggest that you put an isolating capacitor from the slider of the volume control to pin 3 of the LM386.

 

You can use any available value that you have, such as 100nf, 1μf , 10μf etc with the + side going to pin 3. The lower value will simply reduce the low frequencies.

 

You can put this capacitor on the back of the volume control

 

Have a go and please report back to the group.

 

Regards

lawrence


On Mon, Mar 27, 2017 at 8:00 PM, Allard PE1NWL <pe1nwl@...> wrote:
Graham,
you can also try to shield the wires to the volume pot.
In my case this fixed this problem, and it also greatly reduced the tuning
clicks.
73, Allard PE1NWL




tony.vasile@...
 

100 nF in series between wiper pin and LM386...PERFECT!  Thank you! 

 

I found a 1K to 10K resistor in the same place stops the instability.
My first board at 0 volume used to growl !!

Raj

At 19/04/2018, you wrote:
100 nF in series between wiper pin and LM386...PERFECT! Thank you!