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For those who are tired of the pop and noise when starting to transmit, I would like to recommend the simple circuit described here:

/g/BITX20/message/21876

It eliminates the hearing destroying pop when pushing the push to talk button.? It is very easy to connect, one wire to +12V, one to the positive side of the PTT jack, and one to pin 7 of the LM386, which is easy to connect to on the back of the board.

A huge improvement to user comfort for those using headphones.

The circuit closest to the camera is the I7SWX PTT Pop Mute, the circuit to the rear is the VK3YE AGC.

Hi,

thanks for reporting the trial of my muggested circuit and? the positive results
73

Gian
I7SWX

Hi,
I also tried your circuit for the tx noise.
It works very well.
I have tried several but this had the best results.
I use it in conjunction with a relay mod.

Thanks
Joe
VE1BWV

Joe,

Please tell us a bit more about what you figured out.

Does this cure the pop in the headphones when you press PTT??
What I would call "tx noise" is the brief outgoing carrier, which is a totally different issue.
If this fix addresses the outgoing carrier issue, please show us where to find the fix.
?
What other circuits did you try?
In what way did this solution work better?
What is the relay mod?

A full description of what you did would be helpful to many here.

Jerry

Unfortunately I have to withdraw my recommendation for this circuit, it is doing something odd to the transmit.? I am still using it, and have built three of the circuits to find out if it is my construction that is the problem or if it the circuit.? I am coming to the conclusion that the circuit is adding noise or carrier of some kind to the transmit signal.? I don't have a confirmation on this yet.

I assume this is the circuit you are referring to: ?/g/BITX20/message/21876

Dumping 12v into the bypass pin of the LM386 to quiet it down is a bit unorthodox, but apparently works.
The LM386 does not have much of anything to do with transmit, hard to imagine why you are getting this result.
I suspect something else is going on.

Jerry

You might try taking the "+12V" of Gian's diagram from pin 8 of the LM386, which is only at 12v when the LM386 is powered up for receive.
I don't really know what all "+12V" into pin 7 might do, it certainly is not something written up in the datasheet.
But not much point in doing that when the LM386 is not otherwise powered up, which is the case during transmit.

Hi,

The LM386 has no inhibit control so we have to find different solutions, if possible. As Jerry reported "Dumping 12v into the bypass pin of the LM386 to quiet it down is a bit unorthodox". If you look at the internal schematic of the LM386 you will see that taking pin1 to +12V will block the amplification of signals to the output push-pull stage. The relay commutation should take a few tens of mSec to switch and the LM386 gain being zero, as the PTT is de-pressed, will block any pop up plus the RX +12V will quickly go to zero, NO audio output available. No interference with TX stages is expected.

73

Gian
I7SWX

Do you mean pin 7, not pin 1?

I checked my work and found a wrong connection on my part.? I fixed the connection and the circuit now works as described, reducing the PTT pop and noise on initial transmit. ? I did not experience an issue with transmit, it seems to be working normally.? There is a slight click when entering TX and a slightly louder pop when exiting TX, but neither are as loud as the PTT pop before adding this modification. The audio is silent during transmission.

Sorry about the false alarm in #30619. This modification is a worthwhile addition to any BITX40.

/g/BITX20/attachment/21876/0/BITX-40%20PTT%20CLICKS.png

I have spent some time txing with the mod.? I get a buzz or rumble in my headphones that is from my voice.??

Audio reports are good.

Sometimes? frequency bumps down a tenth of a kHz or two when I key down, and bumps up when I key up.

I will try to reroute wires to limit rf into the audio amp.

What type of relay works best for this?

Lots of hits on the web for "LM386 pin 7 mute". ?Most pull pin 7 low, though pulling high works too. ?Allard's now routing PTT through the Raduino and driving a transistor to turn on the relays. ?So we could have a second 2n3904 with base and emitter in parallel with Allards 2n3904 transistor, ?the collector of this new 2n3904 pulls down on LM386 pin 7. ?Add a cap from pin 7 to ground, discharges almost instantly but would take a few miliseconds to charge back up through the LM386's internal 15k resistor?when PTT is released, long enough to?keep the LM386 quiet till after this exit noise is gone. ?So a 2n3904 plus a cap replaces Gian's 2n3906+resistors+diode plus Joe's relay, assuming you are going to implement Allard's "TX-RX Line Wiring" mod anyway. ?Beware, I have not tried this.

Pin 7 is meant to be bypassed to ground through an optional 10uF cap, preventing power supply noise from disturbing the diff-amp input.. ?But the Bitx40 has some serious external 12v filtering going into the LM386, so no large cap on pin 7 is probably fine.

Not being an analog kind of guy, fully figuring out the LM386 from the datasheet's internal schematic is quite the puzzle. ?I think one key is to assume the IN- pin is grounded (as it is in the Bitx40), so the collector of the NPN at the top of that side of the diff-amp is always two Vbe above ground, or about 1.2v. ?The current mirror below the diff amp draws an equal amount of current from the two sides of the diff amp, any excess current on the right side goes off to drive the output stage. ? This guy: ?? says the gain of the LM386 is equal to ?2*Z(1-5)/(150+Z(1-8)), ?where Z(1-5) is the impedance between pin 1 and pin 5 (typically just the 15k internal resistor) and Z(1-8) is the impedance between 1 and 8 (typically just the 1.35k internal resistor, unless you have an external cap across those pins to increase the gain as the Bitx40 does). ? The 150 comes from the internal resistor between pin 8 and the top of the left side of the diff-amp, the factor of two comes in because of the diff-amp. ?That all sort of makes sense, though I'm not convinced my understanding is complete. ?When muting, I assume bringing pin 7 to gnd or vcc upsets the diff-amp to where it's no longer functioning as a diff amp. ?Could be that going in and out of mute like this creates its own pops, I'll have to try driving pin 7 both high and low, see if either works better.

This guy does a dc analysis without just too much hand waving. ?
He's trying to get the LM386 behave like an op-amp, no internal feedback, so he can add an output stage to the LM386 and have the feedback loop go around everything. ? He says that at 125mw driving headphones, the LM386 is clean enough. ?Ask for more power from it to drive a speaker, and it distorts badly. ?The uBitx has moved to a better amp.

Here's an interesting webpage on cheap and simple and easy to understand alternatives to the LM386. ?He gives up on the LM386, drives an output stage with an op-amp with good results, written long before the above article: ? ?

I like how those two articles show the process of figuring these things out. ?Both seem quite competent, and they're not afraid to describe stuff they try that didn't quite work. ?Even the simplest circuits can keep a good engineer scratching his head for days.

Kind of amazing the LM386 is still such a popular chip at a hoary 34 years of age. ?Hobbyist interest on the web seems equally divided between qrp rigs and guitar amps. ?In 20 years the kids will be messing with their DSP algorithms to get a guitar amp that sounds grungy like those badly overloaded LM386's, having no clue how an overloaded vacuum tube amp would sound.?

Jerry, KE7ER?

I added 0.1 uF caps between pin 7 and ground, +12 and ground and PTT and ground with no improvement to the buzz noise on tx.

I put resistors in line of the +12 to the mini board, and found that the mute did not function above 3.3k.? The mute did function at 390 ohm and there was a delay of a second or two before the buzz started.

So no real improvement yet.

I can fix the buzz problem with a relay, which may also fix the keyup pop after transmit.

The unstable frequency on transmit is a bigger problem so I will shift my attention to that.? Any ideas for what to try?

Add a cap to ground at the raduino end of the tuning pot wire if you have not already done so, same 0.1uF as the WireUp instructions say to put at the pot.
Allard has made a number of improvements to freq stability with his sketch, including oversampling of the pot and the flutter fix.
He also disables freq changes while transmitting.
Could go into Allard's settings menu and reduce the size of the pot tuning range, that would help reduce the tendancy to jump around.
Could adjust the sketch to disable any frequency changes (except rit) once you hit the ptt button, this ptt freq lock ?gets turned off when you try to move the tuning knob more than a khz or so.
Or could add a lock switch, adjust the code so when the sketch sees this switch on it ignores the tuning pot and leaves the operating freq where it's at.

If you are running Allard's latest code, frequency should be reasonably stable.
If it's not stable, you should probably address it rather than hide it.
Though once that's cleaned up, the additional measures I mentioned in my previous post are worth considering.

The unstable freq and the buzz are likely related.
Perhaps trash on the power into the raduino, trouble around the raduino regulator, or all them wire connections talking to each other.
If you have long antenna connection wires and/or long tuning pot wires, try shortening those and twisting up each cable so ground and signal are well coupled.
Maybe use coax.?