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I7SWX PTT Pop Mute
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Too much current.
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Goes down through the two transistors on the right side of the diff amp. From emitter to base on the upper one, then from emitter to base at pin 3 and (mostly) emitter to collector (grounded) on the lower transistor. Possibly several amps, at least for a few milliseconds. If you have a beefy enough power supply. Jerry On Thu, Aug 3, 2017 at 12:21 pm, KC8WBK wrote: I have npn transistors on order and will try version 3 below when I get them from Thailand. |
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Regarding version 2:
I am using amunters v14.? The frequency changes happen(ed) as the radio moves from rx to tx, and back from tx to rx.? During TX there is no change, it is only as you keydown and keyup.? I was not able to download v20 this afternoon, perhaps internet connection problems. Disconnecting the PTT mod C150 to the black wire of the Raduino provided relief from frequency changes.? When I reconnected this wire, the frequency remained stable.? ??? I then added filter caps, 0.1uF across the Raduino power regulator, both legs to ground.? I also added a cap across violet to yellow on the tuning pot, which I neglected when I replaced the original with a 10 turn pot. So far, the pop mute works well and the frequency is now stable.? I will need some good band conditions and an evening of making noise on the bands to say that the board is stable and suitable for recommendation to all.? But at this point, it seems quite positive. I had an idea that did not work: I thought I would make a simple AGC that would take input from LM386 pin 3, run it through a diode and variable pot to the base of a 2N4403, with the collector to positive 12V and emitter through a 1.2k resistor to LM386 pin 1.? It shut the audio down and the sketch was corrupted at this time (related?).? It might be possible to shave the audio peaks or work as an actual AGC.? I don't really know what I am doing so I would have to learn a lot to design this, which would be fun. One thing that I discovered is that it does not take 12V to mute the LM386 pin 1.? I put a measured 4V on pin 1 and audio was shut down. Pin 1 seems to run about 1.5V under normal operation. I may try this mod but I decided to work on the VK3YE AGC first, because I have it ready to be installed and I have great hope that it may actually work this time. |
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Do try it if you feel so inclined. ?Should work. ?Though I'm inclined to mess with either pin 1 or pin 7, as that avoids the cap discharge you mention, and is useful in other situations beyond the Bitx40.
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On Tue, Aug 1, 2017 at 11:08 am, Andrew Kasurak wrote:
why do we not just use a transistor to shunt pin 5 to ground when TX is high? Since the 386 will be disabled and only running on capacitors there shouldn't be too much energy to move. |
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That's pretty much what Gian started out with.
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Gian was pulling pin 7 high, the article says hi or lo works. As I say, lots of others seem to be finding that grounding pin 7 (or tying it to VCC) gives a full mute. ?My simulation says grounding pin 1 is more effective. But I still don't fully understand everything going on here. It could be that we could have a cap to ground on pin 1 to slow the unmute, it would also increase the gain much like a cap between pins 1 and 8. ?But probably needs a series resistor to keep the gain from getting out of hand. ? Jerry On Tue, Aug 1, 2017 at 10:52 am, Vince Vielhaber wrote:
Figure 2 shows how to mute the LM386 by grounding pin 7 and feeding pin 6 |
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Vince Vielhaber
Take a look here:
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Figure 2 shows how to mute the LM386 by grounding pin 7 and feeding pin 6 thru a resistor (10-100 ohms). Vince. Driving pin 1 to 12v does indeed put about 8ma through the pnp's on the --
Michigan VHF Corp. |
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Assume an input voltage of 0.1v at pin 3. ?Current through the 50k resistor to ground would be 0.1/50000, and assuming a beta of 100 on those two PNP's the maximum current sucked away from pin 1 would be 100*100*0.1/50000 = 0.2ma. ?Which is considerably less than the nearly 1ma coming in through the 15k resistor from the output. ?So the current down through the PNP's can get significant, but not overpowering. ?Pretty clear now why you want an AC coupled source into pin 3.
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On Tue, Aug 1, 2017 at 09:56 am, Jerry Gaffke wrote: The datasheet describes the LM386 as an op-amp with internal feedback, and the above is pretty much a standard inverting op-amp configuration with a voltage gain of 15k/(1.35k+150). ?However, there is significant current going down into the diff amp from pin 1, which should disturb that model considerably. |
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Hi Jerry,
thanks for your report and data. Yes, 1.35K. Your worry about the transistor driving the push-pull maybe right. KC8WBK has moved from pin 7 to pin 1 and it seems to be working OK. The solution of taking pin 1 to gnd requires an additional NPN transistor, I have attached the V3 schematic. The LM386 maybe OK as spares for KC8WBK ... for the PC I have an HP with keybod smoked ... hi 73 Gian I7SWX |
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Driving pin 1 to 12v does indeed put about 8ma through the pnp's on the pin 2 side.
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More worrisome, nothing much limits the current through the pnp's on the pin 3 side. Driving pin 1 to something between 0v and 1v does mute the chip quite nicely though. No significant extra currents. If we add a cap somewhere to slow down the mute-off, that would have to happen in front of the switch that pulls pin 1 low. Driving pin 7 to 0v ?reduces the voltage gain from 20 to around 3, output is centered on 1.75v. Driving pin 7 to 12v reduces the voltage gain from 20 to around 10, output is centered on 10.9v. That second observation of mine in LTSpice does not quite square with the experience of others that this gives nearly full muting. ? I'm messing around in LTSpice, no need to send any spare LM386's, just a new laptop when I blow this one out. ? ?;-) The LTSpice file is here: ?/g/BITX20/files/KE7ER/lm386jg.asc It's a straightforward entry from the internal schematic shown in the LM386 datasheet. This other Spice model is likely better than mine: ? I'm still not confident I understand the LM386's feedback scheme. ?The 15k + 1.35k + 150 ohm resistors do effectively form a voltage feedback network into pin 3. Pin 3 is always 2*Vbe below pin 1, so IN+ at pin 3 effectively sees a feedback voltage that is an inverted version of pin 1. ? Also, with pin 2 grounded as it is in the Bitx40,, the left side of that 150 ohm resistor is effectively pegged at an AC ground of 2*Vbe = 1.2vdc. ?The datasheet describes the LM386 as an op-amp with internal feedback, and the above is pretty much a standard inverting op-amp configuration with a voltage gain of 15k/(1.35k+150). ?However, there is significant current going down into the diff amp from pin 1, which should disturb that model considerably. ?And while I still don't quite understand exactly what current would be heading off to the right from the diff-amp to drive the output, that may not matter so long as there is sufficient open loop gain.. ?? > ?The only “puzzle” of this solution would be if the 1.23kohm resistor can dissipate circa 100mW due to a current expected of 8mA ? I assume you mean the 1.35k resistor. ?Anyway, I'd be much more worried about the pnp's on the right hand side of the diff amp if you hit pin 1 with 12v. Jerry, KE7ER On Mon, Jul 31, 2017 at 10:57 am, Giancarlo wrote:
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Hi All, ? I see some very interesting comments about the LM386 PTT Pops solution I suggested in february. ?My latest e-mail generated some confusion as I reported Pin 1 in place of Pin 7 to KC8WBK. Please let me explain why. My original schematic suggestion taking to RX +12V via pin 7 was thought as simulating the increase in value of 1.35kohm resistor, between pin 8 and pin1 (gain), to reduce gain of the balance amplifier, due to the increase of current through it. I generated another change schematic where pin 1 was taken to RX +12V, see drawing V2. Next day I was taken to hospital and stayed away from home, between hospital and rehab center for four months. I was convinced to have uploaded such mod and following my memory I commented this mod when I read KC8WBK complain note. ?I was not too shure the simulation of pins 8-1 resistor change would be enough and I addressed my solution to take pin 1 to +12V. This move will block any signal to reach the push-pull stage as + input from pin 3 would never be amplified. The only “puzzle” of this solution would be if the 1.23kohm resistor can dissipate circa 100mW due to a current expected of 8mA, as the PNP darlington transistors, associated to pin 2 - input connected to ground, have a voltage at the emitter of probably +1.4V. If someone will try and demostrate that is not a valid solution and blows the LM386 I will send him a couples as a replacement.... We are experimening .... hi. ?Best 73 ?Gian I7SWX |
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If you are running Allard's latest code, frequency should be reasonably stable.
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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.? On Mon, Jul 31, 2017 at 08:35 am, KC8WBK wrote: The unstable frequency on transmit is a bigger problem so I will shift my attention to that.? Any ideas for what to try? |
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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.
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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. On Mon, Jul 31, 2017 at 08:35 am, KC8WBK wrote: The unstable frequency on transmit is a bigger problem so I will shift my attention to that.? Any ideas for what to try? |
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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? |
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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.
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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? On Sun, Jul 30, 2017 at 05:45 pm, Joe wrote: I also installed the LM386 PIN 7 mod and found it works very well, ?the best noise ?reduction so far. |
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I used a 5v arduino relay module. with 1 normal open ?and ?1 normal closed contact. I used the normally closed contacts so there is audio in rx mode. When tx occurs the contacts open. My power source for the relay activation comes from a tx led mod. The best is a reed relay? Joe VE1BWV? On Sun, Jul 30, 2017 at 11:05 PM, KC8WBK via Groups.Io <cruisenewsnet@...> wrote: What type of relay works best for this? |
Giancarlo - I7SWX
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