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 Post subject: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Wed 02, 2017 6:19 am 
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Howdy Folks,

Well, I've been working on a new receiver recently. I'm trying to make the rig out of
as many scrounge parts as I can. Got the audio amplifier up and running, but man,
does this thing hum :D. Guess I need to use all new electrolytics. Here's a picture of
my progress.

Attachment:
IMG00013.jpg
IMG00013.jpg [ 136.87 KiB | Viewed 1194 times ]


Built the thing on one of those cheap, plastic breadboards, and that thing can be a
pain in the butt and then some. It can be trying at times to make sure you're making
good connections.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Wed 02, 2017 1:58 pm 
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With the "extra" stage of amplification you may have too much gain into the LM386, its more or less a complete amplifier & audio output all by itself... Unshielded wiring is usual cause of hum in battery powered devices... Unless using a AC power supply, bad caps will not likely cause hum...


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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Wed 02, 2017 3:54 pm 
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Howdy 35Z5,
35Z5 wrote:
With the "extra" stage of amplification you may have too much gain into the LM386, its more or less a complete amplifier & audio output all by itself... Unshielded wiring is usual cause of hum in battery powered devices... Unless using a AC power supply, bad caps will not likely cause hum...

I've played with the value of the cap that goes from pin 1 to pin 8. What I'm going to do is leave it at the value that gives a gain of 200 and see if I can solve the issue. Now, I know I could easily put the amp on my scope and locate the issue, but I love seeing if I can find issues via logical analysis and inductive reasoning -- that, and my trusty Radio Shack True RMS DMM :D.

Doing this reminds me of my days in the oil patch, sitting in my logging unit, trying to fix a piece of gear while an apoplectic drilling engineer is yelling his lungs out saying, "Get that damn thing working this instant, or I'm going to run your butt off and get a company out here that can do this!!" I used to love having them run off instead of me and my company. I'd call the main office and say,"This douche bag you have out here is a complete jerk and is annoying the whole rig. It's bad enough that we have to be out here on this stick of dynamite away from our families and friends, and now we are having to tolerate this low-grade moron who is ticking everyone off." I used to get a LOT of free beer and Crown Royal getting those kind of jerks run off the rig when we got back on shore :D. We had discounted-rate contracts with all of the major oil companies when we were on a rig and testing new equipment, so they put us on rigs that were going to be slow drilling, and it wasn't absolutely necessary that our equipment we were testing worked perfectly or if at all. In other words, we were out their debugging software and hardware that was bleeding-edge technology, and the oil companies we worked for had a vested interest in our success, which, in the long run, would save them all a LOT of money, but I digress...

Anywho, thanks for the info, dude, and I'll flip out a few electrolytics and try adding a bit of shielding and see what gives. There are no external sources causing this unwanted "signal," btw.

Most of the electrolytics are pretty old, but they pass muster using my AADE LC-2B meter, although 2 of them have a bit of ESR that doesn't look too pretty :D. I'm also powering everything with 9VDC, which means I can pretty much eliminate common-mode hum. At this stage of the game, I'm more interested in the gain bandwidth product of the LM38X series of amplifiers, chuckling a bit at the lowly LM386 :D. So, let's just say that I'm using the breadboard until I get something I like before I commit the circuit to actual physical construction on copper.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Wed 02, 2017 4:41 pm 
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In my LM386 transmitter I've never seen need for gain cap between pins one & eight...

viewtopic.php?f=12&t=193042


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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Wed 02, 2017 5:17 pm 
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Re-Howdy 35Z5,

35Z5 wrote:
In my LM386 transmitter I've never seen need for gain cap between pins one & eight...
viewtopic.php?f=12&t=193042


I wouldn't imagine you'd have to worry too much about gain in that situation. Unfortunately, it's
something that I do have to worry about with amplified xtal sets. This is due to having all of the
gain of the receiver at audio, of course.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sat 05, 2017 1:59 pm 
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The voltage gain of the LM386 may be increased to over 70 dB by disconnecting the negative end of the 10 uF cap going to Pin 8 and placing a small value resistor in series with it to ground which eliminates the need for an intermediate audio stage.

The single ended gain (not differential input gain) is approximately 15,000/R.


Attachments:
LM386 Gain.jpg
LM386 Gain.jpg [ 10.56 KiB | Viewed 1065 times ]


Last edited by LM386 on Aug Sat 05, 2017 2:07 pm, edited 1 time in total.
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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sat 05, 2017 1:59 pm 
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Double post!


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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sat 05, 2017 7:24 pm 
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Howdy LM386. We were just talking about you, dude! :D

LM386 wrote:
The voltage gain of the LM386 may be increased to over 70 dB by disconnecting the negative end of the 10 uF cap going to Pin 8 and placing a small value resistor in series with it to ground which eliminates the need for an intermediate audio stage.

The single ended gain (not differential input gain) is approximately 15,000/R.


Well, I connected 10 uF from pin 1 to 8 for maximum gain, did a couple of other mods, and I've removed most of the hum, motor-boating, and various oscillations I'd get when I'd adjust the volume control. The main problem I'm having now is that multiple, simultaneous, MW-BCB stations are coming through loud and clear. I have an RF attenuation pot and bandpass filter on the frontend, but when I adjust them, nothing seems to happen. I've triple checked these connections. When I adjust the RF attenuator, the MW-BCB stations eventually fade away in a linear fashion, but then I don't hear anything at all with full attenuation.

I've tried various cap values between pin 1 and 8, such as no connection at all, 10 uf, 47 uF, and 100 uF. This amp is built on one of those cheap, plastic breadboards. I'm guessing that one of those interconnecting wires with the pins on the end might be acting as an antenna (shrug). I'll try your suggestion, LM386, and see what gives.

I've also added a VFO/BFO that runs from 6.9 to 7.4 MHz, coupled the output via 2-turn link to the frontend, and it has absolutely no effect on the incoming signal to the audio amp. I've checked the output of the VFO/BFO, and it's working fine. Now I'm really starting to scratch my head.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sat 05, 2017 9:56 pm 
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Seeing that this is a thread about the LM386 and that you are a radio amateur, I thought that you might enjoy a look at my latest LM386 project, a regenerative receiver for 80 meters.

The receiver works well up to about 5 MHz with most manufacturer's LM386s and over 6 MHz with Samsung chips.

I use the TL431 shunt regulator as an audio amplifier in most of my receivers, but in this case it would seem more appropriate to use another LM386.

Likely best built over a copper ground plane, dead bug style with a ferrite rod antenna.

I use it most evenings listening to SSB nets on 80 meters up and down the East coast of the U.S. with no external antenna.

Using a standard loopstick antenna for the MW AM broadcast band and a bit of fiddling to flatten the regeneration response, it makes a simple broadcast band receiver capable of DX. In a MW broadcast band application, the audio output is large enough that the extra audio amplifier is not required and the LM386 front end will drive earbud headphones directly.


Attachments:
lm386 80m regen 2.png
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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sat 05, 2017 11:24 pm 
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Howdy LM386,

LM386 wrote:
Seeing that this is a thread about the LM386 and that you are a radio amateur, I thought that you might enjoy a look at my latest LM386 project, a regenerative receiver for 80 meters.


I always enjoy circuits that provide an excellent price-to-performance ratio! As a matter of fact, I have a passion for such circuits and have collected them and built them for years and years. :D

Quote:
The receiver works well up to about 5 MHz with most manufacturer's LM386s and over 6 MHz with Samsung chips.


My LM386 collection is mainly from old, busted computer speakers, so there is no telling if I'm going to get a Samsung or not. Maybe I'll run across one soon. (Shrug.)

Quote:
I use the TL431 shunt regulator as an audio amplifier in most of my receivers, but in this case it would seem more appropriate to use another LM386.


I never tried using a shunt regulator as an amplifier, although I've seen them being used for such applications. I was figuring that they would be a bit noisy for such applications, but apparently not. :D

Quote:
Likely best built over a copper ground plane, dead bug style with a ferrite rod antenna.


Cool. That's what I do. Basically, I breadboard a circuit, learn it inside out, make adjustments, and once it is finalized, I ship it out to the "Manhattan Construction Company," located in the Goldman Sachs Building in lower Manhattan :D.

Quote:
I use it most evenings listening to SSB nets on 80 meters up and down the East coast of the U.S. with no external antenna.


Man, that sounds like what I want to use for 80, LM386. Hopefully, I have enough litz wire to make the coil. How many turns are you using for 80? I guess I could make another winding for 40 as well, if I can find a long enough rod. I'll breadboard the thing up after I finish my current project and let you know how things go either here or via PM.

Quote:
Using a standard loopstick antenna for the MW AM broadcast band and a bit of fiddling to flatten the regeneration response, it makes a simple broadcast band receiver capable of DX. In a MW broadcast band application, the audio output is large enough that the extra audio amplifier is not required and the LM386 front end will drive earbud headphones directly.


Awesome. It's been awhile since I've built a regen. Last time I did this, I'm pretty sure I lost hearing because of howls and screeching from not having a smooth enough transition from regeneration to flat-out, full oscillation. You either have a nice circuit or you're very deaf, LM :D.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sat 05, 2017 11:40 pm 
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ab5ni wrote:
I guess I could make another winding for 40 as well, if I can find a long enough rod. I'll breadboard the thing up after I finish my current project and let you know how things go either here or via PM.

It is unlikely that the LM386 will oscillate above 6 MHz which means that 40 meters is out.

To this end I have modified a QRP Kits direct conversion receiver into a superhet using the LM386 as a regenerative IF.

http://www.qrpkits.com/ezseries.html#ezreceiver

Using a 5 MHz LO, a 2 MHz IF and both high side and low side local oscillator injection, it is possible to build an 80 and 40 meter superhet which receives AM as well as CW and SSB on both bands and achieves band switching by simply tuning a preselector in the front end.


Attachments:
QRP SUPERHET REGEN IF LM386 Pin 1 Pin 2b.jpg
QRP SUPERHET REGEN IF LM386 Pin 1 Pin 2b.jpg [ 29.03 KiB | Viewed 1027 times ]
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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 7:25 am 
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Hi Randy. I recently built a one tube radio with an amplifier circuit like the one posted by LM386 above. The only difference, I used a 150 ohm resistor connected to pin 8. Also connected the unused input to ground. Then added a .003uf (not critical) from the active input to ground. This fixed the issues for me that you are describing. Good luck.


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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 3:05 pm 
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Howdy LM386,

Quote:
It is unlikely that the LM386 will oscillate above 6 MHz which means that 40 meters is out.


Yeah. Forgot about that :D. Well, couldn't you add a bit of circuitry and get it to work? Wouldn't a frequency doubler/multiplier be sufficient? I'm thinking that would pick up the 6, 9, and possibly the 10 MHz bands? I wonder if it would be worth the effort? I'm thinking a 5 MHz xtal should suffice. OTOH, at that point, things are starting to become a bit more complicated, and if you start down that path, would it be worth retaining the LM386? Probably not. Running the circuit through my head; I'll have to think about it, or maybe you already have?

Quote:
To this end I have modified a QRP Kits direct conversion receiver into a superhet using the LM386 as a regenerative IF.
http://www.qrpkits.com/ezseries.html#ezreceiver


I love it! Nice! That's one way to tackle that, for sure. :D

Quote:
Using a 5 MHz LO, a 2 MHz IF and both high side and low side local oscillator injection, it is possible to build an 80 and 40 meter superhet which receives AM as well as CW and SSB on both bands and achieves band switching by simply tuning a preselector in the front end.


Awesome! Man, I wonder what you could do with an LM380N(GRIN!)? I sometimes wonder if you could build a semi-decent monoblock or integrated amplifier based on the LM380N. Never bothered to look at the noise figures, but I do know from empirical data (from reading the net, magazine articles, and books) that it does seem to have a better noise figure. I realize that there are some really decent op-amps with some awesome noise figures that would probably be better suited for the job, but I get a kick out of designing and building decent gear out of readily available, cheap parts, especially if I can strip them out of gear that would have normally headed for the dump.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 3:19 pm 
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Howdy Michael,

michael w wrote:
Hi Randy. I recently built a one tube radio with an amplifier circuit like the one posted by LM386 above. The only difference, I used a 150 ohm resistor connected to pin 8. Also connected the unused input to ground. Then added a .003uf (not critical) from the active input to ground. This fixed the issues for me that you are describing. Good luck.


I'll give that a try for sure. I talked with Pete via e-mail, and he said he doesn't have any problems with the circuit at all, and he recommends that I just go straight to copper. Being hard headed, I think that the breadboard offers a perfect opportunity to see what would be necessary to settle down the circuit no matter the environment. :D

What I'm wondering is this: Will doing this allow me to have enough gain for the xtal detector? In Pete's schematic, he used 47uF from pins one to eight, and knowing him for awhile now, he's sure to have modeled the entire circuit in LTSpice. I, on the other hand, have decided to go full bore and throw 10uf in there. I'd really like to see if I can get it to work okay with full gain with the diode detector.

73,

Randy AB5NI

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 6:45 pm 
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ab5ni wrote:
I, on the other hand, have decided to go full bore and throw 10uf in there. I'd really like to see if I can get it to work okay with full gain with the diode detector.
A 10 uF capacitor between Pins 1 and 8 isn't going full bore.

Increasing the value of the 10 uF bypass cap from Pins 1 to 8 has little affect on the LM386 gain.

It is increasing gain by bypassing an internal 1350 Ohm feedback resistor. A capacitor having a reactance of 1350 Ohms at audio frequencies which would decrease the gain by 3 dB would be much smaller (0.12 uF at 1000 Hz).

The differential gain of an LM386 with no capacitor is 20 (Single ended input gain is 10) (15,000/1500).

With the bypass cap from Pins 1 and 8, the single ended gain is 100 (15,000/150).

As I previously mentioned, the gain may be increased by placing a small resistor in series with the 10 UF cap to ground instead of the cap going to Pin 8.

The single ended gain in this case is approximately 15,000/R and if R is 30 Ohms the voltage gain in theory would be 500.


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LM386 schematic.png
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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 7:07 pm 
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LM386 wrote:

As I previously mentioned, the gain may be increased by placing a small resistor in series with the 10 UF cap to ground instead of the cap going to Pin 8.

I am going to try this; what value range would you suggest for the resistor?

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 7:12 pm 
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fifties wrote:
what value range would you suggest for the resistor?

I use a small trimpot of 100 Ohms and vary its value depending on the input amplitude of the source.

Depending on the input signal amplitude, the output with earbud headphones can be earsplitting.

At extremely low resistance values, the bass response of the amplifier will begin to suffer giving a "tinny" sound to the audio. In this case, increasing the value of the 10 uF cap will improve the low frequency response.


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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 8:07 pm 
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LM386 wrote:
fifties wrote:
what value range would you suggest for the resistor?

I use a small trimpot of 100 Ohms and vary its value depending on the input amplitude of the source.

Depending on the input signal amplitude, the output with earbud headphones can be earsplitting.

At extremely low resistance values, the bass response of the amplifier will begin to suffer giving a "tinny" sound to the audio. In this case, increasing the value of the 10 uF cap will improve the low frequency response.

I'm building a receiver using an MK484 TRF IC, connected to an LM 386 amp, with the 10uF mod. The problem I'm having is that when the dial tunes to a strong local or in-house transmitter, the amp seems to shut down. Then I have to turn the gain down, and readjust the dial, to get audio again. Any thoughts?

Apologies to the OP for the hijack.

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 8:14 pm 
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Is the LM386 shutting down or is the MK484 shutting down on high input RF levels?

What size input coupling capacitor do you have for the LM386?

If the cap is too large and there is a sudden change in DC levels at the cap's input from the MK484, it could take some time to re-establish a non DC potential at the input of the LM386.

Also what power supply filtering is used for the LM386?

If the series resistor to the power supply is too large, an increase in current drain from the LM386 on sudden large signal inputs could be temporarily depleting the voltage across the filter cap.

Another factor to look at is the value of the output capacitor and the load resistance.


Last edited by LM386 on Aug Sun 06, 2017 8:33 pm, edited 1 time in total.

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 Post subject: Re: 2N3904 Driving an LM386 Audio Amp Working Kinda
PostPosted: Aug Sun 06, 2017 8:24 pm 
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Re-Howdy LM386,

LM386 wrote:
A 10 uF capacitor between Pins 1 and 8 isn't going full bore.
Increasing the value of the 10 uF bypass cap from Pins 1 to 8 has little affect on the LM386 gain.


Ah. I seem to remember reading or seeing a video on YouTube (more than likely) that said removing the 10uF cap would decrease the gain, so I was figuring that playing with that value must decrease on increase the gain as well. Shows what I know :D.

Quote:
It is increasing gain by bypassing an internal 1350 Ohm feedback resistor. A capacitor having a reactance of 1350 Ohms at audio frequencies which would decrease the gain by 3 dB would be much smaller (0.12 uF at 1000 Hz).


Quote:
The differential gain of an LM386 with no capacitor is 20 (Single ended input gain is 10) (15,000/1500).
With the bypass cap from Pins 1 and 8, the single ended gain is 100 (15,000/150).


Ah! Now I get understand. Thank you very much for that explanation.

Quote:
As I previously mentioned, the gain may be increased by placing a small resistor in series with the 10 UF cap to ground instead of the cap going to Pin 8.
The single ended gain in this case is approximately 15,000/R and if R is 30 Ohms the voltage gain in theory would be 500.


Gotcha. I think I'm going to try this once I remove a few issues I'm having, such as motor-boating and oscillation while adjusting the volume control. First thing I'm going to do is put a choke on the 12V rail and see if that helps the situation. I tried using a twisted pair between pin 5 and the output cap, but that didn't seem to help, so now I'm going to see if I can find a small, shielded cable and see how that works. I guess I'm also going to have to play around with the pin 3 input and that circuitry as well. I'm suspecting the lead lengths on the breadboard are the main culprits, but if I can solve those issues via shielding and bypass, I'll probably leave the circuit as is and go to copper once I'm ready to install the receiver and transmitter in a final enclosure. Once I fix things, I'll post my findings and fixes.

73,

Randy AB5NI

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