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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sat 21, 2012 2:49 am 
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Thanks Bill
That is one crazy website.

-Sam T.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sun 22, 2012 1:07 am 
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Well I got started on building a transmitter this evening, it's working on the breadboard thanks to 35Z5.
Found a 12V/1A transformer in the junk box so this one will have the full power supply in the enclosure, this way I can use a three-conductor cord and have a ground. The mp3 player will be running off a 5V regulator.

Attachment:
DSCN1405.jpg
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This one's going in a see-through box.

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DSCN1406.jpg
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Power supply board

-Sam T.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sun 22, 2012 2:29 am 
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Finished up my 2nd one tonight. Plays great, just like the first one. :D

Larry

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 23, 2012 10:17 am 
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Boy, these things are ridiculously simple to make :lol:
I was ordering some oddball parts off 'surplus-electronics-sales' when, low and behold, they had an el-cheapo M-TRON 1 MHz OSC ($.75) and the LM384N-4 ($.99. They also have the lower power one for $.69) so I figured why not, for a buck 75. Actually, 'shipping' was all that had been holding me back anyway. I used another one of my 'Ebay-deal' ferrite antennas for the 330uH, skipped the 82 uH, and tossed in a 7809 voltage regulator ($.40). I also skipped the 'volume pot' but may add one later because they've actually got a little 'too much' gain for PC sound cards. Either that or add some more external feedback. Anyway, this is my skinny version that I am listening to as I type.
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LM386txsch.jpg
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The LED is a 'big' one, just for fun. Power is from an overpowered but spare 13VDC wallwart which, since it's doing only 15 mA, rides at 19V but with so little power that's no problem for the bare 7809 (also at surplus-electronics-sales).

There were no specs for the M-TRON so I wasn't sure if it would work right but, no sweat. Just wire, power, and listen.
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LM386tx.jpg
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They may be a bit difficult to see in the picture but I use clipped component leads as jumpers to 'wire' the pin board.

It just doesn't get much simpler than this but, of course, that's because 'someone else' designed the voltage regulator, oscillator, and audio amplifier (modulator). I still like doing it myself on the others but you can't beat these things for 'instant gratification' 8)


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 23, 2012 3:26 pm 
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Well I dunno about your osc, but the circuit will draw approx 1/3 less current with the ECS when the coupling coil is used... Even a 47-68 ohm resistor makes a big difference but of course does have some voltage drop...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 23, 2012 7:35 pm 
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I just didn't have a coil to use for the 82 uH nor feel like waiting for one. As a side note, I thought it rather strange there's not a single inductor of any type at surplus-electronics-sales and if one needed to buy a complete parts list you'd be better off getting the lot from Mouser and save a second shipping charge.

I plan to eventually order some but may try winding one on one of these toroids I've got as a substitute since 'efficiency' probably doesn't matter on the audio line, but they didn't work very well for the antenna tank on Beamus.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 23, 2012 8:17 pm 
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The current issue I mentioned got me thinking, so I did a couple tests with & without the coupling coil...

On the present bread board setup, current is same with or without coil, but if there is ANY stray RF at the LM386 input current skyrockets... When I connected a loose clip lead to pin #2, current shot from 11ma to over 30ma... With the coupling coil, current actually fell slightly with the added clip lead... Adding a 500pf cap from pin #2 to ground will help minimize RF entering the LM386, which is a possible issue for those using one of the mini USB/card readers...


Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 23, 2012 8:49 pm 
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35Z5 wrote:
Well I dunno about your osc, but the circuit will draw approx 1/3 less current with the ECS when the coupling coil is used... Even a 47-68 ohm resistor makes a big difference but of course does have some voltage drop...
Something I'd like you to try and confirm but, first, background. I did wind up a quickie 94 uH and it appeared to give similar results. When peaked 'no coil' current was 'around' 20 mA but more like 13 mA with it. I'm not sure of the 20 mA number because when I reverted back to take a better measurement all of a sudden it seemed to *still* be 13 mA. Now, I'm not sure what 'changed' in the interim, lead dress maybe, but had always wondered why a coil on the output made such a difference.

At any rate, after trying a few things it looks to me like a 100 pF on the input (my resistor values) to ground achieves the same result but I haven't done a lot of investigating yet and a 470 pF might be better since 100pF, on mine, is only a 470KHz rolloff. Btw, pin 3 is N/C on mine.

I just thought you might find that interesting and my OSC is different so things might behave different.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 23, 2012 8:54 pm 
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35Z5 wrote:
The current issue I mentioned got me thinking, so I did a couple tests with & without the coupling coil...

On the present bread board setup, current is same with or without coil, but if there is ANY stray RF at the LM386 input current skyrockets... When I connected a loose clip lead to pin #2, current shot from 11ma to over 30ma... With the coupling coil, current actually fell slightly with the added clip lead... Adding a 500pf cap from pin #2 to ground will help minimize RF entering the LM386, which is a possible issue for those using one of the mini USB/card readers...
:lol: We were doing similar things. Did you try it with *just* the input bypass cap?

Btw, what got me on it was noticing my 'finger' near the input cap did the same thing you saw with a clip lead.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Tue 24, 2012 5:57 am 
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I got a chance to do a little more checking and, on mine, current seems the same whether its the coi, input bypass cap, or both. Although, it seems I need to re-peak the trimmer between coil and no coil but I'm not sure if maybe I'm just bumping something.

I checked frequency response at a couple of points and amplitude is right spot on but exhibits a definite horizontal skew at 18 KHz. It's not on the LM386 output so I'm guessing some kind of slew rate issue inside the OSC. Of course, 18 KHz is beyond AM bandwidth anyway but I think it starts to show around 10 KHz. Hard to tell because it's slight and I've got some noise pickup on my scope. Anyway, if its OSC slew rate then it could be my M-TRON is 'different' than the ECS you’re using. Have you looked at the upper end of the spectrum on your output?


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Tue 24, 2012 6:08 pm 
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Yeah I remember seeing some distortion of the waveform at very high input freq... Since few AM radios have more than a 5K bandwidth I never worried about it...


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 25, 2012 5:05 am 
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35Z5 wrote:
Yeah I remember seeing some distortion of the waveform at very high input freq... Since few AM radios have more than a 5K bandwidth I never worried about it...
Yeah, I agree it won't make much, if any, difference for the ubiquitous table radio but it does show that, if one is intersted in 'higher-fi' AM, then the tube jobs have a place.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Fri 27, 2012 9:53 am 
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Well, here's my latest version
Attachment:
LM386txschv2.jpg
LM386txschv2.jpg [ 49.97 KiB | Viewed 2091 times ]
The resistor across D1 accentuates tuning indication since it bypasses the bulk of the idle current but there's still enough for an 'on' light. The added .33uF (metal foil) is to accommodate long wallwart leads per the 7809 datasheet recommendation.

I noticed some 'extra' current draw under extreme noise conditions, like directly contacting the input (admittedly not 'normal') so I'm using the coupling coil too although, under normal conditions, mine works fine with just a wire connection. Almost anything will work, meaning even the toroids I've got that are useless as 'RF coils'. I also tried various 'PC power supply' noise suppression toriods ranging from 82 uH to 1 mH and they all worked. I ended up using a small toroid I wound to 94 uH because there's nothing else to use it for.

C7 on the input shunts RF noise pickup. C2 and C3 are physically "at" U1. C1 is at the regulator.

The 10k mixer into 4.7k is just right for my cell phone output and more than enough for PC/notebooks.

The current range is just what it will operate over and actual current on mine is around 14 mA peaked, partly because my LM386 pulls 7 mA, a little more than 'typical'. The 'max' 20 mA at 30VDC puts around 390mW in the 7809, which the TO-220 should be able to handle without heatsinking, and at the more 'typical' 14 mA it's around 270mW. I'm running mine at 20V, which is only 133 mW.

Here's the current breadboard using the more readily available 'resistor looking' coils.
Attachment:
LM386txv2.jpg
LM386txv2.jpg [ 103.78 KiB | Viewed 2091 times ]


Edit: added breadboard picture


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sat 28, 2012 4:28 pm 
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rocketeer wrote:
I'm finishing up the one I'm building for a friend and the 0-50pf trimmer/capacitor is bad.


Image http://www.sm0vpo.com:800/begin/gimmik-0.htm


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sat 28, 2012 4:51 pm 
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Flipperhome wrote:
Well, here's my latest version


On a final build, I'd recommend keeping the B+ input(audio) and RF output spaced farther apart on the osc... The buss type contacts on that proto-board likely have some coupling action in RF circuits, especially if directly adjacent to one another...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sat 28, 2012 5:06 pm 
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Mike Toon wrote:
rocketeer wrote:
I'm finishing up the one I'm building for a friend and the 0-50pf trimmer/capacitor is bad.


Image http://www.sm0vpo.com:800/begin/gimmik-0.htm

Thanks Mike. Not sure how you would use that to tweak the LED for max output to the antenna.

Larry

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sat 28, 2012 7:19 pm 
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rocketeer wrote:
Mike Toon wrote:
rocketeer wrote:
I'm finishing up the one I'm building for a friend and the 0-50pf trimmer/capacitor is bad.


Image http://www.sm0vpo.com:800/begin/gimmik-0.htm

Thanks Mike. Not sure how you would use that to tweak the LED for max output to the antenna.

Larry


LOL that's easy, more turns = higher capacitance, open it up a bit for less...

Though it could work, would be a bit of a PITA to set as capacitance from the tool or even you fingers would skew the adjustment... Will work just fine where a fixed capacitance is needed(maybe a increase to the trimmer already in place)...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sun 29, 2012 2:22 am 
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Finally finished up my transmitter today, works great.
I intended to use a clear plastic enclosure but shattered it while cutting holes.
So I used a regular RadioShack box.
There's a 12V/1A transformer with bridge rectifier and a 5V regulator for the MP3 player.
Used a three-wire power cord to get a good ground.

That's just the remote control laying on top.

Many thanks to Tom and Bill for setting up the kit!

-Sam T.


Attachment:
DSCN1415.jpg
DSCN1415.jpg [ 39.65 KiB | Viewed 2065 times ]


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 30, 2012 10:51 pm 
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Nice setup Sam, makes for a compact unit...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Aug Wed 01, 2012 3:35 pm 
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Hello Sam,
Where did you get the Mp3 player at


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