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 Post subject: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 4:40 am 
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My gosh, 35Z5's LM386 transmitter works brilliantly! Kudo’s to Mike Toon, 35Z5, Norm, Bill, Rich, and all others who contributed to the design. It is wonderfully simple and clever. Mine is still in the breadboard stage, just a sloppy mess on the bench with an extension cord hanging from the ceiling as an antenna, but it sounds very good.

The transmitter’s audio would benefit from an Optimod front end. No way I can do that. Some folks use apps or plugins on their computers to feed their transmitters for a similar result but I really just want to plug in a phone or a CD player. so I want to see what I can do with the analog audio input. Years ago NRSC suggested a pre-emphasis equalization curve that was intended to overcome the reduced bandwidth of “typical” am receivers. I can do something based on that.

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File comment: Includes previous fixes. R5 added for LM386 overload stability.
AM transmitter LM386 NRSC boost v3.1 sml.jpg
AM transmitter LM386 NRSC boost v3.1 sml.jpg [ 77.06 KiB | Viewed 1195 times ]


Here are my modifications so far. This drawing leaves off the power supply portion but expects the same 6-10v supply, with or without the LED, with the same transmitter current draw. The two “gain” pins 1 and 8, formerly unused , now give upper frequency boost similar to NRSC curve below 10kHz. Optionally, if adjustable boost is desired, a 10k pot can be used in place of a fixed resistor R1. If using a pot (reverse log taper gives better "feel", linear is okay), the frequency breakpoint changes with amount of boost, which may or may not be ideal but sounds okay to me. The boost definitely helps clarity of voice or music through my little receivers here. Although the NRSC curve includes a “brickwall” type of low-pass filter above 10kHz to reduce adjacent channel interference, that is too complex for me but a simple 1 pole low pass filter above 7kHz is easy and I found it noticeably reduced sibilance “splatter” on my receivers. The filter drops a couple of db at 10kHz, a few more at 20kHz, and so on. A 2-pole or even 3-pole filter, with requisite more parts, might help even more (I may try later) but I decided to keep this really simple with a single pole filter. Because the filter uses the LM386's inverting input, audio is now fed to the non-inverting input. I have had absolutely no instability or rf pickup problems due to the pre-emphasis or filter *** edit: instability was later found, R5 added to correct. The bypass capacitor on pin 7 does nothing valuable that I see, so I consider it to be optional * see note below. It might be of benefit if the chip were wired for gain much higher than 20. I also noted, like 35Z5 did, increased current draw if the 150uH choke is omitted.

The boosted high audio frequencies also makes the transmitter more prone to over modulation artifacts. My next addition will be an audio limiter. Even a basic limiter is much more complex than the simple equalizer above. I don’t have any prefab ICs which would make the job easier, such as an SST2166. I also have no spare cash at the moment to buy one. I do have a stash of other parts and spare time. A limiter circuit using parts on hand will follow soon. I hope it is of interest. All comments welcomed, please!

edit 1/11, updated drawing symbols and made minor text changes
* edit 1/13, bypass cap on pin 7 is optional but helps the chip achieve best supply noise rejection. It won't hurt anything so there's no reason not to hang a 1uf or 10uf cap there.
** edit 1/16, corrected drawing error around C4
*** edit 1/24. R5 added for better stability during overload, R1 value slightly lowered to keep pre-emphasis within 1/2 db of NRSC

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Last edited by richfair on Jan Wed 24, 2018 3:20 pm, edited 10 times in total.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 6:04 am 
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Thank you for your interest...

I just did enough to get it to work, never really considered going farther... BUT, once it warms up a bit I'll try your processor mod, still have one on a breadboard so will be fairly easy...

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 8:47 am 
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Are pins 5 and 6 reversed?


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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 3:25 pm 
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Mike, 5 and 6 are not reversed. Probably a visual confusion caused by the power supply symbol I chose, which is showing +9v connected properly to pin 6. I've used the symbol for years (in Eagle schematic and layout software) and didn't realize until now it could be mistaken as an antenna symbol. I'll change it to something less confusing

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 4:03 pm 
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According to the LM386 spec sheet pin 6 is the supply voltage and pin 5 is the output. The physical locations of the pins are correct but the labels are swapped. Chip is labeled; 1,2,3,4,6,5,7,8.

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Last edited by black85vette on Jan Thu 11, 2018 4:11 pm, edited 1 time in total.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 4:11 pm 
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Ah, I see now. There is a mistake in the LM386 symbol from the library I used. The pin numbers in the library part were switched. I didn't see it. Sorry, will fix asap. Thanks for the catch!

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 4:14 pm 
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I have one of these still on the protoboard and will give this a try. Thanks for the mod.

BTW; regarding pin 7 bypass; if I recall correctly it is only needed at the higher gain configuration. I eliminated it on mine.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 5:58 pm 
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richfair wrote:
Even a basic limiter is much more complex than the simple equalizer above. I don’t have any prefab ICs which would make the job easier, such as an SST2166. I also have no spare cash at the moment to buy one. I do have a stash of other parts and spare time. A limiter circuit using parts on hand will follow soon. I hope it is of interest. All comments welcomed, please!


You've come up with a thoughtful and useful improvement. I applaud the simplicity.

Years ago, to keep TV volume at a constant level, I came up with a limiter that should work with this circuit. My design criteria included using inexpensive readily-available off-the shelf parts. Here's the link and schematic: http://www.tompolk.com/hobbies/automati ... r/avc.html

Note that it includes universal input and output buffers so it can be used with any input and output device. Therefore, depending on your source and the interface to the LM386, your use of this circuit might be simplified down to just one IC. I plan to try that on the Class E transmitter I'm working on. The optocoupler (LDR) parameters are not that critical. Shown is this 99 cent option: http://www.goldmine-elec-products.com/p ... ber=g15396

Others who built it substituted an LED for D3 to provide visual representation of the amount of compression.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Thu 11, 2018 7:31 pm 
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Macrohenry,

Thanks for this suggestion! I have already been thinking an LDR type of compressor would be a good addition. That type of level detector sounds good. LDR does not have good peak level control (one reason it sounds good). I have a fet based peak limiter breadboarded into the transmitter already, which does a credible job of peak control. The two approaches (slow LDR, fast fet) should be complimentary when put together. Power supply considerations are important... more on that later.

I'll post my work-in-progress fet limiter schematic this evening. This afternoon temp is above freezing for the first time in a month and I want to take advantage outside!

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Fri 12, 2018 3:16 am 
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Parts count will rapidly rise from this point on :)

My original goal was to control peak AC levels coming out of the LM386, and thus control modulation level. I thought about an LDR circuit but I don’t have any LDRs on hand (thanks again for the link to the Goldmine product. It looks perfect.) Also, LDRs are relatively slow to respond to light changes and are not well suited to follow instantaneous peaks. Fets used as voltage controlled resistors can respond very quickly when full wave detection is used to catch positive and negative signal peaks. Unlike simple resistors or LDRs, fets are notorious for non-linear AC gain (distortion) when used this way. This circuit controls fet distortion reasonably well .

<image removed by richfair>

Sound quality is decent. Could be better and I’ve heard worse. It maintains good peak control but is audibly working when pushed hard. Power supply is another problem. A single supply rail between 6-10 volts is spec’d for the original transmitter. The fet is very responsive to small control voltage changes whether from program detection or power supply change. For this reason I have decided to design for a firm 9 volts supply (for the limiter portion at least). OpAmps are typically pos-neg rails with signals somewhere in between. Since we are using a single supply (in deference to existing transmitters) It is helpful to have a virtual ground for the opamps being used on a single supply.

My use of virtual ground could stand improvement. It requires additional parts. IC2A, another cheap TL072, is not an ideal source and reacts to changes of current draw on its output. It is best to have zero current flow, or at least unchanging flow, on a virtual ground. IC1A imposes no current flow on the virtual ground. The current from detector diodes is what we want to have. My “limit LED indicator” circuit (IC2B) DOES force current into the virtual ground at inconvenient times, a flow that changes when the limit LED indicator turns on and of. Glitches appear on the virtual ground that are imposed to the fet and therefore into the audio. My temporary solution is to disconnect the limiter’s LED when not needed, which is obviously not optimal. Whether a proper “rail splitter” IC would keep a stable output I don’t know. I don't have any here. I sub’d in an OPA2604, a faster chip with larger current drive, with no real improvement. Perhaps my messy breadboard, with random impedences, is a contributor but somehow I don’t think so.

The limiting ratio is presently about 10:1 (a 10 db increase on input results in a 1 db increase on output). Ratio can be altered by changing the impedance of the audio signal present on IC1A’s non-inverting input. Increasing values of R6 and R9 will increase limiting ratio, to a point. Lowering them will have the opposite effect. For strongest peak control, high ratios work better (but are more audible). I have settled on 10:1 as a compromise for now. BTW, there is no magic in the ratio of R6 to R9. R9 was sort of chosen randomly. If lower values of R6 and R9 are used, C9 and C10 may need to be increased to prevent objectionable low frequency attenuation of the audio signal.

The detector diodes see only the LM396 output. Threshold for onset of limiting, and thus modulation levels, is somewhat adjustable via R23, near the fet. I have picked a value that seems optimal on my breadboard, while listening through my awful little radios. Actual instantaneous modulation is hard to judge on a ‘scope. Raising the threshold (more modulation) seems to increase chance of audible distortions, most noticeable on voice glottal and hard consonant sounds, which is already accentuated by pre-emphasis equalization . This needs more experimentation.

All thoughts and derisions are welcomed. I fear this is so complicated already that its value is debatable.

I forgot to mention, this circuit doubles the current requirement. My transmitter (with antenna trim indicator led bypassed) draws around 15ma at 9 volts. 30ma with the limiter.

edit 1/12: Uploaded a corrected schematic, I had made a serious mistake while drawing. It should now represent what is breadboarded when this entry was first posted. Design is still very much a work in progress
edit 1/17: corrected schematic again to show grounded C4
edit 1/21: schematic has been retired, look for design changes

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Last edited by richfair on Jan Wed 31, 2018 1:18 pm, edited 5 times in total.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Fri 12, 2018 4:18 pm 
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There is value in doing some audio processing. Pandora is my usual choice of music but their commercials are way louder than the average music track.

I added the C1/R4 and C2/R1 components. Didn't really notice any difference (by ear) with the C1/R4. But the C2/R1 seemed to have a positive effect on the high end. The "S" sound and the cymbal sizzle picked up a bit to my ear. I no longer have a spectrum analyzer to quantify this. I also noticed that at higher modulation levels I could see some "trash" on the signal on my scope. Not sure what it is or where it comes from but the addition of C2/R1 cleaned it up. So I am adding them to my circuits. C5 did not appear to affect anything.

Since I am feeding the transmitter from a retired iPhone I have started using a signal generator app on the phone. It is handy to have a clean sine wave available for set up and testing.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Fri 12, 2018 11:38 pm 
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black85vette wrote:
There is value in doing some audio processing. Pandora is my usual choice of music but their commercials are way louder than the average music track.
Then I shall publicly proceed! Thank you for your comments.
Quote:
I added the C1/R4 and C2/R1 components. Didn't really notice any difference (by ear) with the C1/R4.
Makes sense. Those two produce a gentle high frequency roll-off that only begins around 8kHz. Most people will have trouble hearing any effect under typical listening conditions. Those of us "of a certain age" probably won't hear any change at all. There is a technical benefit <arguably> to having such a roll-off especially when the C2/R1 components are also used
Quote:
But the C2/R1 seemed to have a positive effect on the high end. The "S" sound and the cymbal sizzle picked up a bit to my ear.
Exactly what the NSRC pre-emphasis recommendation can do.
Quote:
I also noticed that at higher modulation levels I could see some "trash" on the signal on my scope. Not sure what it is or where it comes from but the addition of C2/R1 cleaned it up.
Can you clarify this please? Are you saying you did not see "trash" until some of the new components were installed, and the "trash" then disappeared when all 4 new components were installed?

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sat 13, 2018 4:33 am 
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richfair wrote:
Can you clarify this please? Are you saying you did not see "trash" until some of the new components were installed, and the "trash" then disappeared when all 4 new components were installed?


I don't recall actually looking before trying the new components. With all 4 removed I noticed it. With C2/R1 in place it was gone. I will go back and verify this on the breadboard.

Edit; I confirmed that with all four removed the low area between the peaks gets some trashy look to it. With C2/R1 in I can take it all the way to a flat line between the peaks when over modulated with the trash gone. I will try to get some pictures as I am sure "trashy" is not a very technical nor descriptive term. :lol:

BTW; I am picking up the RF signal by placing a 250 uh inductor next to the antenna wire and connecting the scope probe to that. I could not find another place to look at the signal that did not interfere with it.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sat 13, 2018 2:25 pm 
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black85vette wrote:
I confirmed that with all four removed the low area between the peaks gets some trashy look to it. With C2/R1 in I can take it all the way to a flat line between the peaks when over modulated with the trash gone. I will try to get some pictures as I am sure "trashy" is not a very technical nor descriptive term.
Thank you for clarifying. A guess is you may be seeing a product of stray rf pickup by LM386. Are you using the 100p cap on pin 3? Try a bypass cap on pin 7. Look at the AC waveform on LM386 pin 7. It will be at its most negative during modulation pinch-off. Do you see disturbance there? Is the presence of "trash" altered when you change sinetone frequency?

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sat 13, 2018 7:47 pm 
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richfair wrote:
Thank you for clarifying. A guess is you may be seeing a product of stray rf pickup by LM386. Are you using the 100p cap on pin 3? Try a bypass cap on pin 7. Look at the AC waveform on LM386 pin 7. It will be at its most negative during modulation pinch-off. Do you see disturbance there? Is the presence of "trash" altered when you change sinetone frequency?


First off to eliminate a couple of variables; I did not see any significant difference when changing the frequency of the audio signal or the voltage to the transmitter between 9 and 12 volts.

I found earlier that the cap on pin 3 was necessary. Stray RF problem I suspect.

I had removed the bypass on pin 7 earlier based on no noticeable difference to the sound quality. But after making all these checks at higher modulation levels it seems to provide some benefit over about 80% modulation. Not a big deal but worth including it I think. In looking at pin 7 without the bypass cap it produces a clean audio signal regardless of what the modulation is doing.

Pin 2 was connected to ground during all of this.

Right now it is running really clean. I think the compressor will make an all around improvement by allowing the audio to be run higher on average. This will eliminate the distortion on brief over modulated parts. Plus the radio can be run at a lower volume reducing AM noise and AC hum between tracks when there is no modulation.

I used to work for a large AV engineering company. Back then I had a nice UREI compressor limiter but got rid of it and most of my sound reinforcement gear. Didn't think I would need it again. :oops:

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sat 13, 2018 9:16 pm 
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Good grief I need to proof read myself before hitting SUBMIT

I wrote to look at waveform on pin 7 BUT I meant to say pin 5 (DC and AC drive to crystal osc)! Dyslexia gone rampant again.

Urie 1176 was the first pro fet-based compressor limiter. I grew up with them and owned one for many, many years. My limiter above is inspired by an 1176 in that it uses the same approach to fet gain reduction. As posted, it works effectively even though I intend to re-think the whole thing.

I think now that the pin 7 bypass cap should be installed always. In the worst case it does nothing, but a 1 to 10 uf cap will increase immunity to power supply noise. Cheap insurance, so why not?

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sat 13, 2018 11:03 pm 
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I agree on pin 7 cap.

I was out poking on the Internet and found a couple of interesting circuits. The first is FET based and designed around the LM386. The second is an updated project and has all the adjustments you would expect on compressor / limiter.

http://pira.cz/enlter.htm

http://pira.cz/hyperlme.htm

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sun 14, 2018 3:25 am 
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Thanks for those links! I see good and not-as-good ideas in that author's updated circuit, but my opinions good and bad are not based on a working build so I may be wrong. What I like most is the idea to make high frequency limiting separately adjustable.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Sun 14, 2018 3:56 am 
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It is cold outside and I am still in a DIY mood. I will be waiting for parts a few days so I did a quick project using what I had on hand. Using a couple of 2N3904 transistors and an LED I cobbled together an audio peak indicator. It is set it to light up at about 80% modulation rather than 100% so some LED flicker is a good indicator. It is being fed directly from pin 5. Not as cool as a compressor telling you it is acting on the signal but OK for now.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Jan Tue 16, 2018 4:39 am 
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OK. So I got carried away with audio processing. In digging through my piles of "stuff" I found a new unused Radio Design Labs compressor limiter that I completely forgot that I had! I also have a 3 band RDL equalizer. Tucked in my closet was a 4 channel Shure microphone mixer with both low and high impedance mic/line inputs. So I hooked it all together and went through to set the gain for each stage and it worked really well. The compressor limiter has 3 indicators so you know how much it is attenuating. It has a gentle curve and does not pump or breath. With the EQ I just gave the upper end a little help. Guess I can hook up a mic, turn table, tape deck and run my own station now. :lol:

Does seem a bit over the top to put about $500.00 of processing in front of a $10.00 transmitter. Anyway this was just an exercise to see if it worked. I really want to build the C/L into my transmitter and have it self contained.

Two projects completed tonight. I also finished my Philco Model 20 and was listening to it playing music from the transmitter.


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