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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Sat 10, 2018 4:06 pm 
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Quick update to say THANKS to Rick (black85vette) for sending me one of those LDRs from Electronic Goldmine. It is wired into my existing circuit as a variable shunt load on the audio signal, exactly as my fet had been (now temporarily disconnected). For simplicity I am using the same fullwave detector and surrounding gain stages for both LDR and fet drive. Results look encouraging on the ‘scope but no listening tests yet. Response lag of the LDR looks to be around 2ms (depending). I have not measured recovery time yet but both can be adjusted (slower) during listening tests if need be. In the next day or two I’ll reconnect the fet. Being much faster, it can capture loud peaks missed by the LDR. The challenge is to make them work in complimentary ways. At that time, I can post a new schematic-in-progress.

As soon as the LDR part arrived from Rick I began to chart its resistance curve vs. led drive current starting about 0.1ma. When current was increased to 0.75 ma, the resistance value suddenly shot up from about 1k to nearly 5k. It should have gone lower. The measurements I had just made also showed much higher resistance than only moments before. I assume the internal LED suffered a partial failure. There has been no further change since. I just re-charted the curve which looks reasonable. I have no idea if I am now experimenting with a mutant freak LDR but will push ahead anyway. It will be easy enough to adjust drive current later if necessary when a different part is obtained.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Sat 10, 2018 5:21 pm 
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richfair wrote:
For simplicity I am using the same fullwave detector and surrounding gain stages for both LDR and fet drive.


I'd like to see this ckt. Is it posted somewhere?


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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Sun 11, 2018 8:31 pm 
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Rich: See if this helps. I just hooked up the same LDR with 12.36 volts through a 50k pot and measured:

.25 ma = 61k ohms
.5 = 25
1 = 10
2 = 5.4
3 = 3.6
4 = 2.8
5 = 2.3
6 = 2.0
7 = 1.8
8 = 1.5

Hope that helps determine if the one you have is still OK. This was not done with high precision meters so some fudge factor may be present.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Mon 12, 2018 5:17 pm 
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Macrohenry, a new schematic is on the way later today.

Rick, thank you for this. "Fudge factor" doesn’t matter much. I don’t think I am wasting my time with this ldr but it is not behaving the same as yours. Drive current setting can probably make them behave very similarly to each other.

Graph 1, measured ohms vs. drive current. I included a few measurements before this ldr had its sudden change. Goldmine does not give any specs for this part, and I imagine it is because there is a lot of variance from one to the next. I think they say it could come from different manufacturers, thus there is no way to know what you’ll actually get.
Attachment:
LDR ohms.JPG
LDR ohms.JPG [ 23.55 KiB | Viewed 373 times ]


Graph 2, the same data displayed on a log scale for easier visualization. Log scale is prevalent in audio work. Curve shapes are more important than actual ohms.
Attachment:
LDR log-ohms.jpg
LDR log-ohms.jpg [ 21.53 KiB | Viewed 374 times ]


Graph 3. I hooked up an opamp driver similar to what I intend to use (coming in my next post) to drive the ldr with 1kHz sinetone across a wide range of drive currents. The vertical scale is a relative one, using the dbU output of my generator into the opamp. An ideal curve here would be a straight line. I realized my test driver clipped just above a drive level of -11dbU (I meant for clipping to be above -10dbU but didn’t account for reality). Clipping set -11 as the right-most graph point. It corresponded to about 17ma drive current. The curve is beginning to vertically flatten out by that point so I did not bother to recalibrate and run the graph again. I want to operate within the flattest section. This curve looks reasonably good between -50 and -11, which is about a 40db range and adequate for this use.
Attachment:
log(R) vs DB input,jpg.JPG
log(R) vs DB input,jpg.JPG [ 19.04 KiB | Viewed 374 times ]


These LDRs are sensitive to temperature change. Seems like every time I tried to repeat a measurement I'd get a slightly different result. I can blow across it or heat with a finger and watch the resistance drift. My guess is the sensitivity won't be a practical problem.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Mon 12, 2018 5:49 pm 
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Rich; just thinking that after putting in all the work to design the circuit that it would need to be reproducible. Would it help to source a standard known component from a reliable source like Digikey or Mouser? It would be a shame to publish the schematic and then have Electronics Goldmine run out of the LDR and no way to find another source for it.

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Mon 12, 2018 6:46 pm 
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Yes, that is exactly my intention. This part looks like a good choice:
https://www.digikey.com/product-detail/ ... ND/5039793

I also notice a couple of links there for limiter and compressor app notes, within which they talk about selected components within the NSL32SR series. There will be variances from part to part!

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Mon 12, 2018 7:33 pm 
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Macrohenry wrote:
richfair wrote:
For simplicity I am using the same fullwave detector and surrounding gain stages for both LDR and fet drive.


I'd like to see this ckt. Is it posted somewhere?

This is what I have at the moment. The upper section is the exact same circuit as shown in this thread's first post, but with the additional circuitry wired in below it. It must be stated, THIS IS A WORK IN PROGRESS AND SHOULD NOT BE ASSUMED TO MEET ANYONE'S NEEDS.
Attachment:
AM trans with complimit v4 sml.jpg
AM trans with complimit v4 sml.jpg [ 179.44 KiB | Viewed 368 times ]


I have ignored the limit indicator section (lower right corner) completely. When breadboarded with the fet removed, it works nicely. The compression ratio can be changed by altering opamp gains and the impedence of the buffer's input. Values shown make a nice 2.5:1 using the opto Rick sent me. I ended up removing a 2.2uf capacitor that had been across R40 (475k). You'd expect distortion to go up without it, but I did not detect that, and it handled some music better. In hindsight a small value cap would probably be smart there, something like .1uf. More listening is needed, but I began thinking that maybe it was good enough without a limiter at all. A few brief overages from a part 15 transmitter through old radios is hardly a major transgression. Macrohenry, you have already discovered this approach makes a very useable compressor.

After that is when my day went downhill. As soon as I hooked up the fet portion of the circuit I instantly ran into instabilities, more of the same high frequency ringing that gave me fits earlier. I am sure it is poor layout, dirty breadboard connections, very high fet impedance coupled with high gain (50+db from input to LM386 output). Single supply rejection is much worse than dual supplies, which only makes it all worse. I grew tired of fighting the instability yesterday and have decided my project should not be continued until assembled more properly. THAT will have to wait. Also, after finding API app notes for the above-linked opto isolator, notes which describe fully functioning limiter and compressor circuits, I have to stop and decide if I want to continue down this path. Is my work any better, or as good, as what is in the app notes? My attempt is certainly not any simpler or requires fewer parts. In fact, I need to study the detector scheme shown in the app notes. It is about half as many parts.

What to do?

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 Post subject: Re: Audio processing an LM386-ECS100 transmitter
PostPosted: Feb Mon 12, 2018 10:30 pm 
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Rich; one idea might be to approach it like Macrohenry and have the compressor not integrated into the transmitter. You would not be limited by the LM386 and could control the gain in the design. Then this could be a modular front end processor to a transmitter by putting it in-line between the source and transmitter input. Likely not a bad idea to get it outboard and away from the RF anyway. I experimented some with Macrohenry's circuit by reducing the input gain by half and then eliminating the output stage since it was too much signal for the input to the LM386.

I still believe a compressor/limiter for the LM386-ECS100 transmitter is worth the effort because the transmitter is affected so much by over modulation. Much more so than my tube transmitter.

BTW; here are the application notes for the VTL5C4 which was widely used in guitar effects pedals and is no longer produced. But the notes might be useful;

http://www.perkinelmer.com/CMSResources/Images/44-3429APP_AnalogOpticalIsolatorsAudioApps.pdf

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