Actually I never ran AM except to check my SWR. LOL SSB all the way baby. Now this was back in the late 70's/early 80's mind you. I used either an old Midland 13-898B or a Colt Black Shadow.
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In this case the "mushy" sound has nothing to do with compression and everything to do with the 10 uf cap across the output of the TA-7222AP chip. It's basically a short to all the treble and an open to the base frequencies. Exactly what ones does to create a "high cut" tone control and what we want to avoid if the goal is to provide even compression across the audio range.
Turner mics sound bassey for the same reason Astatic mics sound trebly. It's the frequency response of the element used in the mic. Cheap speech compressors can cut treble for the same reason cheap echo boards do. They use filters to limit the frequency response to communications grade audio.
Light compression can sound as natural as music on your FM radio. Heavy compression can be as annoying as our countries useless emergency alert system. So objectionable that every hearing person in range cuts the volume smack off the very second the tones hit the speaker!
If you want natural sounding compression you really have to avoid just about everything made for this market. You don't have to spend more money either. Cheap compressors geared for the pirate broadcast market are available for $40 and run circles around the fidelity the CB and ham stuff offers.
With respect to me providing the resistor values that could be used to replace the 10 uf cap, that requires experimentation with the circuit to find the optimum values. I also don't feel this loading approach is the best method and was just providing clues to improve it's function across the audio range it's used in.
Loading the circuit harder like this causes more distortion and has an effect on the entire audio signal, not just the peaks we need to address. Extra loading will also cause the output to flat top earlier than it would normally and there is no filtering afterwards to reverse this process.
Even the cheapest compressors don't use this loading approach. They simply chop off everything in excess of the desired level using a limiter. Leaving it as flat topped and distorted as you can imagine. Only to run it through effective low pass filtering that cleans it right back up with nice rounded corners through the peaks.
Turner mics sound bassey for the same reason Astatic mics sound trebly. It's the frequency response of the element used in the mic. Cheap speech compressors can cut treble for the same reason cheap echo boards do. They use filters to limit the frequency response to communications grade audio.
Light compression can sound as natural as music on your FM radio. Heavy compression can be as annoying as our countries useless emergency alert system. So objectionable that every hearing person in range cuts the volume smack off the very second the tones hit the speaker!
If you want natural sounding compression you really have to avoid just about everything made for this market. You don't have to spend more money either. Cheap compressors geared for the pirate broadcast market are available for $40 and run circles around the fidelity the CB and ham stuff offers.
With respect to me providing the resistor values that could be used to replace the 10 uf cap, that requires experimentation with the circuit to find the optimum values. I also don't feel this loading approach is the best method and was just providing clues to improve it's function across the audio range it's used in.
Loading the circuit harder like this causes more distortion and has an effect on the entire audio signal, not just the peaks we need to address. Extra loading will also cause the output to flat top earlier than it would normally and there is no filtering afterwards to reverse this process.
Even the cheapest compressors don't use this loading approach. They simply chop off everything in excess of the desired level using a limiter. Leaving it as flat topped and distorted as you can imagine. Only to run it through effective low pass filtering that cleans it right back up with nice rounded corners through the peaks.
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What a great thread. I thought I knew a lot about audio and I have learned a couple of things. Great work guys.
Agreed. I am also learning a bunch as well. Sounds like in the case of the CP-1 compressor and the Turner mics, there's too much compression and that could explain the muddy-ness of the audio. I've been doing a little research, and it seems that a small amount of compression would make a radio sound really well.
If I were to find something, or modify the radio in a way to provide a small amount of compression, I'd probably talk a lot more on AM.
(Edit) I know that on the Turner mics, the compression could be adjusted so the mic itself really isn't an issue, it's more of an operator thing.
~Cheers~
If I were to find something, or modify the radio in a way to provide a small amount of compression, I'd probably talk a lot more on AM.
(Edit) I know that on the Turner mics, the compression could be adjusted so the mic itself really isn't an issue, it's more of an operator thing.
~Cheers~
Hey Shockwave:
Since the problem with negative compression is dependent upon freq, then a series L/C might be the proper solution and not just using a capacitor alone.
Since the problem with negative compression is dependent upon freq, then a series L/C might be the proper solution and not just using a capacitor alone.
Robb and Shockwave,
How about adding a "X" value wirewound resistor in series with the capacitor? Since wirewound resistors are inductive, it could serve a dual purpose. Also, perhaps installing a switching diode (and possibly a non inductive resistor inline with the diode) between the wire wound resistor and the cap to ground could perhaps create a constant reference? I'm at work right now but I did draw up a quickie schematic so when I get home I can do some research.
~Cheers~
How about adding a "X" value wirewound resistor in series with the capacitor? Since wirewound resistors are inductive, it could serve a dual purpose. Also, perhaps installing a switching diode (and possibly a non inductive resistor inline with the diode) between the wire wound resistor and the cap to ground could perhaps create a constant reference? I'm at work right now but I did draw up a quickie schematic so when I get home I can do some research.
~Cheers~
Perhaps we are barking up the wrong tree here too. For example, in the 3600 chassis a 1N4001 diode and a 100 ohm/quarter watt resistor in parallel with C189 are used to compress the negative peak in the AM Regulator circuit. Not in the audio circuit, as was done in the 8719 chassis. If the audio circuit of the 8719 has the negative compression added there, it will also affect SSB audio too since it shares the same IC. We sure don't want that either, since ideally we just want to affect AM modulation. That would just put our nuts in a bind if we did it in the audio circuit.
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Ok, did some experimenting when I got home from work. I did this on a Cobra 2000 GTL. I got the best results by using a non-polarized .22uf mica dipped capacitor with a 22 uH choke in series. This netted me about 150% positive peaks, about 90% negative peaks with no changes in audio tone or clarity. SSB output and audio were also unaffected.
This time, I DID get audio checks from a neighbor of mine that's about 4 miles down the road from me, and he gave me the reports posted above. Is this the ultimate answer to compressing the negative peaks for AM CB? Probably not. But it sure is fun to experiment.
Just for giggles, I did it the original way, with the 10uf 25V electrolytic cap, and gave my neighbor a holler, he told me I sounded flat and muddy. Removed the cap, and it cleared me up immediately. Positive peaks on the Cobra 2000 with the electrolytic cap were close to 200%.
Also of note, I tried several different values of electrolytic caps, from .047uf to 10uf. They all did the exact same thing, both on the scope and in actual audio checks, all sounded muddy and flat. It seems as though it works better with a non-polarized capacitor, I had better adjustability of the AMC circuit, as well as better performance from an audio standpoint. And while my positive peaks weren't as high with the non-polarized cap (150-160%), that's really no big deal. Who wants to have 200% positive peaks anyhow, when you're sacrificing audio quality? I wouldn't.
If anyone is interested in some scope pictures, I'll gladly take some and post if requested.
~Cheers~
This time, I DID get audio checks from a neighbor of mine that's about 4 miles down the road from me, and he gave me the reports posted above. Is this the ultimate answer to compressing the negative peaks for AM CB? Probably not. But it sure is fun to experiment.
Just for giggles, I did it the original way, with the 10uf 25V electrolytic cap, and gave my neighbor a holler, he told me I sounded flat and muddy. Removed the cap, and it cleared me up immediately. Positive peaks on the Cobra 2000 with the electrolytic cap were close to 200%.
Also of note, I tried several different values of electrolytic caps, from .047uf to 10uf. They all did the exact same thing, both on the scope and in actual audio checks, all sounded muddy and flat. It seems as though it works better with a non-polarized capacitor, I had better adjustability of the AMC circuit, as well as better performance from an audio standpoint. And while my positive peaks weren't as high with the non-polarized cap (150-160%), that's really no big deal. Who wants to have 200% positive peaks anyhow, when you're sacrificing audio quality? I wouldn't.
If anyone is interested in some scope pictures, I'll gladly take some and post if requested.
~Cheers~
Yes please.
Sooooo . . . A non-polarized cap and a choke in series did it?
Interesting . . .
EDIT:
150% modulation done right is still xlnt!
How did you arrive at the choke value?
Robb,
So far, it seems that a non-polarized cap is possibly the ticket here. The choke probably isn't necessary, but if anything, it could help with purging any stray RF that could go thru the cap.
SSB was real good. This particular radio has exceptional output, just a smidge over 20W. My power output after the mod was unchanged, and the audio report came back with no changes in audio clarity or tone.
If you'll give me a little bit, I'll take some pics of the radio set before the mod, with the 10uf cap, and then with the .22 mica cap & choke.
~Cheers~
So far, it seems that a non-polarized cap is possibly the ticket here. The choke probably isn't necessary, but if anything, it could help with purging any stray RF that could go thru the cap.
SSB was real good. This particular radio has exceptional output, just a smidge over 20W. My power output after the mod was unchanged, and the audio report came back with no changes in audio clarity or tone.
If you'll give me a little bit, I'll take some pics of the radio set before the mod, with the 10uf cap, and then with the .22 mica cap & choke.
~Cheers~
If you are still in the mood to experiment, perhaps try a 4.7uf silver-dipped mica or even a tantalum cap and see what happens?
The choke and the cap still makes a L/C circuit, just dunno where it tunes ATM.
As a guess, it just might be bypassing any stray RF - like you said.
If it works out, you will have made a simple solution to a question that has gone around for a long time.
Perhaps RF leaking into the audio chip kept the radio from getting more modulation due to introducing distortion all along?
Still think that modding the AM Regulator might have some possibilities yet too . . .
The choke and the cap still makes a L/C circuit, just dunno where it tunes ATM.
As a guess, it just might be bypassing any stray RF - like you said.
If it works out, you will have made a simple solution to a question that has gone around for a long time.
Perhaps RF leaking into the audio chip kept the radio from getting more modulation due to introducing distortion all along?
Still think that modding the AM Regulator might have some possibilities yet too . . .
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Robb,
Pics are uploading as I write this. You can put a "swing" mod in the 148/2000/Madison chassis by adding a diode in series with R228, it's a 560 ohm 1/4 watt resistor. No idea what this does for any kind of negative peak compression, but I know it modifies the AM pass regulator circuit. However if this is done, you MUST change out the 2SC1419, and replace with a GE-66/NTE152/2SC1061, otherwise you will knock out the pass regulator, it can't handle the extra current pulled by the higher modulation peaks.
~Cheers~
Pics are uploading as I write this. You can put a "swing" mod in the 148/2000/Madison chassis by adding a diode in series with R228, it's a 560 ohm 1/4 watt resistor. No idea what this does for any kind of negative peak compression, but I know it modifies the AM pass regulator circuit. However if this is done, you MUST change out the 2SC1419, and replace with a GE-66/NTE152/2SC1061, otherwise you will knock out the pass regulator, it can't handle the extra current pulled by the higher modulation peaks.
~Cheers~
Ok, here are some pics of the scope from the experimenting I just did.
First pic shows the radio showing a 2W deadkey on my Bird 43A wattmeter.
Second pic shows the 2W deadkey on my scope.
Third pic shows the radio modulating 100% on the scope with a 1khz tone.
Fourth pic shows the radio modulating with the 10uf 25V electrolytic capacitor. It actually still shows some cutoff on the negative peaks because I could not adjust the AMC back far enough to smooth out the waveform on the negative peaks. Positive peaks are about 200%.
Last pic shows the radio modulating with the non-polarized .22uf mica film capacitor and 22uh rf choke. I readjusted the AMC back to where negative peaks are right at about 100% with no cutoff or pinched waveforms. Interestingly enough, the AMC adjusted right back to where it was originally where I had set it for the third pic, at 100% modulation. Positive peaks are right at 150% with no changes in audio clarity on AM or SSB. SSB output stayed the same.
That's my little test/experiment that I did today. Enjoy.
~Cheers~
First pic shows the radio showing a 2W deadkey on my Bird 43A wattmeter.
Second pic shows the 2W deadkey on my scope.
Third pic shows the radio modulating 100% on the scope with a 1khz tone.
Fourth pic shows the radio modulating with the 10uf 25V electrolytic capacitor. It actually still shows some cutoff on the negative peaks because I could not adjust the AMC back far enough to smooth out the waveform on the negative peaks. Positive peaks are about 200%.
Last pic shows the radio modulating with the non-polarized .22uf mica film capacitor and 22uh rf choke. I readjusted the AMC back to where negative peaks are right at about 100% with no cutoff or pinched waveforms. Interestingly enough, the AMC adjusted right back to where it was originally where I had set it for the third pic, at 100% modulation. Positive peaks are right at 150% with no changes in audio clarity on AM or SSB. SSB output stayed the same.
That's my little test/experiment that I did today. Enjoy.
~Cheers~
Makes sense that it would compress the negative peaks. I haven't compared the two schematics (8719 PLL & 3600-14) as yet to see if the mod is the same, but I wouldn't be at all surprised if they were.
But that is another experiment that needs to be looked into.
Thanks for the work, xlnt stuff!
BTW - I use the same scope . . .
Honestly, I am thinking of putting this mod in my President Madison to see how well it does in there. I have modified the mic audio circuit to expand the frequency response range so I am able to use a nice studio mic. I'm really curious to see how AM would sound with this mod, plus my mic.
And about the scope, you just can't beat the venerable Tektronix 465 scope for value and performance. A great all-around scope!
~Cheers~
And about the scope, you just can't beat the venerable Tektronix 465 scope for value and performance. A great all-around scope!
~Cheers~