This is my concern. I'm not in it for the typical "power mod " in looking to experiment with audio. I want to do it like the big timers do but the only info I can get its to put a diode in. I understand the concept of diode clipping as I'm a guitarist as well.What I notice is dramatic flat on the negative side. I want to smooth out that curve. Is filtering the way to do this?
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so.. I tried the npc
- Thread starter 9Lives
- Start date
I don't think 9Lives has a broadcast station? He has a CB radio! So why would you compare the two? I agree that the NPC diode is not a magical mod. What it does do is keep your negatives from pinching off and if adjusted correctly it can have a pretty decent scope pattern without any noticeable gain in loudness or distortion. There will be no harsh transitions! I think that's what 9Lives is looking for?
You are correct Shockwave the objective of the NPC mod is to increase the volume without increasing distortion BUT there is no NPC mod that does this even though most people buy into the snake oil. If you have a good enough spectrum analyzer you can see the carrier modulating with the side bands. So you will never really get more than a 100% modulation.
I have to disagree about volting the final, a high level modulated CB radio will modulate one stage of amplification just fine. There is no watt meter tricks, the radio will be linear with or without the final volted.
We have had this discussion before about the NPC mod Shockwave and I have even posted pictures, even a video. I agree its not a perfect commercial broadcast mod but its still better than having overshoots in the negative direction causing distortion.
Read this 9Lives this 2995 is the same as your 959 just surface mount.
http://www.worldwidedx.com/export-radios/119864-2995dx-modulation-scope.html
As I explained, the objectionable IMD caused by overmodulation has little to do with your carrier being pinched off and everything to do with the shape of the waveform and how long a duration the carrier is cutoff. This is because if the waveform keeps the carrier off for any more than a brief instant, the waveform becomes flattened. That's it in a nutshell. You don't want any part of the waveform to be flat or sharp at the top or bottom.
I only have a cheap spectrum analyzer and it easily displays my carrier and sidebands. Point is, if your mod never runs past 100% modulation (positive peak) the NPC mod did nothing effective. It's you setting the gain control and not automatic compression of the negative peak that allows the positive peak to rise higher.
Not trying to be disrespectful here but there are plenty of people well versed in AM that will tell you differently. You will never reach the same depth of modulation when you modulate just the driver. This is basic theory covered fairly early in any communications course. The exception to this is low level modulation versus the high level type used in most CB radios.
Did you ever put the final on a switch to toggle between feeding it with modulated DC and feeding it directly off the unmodulated 12 volt line. Sometimes you need to HEAR the difference side by side to realize this is a mistake. I did this and immediately went on to the next step in figuring out how to modulate the 12 volt line because the difference is night and day. Modulating a single stage always produces "lighter" audio.
We have had this discussion and I've watched the waveforms of all the sinewaves. Anytime the positive peaks went past 100% we had easy to spot distortion in the negative peak. They never looked rounded like the top. You can clearly see the sharp conduction point of the diode in the negative peak before it begins to curve again under load. The only time it looked real good was when the diode never got a chance to conduct and it was close to symmetrical.
This was all with a simple, stable sinewave as the drive. Where it is easy to set the gain properly without the aid of effective compression. The true test is to hit it with a complex waveform of constantly varying amplitude like the human voice. I suspect things would only look good here with further adjustment of the mic gain.
You can't have it both ways. You either run it below the point of diode conduction or you're forced to deal with the same sharp corners when it conducts as you would if it just clipped the bottom off through RF cutoff. You can play with resistors to control the current through the diode but the end result is the same. If the negative peak causes the diode to conduct, the diode will introduce sharp transitions unless its followed by good filtering.
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I thought that you put the diode,cathode end in the hole left when you due solder and lift the right leg of R264 and not R266 on a 959.
https://www.google.com/url?sa=t&sou...14GoCA&usg=AFQjCNHti_JYa-iGfwHkjY702ThgaXmptA
The Defpom EPT06960Z Galaxy 949 / 959 modifications
https://www.google.com/url?sa=t&sou...14GoCA&usg=AFQjCNHti_JYa-iGfwHkjY702ThgaXmptA
The Defpom EPT06960Z Galaxy 949 / 959 modifications
Agreed!
Well most CB's want more watts and big swing and the NPC mod will do this without being bleeding on adjacent channels. You can get more watts out and have a better wave form. A Galaxy style radio is the only one that this mod really works on. I have played with other swing kits and NPC mods on multiple radios and they sound like crap. The diode fools the meter and scope into thinking your percentage of modulation is higher.
I would say most CB radios have some sort of swing kit installed and most techs do not put a radio on a scope before it leaves. Im just promoting a mod that is the lesser of two. If people are going to install a NPC mod I would rather it be this one.
I do have a 148 that i use quite a bit that's the final is on a toggle switch. I cannot tell any difference and I have done a audio gain comparison between both but only with a tone.
What your saying does make sense, I will have to pay closer attention or do some sort of test under voice modulation.
Most of the distortion you were seeing was the from audio chain, yes It did have some distortion. Never the less if you practice headroom, correct microphone phasing and proper speech compression you can get the waveform to look really good under speech modulation.
There is more than one way to skin a cat! R264 is another way to do a swing kit, I like R266
The old 148 is the exact radio that inspired me to figure out how to modulate the 12 volt line because it was notably lacking in audio if both stages were not modulated. If you set the carrier at 4 watts in both the 6 volt modulated condition and the 12 volt unmodulated condition, the audio lost in the final stage is very obvious to the ear.
If the distortion I'm talking about were in the stock audio chain, it would be more apparent in the larger positive peaks. Those peaks are relatively clean. Its the negative peak when the diode conducts and then opens that we see the sharp transition going in and out of the negative peak.
I don't argue that you can't make the waveform look good but I think what you're missing is once you've done that, the diode is no longer conducting or having any real effect on the negative peak. I agree many people can benefit from checking the microphone phasing on AM. Flipping two wires can sometimes make a huge difference depending on the persons voice.
I know the transition your talking about, I agree that makes sense!
Im going to look into the 148 more since it was the same radio you noticed it on. Now I did my test with a 3/4 dead key, modulating to 3 or 4 watts. I have never switched the final stage with a carrier as high as 4 watts.
Thanks for the input Shockwave!
I thought the audio sounded a little punchier with R266, with a mosfet radio and R266 I could get the positive peaks to really round off. Like a tube when your drive it near saturation. Also I have seen a weird carrier changes with R264 on some radios and the problems were normally fixed after moving the diode to R266 or completely removing it.
R264 does its job, If you prefer the diode in the R264 location you can also change the value of R266 to improve the waveform!
It takes proportionally less AF power to modulate a 0.75 watt carrier than it does to modulate a 4 watt carrier. That being the case the loss in audio may not be so apparent at these low power levels. What may also not be so apparent is that you are "pounding" the driver stage with AF in order to get closer to the audio capability of modulating both stages. This will introduce more distortion than modulating the pair of stages at a lower level.
I don't follow why you would volt the final to only achieve 3 or 4 watts modulated. The stock 6 volt modulator is capable of much more than this. If you really want one of these series pass modulated radios to take off, build the same 12 volt modulator with transformer you see in a 29 LTD. Put just the final on this line and watch what happens. Four times the modulated PEP!
Yes, it does stress puny little finals like the old 2SC1969 but they even took it as long as the match was close. It's not a bad idea to upgrade the final because it is the only part that is stressed by this mod. The series pass modulator only has the driver on it after this and the power from the audio chip is no more than required in the 29 because the series pass method does not utilize the power of the TA-7222AP chip to drive the original Darlington modulator.
PS: Do not think you can install a much larger final in order to put both the final and the driver on this 12 volt modulated line. That's asking too much of all the output components and will burn stuff up. If you replace that and get it to hold together, you'll find you're generating too much stray RF inside the radio that makes it useless without tons of shielding and bypassing.
Shockwave:
Let me ask this....
If there is potential stress by modulating the final at a full +12V, could a person perhaps use a LM78XX regulator IC, say a LM7810, and feed the final with that? Perhaps the current draw might pop the regulator, but the idea of regulating the voltage down a volt or two would ease stress on the final, yet still provide better modulation of both stages over stock?
Just a curious thought...
~Cheers~
Let me ask this....
If there is potential stress by modulating the final at a full +12V, could a person perhaps use a LM78XX regulator IC, say a LM7810, and feed the final with that? Perhaps the current draw might pop the regulator, but the idea of regulating the voltage down a volt or two would ease stress on the final, yet still provide better modulation of both stages over stock?
Just a curious thought...
~Cheers~
Shockwave:
Let me ask this....
If there is potential stress by modulating the final at a full +12V, could a person perhaps use a LM78XX regulator IC, say a LM7810, and feed the final with that? Perhaps the current draw might pop the regulator, but the idea of regulating the voltage down a volt or two would ease stress on the final, yet still provide better modulation of both stages over stock?
Just a curious thought...
~Cheers~
It's not the voltage in this case that will stress a weak final since the breakdown voltage of even the 2SC1969 is well above 12 volts. It's the current that if run too high into a poorly matched output can blow the final open. Dual final radios are not as susceptible to this like the old 148 could be.
Turning the voltage down will reduce the current but then you have to deal with the power and heat dissipated in the regulator. It's much simpler to just reduce the drive by lowering the carrier level. It's the added voltage that allows us to increase the output without exceeding the current limitation of the final. Modulating this voltage makes the PEP increase dramatically.
It's also worth mentioning that the required collector current on a final to produce a 4 watt carrier on a 6 volt series pass modulated radio like a 148 is twice the current required to produce the same 4 watts in the circuit the 29 LTD uses with a 12 volt modulated collector line. That means there is still ample headroom to reproduce some good positive peaks with this mod.
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Ahh yes, thanks for clearing that up, I was thinking of the voltage..... but bipolar transistors are sensitive to current, while the opposite is true for Mosfets. Duh. Brain fart! :blink:
I was thinking of volting the final in one of my 2000's and maybe trying to do a little "hi-fi" work to see if I could open up the TX bandwidth a little while maintaining SSB clarity.
I'm always learning.
~Cheers~
I was thinking of volting the final in one of my 2000's and maybe trying to do a little "hi-fi" work to see if I could open up the TX bandwidth a little while maintaining SSB clarity.
I'm always learning.
~Cheers~