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Author Topic: Two-diode negative peak limiter for Johnson Ranger  (Read 11289 times)
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AB2EZ
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"Season's Greetings" looks okay to me...


« on: December 21, 2009, 03:42:02 PM »

Hi!

I added a two-diode negative peak limiter to my Johnson Ranger. I decided to do this primarily to protect the Ranger's mod transformer on negative peaks. It also sounds excellent.

For the Ranger, this can be done using the 9-pin connector... but one must be very careful with the high voltages that are present.

The schematic is shown in attachment 1. The power supply that produces the keep-alive voltage (56 volts in this case) is on the left. The simplified schematic of the Ranger is on the right. Diode D6 consists of three (3) 1N4007 diodes in series (3 diodes x 1000 volts PIV per diode). Diode D5 is a single 1N4007.

The 2nd attachment contains a simulation of the behavior of the overall circuit, with the modulated B+ set (for the simulation) to be 600VDC + an 800 volt (peak amplitude) sine wave at frequency: 500 Hz.

Shown on the simulation are: the plate voltage V(n005); the keep-alive voltage V(n004); the current through R4 I(R4); the 6146 plate current I(R3); the current through the keep-alive diode I(D3), and the current supplied each half of the 60Hz cycle by the keep-alive supply's transformer I(R2)

Best regards
Stu


* Slide1.JPG (49.53 KB, 960x720 - viewed 793 times.)

* Slide2.JPG (97.05 KB, 960x720 - viewed 674 times.)
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
K3ZS
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« Reply #1 on: December 22, 2009, 09:48:37 AM »

I built the zener type limiter which does not require the keep alive voltage, but is a little harder to adjust.   I used a bunch of zeners, in an external box, and connected it to my AF-67 external power supply jack.   Using a bunch of zeners in series, you can monitor the RF signal and tap down on the string of zeners until you get the negative clipping.   Initially I used the circuit shown somewhere on this forum.    The clipping was very sharp.    Later, an article in ER had one that added a resistor to the series string.   It rounded off the clipping point when I added the resistor.   I think that resistor cut back the higher order distortion products that is produced without the resistor.
As Stu has said, I added it to protect the mod transformer, not to produce super modulation.
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WD5JKO
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WD5JKO


« Reply #2 on: December 22, 2009, 06:09:20 PM »



This is all pretty interesting stuff. If more of you guys in the NE use these techniques, combined with some processing, and EQ, then I could understand and copy many more folks from here in central Texas.

 Something I learned with my modified Heising Gonset G50 project, was that taking G2 of the 6146 RF final, and clamping it such that it cannot go under about 30 volts, I achieved the inability to exceed 100% negative modulation, yet I could come darn close. Without that clamp, I was exceeding 100% negative frequently (bright spots on scope center graticule). With the clamp, I could crank up the audio gain until I saw upward modulation compression.

http://pages.prodigy.net/jcandela/G-50/YEA_AM.JPG

Yes the G2 voltage change with modulation was abrupt, but that was not seen on the plate, or the RF envelope pattern as seen on a scope. All this mod required was a diode between the 6146 G2 and the audio modulator cathode bias voltage.

  If I had an abundance of modulation, such that a mod tranny might be at risk, combined with the g2 clamp, then i'd just use a single diode back biased by the modulated B+, and in series with the diode place a resistor to ground. The resistor would approximate the modulator load impedance, and be about 5-10% of the modulator power rating.

   With these two combined techniques, the rig will not over modulate negative, and the mod transformer will always see the a near constant load impedance. Some messing around with the G2 diode clamp might be required, such as a series resistor with the diode, and varying the diode conducting threshold (30v in my case).

  I'm not saying one way is better than another here. I'm just pointing out that there are several approaches that aim to accomplish the same thing. I think that Hoisy Hoisington, W4CJL would be proud that we are still talking about this stuff so many years after he became SK.

Jim
WD5JKO


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KC2IFR
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« Reply #3 on: December 23, 2009, 12:39:52 PM »

Quote
I never could see why people would want to use a bunch of other junk inline though like EQ boxes and etc. I can take an ordinary D104 and properly configure the audio chain in almost any of the old tube transmitters and make them sound 50 times better every time.

Junk...........I dont think so.
Question..........I consider the diode limiter stuff as high level processing. I have processing in my audio chain that I consider low level processing. My processors are designed for AM radio.....ie: the processors will limit the negative peaks to 100% but allow the positive peaks to reach 125% or so depending on the processor.
I feel that the low level processing is superior to the high level. The only problem with low level is if u crank up the audio AFTER the processor than u defeat its purpose. IF one has a good mod monitor that shows both negative and positive going peaks coupled with a good scope and u know what u are looking at.....than there is no problem

Bill   
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N3DRB The Derb
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« Reply #4 on: December 23, 2009, 12:55:02 PM »

the problem is dudes go overboard trying to get that BIG BROADCAST SOUND and just process the readability right out of the transmitter.

As long as they are 40 over they sound like the big cheese, unless you really have a rx setup that goes down to 30 or 40 hz. then they sound like mud again. The  bass roll off of most comm recievers makes em sound good.

if they are any sort of DX, a stock Apache will be more intelligible.  

 

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KC2IFR
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« Reply #5 on: December 23, 2009, 12:59:06 PM »

Quote
the problem is dudes go overboard trying to get that BIG BROADCAST SOUND and just process the readability right out of the transmitter.

Couldn't agree with u more.. Wink
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N3DRB The Derb
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« Reply #6 on: December 23, 2009, 04:53:35 PM »

ultra modulation is a waste of time. most comm receiver am detectors cant handle it. It's one of those things that sounds uber cool, but in practice doesn't really yield that much value.
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KC2IFR
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« Reply #7 on: December 23, 2009, 05:42:20 PM »

OH boy.....
Here we go............... Grin
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WA1GFZ
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« Reply #8 on: December 23, 2009, 05:51:41 PM »

Right on Derb a bunch of useless hardware that does nothing. set the rig up properly ith enough head room and your done. The diode limiter isn't supposed to be hammered on every voice peak it is there to protect the transformer if you have a transient.
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KC2IFR
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« Reply #9 on: December 23, 2009, 06:19:31 PM »

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We will be the ones having our voices heard around the country while the rest of ya only have carriers Grin

What?Huh
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N2DTS
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« Reply #10 on: December 23, 2009, 07:00:52 PM »

There are plenty of people running stockish transmitters with light audio.
There are also many people with a LOT of low bass that makes it tough copy under anything but ideal conditions.

Not all situations call for the same audio, I suppose if you want to work DX, you need DX audio settings.
Most DX operators use compressors and limit the bandwidth a lot. Not something most people want to listen to all the time...

I think it pays to run some compression all the time, get plenty of average audio power on the carrier, which you can not do with a stock vintage rig except maybe the ones that had clippers built in.

I think, if it is done correctly, a little bit of NCL can boost audio levels a bit without splatter.
Unlike many compressors, its an asymetrical sort of compressor that does not limit the positive modulation.
Some combo of general compression, and some NCL is likely the best bet if not overdone.

There was a big round table on 40 meters today, not everyone was strong, plus i have some sort of impulse noise, some guys were easy copy, others were hard because of the light audio...

Brett
 

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Steve - WB3HUZ
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« Reply #11 on: December 23, 2009, 08:44:15 PM »

Of all the guys I know who regularly work across the USA or into Europe on AM, none of them use any high-level limiting or super-modulation stuff. As Bill said, it's far easier and far more effective to do the limiting in a low-level stage.

In most cases, you probably won't even need to do any negative peak limiting. Most male voices are asymmetrical. Just set up the polarity correctly and you'll get extended positive peak modulation - assuming your modulator/final can handle it.
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WA1GFZ
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« Reply #12 on: December 23, 2009, 09:31:37 PM »

Don't worry that 4-400 won't last long unless you find a way to hold 150% negative peaks. You might review the thermal design of a 4-400 mr expert.
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Steve - WB3HUZ
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« Reply #13 on: December 23, 2009, 09:56:41 PM »

So, you are doing it in a low-level stage.  Grin

Taking advantage of the asymmetry of your voice has nothing to do with your modulator power. I can modulate 150% positive without any negative peak modulation. This is possible completely because of the asymmetry of my voice. An overbuilt modulator has nothing to do with it.
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Steve - WB3HUZ
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« Reply #14 on: December 23, 2009, 11:06:34 PM »

Quote
The asymmetry of my voice is really not that important.


It may not be to you, but it is important. To ignore it is to waste energy and limit your audio more than is necessary.

Look at the three waveforms below. The first is with the audio polarity set for maximum positive peak modulation. The positive peaks extend well beyond 100 percent - entirely due to the asymmetry of my voice (no limiting used), as shown in the second waveform, which is the audio straight out of the microphone. Note the large amount of asymmetry.

Now look at the third waveform. This is the same audio as in the first two waveforms but with the polarity flipped. Notice that now with 100 percent modulation on the negative peaks, the positive peaks do not even reach 100 percent (about 80 percent). This is the undesired setting for the audio polarity.


* positive_mod.jpg (106.66 KB, 800x600 - viewed 518 times.)

* pos_audio.jpg (92.89 KB, 800x600 - viewed 511 times.)

* negative_mod.jpg (105.97 KB, 800x600 - viewed 518 times.)
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