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Negative peak limiter revisited - Better SOLUTION FOUND - will keep this




 
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Author Topic: Negative peak limiter revisited - Better SOLUTION FOUND - will keep this  (Read 47274 times)
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IN3IEX
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« on: November 22, 2011, 03:31:25 AM »

Hi, I am now considering this kind of circuits:

http://www.amwindow.org/tech/htm/3diode.htm

Just for curiosity I have simulated a circuit that includes a 8 kohm resistance and a diode (cathode to ground) to emulate the load of an RF tube. The "tube" is powered by DC voltage in series with a modulation transformer 1:1 - 10H (coupling k=0.98). The audio amplifier, excited with 1 kHz sine wave, is ideal but has an internal resistance of 1K.
If modulation amplitude is below clipping, the audio amplifier "sees" a nearly resistive load, "voltage and current are sine waves", this is good.
When the anode voltage of the emulated RF tube goes below zero the loading circuit is opened by the diode (it emulates the fact that our RF tube does not take any current with negative plate voltage - it is like an open diode) and the amplifier current (ideally sine wave) has a sort of "shortcut" at the corresponding peak. This is not so dramatic, but various spurious oscillations appear, that are caused by the transformer leakage inductance and distributed capacitances. I suppose that those  oscillations (10 kHz - 200 kHz range)are the origin of most "splatters".
Overmodulation zero clipping cannot be avoided at this stage, on the other hand the perfect sine shape of the current in the audio transformer can be exactly restored and spurious oscillation removed by loading the modulation circuit, in parallel to the RF tube, with a series of a diode and a resistor.
The resistor in my simulation is obviously 8 kohm and the diode, anode to ground, must be capable to sustain the full peak anode voltage.
This is NOT a clipper or a limiter, it is a load impedance "linearizer". Exactly what is needed to make the audio amplifier and the modulation transformer work the best they can  under clipping conditions. Obviously, clipping conditions are those of the modulated RF stage, it is supposed that the audio amplifier never clips and never overloads.
The power dissipation of the linearizing load resistor can be about 1/5 of the power of the audio amplifier.

I shall include this circuit in my future AM tube transmitter...
73 Giorgio

Now I see that this circuit: http://amwindow.org/tech/htm/3diodeka.htm is quite elaborate and D1-R1 perform the function of proper audio amplifier loading.
Very good circuit. It takes into account what I think is the main issue.
See also page 25 of http://www.arizona-am.net/PHOENIX/NA7RH/6DQ5_Charlie_Pics_rev3.pdf
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KA2DZT
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« Reply #1 on: November 22, 2011, 12:35:54 PM »

You don't need three diodes to make a neg peak limiter.

You only need a single diode and a keep alive voltage of about 10% of your plate voltage.  The keep alive PS should be built with poor regulation.  The poor regulation allows the voltage to sag as current is pulled from it.  This sag in voltage rounds the corners of the clipping.  The sudden brick-wall clip is where the distortion comes from.

You need a high speed diode, ordinary PS diodes are usually not fast enough.  The PIV of the diode should be 4-5 times your plate voltage.  So, the diode will be a string of diodes to get the needed PIV.  I use a .5meg resistor across each diode (no caps).

The cathode end of the diode string gets connected directly to the RF end of the mod xfmr.  I put a small fuse (about 100ma, more for big rigs) between the PS and the anode of the first diode.

Any load that the mod xfmr needs to see is presented by the internal impedance of the PS.

For 50-100 watt rigs, your can use a backwards connected filament xfmr.  Use a RCRC filter, then add another resistor (about 2-3K 2-5W) and maybe about a .1-.5uf cap to ground.  So you end up with a RCRCRC filter.  I also use a LED is series with the PS (after filter before the fuse).

Circuit works FB. I've been using this set-up for years.

You can experiment somewhat with your PS, but remember it should allow for voltage sag.

The other circuit you posted seems way over engineered.  IMO, my circuit is simple and it works fine.

Fred
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« Reply #2 on: November 22, 2011, 01:10:00 PM »

I suppose this is very relevant to transmitters having no proper audio processing ahead of the modulator input.

Mic to amp to modulator, barefoot, peaks galore.

Those diode jobs sound a lot like a clipper, and clippers do add loudness at the expense of audio crud & distortion.  Even one that 'rounds the corners' a bit.

For higher 'FI', you wouldn't see any such thing in a broadcast rig, nor should one be added to a converted BC transmitter for ham use.

It is all taken care of in a proper audio AGC/compressor/limiter after the mic preamp and before the transmitter, like a DAP310, Optimod, or Volumax/Audimax.  Correctly phased for symmetry advantage.

Yeah, I know, that is a LOT more complex and way overboard for something like an Apache.

But, as we all find out, the 'next step' in quality of anything is at least 10X in effort/money/complexity. 

i.e., You were happy with a bunch of good Canon camera stuff till you played with a pro Hasselblad & Zeiss combo.

Off the soapbox for now, and 73. Cheesy

DG
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KA2DZT
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« Reply #3 on: November 22, 2011, 01:19:27 PM »

DG,

My circuit is designed for us poorer hams who don't have any of that fancy high priced audio gear. Smiley

Fred
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« Reply #4 on: November 22, 2011, 01:26:35 PM »

Chops sufficiently busted.

If I had any $, I'd buy you lunch! Cheesy

I do have plenty of ideas, but those are cheap.... Tongue

73DG
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Steve - K4HX
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« Reply #5 on: November 22, 2011, 04:19:42 PM »

Eh, you can buy most 70's and 80's vintage bc processors for $50-100. Or get a newer vintage Behringer 2496 for about $100 (it has six band processing).

Fred spent more than that on the knobs for his 51 watt rig.  Wink
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k4kyv
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Don
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« Reply #6 on: November 22, 2011, 08:46:01 PM »

I suppose this is very relevant to transmitters having no proper audio processing ahead of the modulator input.

Mic to amp to modulator, barefoot, peaks galore.

Those diode jobs sound a lot like a clipper, and clippers do add loudness at the expense of audio crud & distortion.  Even one that 'rounds the corners' a bit.

For higher 'FI', you wouldn't see any such thing in a broadcast rig, nor should one be added to a converted BC transmitter for ham use.

It is all taken care of in a proper audio AGC/compressor/limiter after the mic preamp and before the transmitter, like a DAP310, Optimod, or Volumax/Audimax.  Correctly phased for symmetry advantage.

I believe that when Steve, WA1QIX worked at the broadcast station he installed the 3-diode ultra-modulation circuit with an added keep-alive feature in the transmitter.  Said it worked well.  Maybe he could furnish more details.

I prefer to rely on the natural asymmetry of the human voice to achieve extended positive peaks.  The additional apparent loudness is NOT due to the added sideband power from the positive peaks, but due to the overall increase in average percentage of modulation, since the additional positive peak headroom allows the negative peaks to hit closer to 100% without flat-topping in the positive direction. The asymmetrical modulation waveform (phased in the proper direction) does generate slightly more average sideband power compared to a symmetrical one, but that additional SB power is too insignificant to contribute much to the apparent loudness of the modulated signal.

I would like to get my hands on one of those Kahn gadgets that uses a passive R-L-C circuit in the audio line to shift the phases in a manner that renders the voice waveform completely symmetrical, supposedly without introducing audible distortion. Theoretically, that would allow full 100% modulation in the negative direction without positive peak clipping, even using a transmitter just barely capable of achieving 100% positive.
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Don, K4KYV                                       AMI#5
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« Reply #7 on: November 22, 2011, 09:42:01 PM »

Well, Don, the next Kahn Symmetra-Peak I come across will go your way. Cheesy

They weren't very successful in the BC biz.  In theory it was supposed to do just what you said.  The last one I saw working in a station was about 30 years ago.

I've passed on several over the years lurking in storage rooms.

In practice they had problems, like audible phase flipping artifacts. Not a good listen when blowing music through them.

One topology popular was to have two audio chains, one for the jock, the other for everything else.  They summed together at the TX input.

We don't usually play music (damn), so one of Leonard's boxes might just be the ticket for PTT voice.

73DG



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Steve - K4HX
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« Reply #8 on: November 22, 2011, 10:01:11 PM »

You can build one with nothing more than a few op-amps, resistors and capacitors. It's just an allpass filter.

W3AM has a schematic on line.

http://www.w3am.com/8poleapf.html
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K5IIA
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« Reply #9 on: November 22, 2011, 10:17:10 PM »

http://www.ebay.com/itm/Audio-All-Pass-Filter-Phase-Rotator-Module-Based-OPAMP-NJM072D-/130604742281?pt=LH_DefaultDomain_0&hash=item1e68a63689

these are some nice people. they also build a little bbe sonic maximizer.

i sent them an email a while back asking if they woudl build a few and it seems like they have sold a few of them.

all you have to do is build a power supply for it.
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73, Brandon K5iia
ve8xj
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« Reply #10 on: November 23, 2011, 08:55:47 PM »

K5IIA

  Hey Thanks for the link . Those guys on Ebay have some very cool stuff for DYI's.

I bought one of the Phase Rotator's today as I was in the market for something just like that. I will let you know how it works . I am trying to reverse my phase here as I am getting about 10% higher neg peaking . If I can reverse that, SWEET!

I also bought a Behringer  tube pre-amp with phase rotation as well . Should be interesting to see which works better.
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ve8xj
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« Reply #11 on: November 24, 2011, 12:10:00 AM »

I've been slowly delving into the audiofile syndrome and came up with this interesting article which delves into the history of audio processing and where it stands now . Found it researching Kahn who Don mentioned in a previous post in this thread.

All said and done trying to achieve perfect audio is a slippery slope to nowhere that could be a lot of fun.

http://www.broadcasttechnical.com/articles/fourtyfive.pdf
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« Reply #12 on: November 24, 2011, 02:18:12 AM »

Mike Dorrough KO6NM is one of the best in the field.

Go forth and Google.

73DG
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« Reply #13 on: December 04, 2011, 11:53:08 PM »

The symmetra-peak patents and info is online. It ought not be hard to make one, a very simple passive circuit for those who don't want op-amps in the path.
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Don
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« Reply #14 on: December 05, 2011, 12:25:10 PM »

Well, Don, the next Kahn Symmetra-Peak I come across will go your way. Cheesy

They weren't very successful in the BC biz.  In theory it was supposed to do just what you said.  The last one I saw working in a station was about 30 years ago.

I've passed on several over the years lurking in storage rooms.

In practice they had problems, like audible phase flipping artifacts. Not a good listen when blowing music through them.

One topology popular was to have two audio chains, one for the jock, the other for everything else.  They summed together at the TX input.

We don't usually play music (damn), so one of Leonard's boxes might just be the ticket for PTT voice.

73DG

Please do keep a lookout for one for me.  Thanks.
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« Reply #15 on: December 05, 2011, 08:12:32 PM »

Anyone looking for the Behringer 5 band compressor should keep in mind a few things:

It is a compressor only; not a peak limiter. 
It is extremely difficult to use (or it was for me at least, YMMV)
From what I have been told, if the lithium battery in one goes kaput, you loose everything, including the program or programs in the chip sets that make the whole thing function. 
The Kahn Symmetra-Peak was not popular because all it did was eliminate the asymmetry in the program material and broadcasters wanted to exploit the asymmetry by keeping the high peaks on the upside.   The Kahn basically reduced "loudness" instead of improving it.   With a big carrier on a vintage rig it might work well.   I am not a fan of super high positive peaks--vintage envelope detectors don't like them--also the occasional ham with super modulation is a PITA in a group because when his turn comes my speaker cones blow if I fail to get to the gain pot in time.   
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« Reply #16 on: December 05, 2011, 10:49:14 PM »

...
I am not a fan of super high positive peaks--vintage envelope detectors don't like them--also the occasional ham with super modulation is a PITA in a group because when his turn comes my speaker cones blow if I fail to get to the gain pot in time.   

Some AGC could be applied to the listening/audio side of things. Even a simple photocell across the amp's input and a #47 or lower voltage lamp across the speaker will do pretty well for the clip lead crowd.
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« Reply #17 on: December 05, 2011, 11:22:22 PM »

Anyone looking for the Behringer 5 band compressor should keep in mind a few things:

It is a compressor only; not a peak limiter. 
It is extremely difficult to use (or it was for me at least, YMMV)
From what I have been told, if the lithium battery in one goes kaput, you loose everything, including the program or programs in the chip sets that make the whole thing function. 
The Kahn Symmetra-Peak was not popular because all it did was eliminate the asymmetry in the program material and broadcasters wanted to exploit the asymmetry by keeping the high peaks on the upside.   The Kahn basically reduced "loudness" instead of improving it.   With a big carrier on a vintage rig it might work well.   I am not a fan of super high positive peaks--vintage envelope detectors don't like them--also the occasional ham with super modulation is a PITA in a group because when his turn comes my speaker cones blow if I fail to get to the gain pot in time.   

no you just loose the settings that you had. the rest is safe.
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73, Brandon K5iia
IN3IEX
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« Reply #18 on: December 06, 2011, 03:38:06 AM »

I see...

The problem is that the envelope cannot go "below zero". Obviously if we want to stay near 100%, it may easily happen that the envelope goes "below zero"
Now, what can we do to avoid these events?
I think that audio processing certainly works but it must be carefully tuned after the full RF tuning of the transmitter. It may not be practical for ham radio operations.
Negative peak limiters may save the life of modulation transformer, but still produce distortion and splatter.
I think that the best approach to the problem is to dected when the modulated RF output of the class C power device goes below say 5% of unmodulated supply voltage. This "detected" signal could control the gain of an audio compressor located after the mike gain control. This is a feedback ALC that will work fine independently of the loading conditions of the power RF tube. Alternatively a special audio compressor controlled by the RF signal produced by the AM transmitter could be used.
Now the question is: are there examples around?

Giorgio
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IN3IEX
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« Reply #19 on: December 07, 2011, 05:20:49 AM »

This may be the solution:

1)Simple
2)Full tube circuit.

First) change the first audio tube (mike preamp) with a variable mu pentode like 6BA6.

Second) install a negative voltage loading circuit at the output of the modulation transformer. Diodes towards the transformer and resistor (=RF tube plate input resistance) to ground. A negative voltage will develop on the resistor when overmodulating. In fact the modulation transformer will see the anode resistance of the RF tube (say 8kohm) if the voltage is positive respect to ground, alternatively if the voltage is negative respect to ground it will see the loading resistor (say 8kohm). With the loading circuit the modulation transformer will always see 8kohm (in this example).

Third) RC filter and "voltage divide" with a pot this negative voltage and give bias to the 6BA6. High pass filter (300Hz) at the output of the 6BA6. This is the ALC that I propose to keep modulation limited to 100% anytime.

Alternatively) because the power available on the resistor of the negative voltage loading circuit is high, a simple RC and LED coupled to a photoresistor shunting the audio signal could easily do the ALC task.  

What do you think?

Giorgio
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« Reply #20 on: December 07, 2011, 07:55:40 AM »

DO NOT USE THE *First* CIRCUIT linked from the first post!  The 2nd one is fine.  The first one works by dissipating more power in the modulator tubes, changes the load on the modulator, and will shorten the life or damage any GOOD modulator.

Why?  Perfect modulators should function as ideal voltage sources - meaning, the modulator should be able to supply whatever output voltage is required, regardless of the load.  Albeit, there are no perfect modulators, but you can get close by using negative feedback and a good design.

So, the very last thing you want to do is to put something across such a modulator to attempt to reduce its voltage output, since the modulator will then work very hard to maintain whatever output voltage it is trying to deliver, and a whole lot of heat will be produced, and stress placed on the modulator components.

Use the 3 diode circuit shown below.  This one keeps the load on the modulator constant, and simply "switches" between the modulator output and the "keepalive" voltage, depending on the modulation level.  I have used this circuit with 5kW broadcast transmitters.

I have a variant which does not require a separate keep alive supply, although having the adjustment available is nice.

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« Reply #21 on: December 07, 2011, 10:38:56 AM »

I'm not sure about the BehRINGer 2496 but the DSP-9024 features 6 bands.
The bands can be frequency taylored by the user. It also has a limiter which works well.

Being able to set band edges can make these inexpensive boxes more useful than broadcast surplus units which are pre-set.

Yes, they are not easy to set up. IMO the work is well worth the effort.

The 9024 can be had on ebay and elsewhere for under 100 bux used. Replace the supply electrolytics. They are a high failure item. Replace the backup battery as the unit will dump your settings. Look for a UPS to run the thing on because fast cycling of the mains power will make the 9024 dump the settings also.

My DSP-9024 dumped all the settings 3 weeks ago due to the power company doing some scheduled maintenance. I forgot to put it back on the UPS which was disconnected during the recent extended power outage from wind storm. So much for K.I.S.S.

Here it is in use. Not a direct connection IIRC a camcorder mic. Thanks to G3YPZ for putting the audio up:

http://www.traditional-jazz.com/mainpages/w2vw1.mp3


Anyone looking for the Behringer 5 band compressor should keep in mind a few things:

It is a compressor only; not a peak limiter.  
It is extremely difficult to use (or it was for me at least, YMMV)
From what I have been told, if the lithium battery in one goes kaput, you loose everything, including the program or programs in the chip sets that make the whole thing function.  
The Kahn Symmetra-Peak was not popular because all it did was eliminate the asymmetry in the program material and broadcasters wanted to exploit the asymmetry by keeping the high peaks on the upside.   The Kahn basically reduced "loudness" instead of improving it.   With a big carrier on a vintage rig it might work well.   I am not a fan of super high positive peaks--vintage envelope detectors don't like them--also the occasional ham with super modulation is a PITA in a group because when his turn comes my speaker cones blow if I fail to get to the gain pot in time.  
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« Reply #22 on: December 07, 2011, 11:31:41 PM »

This may be the solution:

1)Simple
2)Full tube circuit.

First) change the first audio tube (mike preamp) with a variable mu pentode like 6BA6.

Second) install a negative voltage loading circuit at the output of the modulation transformer. Diodes towards the transformer and resistor (=RF tube plate input resistance) to ground. A negative voltage will develop on the resistor when overmodulating. I fact the modulation transformer will see the anode resistance of the RF tube (say 8kohm) if the voltage is positive respect to ground, alternatively if the voltage is negative respect to ground it will see the loading resistor (say 8kohm). With the loading circuit the modulation transformer will always see 8kohm (in this example).

Third) RC filter and "voltage divide" with a pot this negative voltage and give bias to the 6BA6. High pass filter (300Hz) at the output of the 6BA6. This is the ALC that I propose to keep modulation limited to 100% anytime.

Alternatively) because the power available on the resistor of the negative voltage loading circuit is high, a simple RC and LED coupled to a fotoresistor shunting the audio signal could easily do the ALC task.  

What do you think?

Giorgio

That is a very sane starting point.
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IN3IEX
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« Reply #23 on: December 09, 2011, 11:36:41 AM »

It works:


I just installed a negative voltage loading circuit at the output of the modulation transformer. Diodes towards the transformer and resistor (=RF tube plate input resistance) to ground. A negative voltage will develop on the resistor when overmodulating. In fact the modulation transformer will see the anode resistance of the RF tube (say 8kohm) if the voltage is positive respect to ground, alternatively if the voltage is negative respect to ground it will see the loading resistor (say 8kohm). With the loading circuit the modulation transformer will always see 8kohm (in this example).

The negative voltage is filtered by a low pass RC (22 kohm and 47 uF ---- in place of the 47 uF capacitor it is possible to use a series of a 200 ohm trimmer and 330uF capacitor that is a pole-zero compensation) and sent to a high efficiency red LED coupled to a small photoresistor shunting the audio signal. The voltage divider is a 47kohm resistor in series and the photoresistor to ground. This is located after the mike gain pot. The 200 ohm trimmer must be tuned for best transient response of the control loop.


Giorgio
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« Reply #24 on: December 10, 2011, 12:09:24 AM »

Great! Its always good to read about the results and success. The mike AGC is very interesting, good job on that!
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