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

[WD5JKO]

Giorgio,

   This is good stuff! You will find that depending on your R-C time constant, that the first syllable or two will squeak through before the gain reduction takes place. It's hard to get around this without a lot more circuitry.

   I do something similar with my Retro-75 AM QRP rig where I threshold the positive modulation peaks at about 150%. Here I RC filter the pulses that try to cross the threshold to reduce the gain at a lower level. My problem was the initial R-C lag, and when trying to minimize it, I could see the ripple across the C varied at the audio rate when the frequency was < 500 hz. This caused a noticeable audio distortion. So my circuit got a little more complicated.

   You might add another high efficiency LED in series with the one there already and use it to monitor the circuit behavior. Anotherwords, put it on the front panel.

  With your circuit, also try swapping the audio phase polarity. For many of us guys, our voices are highly asymmetric, so polarity chosen makes a big difference.

Jim
WD5JKO
   

[IN3IEX]

The IEX limiter (let's say) is very similar to the ALC of SSB tube transmitters.
For SSB linear amplification by class AB1 output stages, the control grid current is monitored. If grid current is zero, everything is fine. If there is current, it is converted to voltage, amplified, filtered and used to control the gain of an IF tx stage by a variable transconductance tube.
My approach is conceptually the same, but used for plate and G2 modulated AM transmitters. If there is no voltage on the loading resistor everything is fine. If there is voltage, it is filtered and used to control the gain of a modulator gain stage by the LED - photoresistor method. Simple, inexpensive, and sounds good for voice.
Case closed. ... for today....

[Steve - K4HX]

As with any wideband feedback control system, there will be problems with rise time and transients. There is no way around this. Much of the splatter your hear from SSB rigs is due to the ALC. There is a reason why broadcasters went to multichannel processing over three decades ago.

[W2VW]

As with any wideband feedback control system, there will be problems with rise time and transients. There is no way around this. Much of the splatter your hear from SSB rigs is due to the ALC. There is a reason why broadcasters went to multichannel processing over three decades ago.

While Steve is correct an old trick is to apply control voltage to a push-pull stage. This will help with transients.

[K3ZS]

How about delaying the audio using a bucket brigade chip, then processing the audio before it gets to the modulator.

[IN3IEX]

I am experimenting and it sounds good.
I used a 5mm diameter photoresistor in front of a 5mm LED. Both are inside a black plastic tube.
I think of this limiter as a protection accessory. Protection for the modulation transformer, for the audio amplifier and for the RF spectrum.
It is hard-wired in my TX and cannot be disabled.
Audio processing will be added in the future, without removing this circuit.
The measured LDR resistance vs LED current is attached. I am not certain of the first point, maybe there was some leakage of ambient light.

[WD5JKO]

Giorgio,

   I remember an old guitar amplifier that used a #49 lamp coupled to an LDR with a cardboard sleeve. It was used in a tremelo circuit that worked very well. There is a device already made that is more linear for low level AC signals, and this is an opto-fet in a 6 pin dip package:

http://www.fairchildsemi.com/ds/H1/H11F1M.pdf

These are getting scarce these days, but if you can get some, they are very interesting to experiment with.

Jim
WD5JKO

[IN3IEX]

This is really interesting. The LDR is slow, it has 30ms response time.
Maybe I can put a pole-zero compensation to compensate for the slow response of the LDR: previous post updated.
Thank you.

[steve_qix]

LDRs are really slow.  One type of "gain cell" you could use is a 4-quadrant multiplier.  I have some here, and they work very well as a gain cell - and they are quite fast.

THAT corporation (that is really the name of the company) also sells a whole line of ICs, including some very linear gain cells.

[IN3IEX]

We like Tubes in this context

LED: hmm.....a strange lamp, better not to investigate...

chips: oh no !!

LDR are ok because they are older than me.

Anyway the audio limiter works in the band DC - 10 Hz so 30ms is ok (faster will make no difference), then the pole-zero compensation will remove this time constant from the transfer function and the dominant pole will be the RC (22k and 330uF) = 7.26 seconds. Then this time must be divided by the open loop gain of the control loop and should fall in the 100ms region....
Because of the LED (that emits as a function of the current) R (22k) will affect the dominant pole RC and the open loop gain. C (330uF) will only affect the dominant pole RC.

Yesterday I played with the circuit and tuned the 200 ohm trimmer. I started with zero resistance. With audio bursts over 100% I got about 100ms of overmodulation, then 100% (330uF capacitor with "equilibrium" voltage on it). I increased the resistance until the response of the control system was smooth, never overmodulating.
Resistance was about 30 ohm. When this happens the time constant of the LDR is compensated, the control loop is now first order (like an OPAMP with dominant pole compensation (741), and has no overshoot.
Further increase of the resistance of the trimmer makes the control loop more sensitive to audio frequencies.

Giorgio

[IN3IEX]

Here we have the suggested schematic:

http://www.ing.unitn.it/~fontana/AM%20ALC.pdf

It should work in any transformer plate modulated transmitter. Put enough diodes according to maximum anode voltage.

Please report success or failure ....

Giorgio

[Gito]

Hi

A low level negative peak limiter circuit from an old hand book,maybe looking at the basic circuit we can modify our audio circuit .

Gito.N

[IN3IEX]

After extended experimentation and after reading this article:

http://www.w8ji.com/amplitude_modulation.htm

The circuit has been improved, while keeping its simplicity.

http://www.ing.unitn.it/~fontana/AM%20ALC.pdf

As you can see with the two additional resistors and the 250V supply, the diode network will conduce "before" the anode voltage of the RF tube reaches zero volt. You can decide at which voltage. In the schematic with 250V, 3.3k and 14k we have a threshold voltage of +47.6 - about 1.2V = +46.4V. Approximate solution because of RL. When the anode voltage will go below  +46.4V the LED (it takes about 1.2V) will start to emit light and the low level limiter will be activated.
Due to the finite gain of the control loop the voltage might drop below 46.4V, but not too much. Trim here to have 99.9% with your TX !

With this low level (high level activated) limiter/compressor most modulation enhancers can be used in combination. The gain will be automatically adjusted to avoid overmodulation, as defined for this class of modulators and measured exactly where it manifests itself.

Audio quality is top.
Modulation % is kept always slightly below 100% under any tuning condition.
Threshold voltage of loading resistor and audio limiting circuit can be adapted to any RF tetrode.

No transistor or IC......could be made in 1960 with vacuum or selenium diodes and a filament lamp.

[flintstone mop]

You are a lucky guy DAVE.......
Dave Said::::
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.

I SEE:
I see DSP 9024's going for $325 on the BAY....nice box

[W2VW]

You are a lucky guy DAVE.......
Dave Said::::
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.

I SEE:
I see DSP 9024's going for $325 on the BAY....nice box

Not to disrupt this fB thread but:

http://www.ebay.com/csc/i.html?_nkw=9024%20behringer&_clu=2&_dlg=1&LH_PrefLoc=0&_fsct=&LH_Complete=1

[W2NBC]

Also not to disrupt Dave's FB used Behringer price list..
 
As mentioned, asymmetrical peak limiting is a different animal from any of these low priced boxes. Tight multiband compression control they do well, and when set up right can make you sound louder.. Seems like Behringer still might have power supply issues in even their newest stuff..

Interesting video on NEW DEQ2496's "do it all processing"
 (2 out of 3 isn't THAT bad)


http://www.youtube.com/watch?v=H-l_P8hROLY

[flintstone mop]

You are a lucky guy DAVE.......
Dave Said::::
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.

I SEE:
I see DSP 9024's going for $325 on the BAY....nice box

Not to disrupt this fB thread but:

http://www.ebay.com/csc/i.html?_nkw=9024%20behringer&_clu=2&_dlg=1&LH_PrefLoc=0&_fsct=&LH_Complete=1

OK OK you gots me!!!

[K5UJ]

I'm in the process of trying to eventually become Behringer-free.   Their gear is either too complex (or else the manuals are too vague) or if nothing else the power supplies are apt to blow out all of a sudden.  I use the ultracurve-pro box but once I no longer need a noise gate (it has a great one in it I have to admit) I'll just use an old analog graphic eq.:  No memory, no batteries, no delicate microchips to get zapped, no settings that can suddenly get lost.   I had one of those 9024 compressors and gave it to Derb.   It was too much of a hassle to figure out and I thought he might have better luck with it on Ray.     Anything with "menus" and nothing but push buttons...forget it.  

[Opcom]

"I'll just use an old analog graphic eq.:  No memory, no batteries, no delicate microchips to get zapped, no settings that can suddenly get lost."

Darn tootin', Analog knobs are real knobs and stay where they are put. dang it.

[ke7trp]

After 3 years. I am Behringer free here.    Some of the B gear is ok. But most of it, failed or was complex.

Now you can get a real voice channel at a good price. I first saw this unit in the BHW catalog.  Now that I have one, I wont go back. The entire focus of this unit is for exactly what we are doing, Speaking into the mic. The EQ is perfect for voice and there was no reason to keep the 32 band I had so I ditched it.  The USB connectivity is a very nice feature also. It plugs right into your computer.  I would rather have one complete unit rather then a bunch of crap daisy chained together.

http://youtu.be/XOdi50HRqAk

Sorry for the OT

[WBear2GCR]

One possible solution, it seems to me, might  be to take the incoming audio, run it through a "precision rectifier" (a diode wrapped around an opamp), one in the + and the other in the - , then run each half through one of those nice IC chimps that do compression/limiting, then recombine at the output end...

The idea of the precision rectifier is to avoid the "dead" zone around zero crossing...

with a set up like that, and knowing which phase modulated upward in ur shack you could limit the negative going to 99% and make the upward direction whatever ur rig wants or can stand...

I have no idea if this has been done or not...

                       _-_-bear

[W8IXY]

My $0.02 from a long time (and still at it) AM broadcaster.   "I know its been said, many times, many ways...."   Negative AND positive peak control should be done before the audio is ever supplied to the transmitter.   The modulator should be able to reproduce and impress on the carrier the EXACT audio waveform fed to the input of the transmitter.  Attempting to modify the audio waveform AFTER the input stage of the transmitter is a compromise at best, and can be a disaster at worst.

Make the transmitter as linear as possible (not to be confused with a linear amplifier) from audio input to modulated output.  Treat the audio BEFORE not during the transmitter's modulation process.

73
Ted  W8IXY

[flintstone mop]

My $0.02 from a long time (and still at it) AM broadcaster.   "I know its been said, many times, many ways...."   Negative AND positive peak control should be done before the audio is ever supplied to the transmitter.   The modulator should be able to reproduce and impress on the carrier the EXACT audio waveform fed to the input of the transmitter.  Attempting to modify the audio waveform AFTER the input stage of the transmitter is a compromise at best, and can be a disaster at worst.

Make the transmitter as linear as possible (not to be confused with a linear amplifier) from audio input to modulated output.  Treat the audio BEFORE not during the transmitter's modulation process.

73
Ted  W8IXY

AGREED and passed......As Steve (HX) said .............about $150 and you're there.
The negative loading possibly strains the modulator in those desk top TX's

fred

[Gito]

Hi

What about this high level negative peak limiter ,that used a diode and a low pass audio filter,design with an input impedance = impedance of the transmitter.

Gito.N

[Steve - K4HX]

A sure way to destroy your modulation transformer.