Negative Peak Limiting Circuit Single diode

[The Slab Bacon]

Believe me, the am crew over on this side of the world wont hesitate to bust your BAs if you have bad sounding audio. they definately dont pull any punches. And I like it that way, it keeps you sounding good.

I like the bass response (turkey) but I want the full meal as well. (gravy and all) I cant stand Ear bleeding tinny audio. You need to have bass, but you also need to have presence rize and a little syballance as well to sound good. Again WELL BALANCED audio is the key to sounding good. It is not that hard to achive if you want to.
I run a minimal amount of processing gear, but I do run some.

                                                     The Slab Bacon

[Jim KF2SY]

I've used both the single and three diode versions of the circuit
with my DX100.  Not because of any huge potential of positive peaks for
the transmitter (which is nil), but because of similar reasons that Sam did.
My voice is highly asymetrcial and I wanted to have some control of the negative
peaks before I was even approaching 100% mod.  I've seen absolutely no
difference on the scope or could detect any difference in sound with either circuit.
That said, the 3 diode circuit does offer some loading for the modulator when hitting the baseline (which is above zero w/ckt.), which Steve points out is a safer way to go for your mod. xfmr.  Both I suppose in the strictest sense do contribute to distortion, as it "flat-bottoms" on negative peaks. 
Recently, I have not been using either circuit any longer and use a fair amount of compression (and swapped plate caps for phase) to control things without too much negative peaks.
--my 2 cents.

Jim

[W7XXX]

Jim, The single diode circuit does offer the same amount of protection as the 3. When you exceed 100% modulation, your mod xfmr sees infinity and this is when burn out occurs. The single diode puts the load of the resistor across the secondary and prevents this. The 3 diode was originally designed to achieve higher amounts of positive peaks than the single will allow. In the case of your DX-100 no difference should show as you experienced.

I used a trapezoidal pattern when I originally adjusted my circuit, but use a full envelope pattern for monitoring. I will suggest that anyone building either circuit should adjust it with the trapezoidal pattern. To the newcomers ... these circuits will NOT make poor audio sound better. Good audio starts with the mic and tayloring should be done from there forward. My preference is triodes and transformer coupling.

[The Slab Bacon]

From what I have seen in the past, I have noticed an odd phenomenon. When just using a straight mike into the speech amp
(dx-100 / d104, etc) Assymetry of the voice and the phazing of the audio doesnt seem all that critical. There is usually not a very big difference in the scope pattern. And if there is, like Jim said, just switching the modder plate caps is all that is necessary.

But..........Howeva............ When using audio processing gear this assymetry becomes real apparent both in the scope pattern and in the overall sound. I have also noticed that this oddness in the scope pattern and assymetry is also changed as you dial around on the frequency response. So apparently the assymetry of the male voice is different at different frequencies. Especially down in the bassy regions. I have even noticed that dramatic changes to the EQ settings sometimes require a phaze reversal to sound right. Yet I have never had to swap the phaze on my audio for another voice at the mike. Go Figger??  You can definately hear the difference between phazed correctly or not when you are using audio processing, but it isnt very noticable when you're not.

Its all just friggin magic  

                                                  The Slab Bacon

[flintstone mop]

Yea Mack,
Say What to the POS peaks??
It's the negative peaks that are the technical limitation and legal operation of AM. As far as I know we are not under any regulations for pos peaks. But good engineering says that 130% POS peaks is all that a trypical detector in a receiver can handle before IT distorts. It is the AM B'cast folks that are under regulations for 125% pos. peaks not us.
The processors, even though it's an outlay of hard earned cash, make the transmitter happy and enable bigger POS peaks. But I have heard awesome audio from Hams with the right mic for their voice and little or no processing. Just fully modulate the carrier and watch your 'SCOPE. Looking at the metering on a typical Ham transmitter is like flying at 30,000 feet with your flaps down.
Interesting threads a-happening on AMFONE


Fred

[WD8BIL]

I like the bass response (turkey) but I want the full meal as well. (gravy and all).....

Well Frank, fron the looks and sounds of it neither one of us have missed too many meals !! ( pass me the sour cream, will ya ??)

[W7XXX]

Thanks for pointing that out. I meant when 100% "modulation" is exceeded. The RF output cuts off during the negative cycle, the mod xfmr sees infinity and burn out can occur unless a load is provided as in the case of this circuit or the diode & series resistor to gnd combo that Don mentioned.

Sometimes I type something different that what I am thinking. I can even reread it and not see it. this is why I stopped writing novels. The editing was getting too difficult. It all started from a virus that destroyed a few nerves in my brain causing serious equilibrium issues and other problems. Anyway appreciate your pointing that out.

[WBear2GCR]

Sam,

I'm still quite puzzled as to why what amount of bass a station is transmitting has much of any effect on how your receive audio sounds??

The simple solution is to remove the bass?

As I mentioned in an earlier post, there are at least three or four distinct places that you can "tailor" the recieved audio so that the bass frequencies from a given station is essentially non-existant!

What I do is to simply employ a speaker in a cabinet that rolls off at 12dB/oct below about 100Hz. That would done by chosing an enclosure of the correct type and size for the given speaker. In this case and speaking in general terms, that means a cabinet that is somewhat "too small" for the speaker. Although it is best to actually use the T/S parameters of a given driver, and a simulation program to get the best results, it is sufficient in most cases just to use a box that is too small.

Then, if that is not sufficient you can add an external highpass filter at "speaker level". That would be in its simplest form, just a cap in series. You can of course increase the roll off by adding a coil to ground (2nd order, 12db/oct) and another cap (3rd order, 18dB/oct) and even another coil (4th order, 24db/octave).

Then too you can run your receiver audio through an external amplifier and employ line level "equalization" or just a tone control or highpass filter (bass cut).

One can "tailor" the response of the receiver by altering the coupling caps so that the receiver has one or more stages of "high pass" built in, reducing the low frequencies.

You could use "studio type" equipment, available on the cheap from companies like Behrenger (under $100 new) to do the "inverse" of transmit audio processing - causing the output of your reciever to be forced to a set of relationships that you prefer.

Similarly, you can choose to increase the "presence" frequencies or do that and the things above!

You can make any and all of the above switchable, if that's what you need or want.
Seems to me it's pretty much the same thing as switching in and out IF filters, for example...

So, I'm puzzled still as to why the recieved bass frequencies should be much of a problem, if you don't want to hear them??

             _-_-WBear2GCR

[KA1ZGC]

Part of my beef with the new wave of bass dominated AM is I have an inner ear disorder and can't hear many lower frequencies with any amplitude. When these bass dominated guys come into the group, I can't hear half what they are saying because they have mask their midrange and highs. If they practice well rounded audio, then I can hear them fine and they still have those mellow lows for those that can hear.

Okay, this begs the question: what do you do to compensate for the audio of broadcast AM talk stations? Many of us are broadcast professionals (I can't speak for the guys you're talking about, because I don't know who you're talking about) and set our audio tailoring exactly the same as we would for a client.

If the answer is "nothing, AM broadcast sounds fine to me", then "hi-fi audio for AM" isn't your receiving problem, it's somebody mis-equalizing their transmit chain. There's a huge difference there. "Hi-fi" does not mean "tons of added bass", but when you first showed up here you were cursing true hi-fidelity audio on AM (and AM operators as a whole) because a few guys in your area have their bass cranked up too high, as though that's what everyone thinks "hi-fi" means, and we're somehow to blame for that.

Don't blame AMers in general for the misconceptions of a few. Likewise, don't blame someone else's audio for your compromised hearing. If you have specific requirements for received audio spectra, you should consider tailoring your receive audio to meet those requirements. A simple parametric equalizer would go much further in helping you than your previous rant about how people you have never heard in your life are doing their audio all wrong.

If, on the other hand, AM broadcast talk stations (in general, not just one or two) also sound bad to you, then you certainly need to do something about your receive audio. If too much bass is an issue for you, roll it off.

I have a serious high-end rolloff in my left ear (20dB/octave above 4kHz, last checkup) from having my eardrums repeatedly lanced as a child to drain the fluid from multiple ear infections, so almost every piece of audio gear I own has treble that makes dogs cringe to compensate for that.

My transmit audio typically has a slight midrange sag (when I've got EQ in the line, which I don't for now), but otherwise perfectly normal like everyone else's. I don't expect anyone else to wind the sibilance to their transmit audio to compensate for my disability, that just isn't fair to everyone else. It's my hearing problem, so I take care of it at my end.

Sorry that your ears can no longer handle much bass, but please don't take it out on those who run added bass. Your hearing issues are not their fault any more than mine are.

Food for thought.

--Thom
Killer Aircraft One Zeppelin Goes Crash

[ka3zlr]

I like Derb's idea..."run what ya brung"....what the heck....LOL.....

[W7XXX]

AM broadcast stations sound perfect on my homebrew receiver as it is tailored for well rounded audio. My point was concerning the muddied signals where bass overpowers the midrange and are hard to understand under all but ideal conditions. This may effect me more because of hearing loss, but still to compare these bassy signals to broadcast talk AM radio are two different things. AM talk radio wouldn't be around long if they transmitted audio void of any highs and low amplitute midrange. If you have well rounded audio, then my comments don't apply. The unfortunate thing is this muddy audio is called hi fi or broadcast by many and many praise it to avoid conflict and get along with the big AM groups. Anyway this is off the topic here and we have beat this old deceased pony enough.

[W1EUJ]

> Whomever pulled my post, I hope you're damn proud of yourself.

Usually your argumentative posts stay up - your second post must have been a doozy!

David Goncalves
W1EUJ

[KA1ZGC]

Usually your argumentative posts stay up - your second post must have been a doozy!

That's okay, I have a copy. I always make copies in case someone gets the idea that they can remove my words from existance by removing my post from the board. I simply repost it somewhere they have no control over.

No sweat, I made my point, and I stand by it, and I'm sure Sam got the message.

[WU2D]

I used the 3 diode circuit back in 1986 on my first AM rig. Steve KA1SI (now QIX) gave me the circuit over the air and I carefully wrote it down. My first rig was a souped up TCS with an external modulator with a pair of 1625's. The original tranny was pulled out and used for the driver XFMR! Worked slick. I called it the mini-Ranger.

Mike WU2D