The AM Forum

I now have a tube preamp. Presonus.  This made a HUGE difference in sound quality over the behringer junk. 

This preamp has a phase reversal switch.  I am having trouble choosing which way to run it.  I went back to my handbook and cant find any real hard data.

NORMAL has more midrange.  A fuller scope but LESS peak mod and less peak power.
Reverse has  LESS midrange tone, BIG peaks and MORE peak power.

Normal sounds to me well Normal.  Reverse has a flatter tone with more peak power.

If I DO NOT touch the gains.  REVERSE is clearly higher on the scope.  Normal is less. 

Can someone school me?

C

Generally, you'll want to go with the phase or polarity that produces the largest positive peak modulation.

I have a preamp with a phase switch. Below you'll see a scope photo of the preamp audio with the largest peaks negative going (attachment 1). The second attachment shows the resulting modulation envelope. Notice the positive peak modulation is not that great but the negative modulation is near or at 100 percent.

Continued next post.....

In the shots below, I've hit the phase switch and flipped the phase. Now the audio is positive going and the positive peak modulation is much greater.

You should be able to see similar differences when looking at your modulation envelope. Don't worry about the phase or polarity of the audio out of your preamp, mixer or whatever. It could be positive or negative for the best positive peak modulation of the transmitter and it may vary from one transmitter to the next.

Thanks alot steve.  I do not see the ratio change much from pos to neg.  What I do see is more like the second photo when switched.  I have larger peaks up and down.  The transmitter makes more modulation with it reversed.

The thing that got me was that the midrange when way down when reversed.  To compensate, I just added a bit more midrange on the EQ.   I think I have it the right way for me and my transmitter.

C

I've tried this same thing in the past with my tube mic preamp and when I do it I see no difference on the scope.  I wonder if that means I have a symmetric voice.

Rob

When you listen to yourself with headphones, while you are talking, you actually "hear" two (2) signals:

a. What is coming out of your headphones
b. A signal following a direct acoustic path ("bone conduction") from your vocal tract to your ears

Other listeners hear only one signal

The low frequency content in the sum of signals a and b is particularly sensitive to their phase relationship. If you flip the phase... so that the signals subtract... what you hear (not other listeners) is a loss of low frequency content.

The subjective effect on higher frequencies (what you hear) is less, because of a number of physical and psycho-acoustic effects.

The issue of how asymmetric (positive peaks v. negative peaks) the voltage waveform of your voice is depends very much of the phase v. frequency (not +/- phase, but phase in radians) of your audio chain... as well as the characteristics of your vocal tract. If you had an "all-pass" filter that could be used to adjust the phase v. frequency (not the amplitude v. frequency)... you could make the asymmetry of your voice waveform (the output of your audio chain) larger or smaller.

In fact, when you make changes to the settings of your equalizer, you always introduce some changes in the phase v. frequency characteristics (even though you are attempting to adjust only the amplitude v. frequency characteristics). As a result, the amount of asymmetry in your voice waveform (at the output of your audio chain) will get larger or smaller (and can even reverse "polarity") when you change the equalization settings.

Stu

Clark,

Whenever you change the phase of your transmitter, you also need to change the phase of your heaphones. Otherwise you will be monitoring in the wrong phase from optimum for your particular voice.

I have a switch on the mod monitor here that lets me flip the monitoring phase. It makes a big difference in the sound you hear. One phase always sounds better than the other. The wrong headphone phase sounds muddy.

One way to combat the bone conduction monitoring problem is to use "open" headphones that don't seal the air with tight cushions. There are not many sets out there anymore.  Even my new Sony 7506 headphones are tightly sealed units, but when adjusting my audio I try to put them on half-way to keep the bone conduction to a minimum.

I'm surprised you are having trouble with the Behringer gear and you find a big difference with the tube stuff. I have all Behringer audio gear and love it. I might even buy a B1 mic too. The other day on 160M QIX ran a blind test between the RE-20 and B1 - everyone in the group, including me, thought the B1 sounded better... :o

BTW, the polarity (phase) choice of you rig will have a lot to do with how clean your deep lows are and how you EQ the low end too.  Some have found just by cutting the deep lows they required a different polarity. But in general, pick the polarity where your PEAK power wattmeter goes the highest when saying "OOOOONE." This usually works.  Some guys with scopes can just look at your signal and tell you which is correct too.

T

The behringer gear has failed on me now twice.  Two mixing consoles in 2 years.  That is why, I dont care for it.  ITs real low quality. I like the price but dont like the quality. Several people told me that preamp in the Behringer mixer is junk. So I decided to try a preamp that was cheap and got good reviews. 

The tube preamp made a great difference through an audio amp and speakers.  Very very noticable. I am not sure how much through the T368. Probably not much.  Its limited up there around 8 to 10K range.

The Preamp has an 80hZ cut which I like. Below 80hz on the T3 struggles. It was on sale with an Sterling audio ST51 condensor mic for one hell of a price. I have the Sterling on on a boom as a "guest" mic.  Now if someone comes over, They have a nice high back chair, A mic boom, sterling mic and a switch to key the transmitter :)

By the way, The sterling audio mic is not that bad.. It has ALOT of lows in it. So much I had to cut the lows on its channel. Once I did that, It was very nice and smooth. A great bargain. Its very heavy and well made.

Nothing has so far compared to my old MXL V57m. Its a direct copy of a U87 and just kills anything we try here.

I dont use headphones. I dont care for them. I have Senhieser and Beats headphones here. Maybe I will try them sometime.

What we where doing to monitor is taking an ETON XM1 down into another room of the house, Then talking and making changes on the station.

In reverse, The transmitter makes more mod and more peak power. I get a smoother wide larger sound. But I loose midrange punch. So I just upped the midrange a bit..  So an extra half division on the scope when modulated is not a bad thing :)

C

Hot news straight from the shack  ;)  I just tried the phase reversal switch again on the mic preamp (bellari RP220) and this time there was a very noticeable change in modulation level.  As it happens, I had it in the right phase all along fortunately (I would have been p.o. if I had not) but I'm certain last time I did not see such an obvious difference.  Only thing I can think of is I'm using a better scope now, and I have made a few changes to the audio boxes in the chain.  I'm temporarily running a PMC400A peak limiter instead of the inovon.222 and last time I don't think I had the Orban 422A compressor in the line.  I was doing something different before with the eq'ing too I think.  I'll bet the big difference is the 400A as I have it set up for 150% positive.  This time, one way I'm almost pinching off the carrier and the other way it looks like I'm barely hitting 50%.

Rob

Polarity, equalization and bass phase are interesting.  I found that advancing the phase of lows a little bit improved voice polarization.

Improving Polar Asymmetry - The Phase Shift Technique
AM Press/Exchange issue 62, July, 1988
http://amfone.net/AMPX/62.html

I am undecided on this one.  If I hook the Rack to an audio amp.. Clearly, It is better with the tube pre reversed.  I sound BIG and wide.

On the air, HALF the guys like it normal. Half like it the other way.

I have lots of meat on the scope with wide peaks. Harder to flat top in normal.  On reverse, These peaks narrow up and go higher. Easy to flat top. 

I guess its really a toss up.

C

I've found the best solution to this problem is to get it sounding the way YOU like it. If you have a buddy that can make decent air-check recordings that helps.

But it's your station. Make it sound like you.

Receiver AGC can affect receive audio quality.  Receivers with peak detecting AGC, really meant for SSB, can produce a very compressed sound, especially when a signal has substantial positive modulation peaks, and the effect is worse when the AGC is fast.  AGC should generally hold steady with modulation, but with a peak-detecting AGC, it does not.  A peak oriented AGC might drop 6 to 10 dB of gain when there is modulation, and this produces an unpleasant compressed sound.

It is not good for the sounds having the greatest asymmetry to be selectively attenuated - but with many crude automatic gain control systems (both transmit side and receive side), that is exactly what happens.  Usually the sounds with the highest asymmetry are critical and emphatic, and reducing them more than other sounds is really inappropriate.  But simple peak-oriented compression does exactly that.

On a receiver with peak detecting AGC, reverse audio polarity (larger peaks downward) sounds better, because that way there is less positive modulation, so there is less AGC pumping with modulation.

If the peak-detecting AGC is switched to manual, the received audio is usually a lot better, but of course you lose the AGC

The simple answer is to use average AGC on AM signals.  The old diode detector and the resistor - capacitor AGC filter worked pretty well.

Welllll when it comes to phasing you want the most pos peaks, that's going to be your "loudness" over the air. More pos peaks adds a little more punch to the audio on the RX end.
The EQ is a personal taste for that 50KW radio station voice or telephone quality audio option.

The un-touched T-3 will modulate near 100 cycles and the high end might be 3kc if you're lucky. The excessive capacitance in the final tank circuit will kill a lot of your high end freqs. This was Timtron's mods that can get a T-3 to be near B'cast quality..........but I know Clark does not want to modify his T-3. So, it's stock telephone quality audio.

Fred

I guess your telephone goes from 80 to 10K?   Better have a listen before you make judgements like that Phred you little smugster..  :)

I love it when people give information about something they dont even know about :)

I have spent more time pushing that reverse button.  The peaks are higher reversed with my voice, But its not as loud and the AVG power is less as normal phase.  Thin tall scope pattern is the result of revese mode.. So its not a rule that more peaks are better. Either way, I cant see the ratio of neg to pos peaks being changed much. Neg and pos go higher in reverse.  Less in normal.


C

Indeed. Of course, you don't know that Fred previously owned a T-3 and did extensive mods on it.  :P

Serious question, here: what are you describing as "negative peaks"? Negative peaks appear in the center of the RF scope trace, positive peaks appear at the top and bottom of the RF scope trace. Sounds to me like you're talking about the top of the scope trace as positive peaks, and the bottom of the scope trace as negative peaks. That's not how it works.

I knew that. I was typing about him never hearing mine :)   He was poking fun to me saying I have telephone audio.

C

No, he wasn't. Fred was saying that stock T-3s have telephone audio. Go back and read it again. He's right, they do. He'd know, he has experience with them.

Relax, Clark. Not everything written on the internet is an attack on you.

Having heard Clark's T-3 on the air....It does not have telephone audio. Maybe the operator,but not the rig ;)