A high-fidelity interface for coverting line level audio (600ohm) to high-imp

[W2WDX]

Not necessarily.  Sometimes the distortion in one stage partially cancels distortion in another stage.  If one stage in the audio chain, for example, has 0.3% THD, and a following stage has 0.25%, the combined THD of the system may end up less than exactly 0.55%, since non-linearities may be subtractive as well as additive.  This is exactly what happens in a push-pull class-A amplifier stage.  Each side inevitably has a degree of non-linearity, but the non-linearity in each side that would tend to generate even harmonic distortion should largely cancel the  corresponding non-linearity at the opposite side. That is why a push-pull amplifier stage has lower distortion than a single-ended stage using otherwise the same design parameters, despite claims made by the audiophool community that a single ended output stage is superior and the big bucks they spend on single ended output transformers.

That's right on, Don. Thank you for mentioning that. What I meant was more generally related to tonal characteristics and audio quality, and not as specific as your comment.

I'm still very interested in hearing about asymmetry in vocal signals as seen on the outputs of transmitters. A few questions. Is this related indirectly to what Don mentioned above? Would a push-pull configuration cause asymmetry in amplitude if an inbalance of some kind exists, either in the modulator or the final or both? Is it the nature of modulation transformation? Could it be something as simple as different gain within the tubes in a push-pull? What about supply voltage differences or capacity variances of such. Would a pure sine wave show as asymmetrical on a transmitter showing this asymmetry on male voices? I'm very curious about this. I am quite positive about my assertion that audio of a vocal is symmetrical prior to going through a transmitter, so I would like to know what could cause it to change. I'm not seeing this effect in my K1JJ 813 style transmitter. So I'm a little confused and even a little concerned. What am I missing?

It's getting off the original topic, but .......

[k4kyv]

Even harmonic content tends to produce asymmetrical waveform.  I can look at my own voice with an oscilloscope connected directly to a microphone, and see a certain amount of asymmetry.  I see a very similar asymmetrical waveform regardless of the microphone I use, except for a very poor quality one, like for example a WW2-era military surplus carbon mic.

Remember, capacitors and transformer cannot pass DC, so the area under the curve, which represents total energy, on the positive half of the audio cycle must equal that of the negative half unless the stages are direct coupled.  If a transformer or coupling capacitor is used, the half of the waveform with the higher amplitude ends up of shorter duration, while the lower amplitude half remains above or below the curve for a longer period of time.

Distorting the waveform, deliberately or unintentionally, so that there is a high degree of even order harmonics will allow an asymmetrical  waveform to pass through a transformer or coupling capacitor, and a good modulation transformer will pass the natural asymmetry of a voice waveform .  But if that asymmetrical wave is used to modulate a transmitter, there will still be zero carrier shift.  OTOH, with schemes like ultramodulation and negative cycle loading, rectification occurs, and the resultant DC adds to the DC from the power supply, so there is a positive carrier shift.  But the rectifier circuit must be placed between the modualtion transformer and final PA stage, or else the audio chain must be DC coupled all the way to the final to maintain the carrier shift.

If the waveform is hard clipped, extremely good low frequency response must be maintained or else direct coupling must be used, to avoid tilting the clipped waveform.

[KA1ZGC]

Okay, just to show what a good sport I am, I'll bite on this one.

I'm still very interested in hearing about asymmetry in vocal signals as seen on the outputs of transmitters.

It's seen on the inputs of transmitters, too. It's seen on all male voice audio.

Take a look at the scope shot that WB3HUZ posted. It's fairly typical of a male voice.

Look closely at the waveform. Pay particular attention to where the zero line is. Notice how every nth peak shoots way up above the zero line, while the others stay mostly on the negative side? That's the asymmetry of the male human voice.

Now plug one of your mics into a scope and speak into it. No processing, just the mike. You'll see exactly the same thing.

The fact of balanced vs. unbalanced is irrelevant. The only difference there is one conductor carrying one waveform vs. two conductors carrying one waveform in two different polarities (notice I didn't say "phases").

The male voice consists of low-frequency pulses and high-frequency semi-sines mixed together. Because those pulses are always in the same direction, you get asymmetry in the waveform. To make up for this, most of the lower-amplitude waves spend more time in the negative, so there is no DC component.

If you get your mic wired backwards, those pulses will travel negative, and the tube or transistor will get slammed shut, causing splatter. That's why we care about the phasing of our mics.

Forget about everything else you asked about. You're overthinking the issue. The waveform of the male voice is asymmetrical. Period. This has been known since humanity first developed the means to observe these things. It's not something a transmitter or processor does from bad design, it's something our voices do because they work as designed.

[KB3DKS]

  And if you really look at waveforms from other sources as well you will find that most sonic waveforms are non symetrical. That is one of the main characteristics
that give an acoustic instrument or even a synthesizer for example, it's sonic signature since the assymetry effects harmonic content of the sound.
  Even something like percussion instruments have a greater positive than negative going initial waveform.
  This critical detail is usually lost due to excessive, or even just normal negative circuit feedback. This is why single ended 0 feedback amplifiers claim to sound beter even tho the overall performance specs are poor.
  But even the best low to high matching device will still be subject to the following amplifiers response characteristics that in vintage amateur transmitters usually needs serious modification to equal the input sources quality.
  Just my 2 cents saved up from 35 years of audio engineering work.

Bill,
KB3DKS in 1 Land

[W2WDX]

Ok ... more questions.

Even harmonic content tends to produce asymmetrical waveform.  I can look at my own voice with an oscilloscope connected directly to a microphone, and see a certain amount of asymmetry.  I see a very similar asymmetrical waveform regardless of the microphone I use, except for a very poor quality one, like for example a WW2-era military surplus carbon mic.

Remember, capacitors and transformer cannot pass DC, so the area under the curve, which represents total energy, on the positive half of the audio cycle must equal that of the negative half unless the stages are direct coupled.  If a transformer or coupling capacitor is used, the half of the waveform with the higher amplitude ends up of shorter duration, while the lower amplitude half remains above or below the curve for a longer period of time.

Distorting the waveform, deliberately or unintentionally, so that there is a high degree of even order harmonics will allow an asymmetrical  waveform to pass through a transformer or coupling capacitor, and a good modulation transformer will pass the natural asymmetry of a voice waveform .  But if that asymmetrical wave is used to modulate a transmitter, there will still be zero carrier shift.  OTOH, with schemes like ultramodulation and negative cycle loading, rectification occurs, and the resultant DC adds to the DC from the power supply, so there is a positive carrier shift.  But the rectifier circuit must be placed between the modualtion transformer and final PA stage, or else the audio chain must be DC coupled all the way to the final to maintain the carrier shift.

If the waveform is hard clipped, extremely good low frequency response must be maintained or else direct coupling must be used, to avoid tilting the clipped waveform.

Is the asymmetry you see on your scope possibly being caused by the compensation on your probe? Don't be offended by that question ... please. I know that in the case of what I see at studios, on microphones like vintage Telefunken U-47 in good shape and new Neumanns like the U-87, the waveforms are very symmetrical in terms of amplitude. But also I have seen this type of asymmetry coming out of some tube microphone preamps, like some of those made by Manely and Avalon. Some of these preamps use a single ended designs (more common) and others use push-pull (less common), but both types show similar asymmetrical output. I'm not sure if you are familiar with modern recording systems, like digital recording on computers using software like ProTools, but you can see the waveforms as you record them, which is a great aid for engineers to see at least symmetry.

So I guess my question is ... is this asymmetry a factor of the electronics and how it works or an analog of the acoustic wavefront presented to the mic diaphram. I can't answer the latter, but the former is something I have yet to observe in practice unless there is something inconsistent in the microphones or cables.

This goes to you Tom as well. I have not observed this asymmetry in the studio on male voices, except under the circumstances I already mentioned. Now don't get me wrong, simply because I haven't seen it doesn't mean it doesn't exist. I'm open on this at this point, and after hearing more points of view. I am aware of the glottal force differences between male and female voices, and also phase rotators in broadcast applications (which I always assumed were used to reduce the peak-to-average ratio of voices by 3 or 4db). Are they actually being use to make voice waveforms more symmetrical? It's seems like something they could be doing since they are simply a chain of filters (allpass I believe) whose group delay is very non-consistant as a function of frequency. Ok ... now you guys have me thinking ... Am I wrong about this?

Now mind you, many of the vocalists I work with are very trained and experienced, so their glottal volume velocity waveform characteristics may be different due to that training. Have I tricked myself and made an incorrect assumption? I have to look into this now.

Thanks guys,

John

[w4bfs]

congrats John ...you have stirred the pot with a superior hand ...got folks talkin !! ... on microphone symetry ...satisfy yourself with a simple test ... I hooked up a Sennheiser 421 directly to oscope input ... when you talk into the mike the amplitude assymetry is immediately obvious .... may not be a condenser but is reasonable quality ... hey Thom ... this seems to have been said before ? .... my dega Vu moment ....javascript:void(0);

[W2VW]

You are in good company John. I've heard this issue argued by several seasoned engineers over the years. In the end it gets proven on a blackboard.

[W2XR]

Ok ... more questions.


I am aware of the glottal force differences between male and female voices, and also phase rotators in broadcast applications (which I always assumed were used to reduce the peak-to-average ratio of voices by 3 or 4db). Are they actually being use to make voice waveforms more symmetrical? It's seems like something they could be doing since they are simply a chain of filters (allpass I believe) whose group delay is very non-consistant as a function of frequency. Ok ... now you guys have me thinking ... Am I wrong about this?

John,

The phase rotator circuitry used in AM broadcast audio speech processing equipment is utilized for purposes of making the audio waveform more symetrical. The effect is significant; I can see it every time I use my Orban 9100A AM processor with my transmitter. The modulation characteristics as viewed on my modulation monitor are very much more symetrical in nature when the phase rotation is switched in. You are correct in that the circuit is really nothing more than an all-pass filter, but I'm sure it is also optimized for controlled group delay characteristics, overshoot, etc.

I suggest that you take a look at the Orban website. Bob Orban has written extensively on this very subject, and if anyone is qualified to discuss it on an engineering basis, it is he. I believe the subject at hand was covered in one of his technical white papers as posted on his site. Lots of other good technical information available there as well.

73,

Bruce

[Steve - WB3HUZ]

John, did you look at the scope photo I posted. It is my voice directly from a 1" diameter, high quality studio condensor mic, through a preamp into the scope. That's it. The waveform is obviously asymmetrical.

The energy contained in most male voices is asymetrical. Female voices are generally more symetrical in nature.

Bruce, this is nitpicky but important. The energy is usually (maybe always) NOT asymmetrical. The voltage waveform is asymmetrical but the actual energy of the two sides (positive and negative) will be equal (integrate the positive portion and the negative portion and the result will be equal).

[flintstone mop]

Interesting thread indeed. We're not a group of old cranks, but look at audio and the nice sound of A.M. with practicality and simplicity. We're interested in an audio range of 50 or 60 HZ to about 8K. Nice full clean sound for A.M. The Hi-Fi bassy  sound does not propagate well in poor conditions. Too many people lsitening on a narrow banded receiver or on LSB on their SSB transceiver.
I dabble with broadcast with a radio show on a shortwave station and love the A.M. mode in Ham radio, and have often had to reverse my balanced audio going into the transmitter to get the assymetry in my voice to make pos peaks out of the transmitter.
And I have an on-going problem with my radio show. It all depends on what CD player they use or what computer is playing the MP3's. Apparently there's inconsistencies with hardwiring these devices to the audio console at the station, or something weird in my recording computer in my studio BUT I have to reverse the polarity of the audio from my mic to create the pos peaks from my voice. And their audio processing to the transmitter must not automatically shift phase. (I found that annoying, listening closely to an A.M. station playing music, I could hear the phase shifting from the processor) Music is constantly shifting in phase.
By viewing the modulation enevelope of my Ham station or the broadcast station, when they play my CD, I can see when the phasing is proper, when I open my mic.
When the phase is wrong, there is severe negative limiting and there will be reduced modulation of my voice. Ugly pos. peaks

As for that special sound from guitars, I thought there was a "dummy Load" that the musician connects his/hers amp to to get that "overloaded sound" to the audio console to preserve the sound from the speaker and cabinet. Still miking the cabinet. It seems like "Re-amping" creates 10 more steps to record a certain sound from a guitar amp. Computers, human interpretation of "is this the sound we want? or "does it sound like what comes out of the cabinet when it's cranked?" Seems like more labor.

Fred

[W2XR]

John, did you look at the scope photo I posted. It is my voice directly from a 1" diameter, high quality studio condensor mic, through a preamp into the scope. That's it. The waveform is obviously asymmetrical.

The energy contained in most male voices is asymetrical. Female voices are generally more symetrical in nature.

Bruce, this is nitpicky but important. The energy is usually (maybe always) NOT asymmetrical. The voltage waveform is asymmetrical but the actual energy of the two sides (positive and negative) will be equal (integrate the positive portion and the negative portion and the result will be equal).

Yes Steve, you are correct. I am speaking in strictly general terms here.

Thank you for the clarification!

73,

Bruce

[K1JJ]

The voltage waveform is asymmetrical but the actual energy of the two sides (positive and negative) will be equal (integrate the positive portion and the negative portion and the result will be equal).

That's quite a message. (Really)  Certain math relationships are quite revealing.

I see the effects of asymmetrical audio in my voice when using a clean AM transmitter that will pass below ~ 140 hz to DC faithfully. (A balanced modulator or class E rig)   A switch of the audio phase creates either a high pos peak AM modulated "shark fins" pattern (correct phase) or a more symmetrical looking waveform that hits negative 100% prematurely and has lower pos peaks.  I get some "false" but effective low end by boosting the deep lows using the EQ when in the correct phase.

Interesting thing is IF the transmitter will not pass DC-140hz cleanly, (using modulation iron for example) then the opposite phase gives higher positive peaks cuz the low frequency "shark fins" pattern become distorted and limited in positive amplitude.

I notice very little asymmetry in my voice at higher freqs above say, 800 hz. In fact it appears to be of opposite phase to my low-end voice freqs.

BTW, hello to John. Your 813 schematic has gotten lots of use - tnx again.

T

[kc2ifr]

The phase rotator circuitry used in AM broadcast audio speech processing equipment is utilized for purposes of making the audio waveform more symetrical. The effect is significant; I can see it every time I use my Orban 9100A AM processor with my transmitter.

I disagree to a point. The fcc allows broadcast stations to modulate to 125% percent. Of course this is in the positive direction. A good AM compressor limiter will ignore positive going peaks untill they reach 125% BUT will limit negative peaks at the base line. There is no other way to reach 125% modulation without splatter. Im sure u realize 125% modulation in the negative direction doesnt exist...at least in normal AM modulation.

JMHO,

Bill   

[WBear2GCR]

Thom, I'm sorry you took offense or thought I was being patronizing. But the truth is it's counter-intuitive for some who do not like rock or not musical in general, as I'm sure many here do not or are not, <snip>
As far as the negative vs positive peaks on voice you are incorrect. On the output of a transmitter yes, but not in the balanced line level audio world. I spend my days looking at waveforms and I can tell you with all certainty (with the exception of bad cables in a balanced signal situation and some tube processors) the positive and negative peaks are equal.

<snip>
The device in question uses Jensens, about as good as it gets in audio transformers. And the rest of the audio chain throughout is Class-A. This device is not only used for guitars.

John
W2WDX

John,

Better look again on a scope at the envelope of a typical voice that you record, BEFORE you run it through some processor. It is going to be asymmetrical.

I think a whole lot of us know about Jensen transformers.

You can do the same trick using high quality opamps and convert balanced to single ended. Your guitar guys like the transformer for various other reasons - not "fidelity."

I have yet to run you in QRZ.com so I don't know your last name. Or your "level" of "fame."

Many on here either know what you know and more, or know about or are in the broadcast and/or recording industry.

Fact is that you are actually talking about converting BALANCED low Z to Single Ended high Z. You don't actually "need" anything for a low Z line to drive a high Z input - you can just throw away one phase if you want. Noise and hum from a long run, Common Mode of course will not be cancelled out if you do that. But in a ham radio situation, that may or may not be an issue. And besides, no one had best be running an AM transmitter with response out to 20kHz that he USES as a matter of course.

You do know why that is? 

So what this means is that an old UTC A series transformer will do the job just fine. Or for that matter a whole range of other transformers that have less prestige or pedegree...

My comments come after reading only your second post... maybe all this has been covered already.

               _-_-WBear2GCR

Edit: Let's talk acoustics and transducers for a minute. Technically none are symmetrical. Not one. Nor is the waveform produced technically symmetrical, that is because air is asymmetrical even when driven "without distortion by a sine wave". Compression and expansion are not equal. We can say that it is insignificant, but it is so far virtually impossible to make anything that moves air be truly symmetrical. This includes the Neumann and Telefunkens...

[W2WDX]

Ok,

Just got back from the studio today. I did some tests, using several microphones and a nice recently calibrated Agilent scope. I also called several top engineers, both from the audio world and BCE's.

Asymmetry indeed! I went through quite a bit of ribbing today. Comments like, "Aw .. come on John, your kidding this is fundemental. YOU didn't know this?" and various similar comments from my engineer friends. My only defense is that it is not so noticeable in the waveforms in Protools, but on the scope it was prevalent. Not an excuse. It has been so many years since I bothered to use a scope in that environment.

I went on at that point to start reading the information available about why that is. It turns out, as it is obvious many of you know this already but I'll mention it anyway, it is in the nature of the mechanical folds in the larynx and how they effect airflow and subsequent wavefront formation in the mouth. Not to mention why there are differences in the larynx, especially when you compare male and females. Incidentally, there is some data out there that talks about the differences between trained and untrained voices, due to the stronger and more developed muscles in the trained voice, both in the larynx and diaphram. As it was explained to me by Alan Parsons's this afternoon, glottal volume velocity waveform characteristics are such that the acoustic wavefront produced by the stronger forces in the male voice produce stronger positive forces in the air in the acoustic wavefront leaving the mouth. These are transduced accurately by the microphone as stronger electrical positive peaks or negative peaks, depending on the phasing of the microphone. As Thom mentioned, and confirmed by my BCE buddy's, this is why phasing of microphones is an important aspect at a commercial AM radio stations, as one example.

I am humbled and yet excited about learning something new, at least for me. This is fascinating stuff. That's what great about forums like this.

I did something like this before on this board, didn't I? Thom is that what you were referring to? LOL!!!! That's funny. I'll probably do it again!!!  

John  

[KA1ZGC]

I did something like this before on this board, didn't I? Thom is that what you were referring to?

Eureka! Give that man a medal!

Since you asked (and he did ask, folks): yes, John. A few times, actually. You rubbed quite a few of us the wrong way when you did it, too. That's okay, though... admission is the first step to recovery, as they say in those insipid twelve-step programs.

Part of it was that, but part of it was the way you blew your own horn as loudly as you could, not really knowing who your audience was. In the process, you were also showing that you actually didn't understand certain fundamentals of audio. It's impossible to do both without coming off as fraudulent.

Obviously, you didn't realize you were doing it, but that also rubbed a few of us the wrong way all those years ago.

While some don't approve of my tactics, your wheels started turning pretty quickly when you discovered I was doing it deliberately. Took about 24 hours, right? That's quicker than 10 years. We got nowhere with you being nice guys the last time around.

A word of advice: you and I are about the same age, and much younger (and far less experienced) than most on this board. Listen to them, and don't make any assumptions, pro or con, of your knowledge base vs. theirs. As we say on usenet: read, learn, evolve.

There, now that's settled. Welcome back, John, and don't be afraid to have any preconceived notions you walked in with uprooted. It's good for the soul.

Now, back to your regularly scheduled thread about audio.

--Thom
Kilowatt Amplifier One Zero Grid Current
See, guys? Now, was that so hard?

[Opcom]

Nice results can be had by operating all the audio iron at 1/2 to 1/3 of its ratings. overbuild, and loaf along. Crank up when necessary.. I use the typical old iron for converting those levels from line to 600, etc.. no active components.

The only thing about the product I want to disagree with is the claim that the board is mil-spec. It does not look like a glass expoxy board - looks white on the end/side, like the consumer grade material, instead of the thick glass epoxy boards I usually see in the fighting stuff.

[KA1ZGC]

White? Wouldn't that be G-10? I thought that was compliant.

Then again, I tought they were slipping so bad as to let some of that RoHS garbage into the system.

[WBear2GCR]

John,

Do you wish to comment on my post, especially the part about the balanced to single ended conversion??

             _-_-WBear2GCR

[Opcom]

White? Wouldn't that be G-10? I thought that was compliant.

Then again, I tought they were slipping so bad as to let some of that RoHS garbage into the system.

Maybe. The picture is too poor. It reminds me of the whitish phenolic-like material instead.


On the question of symmetry, more exactly the symmetrical action of using a push-pull modulator to plate-modulate a class C RF stage, doesn't one modulator tube work harder that the other? Maybe it is my imagination.

and for phase rotators, nothing beats a symmetra-peak! for age and weight anyway.

[KA1ZGC]

On the question of symmetry, more exactly the symmetrical action of using a push-pull modulator to plate-modulate a class C RF stage, doesn't one modulator tube work harder that the other?

Yep. Even when I still had a well-matched pair of RCA 811s in my modulator, the positive-peak tube showed a bit more color on the anode than the negative-peak tube.

The difference isn't as great with a pair in class AB_whatever compared to a pair in class B (as the vector sums are closer in AB), but it's still noticeable.

[DMOD]

I see nothing new or extraordinary about this circuitry.

The input is balanced by a solid state idifferential amp circuit followed by a Jensen output transformer which does have good phase specs.

IMHO, if you look at the Jensen app. circuits, it appears this amp is a combination of various app. circuits.

Phil - AC0OB

[W1DAN]

Hi John!

Welcome to the board!

The Orban phase shifter is an all-pass phase rotator (using caps and an op-amp) that shifts mainly the low frequencies. So the low frequency peaks get tilted (err rotated) and end up being more symmetrical than the input (if they were assymetrical). This circuit does affect the sound a little and many people on this board end up using a mic polarity switch, which is more faithful.

You would not see the assymetry in a vocal track in Pro-Tools unless you expand the horizontal time way in to where part of a word fills the screen (so you see maybe 10 cycles of the waveform). Maybe someone singing will tend to be more symmetrical that spoken word??

As mentioned here, some modulators can be VERY linear..tube or solid state, and thus are able to impose the exact waveform on the RF carrier that is fed into it. Steve's two modulators...PDM and linear are flat within the audio range of near DC to 7kc or higher...able to pass a square wave pretty accurately. Those tube modulators with just a modulation transformer (no grid transformer) can get close to this.

As stated earlier here, for ham AM, the goal is to get the highest peaks of a voice going in the positive direction and not negative so that the carrier is not cut off by the extended peaks. Some people use analog soft limiters and/or clippers to limit the negative modulator excursions.

Please continue to chime in, ask questions and tell us of your radio exploits.

I have learned a lot from this board and enjoy reading all the opinions.

73,
Dan
W1DAN
(A broadcast engineer)

[Opcom]

Here is a Bogen passive that's not expensive. It does not have all the high spec for frequency response, but looks OK for comunication use.

http://lib.store.yahoo.net/lib/cam-electronics/WMT1As.pdf

[flintstone mop]

YUP OPCOM,
I love those WMT-1's from Bogen. My former employer always had many, many of those. Notice HAD!!!  Great z match. Made a big differernce in the TX audio in the K7DDY Class D transmitter.

Fred