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Author Topic: Over-the-air modulation monitor schematic  (Read 10649 times)
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ka1tdq
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« on: July 21, 2014, 10:44:23 AM »

I'm looking for an over-the-air modulation monitor schematic.  I've seen one before while browsing stuff on the web, but I can't find it again.  It was something like an unpowered, diode detector circuit that could be used to feed an audio chain. 

I'm thinking about feeding that into my headphones (via an audio amp) during transmit and then switching to normal receive audio during RX.

Jon
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« Reply #1 on: July 21, 2014, 11:29:53 AM »

Jon

[Schematic of an off-air AM monitor (not a "modulation monitor") attached]

There are several ways to build a simple off-air monitor (most people would not refer to this as a modulation monitor).

I've tried them all. They all work fine (if implemented properly). Some employ a resistive voltage divider, bridged across the transmitter's output coaxial cable. E.g. the pick-ups that Steve (WA1QIX) uses with his REA modulation monitors. Some employ a capacitive voltage divider. Some employ a magnetic pickup loop, located near the transmitter's output tank circuit.

I generally use a simple current transformer design because it can (if you wish) drive a balanced audio cable, feeding the balanced input of an audio amplifier. Also, for plate modulated transmitters, this approach avoids modulated B+ (direct audio) feed through at the RF output of the transmitter.

Note 1: A current transformer, in this application, is (in effect) a magnetic loop pick-up with more predictable and controllable coupling characteristics (to the RF source of interest) than a magnetic pick-up loop placed in proximity to the transmitter's output tank circuit.

Note 2: One can eliminate modulated B+ (direct audio) feed through at the RF output of a plate modulated transmitter by including a "safety choke" (an RF choke) across the transmitter's RF output. That is highly recommended, just for safety purposes... but this pick-up will work fine even if the transmitter doesn't incorporate a safety choke across its RF output.

For good fidelity in the monitor, it is essential to employ an audio amplifier (as you propose to do) between the output of the diode demodulator, and the headphones. This has been pointed out by Steve (WA1QIX) and others.

Attached is the schematic.

Note 3: The attached schematic is useful for a transmitter whose output power (at carrier) is between 37.5 watts and 375 watts. For power levels between 3.75 watts and 37.5 watts: use a 150 ohm non-inductive resistor instead of a 50 ohm non-inductive resistor. If you use a 150 ohm non-inductive resistor, its power rating must be 1/85th (i.e. 3/256) of the output power of the transmitter.

The ferrite core that I use is the same core that many people use to build transformers for class E transmitters: 1.125 inches long, 1 inch O.D., 0.5 inch I.D. Type 43 (I use those because I have lots of them on hand... but none of the details are critical for this application)

The insulated center conductor (but not the shield) of a piece of coaxial cable passes through the core (i.e. equivalent to 1 turn).* The secondary consists of 16 turns of insulated hook-up wire. Since this is a current transformer... the current that flows in the 16 turn secondary will be 1/16th of the current that flows through the coaxial cable's center conductor. The voltage across the permanently-connected, non-inductive, 50 ohm resistor (neglecting the effect of the amplifier's input impedance, and assuming that the coaxial cable is looking into a 50 ohm load) will be 1/16th of the voltage across the coaxial cable (center conductor-to-ground). The power dissipated in the permanently-connected, non-inductive, 50 ohm resistor, will be 1/256th of the output power of the transmitter.

*Note 4: If you wish (but it probably won't make much difference) you can implement a "better" version as follows: Push a short piece of coaxial cable... including the center conductor and the shield... through the ferrite core. At one end, connect the center conductor of the coaxial cable to the center pin of the RF connector, and connect the shield of the coaxial cable to the ground of the RF connector or to the metal box in which the coupler is built. At the other end, connect only the center conductor to the center pin of the RF connector. Leave the shield unconnected (trimmed off, but not grounded to the box or the connector). This approach will allow the intended inductive coupling to take place between the coaxial cable's center conductor and the 16 turn secondary winding... but this approach will reduce capacitive coupling between the coaxial cable and the 16 turn secondary winding. In my own experience, this "better" version has little or no effect... particularly if the transmitter incorporates a safety choke at its RF output.

Note 5: It is essential to include a non-inductive 50 ohm resistor (the resistance value isn't critical), permanently connected across the secondary of the RF current transformer (as shown). This is in addition to the impedance looking into the amplifier. If you leave the secondary of the RF current transformer "open", a very high voltage will be produced across it.

Stu


* Envelope Monitor Floating.jpg (63.65 KB, 960x720 - viewed 684 times.)
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« Reply #2 on: July 21, 2014, 11:46:25 AM »

Thanks Stu

This works out perfect since there's room on my homebrew linear at the SO-239 output to put a ferrite core for this.  Plus this is simpler to build than what I was thinking.

I was going to build basically a crystal radio with an antenna and feed that into the audio amp.  This just samples the RF out of the linear.  Cleaner, neater...

People are telling me that my audio sounds great, but I want to check for myself to be sure.  Not to be critical, but I've heard some nasty audio down in this part of the country compared to the northeast.  I want to compare apples to apples.

Jon
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« Reply #3 on: July 21, 2014, 12:34:12 PM »

People are telling me that my audio sounds great, but I want to check for myself to be sure.  Not to be critical, but I've heard some nasty audio down in this part of the country compared to the northeast.  I want to compare apples to apples.

Jon
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Hi Jon,

Yes, sometimes getting audio opinion reports off the air can be very misleading. There are so many variables - the other guy's audio desires and standards, his receiving gear, your own audio desires, etc.

If you monitor yourself with headphones and a diode detector, bear in mind that head bone conduction will color it too.  The best method is to record yourself off the monitor using a good, direct recording system that has been tested.

Another good method is to have someone with a good working SDR (Flex, etc)  make a recording off the air and send it to you in an .mpg file via email.  Make sure there are a few other stations on the recording who have good audio so you can see if they sound accurate compared to what you hear of your own audio.

As for on-air opinions...just like a finding a good mechanic, once you find a few guys who can give out accurate audio reports that match your own goals and desires, stick with them.  Some, even though they are capable of giving out accurate reports, tend to "hi-hi fine biz it" and just say the audio sounds fine - trying to avoid conflict.  You want someone who will tell it like it is.  Here in the Northeast, TimTron is someone who will tell us we have a sock in our mouth if it sounds like crap. And there are a few others. We need someone with BAs and recording gear to tell us what it really sounds like.  Nothing worse than hearing someone with obvious audio problems...clipped and distorted audio, getting a FB audio report.  How is the poor guy supposed to help himself with no direction?

For example, just adding 6 dB of 150 Hz "mud frequencies" can take great sounding audio and turn it into a boomy mess.  Someone with a good ear can actually hear this and tell us to reduce the 150 hz EQ slider. I've heard the Tron do this many times.

If you are particular, getting your audio settings optimized for your own voice can be a long and difficult road until you yourself develop a good ear for it.   There are some AMers who have worked hard to develop these skills and are always getting great unsolicited audio reports.   And some guys just get their audio sounding "OK" and are satisfied with that.  It all depends on your own goals.  

T
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« Reply #4 on: July 21, 2014, 01:17:55 PM »

On the air recordings from someone with an SDR would be great, provided the band is in good shape with little QRM.  I was planning on having my wife talk on the air while I listen in the other room.  Her voice doesn't have as much bass as mine, but it would be a fairly good indicator if something was majorly wrong.

Sometimes simple "nice" honesty goes a long way.  I was talking with a guy in CA who was upping the drive to his linear little by little with a rice box.  It sounded ok at first and then got to a point where it sounded pretty bad.  TimTron probably would've said much worse.  I nicely said that the extra drive was causing audio problems and that I could hear him fine at the lower drive level.  He was glad to know and reduced his drive.

I heard a station in Utah that was completely overdriving his linear.  The transmitter was a boat anchor style (Johnson model something).  He was overdriving so bad that the distortion completely clipped everything.  I didn't get on the air though to tell him.

Bottom line I guess is that less power is better than crappy audio.

Jon
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« Reply #5 on: July 21, 2014, 01:46:25 PM »

I was planning on having my wife talk on the air while I listen in the other room.  Her voice doesn't have as much bass as mine, but it would be a fairly good indicator if something was majorly wrong.


Jon,

Yes, your wife's voice will tell if something is wrong, but you need to go further.

Optimize the audio settings for your own voice.  Your wife's voice might require entirely different audio settings.   I've heard guys as "guest ops" on others' rigs and it hardly sounded like them.  If you are going for the best YOU can sound with your particular voice, warts and all, then use only your voice for tests.  In fact, a good recording of your voice piped thru the system as program material can eliminate the "bone conduction" problem, though, the mic is not used.

As for testing for rig fidelity and cleanliness -  use audio tones to run audio sweep, IMD and THD purity tests. Even a simple triangle audio waveform at 200 Hz, looking at the scope modulated waveform, can tell you a LOT about the linearity of your rig.  Once you get it clean, then start playing with EQs, processors, mics, etc.   Once you are sure the rig is clean, setting up the audio is easier - and you have the confidence to proceed.  There are a few good threads on the forum about purity testing.

T
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« Reply #6 on: July 21, 2014, 03:50:32 PM »

 " your wife's voice will tell if something is wrong "


This is very true. With my wife, I don't even have to ask.


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« Reply #7 on: July 21, 2014, 05:15:15 PM »

Jon,

Keep in mind when your listening to yourself on an off the air monitor you will likely sound brighter in the monitor than as heard on the others persons receiver.  Some folks listen to themselves over a off the air monitor and dial in the EQ and that seldom works.  The objective is to sound good to most people you talk to.  The average BW of a lot of receivers is 6kHz.  Yes, some will have wider bandwidths but a lot will not!  After awhile you will get use to the extra brightness you hear on your local off air monitor and not think anything more about it! 

I set up my EQ while listening to myself on a 6 kHz portable receiver and then I heard what that sounded like on the wide bandwidth local monitor and then that sound became the norm for my station.  As Tom said, depending on your voice, it is often necessary to do some notching around 250 Hz.  When doing that you can have the frequencies below that flat and not sound at all boomy.  Based on your mic, some mid range pre-emphisis tends to really improve things as well.  Doing only that with a good clean audio chain within the transmitter will produce a great sounding signal on the air.

Joe, W3GMS                 
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« Reply #8 on: July 21, 2014, 09:29:53 PM »

The sdr recordings are the way to go.

I listen in the mod monitor but I sound good to myself even when I sound crappy to others.
A nice long recording is very nice to give you an idea about other things besides the tonal qualities.
A few have sent me recordings and I sound like I have mad cow or something, I have a hard time speaking like a normal person.

Besides nice sounding voices and rigs, some guys have something interesting to say, and say it well.

Some others may sound really good, but after 5 minutes you know as much as you ever will, and more then you want to know.

 
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« Reply #9 on: July 22, 2014, 12:12:43 PM »

Agree with all comments that point that live monitoring needs to be taken with this thought (my opinion):  Most folks with BA receivers1 will tune for peak.  This will always result with a roll off of the highs  and a boosting of the bass.  Combine that with the addition of atmospheric noise there is a need to overcompensate in the high frequency end.

SDR recordings, or the like, to check for audio performance are in my opinion the way to go 'cause there is little opportunity to color the response with Bell curve IF filters of many receivers as illustrated with the attached PDF(with apologies to the Forum - I have previously posted this attachment).

That said, I use trusted sources for on the air recordings or will hook up my high power coupler directly to my SDR and make a recording.  When listening to your own voice live, remember to have a means to switch the phase to your high quality headphones as one phase will tend to cancel out the bass end - resulting in your boosting up your bottom end response.

My 2c, Al

_______________________________________

1 Rectangular mechanical filters may be an exception as long as the signal is tuned to one edge of the filter or the other

* PROPER BA RX TUNING OF AM SIGNALS.pdf (16.38 KB - downloaded 169 times.)
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« Reply #10 on: July 22, 2014, 01:14:46 PM »

Agree with all comments that point that live monitoring needs to be taken with this thought (my opinion):  Most folks with BA receivers1 will tune for peak.  This will always result with a roll off of the highs  and a boosting of the bass.  Combine that with the addition of atmospheric noise there is a need to overcompensate in the high frequency end.

SDR recordings, or the like, to check for audio performance are in my opinion the way to go 'cause there is little opportunity to color the response with Bell curve IF filters of many receivers as illustrated with the attached PDF(with apologies to the Forum - I have previously posted this attachment).

That said, I use trusted sources for on the air recordings or will hook up my high power coupler directly to my SDR and make a recording.  When listening to your own voice live, remember to have a means to switch the phase to your high quality headphones as one phase will tend to cancel out the bass end - resulting in your boosting up your bottom end response.

My 2c, Al

_______________________________________

1 Rectangular mechanical filters may be an exception as long as the signal is tuned to one edge of the filter or the other


Hi Al,
All good points you raise and the SDR based radios with the square shaped filter response is a great way to hear what you sound like under an ideal receiving set-up.  In fact, I would expect the sound to be very similar to whats heard on a standard high quality off the air monitor providing the SDR receiver is set to a wide enough bandwidth. 

Now we get into a philosophical point.  Since most guys that I talk to seems to be using non-SDR based receivers, how best should I EQ the audio chain to sound reasonable to both camps?  Some folks will say, if they don't have a wide bandwidth receiver then the heck with them!  I have taken somewhat of a compromising position.  So the final EQ may not be perfect for an SDR transparent receiver and it may not be perfect for the guy using the old boat anchor receiver, but it sounds pretty darn good to both.  So for me the goal is to try and sound reasonable to both camps! 

When people first get into AM it seems like the first thing many want is a boosted low end and when they obtain it, it really does not sound very good because of the typical receiver used at the other end.  Even worse is those that do not try to obtain a balance so articulation is projected in ones signal.   One's voice has a major factor into the equation as we all know.  Take a soft spoken guy with a bassy voice his EQ can be a great friend in bringing some articulation into the signal.     

At the end of the day people like all kind of different audio so find what your comfortable with a go with it! 

73,
Joe, GMS     
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« Reply #11 on: July 22, 2014, 01:36:58 PM »

To hear myself over the air, I plug a set of earphones into an adjacent receiver. I like things simple.
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« Reply #12 on: July 22, 2014, 09:26:55 PM »

Joe

I think most folks would agree that making sure a "presence rise" is present should be pursued.  A look at the typical frequency response of the venerable Astatic D104 would be a good place to start.  I refer to that frequency response because it enjoys a time-tested reputation.  After that one can decide whether or not to go for the so-called HI FI response.  This is where I and some of my friends may part ways because I believe a well adjusted AM transmitter that may not have the "bottom end" response of a HI FI station is quite acceptable and pleasant for me to listen to.

My main point in my previous post was to point out that some lack of high frequency audio component or a boomy bass report may not be entirely the problem at the transmit end. 

I do not agree with the use of a adjacent receiver for an accurate assessment of how one sounds like.  Frequency response of the receiver audio, ground loops, stray coupling and other problems that arise from that approach may get in the way.  There are good suggestions of assessing one's audio on this forum. 

This brings me full circle to the topic of this thread that seeking out a good way to monitor one's modulation is a good practice and that while one is at it inclusion of a way to monitor one's audio live is a good thing also.  My most recent learned lesson is the point that I made in my previous post - that the monitoring headphones should have the means of phase reversal to eliminate the tendency to boost the bass higher than is real because of bone conduction issue with the use of headphones.

Al


* ASTATIC_Base_D104_Frequency_Response.gif (4.86 KB, 490x178 - viewed 337 times.)
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« Reply #13 on: July 22, 2014, 10:15:50 PM »

Al

Hi!

Note the (audio) phase reversal switch at the output of the off-air monitor schematic that I posted.

Best regards

Stu
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« Reply #14 on: July 23, 2014, 12:46:09 AM »

Al

Hi!

Note the (audio) phase reversal switch at the output of the off-air monitor schematic that I posted.

Best regards

Stu

Yes Stu!  I did.  I'm thick but I get there eventually.Timmy, Nick, and Steve suggested it and what a revelation when I built it and tried it for the first time I flipped the switch.

Havent worked you lately, we'll have to hook up one way and another

Al
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« Reply #15 on: July 23, 2014, 07:48:08 AM »

Joe

I think most folks would agree that making sure a "presence rise" is present should be pursued.  A look at the typical frequency response of the venerable Astatic D104 would be a good place to start.  I refer to that frequency response because it enjoys a time-tested reputation.  After that one can decide whether or not to go for the so-called HI FI response.  This is where I and some of my friends may part ways because I believe a well adjusted AM transmitter that may not have the "bottom end" response of a HI FI station is quite acceptable and pleasant for me to listen to.

I totally agree.  My goal has always been to sound natural on the air.   It must work since when I meet people in person the first thing they tell me is I sound exactly like they hear me on the air.  The D-104 as we all know can sound excellent on the air.  One tall ship station that uses only a D-104 is Bob, W2ICQ.  His audio is transparent and having met Bob several times in person that's been proven over and over again.  In fact, his audio sounds better than some AM BC station that I listen to.  The worse situation is when someone EQ's a transmitter by jamming audio into it which it is not capable of passing without distortion. 

My main point in my previous post was to point out that some lack of high frequency audio component or a boomy bass report may not be entirely the problem at the transmit end. 

Technically that is a fact!  My point is sometimes you can compensate at the transmitting end to sound good to those that are using less than ideal receiving setups while still sounding good to those with better receiver setups.   As long as at least a 6 kHz bandwidth is used, I have found a compromise can be reached.  A lot of stations we both talk to are using less than ideal receiving set-ups and I would like to sound as good as possible to them. These folks are satisfied with the receiver they are using and likely will not change a thing. 

I do not agree with the use of a adjacent receiver for an accurate assessment of how one sounds like.  Frequency response of the receiver audio, ground loops, stray coupling and other problems that arise from that approach may get in the way.  There are good suggestions of assessing one's audio on this forum. 

This brings me full circle to the topic of this thread that seeking out a good way to monitor one's modulation is a good practice and that while one is at it inclusion of a way to monitor one's audio live is a good thing also.  My most recent learned lesson is the point that I made in my previous post - that the monitoring headphones should have the means of phase reversal to eliminate the tendency to boost the bass higher than is real because of bone conduction issue with the use of headphones.

The first time I had a phase reversal switch on off the air monitor was when I purchased a REA AMM-HF1 and it sure does make a difference to have the monitor phased correctly. 

Al


See my comments above.  This is just my opinion!  I hope I have clarified my position that you can sometimes have the best of both worlds with a very good sounding on the air signal even when the receiver used by the listener is less than ideal but is a reasonable receiver. 

Joe, W3GMS         
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« Reply #16 on: July 25, 2014, 01:15:59 AM »

I modified the output of my linear to include the AM monitor.  I didn't include the phase reversal switch for simplicity.  Now I just need to add an LM386 circuit afterward as an audio amp.  I will feed this into the back of my powered Bose speakers through the RCA jacks.  The speakers have a headphone jack. 

Plugged into the other set of RCA jacks on the Bose speakers is my Alinco receiver.  This way I can listen to myself during TX and the radio during RX.  The radio is muted during TX.

Jon
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* amp am monitor.jpg (1907.26 KB, 3264x1836 - viewed 438 times.)
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« Reply #17 on: July 25, 2014, 08:27:13 AM »

Jon

Some things to think about:

The audio output of the diode/capacitor/resistor detector is going to have a level of around 20 volts, peak-to-peak, provided that it is connected to a load having an impedance of 10k ohms or more. This assumes that the power output (at carrier) of the RF amplifier is 256 watts, and the modulation index is 100%.*

The LM386 provides high impedance inputs (50k ohms, balanced, on the + and - inputs)... which is good. But the LM386's voltage gain of 20 is not compatible with the 20V peak-peak output of the detector.

You will need to employ a suitable resistive voltage divider at the input of whatever amplifier you use to match the high impedance output of the detector to the low impedance load presented by headphones or a speaker. I.e. you need current gain, but you don't need voltage gain.

The secondary of the current transformer, including the permanently-connected load resistor and the diode/capacitor/resistor detector circuit, is floating with respect to the RF amplifier's ground... provided you don't connect either side of the transformer's secondary winding to ground... and provided you don't connect either side of the detector circuit to ground.

There are strong RF fields near the output tank circuit of the amplifier. Also, there may be a non-negligible 60Hz voltage difference between the RF amplifier's ground and the audio amplifier's ground. This is a potential source of hum (if you create a ground loop between the RF amplifier and the audio amplifier).

Consider using a shielded, balanced cable (like balanced microphone cable) between the (audio) output of the diode detector circuit (located inside the RF amplifier) and the input of the audio amplifier (located outside of the RF amplifier). Ground the shield of this cable only at the RF amplifier end. Leave the shield at the audio amplifier end of the cable floating.

If the audio amplifier has an unbalanced input (like your Bose powered speaker), connect one wire to the center pin of the RCA connector, and the other wire to the shield of the RCA connector. Leave the shield of the cable floating (not connected to anything) at the side where the powered speaker is attached.

* If the RF amplifier is delivering 256 watts into a 50 ohm load, then the RF current flowing through the center conductor of the coaxial output cable has an amplitude of 3.2 amperes (peak, not rms). The RF current flowing through the secondary winding of the current transformer has an amplitude of 3.2A/16= 0.2A (peak, not rms). The peak of the RF voltage waveform across the 50 ohm resistor that is permanently connected across the secondary winding of the transformer is 0.2A x 50 ohms = 10 volts. The 0.001uF capacitor of the detector will charge up to a voltage (across the capacitor) of a little less than 10 volts. With 100% modulation, the voltage across the capacitor will vary from a little less than 20 volts to 0 volts.

Stu
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« Reply #18 on: July 26, 2014, 11:46:28 AM »

I just tested it this morning and it works fine.  I do get some distortion in my audio when I turn the audio drive above about half way.  When I do that, my PEP meter doesn't budge above carrier level.  I'm thinking that the extra audio drive is overdriving my Bose RCA jack inputs.  I'll put a potentiometer in line to see if that clears up the problem.

For construction ease, I just fed the audio line from the AM monitor to the Bose speakers with unbalanced regular RCA audio line.  I broke it out into left/right feed with a Y-splitter.  At carrier I'm hearing crystal clear audio, so I don't think I have RF or AC hum getting into the line.

Jon
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« Reply #19 on: July 26, 2014, 12:43:06 PM »

Jon

Good!


What is the power output of your amplifier, at carrier level?

Stu
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« Reply #20 on: July 26, 2014, 01:08:56 PM »

I used it at both 200 and 250 watts carrier with the same result. We're out and about today, so I'll work my way to Radio Shack for an enclosure and a linear 100k pot.

Jon
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« Reply #21 on: July 27, 2014, 12:31:15 PM »

I put a 100k pot in line to the speakers, but I still got distortion.  I think the problem is RF getting into them.  It's a simple solution, but I listened to myself through the receiver unmuted and it sounds fine.

I did talk this morning with WB6QKO in Santa Monica this morning and asked him to be super-critical of my audio.  He said that when I put the drive at 10 (maximum) it got a little rough around the edges, but 9 was fine. 

Not to get too technical, but I left it at 8.

Jon
KA1TDQ
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« Reply #22 on: July 27, 2014, 12:49:25 PM »

Jon

As a test of whether RF is getting into the Bose powered speaker system (via the wall wart):

Unplug the audio cable from the Bose speaker system's RCA input jacks. Leave the Bose speaker system powered on, with the volume up. See if there is a transmitted signal modulation level at which you begin to hear (probably distorted) output from the Bose speakers.  

Try a battery powered amplifier. You may still need the 100k ohm pot at the input because of the 20V peak-to-peak audio output level of the off-air monitor (at 100% modulation). You could use the LM386 with a 9V battery, with the 100k pot's output from the + input to ground... adjusted to keep the input voltage level at no more than +/- 0.1 volts... and the other input floating/unconnected. LM386 can easily drive headphones or a small 8-ohm speaker.

Even some AC-powered speaker systems, that employ AC-DC "wall wart" power supplies, can be powered (at least briefly) from a 9V battery.

Separately, you might try putting a ferrite choke on the cable between the power converter (the wall wart) and the Bose powered speaker system. If the DC connector is not too large, the best way would be to run the cable through, and around, one of the 1 inch OD, 0.5 inch ID ferrite cores. Even one turn may be sufficient, but a few turns would be better. A snap-on ferrite, with a few turns wrapped around it, may also work.

The Harman/Kardon, powered computer speaker system that I use in my shack (connected to a separate computer that I use for E-mail and Web access) is so susceptible to RFI, that I have to completely turn off the power to it, when using my transmitter.

I'm using an AC-powered audio mixer, feeding AC-powered audio amplifier, for audio monitoring in my shack, including off-air monitoring of my own AM transmissions. It is (fortunately) well-protected against RFI.

Stu

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« Reply #23 on: April 28, 2015, 02:25:52 AM »

Here's to reviving an old topic...

I built the circuit for my amplifier that Stu posted, but had a problem with RF getting into the audio chain.  So, I shelved the project until now.  I revived it because I found that monitoring myself through my microphone preamp headphone jack produced HUGE amounts of RF hash into the headphones.  It was so much and so loud that I threw the headphones off.  

AND, the reason why I decided to change to that way of monitoring myself was so that I could use the SDR feature of my receiver.  When in I/Q mode, I can't monitor transmissions because there's no audio out of the headphone jack on the radio.  

To get this circuit working for my situation, I found that by simply connecting the headphones to it with a parallel resistor at 7 ohms, it sounds really good!  There's only a touch of hum from RF getting into the headphones.  From the picture, you can see all the different forms of testing I did.

I'll neaten this up with a nice box.  I also built a separate switching box that feeds the audio from my computer to the headphones during RX and then to this circuit for TX.  

The audio is nice and loud too.

Jon
KA1TDQ


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« Reply #24 on: April 28, 2015, 03:16:36 PM »

Like using an sdr, once you start monitoring yourself, its hard to operate without doing so.
I want to have some idea about the audio going out, things can happen and you can get distortion or a crap out and its nice to know about it.

I am not sure why everyone has to sound fantastic, clean and normal is good enough for me.
I actually enjoy a bit of the ear bleed sound, its easy to copy!
Most people on AM sound quite good to me, I clue in more on distortion and noise then frequency response.
Some bc610 or globe king power hum is fine, sounds cool, but blower noise and distortion are a turn off, as is excessive or low modulation.


I have RX audio fed into the mod monitor with a relay hooked up to the PTT keying/muting circuit.
RX audio in the headphones and speakers through the Marantz and mod monitor audio on TX.
I have a hard time operating without it now.
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