Homebrew AM Monitor

[W1AEX]

I'm just curious what people are using to monitor their own audio while transmitting. I searched the forum but did not see any schematics or much discussion other than a few posts about commercial monitoring stuff. I have been using a junkbox lightly coupled variable LC network to a diode bridge detector that dumps the signal into an audio transformer to drive some cheap 8 ohm stereo headphones. It works, but it's fussy to adjust and tends to distort/clip the detected signal at times. Anyone got something simple that works?

Rob W1AEX

[W2NBC]

Hi Rob,

This link is for a simple and effective general purpose detector, works OK fine..

http://amfone.net/ECSound/RFsampler.htm

Jeff

[w3jn]

I use a HP 8901A modulation analyzer.  Has all kinds of cool features (freq counter, power meter, mod %, various audio hi/lo pass filtering, etc), but the best of all is a AM detector with about .2% inherent distortion.  THAT is a pretty good trick.
$120 bux with shipping on eBay.

[Pete, WA2CWA]

I use earphones plugged into the receiver headphone jack.

[WBear2GCR]

Heresy, I use an un-muted R-388 receiver, with the station speaker on, and about 10 feet away... Dow-key switches the receiver out, transmit in, the signal level to the receiver seems ok, fine and the AGC levels the output so that the differential between my receive audio and transmit audio is nil. Serindipity. 

Ymmv.

            _-_-Washers Bolts Two Grease Caps Rivet

[ve6pg]

...on east coast sound is a simple audio circuit, which includes a scope pick-up...werks fer me....sk...

[AB2EZ]

Agreed

The East Coast Sound design is a good example (among many others) of an off-air monitor that will work fine: with one caveat

The circuit shown is good for monitoring an AM signal with around 16 watts of carrier:

I.e. 16 watts into 50 ohms corresponds to a peak rf voltage of 40 volts (at carrier), and 80 volts on 100% modulation peaks.

Even with the 2700 ohm resistor in series with the 100 uH choke (whose impedance will be somewhat unpredictable at frequencies above 1 MHz, but perhaps as high as 2k ohms at 3.885 MHz... depending upon what its self-resonant frequency is), there is probably too much reverse voltage across the diode if one were to use this circuit with a higher power transmitter.

Also, at higher frequencies (well past the self-resonant frequency of the 100 uH choke), the 2700 ohm resistor will dissipate about 1/50th of the transmitter's output power... so the 1 watt value will limit the carrier level as well.

I would suggest that one use the circuit shown with the following changes (again there are many alternative designs, but these changes will enable this circuit to handle the power levels):

1. Place another 2700 ohm 1 Watt resistor in parallel with the 100 uH choke. This will form a voltage divider with the existing 2700 ohm resistor of at most 1:2 (at most, 50% of the rf voltage will appear across this new 2700 ohm resistor)

2. For carrier power levels between 1 watt and 16 watts, leave the existing 2700 ohm, 1 watt resistor as is.

3. For carrier power levels between 10 watts and 160 watts, change the existing 2700 ohm resistor (not the new one you added in step 1. above) to a 10k ohm 2 watt resistor.

4. For carrier power levels between 100 watts and 375 watts, change the existing 2700 ohm resistor (not the new one you added in step 1. above) to a 27k ohm 2 watt resistor

Other comments:

The purpose of the 100 uH choke is to bypass any audio frequency voltage (not modulated rf) that may be at the output of your plate modulated transmitter.

The circuit would work even better (better fidelity in tracking your modulated envelope) of you changed the .01 uF capacitor to .001 uF, and changed the 1000 ohm resistor to 10,000 ohms. This will reduce the amount of current/charge that the diode has to deliver to increase the voltage across the capacitor... thereby reducing the distortion introduced on negative peaks due to the voltage drop across the diode.

[W3RSW]

I assume the orig. 1k load resistor from the diode was so that you could feed 500 ohm, reasonable 'fi' headphones directly. With the 1k changed to 10k, might you now have to feed some sort of transformer or amplifer to get a decent match?  Especially with 8 to 16 ohm phones?  Sort of defeats the very simplicity of the original circuit.  Might be workable to change the final 1k to a 5k pot, the arm of which could go to the phones.

Which would be better, putting a pot in the input to the diode or in the output for variable volume? Probably at the input divider and keep a more constant load on the output.  Oh well, with the inherent distortion on a typical, even east coast, transmitter probably chasing phantoms anyway.

[AB2EZ]

Rick

Yes...

If you want to put together a decent off-air monitor that is free of RF artifacts, and which produces a high fidelity replica of what your signal is... then it is likely that you will need to use an amplifier between the pickup and the headphones. It's a trade-off. The output power from the pickup is ~ 5 volts x 5 volts / 10,000 ohms ~ 2.5 mW. This is enough to drive a pair of headphones if you have a high impedance-to-low impedance matching transformer.... but it is probably more practical to use a headphone amplifier with a high impedance (10k ohms or more) input.  Fortunately, you can build a headphone amplifier from a plain vanilla, single voltage audio op-amp (the kind Radio Shack sells for around a dollar) and run it off of a 9-volt battery. In any event, one should take some simple steps to keep rf out of the op-amp (bypass capacitor between B+ and ground) and the headphones (a 100 ohm resistor in series with the op amp output, and a .1 uF capacitor to ground after the resistor). Using a 10k pot instead of a 10k fixed resistor to feed the op-amp is a good idea. That way you can adjust the headphone volume. You definitely don't need voltage gain... just current gain... so set the op-amp up for a voltage gain of unity.

I find that getting the off-air monitor working well (low distortion and free of rf) is a very worthwhile investment in putting you in a position to adjust your audio properly.

Stu

[flintstone mop]

WOW John,
That HP gadget makes my mouth drool..............$120 from epay?

Fred

[Ian VK3KRI]

An inline monitor is great for monitoring what you transmit, but don't forget it will have an audio bandwidth MUCH wider than any real receiver.  If you listen to a signal with pre-emphasis, it will sound excessively bright off the direct monitor, but it might be spot on for real receivers with 5khz or less bandwidth.
                                       Ian VK3KRI

[w3jn]

WOW John,
That HP gadget makes my mouth drool..............$120 from epay?

Fred

There are 3 on eBay right now at $99 with no bids.  Several of these are re-lists, they didn't sell the first time at $99.

[AB2EZ]

Ian

Agreed!

What comes out of my off-air monitor is not what comes out of a typical receiver. Each receiver... some with a wide bandwidth, some with a narrow bandwidth, some with a fast-acting AGC circuit (which often interacts with AM in an unfavorable way)... will produce a somewhat different sound. Listeners, of course, have a wide variety of preferences with respect to what they think sounds best.

What I find particularly useful about my off-air monitor... which is free of artifacts (perceptible distortion, perceptible rf pickup interactions, perceptible hum)... is that I can easily detect problems or changes that may arise in my own signal. I can easily hear the effects of distortion that is introduced if my "linear" amplifier is not faithfully following the modulation on the input signal (e.g., by doing an A/B comparison between a pickup monitoring the envelope of rf input to the linear amplifier and a pickup monitoring the envelope of the rf output of the linear amplifier). I can easily hear differences among microphones and audio channel processor settings... and even with the effects of bone conduction... I have learned what I sound like in my own headphones when others say that they like the way my audio sounds.
 
Thus, between my off-air monitor (headphones) and my scope, I believe that I can do a pretty good job of consistently adjusting my signal to sound good to others... even though it may sound different to them than it does to me.

As an aside, it would be easy to add an audio filter to one's off-air monitor (a few resistors and capacitors and an op-amp) to limit the bandwidth to... for example, 100 - 3500 kHz 3dB roll off points, in order to emulate a band-limited receiver.

I run the output of my off-air monitor through a 2/3 octave parametric equalizer... with the high frequencies typically rolled off by 3dB at 6300 Hz, and more beyond that. This same equalizer works with the audio I get when I listen to others... and I sometimes use it (for example) to roll off the low frequencies on signals I receive from operators who like to send out a lot a "lows".

I find that as long as the signal-to-noise ratio is reasonably high, I can tailor the audio on received signals to suit my ears... just as I can tailor the audio on my outgoing signal.

Best regards
Stu 

[steve_qix]

Cheap, Simple, Works Well - PICK 2   

Well, it depends - how serious are you about really hearing your audio?

If you *really* want to hear your audio, you won't be able to use a detector that can power headphones directly.  You'll need a low distortion detector, fed into an audio amplifier (with bass/treble adjustments), driving very good quality headphones.

I have quite a bit of experience with off-air monitors, both from broadcasting, ham radio, and desigining the AMM-HF1 commercial modulation monitor.  It seems like a simple problem, but it's actually not 

The detector is fairly straight forward.


The output of this detector should feed another filter just like the one in the detector (an input 220pF cap to ground, a series inductor, another 220pF to ground and a 27k to ground.

Feed this into an audio amplifier with bass/treble controls and high quality headphones.  An added bonus:  As part of your transmit/receive system, include a relay to switch between the off-air monitor and the receiver audio (from the audio level control, fed into the headphone amp).  Switch the input to the headphone amp with the relay.  Makes a *nice* receiving/monitoring setup.  This is what I have in my own station.

Oh, do not bother trying to emulate a poor receiver.  You can't possibly predict the receiving end.  Just make your audio sound the way YOU like it, ensuring that your audio balance is not so far out of whack that you lack presence rise or have way too much bass.  If the guy at the other end has a poor receiver, so be it.  There are plenty of other folks with really GOOD or GREAT receiving setups who deserve to hear the best you can deliver     If you need to punch through the crud, cut the lows a bit, crank in the negative peak limiter and let the positive peaks fly !!!

As an aside: when I used the first prototype of the modulation monitor (with a super-low-distortion detector), I heard distortion in my signal that I never knew was there, and that NO ONE HAD EVER REPORTED !!!  It had been that way for several years.  It was not terrible distortion, but it was definitely noticable - but ONLY with this very low distortion detector and amplifier system.  Of course, I fixed it!!

Anyway, what to use is largely dependent on how accurately you want to hear what you are transmitting.  If you have a really good transmitter, the monitoring system has to be better 

Regards,

Steve

[N6WDR]

Just thought I would throw my 2 cents in. 

I took the simple route and used my Kenwood ts2000 as my monitor just plugged in a good set of headphone.  I also use it to record my side of the audio when I am recording the AMI net here on the left coast.

[W1AEX]

Thanks for all the replies and suggestions. Since my junkbox runneth over I'll give all the schematics a try!

Rob W1AEX

[Steve - WB3HUZ]

Another thing to remember is that off the air monitors are only use for hearing how you sound locally or in a fairly absolute sense. They will not let you know how you sound on the other end (i.e. how you sound coming out of the receiver of the person you are in QSO with). Noise, lower SNR and bandwidth limitations of the far end receiver will modify your audio, most noticably the higher frequencies. Keep that in mind when using an air monitor.

[WA1GFZ]

look out the East side window and if you are looking at the sun it is AM

[W1AEX]

Heh, I remember what George, W1GAC said to me (25 or so years ago) when I offered to record him on 75m and play it back as he swapped out a few components:

"I know what I sound like and I sound DAMN GOOD so never mind that stuff!!!"

[K1JJ]

Heh, I remember what George, W1GAC said to me (25 or so years ago) when I offered to record him on 75m and play it back as he swapped out a few components:

"I know what I sound like and I sound DAMN GOOD so never mind that stuff!!!"

 
    Yep, good ole George.

How about:  "Those darn young blondes driving red cars - anytime someone's tailgating me it's one of them.... I got back-ended last month... sure enough, it was a young blonde in a red car!" 


About the off-air monitor....   I have a HB one here that uses a simple L/C pickup circuit, a diode and a 5W audio amp going into headphones.  I contacted Steve/QIX and ordered the parts for his low distortion diode front-end. Just the board parts pictured above. Think I'll replace mine with that front-end and use the same audio amp, etc..

With the additional filters and better diode circuitry, what have I got to lose but a few % of rotten IMD?   I'm a believer that the monitor has to be better than the transmitter to hear the distortion products. It only makes sense. Using the best detector, audio amp and headphones is a good step.


**  Hey Big Country!  I'm about ready to fire up GirlieMan, (4X1 PDM rig) on the air here soon, so lets make a sked.  He's all finished except for connecting up the "white room" supply.  I'm too scared and running the Fabio 3.5kv supply for now...caw mawn.   Frank/GFZ, maybe you can join us... I'll bet you'd like to hear that rig for the first time after all the time we spent talking through it's problems....  I'm thinking this weekend sometime, 75M of course.

T

[Steve - WB3HUZ]

Saturday, say 5-6 PM around 3725 or so. It will be good to hear you back on Mr Mucas.

[w3jn]

Damn, it'll be great to have you back on, T.  Bunch of us were talking at Phrank's about our #1 most-missed on the air personality.

[K1JJ]

Saturday, say 5-6 PM around 3725 or so. It will be good to hear you back on Mr Mucas.

Sounds good, dudes....

We'll check ya out around that time tonight down the band.

Yes, it'll be fun getting back on.

Johnny, I saw your post about the Sherwood sync detector you bought from Chas.  Sounds like a great addition to the shack. I'd like to hear more about it and where I can buy a used on too ... :-)

Maybe we can talk about how it compares to a software sync detector box, etc. Maybe all the same performance, just different hardware.

I'm still trying to figger out which new rig to drive tonight ; the Blown 813's,   Fabio - the 4X1 X 833A's  or  GirlieMan the PDM.. Oh, decisions, decisions.  Which present to open first...?  They're all waving their hands.  Pick me, me, no me!!


Update:  I'm ready and tuned up on 3725 with Fabio pictured below:

T

[w3jn]

Look forward to it.  Wife's birthday and we're going out so I can't stay too long.  Plus, I'm up at the lake this weekend so I'll be kinda pissweak.  I'm sure The Little GTO will be able to pick me outta the noise though 

[WA1GFZ]

I'm working in the yard and will try to get on later. XYL is odering a pizza so I'll
try to get in after that. 3.5 KV is a lot easier than 13.5 Kv Mr vu.