Boom Mic and Equalizer

improving transmit audio quality…

© 2018 by KV5R. All Rights Reserved. Rev. 2/10/2019.


Amateur radio is generally not known for high fidelity audio. With most voice communications being single sideband (SSB), and restricted to a 3 kHz bandwidth, fidelity has always been, well, about like a land-line telephone. In recent years, some Amateurs have expanded the hobby by taking an interest in more sophisticated audio systems using broadcast-type audio equipment such as studio microphones and various audio processors. Radio manufacturers have finally started adding audio processing to some models, ranging from basic bass & treble controls (Icoms) to parametric equalizers (some Yaesus).

The purpose of this article is not to fully explore the subject of high-fidelity audio on Amateur radio, but rather to provide an example of how to significantly improve your transmitted audio without breaking the bank. At minimum, this requires a mic equalizer and a good vocal microphone.

Audio Types

Generally speaking, there are four types of audio on ham radio.

Stock Audio

This is the default, new-out-of-the-box hand-mic audio designed and provided by ham radio manufacturers. It is typically peaked at mid-range frequencies, rolling off on both high and low ends. This response produces a clean RF output and helps manufacturers pass spectral purity tests needed for FCC Type Acceptance. The sound of stock audio is bland and unimpressive. It is designed to be good enough for any mode of operation, while excelling at none. To sound more distinctive, manufacturers hope you will purchase one of their expensive desk mics. But doing so is usually disappointing, because such mics are usually very little better sounding than the stock hand mic. They usually include filtering of both high and low frequencies, again, to provide a clean but generic-sounding audio. Some of the better desk mics do provide limited audio adjustment capabilities, such as a switch for enhanced treble (DX) or flat response (rag-chew) settings.

DX Audio

For DX and other weak signal work, the emphasis is to concentrate most of the transmitted energy at audio frequencies that enhance intelligibility in low signal-to-noise conditions. For such weak signal purposes, “good” DX audio is enhanced in the upper-mid-range of the audio bandwidth, making the voice sound “brassy” and rather harsh, with no lower-mid-range or bass components. It is not pleasant to the ear, but it breaks through the noise much better than audio with a broader and flatter frequency response.

Rag-chew Audio

This is audio with a broader, flatter, more natural response than DX audio, and is much more pleasant to the ear for long conversations in good signal conditions. Every quality of audio from stock hand mics to desk mics to enhanced audio systems are acceptable for rag-chew communications. Operators with better than average audio tend to stand out from the crowd a bit, since their audio clearer and more pleasing to the ear. They are usually running a high quality microphone, such as a Heil PR-40 or similar, and have some audio enhancement, either in the radio or as outboard equipment.

Enhanced Audio

Also known as enhanced SSB or ESSB, stations with enhanced audio typically have studio microphones and several pieces of audio processing equipment as used for commercial broadcasting and recording. This expensive equipment typically includes mixers, equalizers, compressors, and noise gates. The ESSB operators usually also operate radio models that allow the user to expand the audio bandwidth of the transmitted signal beyond 3 kHz. Some of them sound fantastic, while others sound, well, over-processed. A common newbie fault is adding too much low bass, producing an unnatural rumble in the signal. This sounds great in their monitor headphones, but not so great on the air, except to the few that have enhanced audio equipment or studio headphones on their receivers. Another fault is pulling the mid-range too low, to the point that good voice intelligibility is compromised. What I have observed is the more experienced ESSB operators, with help from others on the air, have migrated from over-processed to more natural-sounding settings.

ESSB is somewhat controversial, as many hams resent the wider bandwidth, and equalization that may not play very well on the typical radio and speaker.

One thing I have observed is that the Rack Boys do tend to be endlessly adjusting things, trying to get that perfect sound, which is hopelessly elusive, since sound quality perception is rather subjective. The new ham goes from “can you hear me?” to “how do I sound?” and moves on to “do I need a bit more bass or treble?” and finally to “should I bring the 125 down 3dB and increase the compression 2dB?” And again, that’s all fine, as long as we are learning new things and having fun.


Once we move beyond the desk mics provided by the radio equipment manufacturers, there is a wide array of options, ranging from consumer to commercial grade equipment. As they say, the sky is the limit. But my focus here is how much is enough, at a reasonable price? And what equipment is just ridiculously overpriced?


Vocal microphones range from about $20 to thousands. In my opinion, there is no reason for any microphone to cost over about $100, except for professional studio and lab reference mics. When I started in audio back in the early 70’s there were only three general types of mics: Cheapo mics that came with your cassette recorder, vocal mics as used by speakers and singers on a PA system, and studio mics as used for professional recording and broadcasting. But it’s more complicated now.

There are now cheapo mics that cost under $20 that look like, and are called, studio mics, while there are still real studio mics that cost hundreds to thousands of dollars. There are decent vocal mics at $20 that rival ones that cost several hundred. There are tiny electret mics that cost $1.50 and sound pretty good. There are cheap computer headsets that sound almost as good as the expensive TV studio headsets. And last but not least, there are mics of all types and purposes that are just WAAAAAY overpriced, with no real justification. Microphones are just one of those things where people will throw gobs of money at it and hope it will suit the purpose, sort of like camera lenses.

Amateur operators that want to improve their audio will usually do what their peers did, buy a $400+ mic and assume that, at that price, it will be more than satisfactory. I do not consider it satisfactory, because a $400 microphone is typically no better than a $100 microphone, at least to the ear. Like so many other things, the last 1% of performance costs ten times the price, nine-tenths of which is just bragging rights.

Microphone quality assessment entails an objective understanding of exactly what constitutes quality. The factors include durability, pattern, output level, distortion, proximity effect, and frequency response. While most high-priced microphones will excel in all of these quality factors, the question is what should these factors cost, and how good is good enough for Amateur radio?

In my case, I looked for a good vocal mic with a response that’s nearly flat but rising a few dB at 3kHz, and I found the Shure SM-58, an excellent vocal mic used by millions for decades, that currently costs about $100. Then I found a clone of the SM-58, with the same appearance and frequency response graph, the Behringer XM-8500, for a mere $20. Yes, I run a $20 microphone, but it’s a darn good one, probably the best vocal mic you can buy for under $100. There is a Youtube video where someone compared the SM-58 and XM-8500, and the reviewer found no difference except for a bit more distortion at very high volume levels (he put them right in front of an electric guitar amplifier).

I did not consider any of the condenser microphones because they require 48V phantom power, and are just too large, with no real benefit in additional audio quality compared to a good dynamic vocal mic. Yes, a large diaphragm condenser mic has more low end, but then, you can add low end to a dynamic vocal mic with an equalizer.

Any microphone with a reasonably flat response and low distortion can be made to sound great with an equalizer, which may be set to compensate both for the mic’s response characteristics and the user’s voice. So I consider the minimum equipment required to provide a significant improvement in transmitted audio quality to be a decent vocal mic and an equalizer. Throw in a noise gate and a low-distortion compressor, and that’s the cat’s meow.

All-in-One Audio Processors

These are little boxes that combine the functions of low-noise preamp, equalizer, compressor, and noise gate. Various models range from about $135 to nearly $400, and are made by W2IHY, UR6QW, and MFJ. I own a UR6QW, because it is the lowest priced of the three, and it does a very good job, with the exception that its equalizer only provides ±6dB, which is sometimes insufficient. Most equalizers provide ±12-15dB, which is more than enough. All of them will interface any mic to any radio (the UR6QW is built to order) and provide all the audio processing needed.

I cannot comment on the W2IHY units, but have heard many of them on the air, as they are very popular, and they always sound good when properly adjusted. I would have one, but I consider them to be overpriced. The same for the MFJ units. Looking inside any of them reveals that the processing functions are provided by a few inexpensive ICs. Add to that a PCB, metal case, controls, and connectors and I see perhaps $60 in parts, not $300. But folks must make a profit, and these are low-production specialty items, so I suppose the price is justified. Still, when you can buy a whole transceiver for $800, a little audio box for $300 seems, well, a little out of balance, as would a $400 microphone.

Rack Gear

This audio processing equipment generally comes in 19-inch commercial rack-mount components (hence, the name) and ranges from moderately-priced consumer-grade products to high-end recording and broadcasting equipment. Usually each piece provides a discrete function, so the user will purchase a pre-amp or mixer, equalizer, compressor, noise gate, and various other sundries that are then combined into a working audio processing chain. I consider this type of equipment to be over-kill for Amateur radio, but if the user has the money and likes to play with lots of fancy audio gear, then it’s all fine. It’s no more or less justified than any other equipment.

My Audio Improvements

I determined to accept the challenge of getting a significant and satisfactory audio improvement for my station for under $200. That ruled out any expensive microphones and audio processors. Can it be done? Yes!

After much research, I ordered the Behringer XM-8500 mic ($20), a 13x13-inch boom ($13), and XLR cable ($8) from Amazon, and also the UR6QW 8-band EQ Compressor Noise-Gate from its maker Sergey in Ukraine. He custom-builds the units according to your needs and wishes, and is very nice to deal with. I ordered it with Icom modular plugs (for the IC-7100), and requested he add headset and PTT jacks, and bring the cable out the side, instead of the back, which he cheerfully did. We exchanged several emails as he wanted to get everything just right. Air-mail shipping then took about 8 days from Ukraine to Texas. The delivered price was $160, which I see is now down to $150 — half the price of other comparable units.

The combination of the XM-8500 and UR6QW gets very good audio reports. For initial settings and testing, I transmitted with low power into a dummy load and recorded the signal with an RTL-SDR dongle and HDSDR software. Note that you cannot properly set your EQ with monitor headphones, as the radio’s transmit filter reshapes it, so you need to actually transmit and use another receiver and recorder. If you want a more distant recording, find a web SDR (on some distance away, set it to your frequency and bandwidth, start it recording, start a QSO with someone, then download the recording. You can get your audio settings pretty close to what you want before ever asking for a report.

One frustrating problem is finding hams with an ear for good audio. I’ve heard many people ask for an audio report and receive “Oh yeah! You sound great!” when in fact they sound pretty terrible, and really needed an honest report. (But then, there are some that ask for a report and then become offended if you give them anything less than “Great!”) So if you’re tuning up a new mic and equalizer, and actually need useful feedback, go find the Rack Boys! Most of them have played with equalizers and such things long enough to know what good audio should sound like, and can provide useful pointers. Some are professional broadcasters or have experience in a recording studio.

I found the UR6QW’s equalizer range (±6dB) to be a little less than desired, so that the mid-ranges are at minimum and the trebles are at maximum. But the noise gate and compressor are simply excellent. The noise gate transparently mutes the mic between words and, unlike some others, does not have a little tail of digital artifacts as it closes, and it does not pop when it opens. Other than knocking out the background noise, you just can’t tell it’s there. The compressor is not like the typical ham radio compressor that raises the whole signal and clips off the tops, but is like a broadcast compressor that amplifies the lows without raising the peaks. Set to 2:1 or 3:1, you can talk very loud or very soft and the radio puts out the same power, which significantly increases intelligibility without sounding compressed. Because most hams think that compression means distortion, they don’t think you’re running any compression! The Icom IC-7300 also has this type of low-distortion audio compressor.

The UR6QW box also has an echo processor. Unfortunately, it is not a subtle reverb or ambiance effect like broadcasters use, but a real big looping echo like a gymnasium or canyon. So it’s useless on ham radio, unless you want to severely annoy people and sound stupid… Oh well, he sells to the CB market, too, so it’s understandable. If Sergey would reduce the minimum echo time to around 5-10 milliseconds it would be usable for an ambiance effect. If you don’t know what that is, listen to the TV news and notice there’s just a tiny bit of reverb, which keeps the announcer from sounding too flat. We do not normally listen to people talking two inches from our ears, but several feet away, and thus hear a little room ambiance. But microphones work best at 1-4 inches, so adding a tiny bit of ambiance just sounds more natural. Also, turning up the mic and backing away sounds bad, because we are adding our room’s ambiance to the listener’s room ambiance, and the two differing room sounds tend to create a subconscious conflict in the brain. So then, a bit of spatial ambiance is better than either none or too much.


Here’s my audio setup.

The Behringer XM-8500, an SM-58 clone for $20

It comes with a holder and a nice travel case, but does not include a cord.

The frequency response is ideal for SSB, as it rolls off below 100Hz, is nearly flat to 1k, then sweeps upward about 6dB at 3k. This is almost identical to the SM-58.

I got this unbalanced XLR cable because it has a right-angle mini-jack on the other end. It’s not commercial grade, but it works fine. Note that if you get any RF into your mic, just wind the cord a few turns through a ferrite toroid.

A foam windscreen (aka pop filter) is almost always used with this type of mic. I found that my mic sounds better if I run it 4 inches away and about 30 degrees to the side, so the windscreen is not needed. It does reduce the treble just a bit.

The stuff, unpacked.

The InnoGear MU01 mic boom.

This boom is superior to similar ones because it mounts the top lift spring at a higher angle and thus supports more weight. It’s just right for the XM-8500 or even heavier mics. Be sure to grease the threads on the clamp so the screw won’t wear out the threads in the cast aluminum.

Since the mic is not shock-mounted, the boom’s four springs transmit a lot of spring noise into it. I cut up an old black sock, rolled up the pieces, and pulled them through the springs with a stiff wire. Problem solved.

Since the EQ puts out 8V phantom power, I did as shown on the Heil site and added a 4.7μf tantalum capacitor in the mic to block the DC. But it made a very low sound like crumpling paper in the audio (like you might hear with a slightly corroded mic jack), so I removed it,

and tried an electrolytic capacitor as a test. It made exactly the same crumpling sound, so I took that one out, too. Normally, 5-8VDC phantom power will pin down the coil in a dynamic mic, making it put out very low and tinny output. I don’t know why, but it works perfectly well without the DC blocking capacitor. I’m pretty sure the UR6QW box has a current-limiting resistor on the phantom power at the mic input, so the mic coil is unaffected. The EQ box runs on the 8V phantom power from the Icom, so if the mic coil was pulling any significant current, the box wouldn’t run. I think Sergey did state that it could be used with electret or dynamic mics without modification. And it might have been just my particular mic or cord that made the crumpling sound, which a bit of DC bias masked.

The UR6QW V.5 8-band EQ Compressor Noise Gate

The internals. Notice that Sergey takes extra care to control noise and RFI by twisting wires and grounding everything.

I mounted a little aluminum flat-bar on the back of it, which is why I asked Sergey to bring the cable out the side, not the back…

…so I could mount it like this.

My operating position. The boom mic and the arm with the radio and the EQ folds back against the wall when not in use. Where’s the PTT? I just use VOX, but later I did add a push-button to the EQ box, just for convenience, if needed. I have another button plugged in to the radio’s main body, for tuning up.


Do you really need a boom mic? Well, a hand mic always ties up one hand, and a desk mic is always a little too far away, so a boom mic or headset is the only way to go. There are good reasons that all professional radio broadcasters use them—a boom allows you to place the microphone at the perfect position, and it doesn’t occupy your hand or waste desk space. Another viable option that many hams use is a wireless telephone headset. They allow you walk around the house and keep on talking. I’d consider one, but I just don’t like wearing headsets.

Do you really need an equalizer? Well, not if the mic you buy has the right output level for your radio, and the right frequency response for your voice—both are very unlikely. The equalizer allows you to make any decent mic, and any voice, sound as good as possible, thus giving you the ability to optimize your audio quality.

So there you have it! Is improving your transmit audio worth $200 to you? It is to me. Is it as good as an expensive studio mic and rack gear? Probably not, but it’s very close, yet costs a small fraction of that. And, as you can see, I don’t have the room for fancy gear at my operating position, which was another consideration.

73, — KV5R

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