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Find your modulation “sweet spot.” Build a solid-state mic preamp. Plan a visit to the WBA Broadcasters Clinic. Create a YouTube channel. And take a look inside the new studios of America’s top-billing radio station.

BROADCAST LAW
Radio Eyes Advantages of Deregulation

With the commission backing off numerous requirements, how will broadcast ownership groups respond?

FACILITY PROFILE
Inside the Glass-Enclosed Nerve Center

Two dozen photos from Radio World’s recent video webcast tour of WTOP in Washington.

ALSO IN THIS ISSUE:

• Deter Stinging Insects at the Transmitter Site
• “THAT Thing” — A Solid-State Mic Preamp Project
• National VOA Museum Asks for Your Support

 

The post Inside the October 9th Issue of Radio World appeared first on Radio World.

“Today, wiring changes are done with a mouse, making and breaking crosspoints in software.” If you are nodding in agreement, RW Engineering Extra is for you! Find out who said it by browsing your latest issue.

DIGITAL RADIO
Upgrading an AM to All-Digital: Why, How and Lessons Learned

Dave Kolesar and Mike Raide update you on the WWFD project.

YOUR CAREER
What Does “Value Engineering” Mean to You?

Sometimes, plans need to be adjusted to fit the available dollars.

ALSO IN THIS ISSUE:

• What Does a Broadcast Engineer Do, Anyway?
• Phasing Quadrature Amplification
• Make the Most of Your Uncompressed Opportunities

 

The post Inside the Oct. 16 Issue of RWEE appeared first on Radio World.

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A new round of construction permits are about to be up for auction.

The Media Bureau at the Federal Communications Commission introduced Auction 106, a new FM broadcast construction permit auction scheduled to commence on April 28, 2020.

Auction 106 will offer 130 construction permits for FM broadcast allotments, including 34 permits that were either defaulted upon or not sold or in earlier FCC auctions. The commission released a list of those vacant FM allotments, which include teeny Texas hamlets like Milano, Texas, population app. 400, as well as populous urban thoroughfares like Coalinga, Calif.

The bureau plans to follow the commission’s standard auction procedures — a multiple-round auction format that offers every construction permit bid at the same time and consists of successive bidding rounds in which qualified bidders can place bids on individual permits. Bidding usually remains open until bidding stops on all permits.

The auction will be conducted over the internet using the FCC auction bidding system, although bidders will also have the option of placing bids by phone. The bureau is also proposing to stop, slow or speed up the bidding if the process is proceeding at either a sluggish or a too rapid pace.

As in earlier auctions, the bureau proposes that applicants submit upfront payments as a prerequisite to becoming qualified to bid.

But before things kick off, the bureau and the Office of Economics and Analytics is seeking comment on a variety of auction-specific procedures relating to Auction 106 — including the proposed open bidding process, how much upfront payment should be required for each CP and the proposed opening bid amounts.

The price range for construction permits in Auction 106 vary wildly. On the low end sits permits for $750, such as ones in Wamsutter, Wy., and San Isidro, Texas. Compare that to the upfront payment of $100,000 — and the subsequent minimum opening bid of $100,000 — for a CP in California’s capital city of Sacramento. Mid-level bids include Huntington, Ore., for $45,000; West Rutland, Vt., for $25,000; and Gackle, N.D., for $15,000.

The initial bidding schedule will be announced one week before bidding starts via a public notice.

Comments on Auction 106 can be made through the FCC ECFS filing system using AU Docket No 19-290. The commission is also requesting that all comments be submitted electronically via the email auction106@fcc.gov.

FM broadcasters who have questions can reach out to the Audio Division within the Media Bureau at 1-202-418-2700.

 

The post New FM Construction Permit Auction Set for April 2020 appeared first on Radio World.

Mike Dosch will be leaving his role with equipment manufacturer Lawo and focusing full-time on his recently launched company Angry Audio.

Separately, his new company also will acquire the StudioHub wiring infrastructure line.

Dosch joined Lawo in 2014 with the title of director of virtual radio projects and later was named senior product manager radio. Prior, he was president of the Axia Audio division of the Telos Alliance; for 10 years before that he was with Pacific Research & Engineering, where he started as a console designer and worked his way up to VP and COO.

His company Angry Audio makes small problem-solving devices that it happily refers to as “gadgets and gizmos” targeting audio needs of the radio broadcast market. Products are sold through a number of U.S. and international dealers. Examples include the Guest Gizmo and the Bidirectional Balancing Gadget. A recently introduced Bluetooth Audio Gadget is intended to make it easier to put a smartphone on the air.

Separately, Angry Audio is acquiring the StudioHub product line, which it currently resells, from Radio Systems and developer Mike Sirkis.

“Angry Audio is buying StudioHub and will soon begin manufacturing the entire StudioHub line including cables, adapters, panels, breakout boxes and hubs, matching amplifiers, etc.,” Dosch told Radio World in an email.

[Related: “Radio Systems Turns a Business Page”]

“Additionally, we will be providing spare-parts support for products previously manufactured under the Radio Systems brand. Millennium consoles for example will soon be supported by Angry Audio. We’ve moved into a bigger space to accommodate the expanded product line and hope to have operations humming along next month.”

Dosch said his last day with Lawo will be next week.

The post Dosch to Devote Full-Time to Angry Audio appeared first on Radio World.

Tom Johnson and I were talking about pests.

Fig. 1: Eaves and overhangs at unmanned transmitter buildings can attract stinging insects.

We met at the Alabama Broadcasters Association and Larry Wilkins’ Engineering Day seminar; and as we talked about bugs infesting transmitter sites, Tom shared the picture in Fig. 1.

I don’t know many engineers who care for wasps, hornets or other flying, stinging insects. Tom’s photo is a great reminder to spray under eaves, and around door and window frames — any place that’s protected from the weather.

Unfortunately, with cold weather approaching, it’s not just vermin that seek shelter from the cold. An unoccupied transmitter building or AM antenna tuning unit is an ideal home for insects and rodents. Make sure it is sealed.

ATUs in particular can attract unwanted guests. If your ATU has a light fixture inside, wait til it’s dark and then  turn it on, then walk around the ATU looking for any escaping light (remember to look underneath, too; and also watch out for the “hot” tower). Any holes you spot, perhaps where bolts once held coils or other components, are “welcome signs” for insects, and usually the proper diameter for these insects to squeeze through. Plug those holes with RTV or caulk.

Remember also that before opening the ATU door, pause to watch whether stinging insects are flying around. They may have infested your enclosure already.

Check that entry panels or doors to the ATU also fit tight; again look for light leakage. Remember that field mice can squeeze through amazingly small crevices.

Tom waited till dark, then sprayed that nest and its occupants.

When I did contract work, a client was losing their satellite signal every day at dusk. I stood on a ladder and unscrewed the LNB. A swarm of angry wasps escaped the nest they built inside. How we didn’t get stung —  or break a leg, frantically jumping off the ladder — still amazes me.

As I mention in my Workbench sessions, a one-liter clear plastic water bottle fits nicely in the throat of a satellite feed horn and will prevent infestation. As for your building, a good spritz of wasp and hornet spray applied under all the overhangs on your building is good preventive maintenance.

 

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San Francisco contract and project engineer Bill Ruck writes, “Been there, done that.” He was referring to the electrolytic capacitor woes we described recently.

Back around 1967, Bill learned about electrolytic capacitors working at a hi-fi store. Rule of thumb in those days was (1) if they’ve “puked their guts” by exploding, replace them; (2) if not, replace them anyway.

Since then, Bill’s experience is only worse. Many times he has traced spurious outputs of an FM exciter to the power supply oscillating and modulating the carrier.

Recently, Bill had two BE FX-30 exciters with that problem. The issue was traced to the FMO module. The problem was that the FMO is potted and to dig out the potting compound to replace the capacitors would take a lot of time and was no longer cost-effective. The group owning the exciter preferred to purchase a new exciter rather than put a lot of money into reconditioning something that was over 30 years old.

Bill adds a few more nuggets to consider:

1. Although high ESR (equivalent series resistance) doesn’t cause “ringing,” it does let an unstable amplifier oscillate. Furthermore, most three-terminal regulators can be defined as an “unstable amplifier” and will oscillate. Bill learned in his own home-built power supplies to put a 1 uF tantalum bead capacitor and a 0.1 uF ceramic disc capacitor as close to the regulator IC input pins as possible.

2. Always put in 105 degree C electrolytic capacitors. They’re slightly larger and slightly more expensive but they last a lot longer.

3. It takes the same effort to remove capacitors from a printed circuit board to measure them, than to just replace them. Yes, Bill can measure ESR and capacitance, but he does that only to confirm his suspicions, after putting in new low ESR 105 C replacement capacitors.

In summary, Bill writes that these days, component level repair is less cost-effective than during his misguided youth; but if you do make these repairs, replace!

 

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Fig. 2: Find this DIY rat trap at the YouTube link in the text.

Our Workbench Malaysian connection, broadcast engineer Paul Sagi, found an interesting YouTube video that we’ll call “Curiosity Killed the Rat!” Here’s the link: https://m.youtube.com/watch?v=T-KJMM55A9A

Paul comments that it appears that clear box sealing tape was used to hold the grain, and the “ramps” appear to be floor tiles, placed so the underside faces up. Placing two ramps on opposing sides permit some rats to balance out each other, a single ramp may be better. Finally, for remote locations, Paul suggests affixing the ramp to the bucket, so it doesn’t fall.

My comment? I sure hope this isn’t someone’s transmitter site! That’s a lot of rats.

I also hope you’ll contribute to Workbench. You’ll help your fellow engineers and qualify for SBE recertification credit. Send Workbench tips and high-resolution photos to johnpbisset@gmail.com.

John Bisset has spent 50 years in the broadcasting industry and is still learning. He handles western U.S. radio sales for the Telos Alliance. He holds CPBE certification with the Society of Broadcast Engineers and is a past recipient of the SBE’s Educator of the Year Award.

The post Deter Stinging Insects at the Transmitter Site appeared first on Radio World.

The author is membership program director of the National Federation of Community Broadcasters. NFCB commentaries are featured regularly at www.radioworld.com.

This week’s big news in community radio was all about layoffs at Pacifica radio station WBAI(FM) in New York and termination of its existing programming. The seriousness of the situation is a bellwether to conversations community media must have about relevance.

WBAI is certainly an iconic noncommercial radio station. It has hosted a veritable who’s who of cultural vanguards, especially in the 1960s. From Bob Dylan to Malcolm X, WBAI has been fondly remembered by fans for such history. Unfortunately, those glory days are long gone.

When I wrote for Radio World about the Empire State Realty Trust $3 million judgment against WBAI and Pacifica in 2017, matters were already quite dire. Pacifica audits noted a listener support decline between 2007 and 2017 in the millions. The California Attorney General’s Office and the Corporation for Public Broadcasting Inspector General launched probes in 2015 and 2011 respectively over management issues. WBAI had seen crashes in 2009 and 2013, and its internal strife, inability to make payroll and subsidization by other Pacifica outlets had been in public circulation for years. WBAI folks, a smart and interesting bunch to be sure, have long contended what may seem to be farfetched theories, from essentially embezzlement by its licensee to its owner kneecapping the station staff and volunteers for their liberal orientation and willingness to host marginal programming under the free speech banner.

[Read: Community Broadcaster: Changemakers]

Indeed, any publication that termed this week’s action stunning or a shock clearly must be excused for not paying attention to a crisis decades in the making.

Pushback to Pacifica’s decision was swift, including a state court intervening in the situation. However, it may be unclear how influential a New York court injunction to prevent layoffs and program changes will be. After all, Pacifica is a California nonprofit organization. The network’s payroll and WBAI’s federally assigned license (and thus programming placed on it) are both based far from said court’s jurisdiction. This story is developing.

Regardless of what happens to WBAI, the health of community radio as a whole is always a concern. What can stations learn from this issue?

As I conveyed to radio station WORT this week, community radio stations should always ask themselves about how they are truly listening to and serving local listeners with content they are passionate about and rely on. It is not difficult to figure out why WBAI, at 99.5 FM and in a city of eight million people and with one of the nation’s top median household incomes, could muster only 78,500 weekly listeners (according to Nielsen Audio) and not cover basic expenses. An old friend at Radio Research Consortium, noncommercial media’s data clearinghouse, once shared, listeners tell you what they think of your programming with their ears and wallets. Every station should track what their local fans think, and be responsive to area needs. That can be hard, given the many perspectives that are part of a station, but centering listener experiences with our stations must always be a priority.

Careful financial monitoring and adopting an approach to problems that sees them as a shared responsibility should also be a part of any station’s ethos. When tensions arise, the easy way out is to cast blame on others. Difficulties such as those experienced by the community radio space this week might have been averted with more swift, productive actions, and buy-in from everyone. All community stations might be wise to unify in times like these.

And finally, every station would be well served to take the lead in telling its story. In times of trouble, explaining one’s vision and where one hopes to be reassures listeners and tells donors you have a solution. Once a station loses control of its narrative, it is hard to regain credibility. Pacifica’s message posted on WBAI’s website is a start, in this instance.

Noncommercial radio observers may recall the WBAI move is not without precedent. In May, Humboldt State University shuttered its volunteer-based programming and laid off staff at KHSU in a bid to reorganize. A Humboldt State University advisory review identified a need for financial review of university investments, a realignment of the station’s operations and other issues for the licensee. One can hope WBAI’s reorganization is successful, and that all of community radio takes a cue from what’s happened to make the best media possible.

 

The post Community Broadcaster: A Cautionary Tale appeared first on Radio World.

The author is the head of technical and infrastructure department at German national public broadcaster Deutschlandradio.

The reception of radio programs with smartphones is becoming increasingly important for radio makers, particularly due to young people’s tendency to use their hand-held devices for a wide range of purposes — information and entertainment, social media networks, smart home and smart speakers, amongst others.

Chris Weck

There is no doubt that broadcasters have to be present on that platform with both linear and non-linear audio, with social media and the various functions of the internet.

At first glance, 5G broadcasts seem to be a promising solution for the future of broadcasting, and a viable solution to bring radio to the smartphone — one device and one transmission standard on one transmitter network. But who will benefit from this — the user, the mobile network operators, radio broadcasters or the industry as a whole?

Physical laws for radio communication are still valid for 5G as for DAB and all the other broadcasting and telecommunication schemes. From the well-known Shannon limit of 1948 we know that a minimum of energy per bit is necessary in order to provide an error-free transmission over a channel with a certain bandwidth (Eb/N0 = −1.6 dB in AWGN-Channel).

New and very efficient transmission systems like 5G are able to transmit very high data rates in a channel of a certain bandwidth, however, the energy per bit will never fall under the minimum defined by the Shannon law. With other words, the higher the data rate of a transmission system, the higher the signal-to-noise ratio required. This means in practice for a certain transmitting power the size of the transmitter cell will be reduced for higher data rates accordingly.

Now, from a theoretical point of view with respect to the energy per transmitted useful bit (including all the overhead), there is no significant difference in performance between 5G modulation schemes compared to the still very robust system of DAB+.

The 5G broadcast mode provides also a robust QPSK modulation to make use of bigger cell sizes. However, the expected performance compared to DAB especially in a single frequency network is rather the same. In fact, there are no results of a system comparison in the field available and therefore it is reasonable to focus on other basic differences between the idea of 5G broadcast and conventional DAB+ broadcasting.

Today, DAB radio receivers have an external antenna as well as car receivers. In comparison to a smartphone with a less sensitive built-in antenna, the link budget for the required field strength differs at minimum of 15 dB or even 20 dB and more.

This means that in order to achieve the same coverage for radio reception by smartphones, 10 dB more transmitting power is required. This is also true for 5G broadcast networks, so that 5G broadcast networks for smartphone reception have to aim for 10 dB more transmitting power compared to a conventional DAB+ network. In practice, this means that a significantly denser transmitter network is required for 5G broadcast to smartphones than for conventional DAB+.

Radio reception differs for smartphones compared to conventional radio receivers.  The field strength required depends on the effective antenna size, and has to be higher for smartphone reception.

The reduction of the transmitter distance can be anticipated easily from the CCIR propagation curves. For example for VHF propagation a loss of field strength of 20 dB corresponds to a reduction of the distance to the transmitter from 30 km to 10 km.

With the basic transmitter distance of about 60 km for DAB+ networks, the average transmitter distance for 5G broadcasting to smartphones has to be around 20 km. In fact this means that the transmitter distance has to be reduced by a factor of three in order to overcome a loss of 20-dB field strength. This means nine times more transmitters in the area are required in order to achieve the same coverage as a conventionally planned DAB+ network. Can radio broadcasters really afford this? In fact round about 10 dB more transmitting power results in 10 dB more money.

For the time being, the national DAB multiplex in Germany comprises of 130 transmitters in a nationwide SFN. Today, coverage stands at around 95% for mobile reception, but in order to reach 99% coverage, the number of transmitters has to be increased to 250 at least and may be around 400 (including small gap fillers) in the long term.

With 5G Broadcast round about 10 times more transmitters will be required which might sum up to 2,500 or even 4,000 transmitters in Germany. The mobile network in Germany comprises already 40,000 transmitters today and everybody experiences that this is rather not enough. Concerning 5G mobile networks, experts anticipate that future high data rate networks will be based on a cell size of less than 1 square kilometre, which would sum-up to around 400,000 transmitters in Germany for nationwide area coverage.

CCIR 370 Propagation Curves

What can we learn from these facts?

1. The DAB+ network with its low number of transmitters is the most efficient network to realize a full area coverage
2. The 5G broadcast networks, the mobile network and future 5G mobile networks require far too much transmitters for a full area coverage that nobody can expect the same area coverage as for DAB radio services

Assume e.g. transmitting costs for a full area DAB network in Germany of about €25 million per year. In order to gain 10 dB more transmitting power for smartphone reception, the network will cost a nationwide broadcaster approximately €250 million per year, as opposed to €25 million a year for conventional DAB. In Germany, no broadcaster is in a position to afford this amount of money — the price for this purpose to reach smartphones with radio is incredible high, and quite frankly, out of reach for any public broadcaster.

If one says that 5G would only be applied in cities as opposed to rural areas, the additional costs would indeed be lower. However, setting aside a budget of €10 million a year for this purpose is also unrealistic for a broadcaster and, should this sum even be available, it would certainly make more sense to spend it on the DAB network, where coverage gaps could be closed, and where broadcasters and consumers could benefit from it.

What’s more, it wouldn’t make sense for a broadcaster to give up nationwide DAB coverage. In order to supply 10% of the area with 5G broadcast to mobile phones for the same amount of money.

So, if broadcasters are far from being able to afford 5G broadcasting, who would pay for this? Mobile network operators will never provide a 5G-radio service for free, and broadcasters will not pay for 5G broadcasting either, so there really is no business model for either.

The one and only solution is that the user pays for the broadcasting service to his smartphone — this could be done by a contract with the broadcaster or with the mobile network operator, something that is already being done today with 3G/4G.

The smartphone user has a mobile contract and pays for the data volume on an individual basis. This enables the mobile network operator to set up very dense mobile networks that have enough power to be received by small smartphones. This works perfectly for radio with LTE and even UMTS, so why wait for 5G broadcasts?

Users already have radio services available on smartphones today, and it works well, so long as the user has enough high-speed volume on his contract.

Today, hybrid radio with DAB+ and Internet via mobile networks or via Wi-Fi at home provides the most suitable solution. Hybrid radio is the perfect fit for all broadcaster and user requirements, as with DAB+ it allows broadcasters the proven and most efficient radio network at an affordable price for area-wide coverage. It allows for free access of the users to radio and information, regardless of whether they live in cities or in rural areas, and whether or not they can afford a high-volume data contract for their mobile phones.

Hybrid DAB radio provides broadcasters with a content distribution platform directly linked to the customers, and independent of the commercially driven infrastructure of mobile network operators. This may be an advantage for emergency warnings, too.

On the other hand, users already have audio streaming and additional non-linear services available on their smartphone via the Internet. So, the only need for radio broadcasters today is to think about attractive hybrid radio services, and an impactful marketing strategy for their brand.

I cannot comprehend why broadcasters and politicians would want to switch a system running with DAB and IP with the more expensive, and in practical terms less efficient system that is 5G. Instead, why not use and extend the existing and approved technology? Hybrid radio is the best approach both economically and in terms of efficiency, and this is unlikely to change in the future.

Diversity between broadcaster networks and mobile phone networks will result in better efficiency and will offer more advantages than disadvantages for broadcasters as well as for users — so proceed with Hybrid DAB and IP. There is no need for 5G for radio broadcast.

[Read: Using Digital Radio to Boost Listening Figures and Revenues]

 

The post Does 5G Make Sense for Radio? appeared first on Radio World.

Six industry professionals have already made their plans to attend the 2020 edition of Radiodays Europe in Lisbon, Portugal, as the conference has announced the first batch of its planned speakers.

Those confirmed to speak at next year’s conference are Cilla Benkö, director general and CEO for Swedish Radio; Cathrine Gyldensted, co-founder and director at the Constructive Journalism Network in the Netherlands; Yagmur Özberkan, journalist and presenter for YLE, Finland; Torben Brandt, Danish radio legend; Ole Hedemann, content developer and head of formats at NRK in Norway; and Susani Mahadura, journalist for YLE, Finland.

Radiodays Europe Lisbon 2020 is going to take place from March 29–31, 2020. For more information or to register for the event, click here.

 

 

The post Radiodays Europe Announces First Group of Speakers appeared first on Radio World.

Over the years I’ve become a student of mic preamp design, building and modifying several along the way and learning a little more each time. Usually, I worked from a kit or published set of plans. Recently, I’ve tried some designs from “scratch,” researching various components, studying earlier designs, and incorporating them into raw schematics, followed by circuit layout, design tweaks and final fabrication.

Since my last two builds were vacuum tube devices, I wanted to do a simple, solid-state design this time. I came across some old preamp ICs in a parts box and almost used them but discovered they had been obsolete for years.

Was there a viable updated replacement? Enter THAT Corp., a relatively small IC manufacturer that specializes in chips for audio applications. THAT makes a few chips that are direct replacements of some popular preamp ICs like the Analog Devices SSM2019 or Texas Instruments INA163. If you’ve ever cracked open a broadcast console, you may have seen one. THAT’s website is a treasure trove of design notes and white papers on mic preamp design, with plenty of ideas to get a project going.

This project uses two ICs from THAT: the 1512 Low-Noise Audio Preamp, and the 1646 Balanced Line Driver. Using design notes from THAT and other sources, including advice from several more experienced DIYers, I was able to come up with a relatively low-cost design that has plenty of gain and good performance numbers for most applications.

The mic preamp can make or break a recording. Aside from the microphone, it’s the first stage in the signal chain before the recorder, and in some cases the only stage. It has to be clean and have ample headroom (unless noise and distortion are your thing), yet have sufficient gain to handle a wide variety of microphones.

Professional microphones have a balanced output, so the preamp will have a balanced input. Normally this is accomplished either with transformer balancing, which is expensive, or by using a standard op-amp as a differential amplifier, usually involving two op-amp stages with their attendant gain feedback loops, etc. The THAT 1512 takes care of this within the chip, providing its own balanced input. All that’s needed is a pretty standard input stage that can provide phantom power. The phantom power is sent to Pins 2 and 3 of the input XLR jack through a matched pair of 6.81K resistors, R1 and R2. These limit the current of the phantom supply.

The phantom power section

In order to preserve common mode noise rejection, any components that are mirrored between positive and negative signal paths must be matched in value as closely as possible. SW1 [switch] allows for turning off phantom power when it is not needed, and LED1 illuminates to show the actual presence of phantom voltage. R9 limits current through the LED to keep it from going “poof!” Capacitor C13 is there to smooth out any ripples from the 48 V supply. Between Pins 2 and 3 of the input jack and ground, ceramic capacitors C1 and C2 shunt any RF noise that might hitch a ride on the mic cable. Bad mic cables make good radio antennas!

Keeping stray static at bay is the job of the diodes.

Obviously, we need to keep 48 VDC out of our audio circuit. In a transformer-based design, the transformer would handle this, as transformers only pass AC. Likewise with capacitors, which are much cheaper and take up less space. This is why inexpensive designs use them. The problem is that inexpensive designs tend to skimp on these coupling capacitors. Years ago, I hot-rodded a mic preamp that originally had 4.7µF tantalum capacitors in the coupling stage. I replaced them with nonpolar electrolytics of a much higher value, and performance was improved.

Here, for C3 and C4, I use the same ones. At 100µF it’s overkill, I’ll freely admit, but the higher value reduces low-frequency phase shift (the LF response here is in the single-digit Hz range). Anything around 22µF or greater will work. Besides, it’s very difficult to match capacitors to such tight tolerances.

The high-pass filter is engaged by a switch — SW2.

Here’s where R5, R6, and R7 come in. They form what THAT calls a “T-bias” circuit, which boosts low-frequency common mode impedance. C14 is another ceramic capacitor across the inputs to clean up any remaining RF noise. By the way, R3 and R4 are there to limit any fault currents that might sneak by the capacitors. Their low value prevents input impedance issues.

Additional protection from stray static charges and other voltage transients is provided by diodes D1 through D4. This is a simplified version of a number of protection circuits I’ve seen. Anything ugly gets shunted to ground.

Now, it’s on to the preamp IC, which does the heavy lifting in terms of gain: up to 60 dB of gain, in fact. While a lot of designs will set the chip at a fixed gain level and introduce level controls somewhere between subsequent stages, ours is a simple mic preamp. It would be a simple matter of just inserting a potentiometer (VR1) across the gain setting pins of the chip, right? Not that easy!

Rapid changes in that resistance can introduce DC offset in the chip, which translates to thumping and popping on the output. This is where C5 comes in; a very large capacitor to kill DC offset. Why so large? Because VR1, R8, and C5 comprise a high-pass filter, so the capacitance has to be large enough to bring the low-frequency response down. In this case, it puts it around 5 Hz at maximum gain, keeping any rolloff well below 20 Hz. VR1 is a reverse-log pot, which provides the correct gain vs. position curve.

Capacitors C7 through C10 filter RF gunk out of the power rails to each chip.

Speaking of high-pass filters, I included one here to roll-off any mic or room rumble. C6 and SW2 provide a HPF, but this one has a twist. (Special thanks to the folks at www.groupdiy.com for this idea.) Because the changing resistance of VR1 naturally changes the characteristics of the HPF, this filter’s rolloff actually increases somewhat at higher gain settings. At first, this may seem undesirable, but think about it — low frequency artifacts are more likely to be a problem at higher gains than at lower gains. At any rate, C6 is small enough to rolloff the low end, but not to the point of sounding thin.

Now on to the output stage, handled by the THAT 1646. It’s one of the simplest I’ve ever seen. One IC and a couple of nonpolar capacitors. Caution must be used if inserting any other stages or components before the 1646, as it is very sensitive with regard to impedance. C11 and C12 are there to address any common-mode DC offset on the outputs. From there, it’s on to the output XLR jack, passing through a simple polarity switch, SW3, to reverse phase if needed.

Finally, capacitors C7 through C10 filter RF gunk out of the power rails to each chip, a very important consideration in any design. Clean audio has to have clean power.

Since this whole thing is built around THAT ICs, I decided to simply call it “THAT Thing.” Tune in next time, and we’ll talk about the power supply, breadboarding the prototype, and putting it all together.

More information about the THAT 1512 and 1646 ICs, as well as design notes and other information can be found at:

• www.thatcorp.com/Design_Notes.shtml
• www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf
• www.thatcorp.com/datashts/THAT_1606-1646_Datasheet.pdf

Curt Yengst, CSRE, is a contributor to Radio World and an assistant engineer with WAWZ(FM) in Zarephath, N.J.

Email us with your own DIY ideas at radioworld@futurenet.com.

The post “THAT Thing” — A Solid-State Mic Preamp Project appeared first on Radio World.

First Meeting and Chairs of Advisory Committee on Diversity and Digital Empowerment

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Petition for Declaratory Ruling and connected assignment applications granted

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