A Tribute to Bob Heil and His Classic Microphone Equalizer

Bob Heil’s Classic Microphone Equalizer

An Audio Giant Passes

I learned in early March, 2024 of Bob Heil’s passing. The news came from a tribute video on the Ham Radio Crash Course YouTube channel, and summarized Bob’s remarkable career in music, Ham Radio and sound engineering. Although I never got to meet Bob Heil in person, I feel his influence in my own enjoyment of the Amateur Radio hobby.

Rekindling the Amateur Radio passion

In an earlier post, I wrote about my own ambivalence toward the hobby, and how a chance introduction to the Ham Nation podcast in 2012 changed my view. Bob was the inaugural host of the show, and his enthusiasm for the hobby reignited my own interest. I felt a further kinship with him as a fellow musician and sound engineer.

The tribute video mentioned his own return to Amateur Radio, after a 12-year hiatus. His discovery of the poor state of audio quality in the radios of the era led to his design of a simple microphone equalizer. An article was published in QST in July of 1982 and began a renaissance in Amateur Radio audio that we are still building on today.

In thinking about his contributions to the hobby and sound engineering more generally, I could think of no better tribute than to resurrect that 1982 design and see if I could build one. Fortune smiled on this enterprise, and I will show you how I did it.

Equalize Your Microphone and Be Heard!

The article wasn’t hard to find. The Amateur Radio Relay League (ARRL) maintains a library of the articles published in QST between 1915 and 2011. I’m an ARRL member, so I have access to that library. The article is entitled “Equalize Your Microphone and Be Heard!” Incidentally, all of the QST magazines from 1916 through 1969 are also available on  worldradiohistory.com without an ARRL membership.

I am unable to  reproduce the schematic here since it’s under copyright. If you’re not an ARRL member, ask a fellow ham who is to help you get it. I will be making some references to the schematic in the discussion below.

A Short Course in Transmit Audio

The article discusses several things:

• a method of evaluating your transmitted audio with the help of a fellow ham
• characteristics of good communications audio
• discussion of passive and active equalization
• circuit theory for the active filter described in the article

Construction methods, the circuit diagram and an etched circuit board layout are also included. I was well on the way.

Construction Method

I then remembered that years ago there was a service that offered etched circuit boards for many projects found in ARRL, 73 and CQ publications as well as many others — FAR Circuits.  They’re still there and the web site, although bare bones, appears to be regularly updated.  The “EQ200 by Heil” is listed on the “Audio Filters and Audio Processors” page. This site is a treasure trove of circuit boards and well worth a look. I paid a little over $10 for the board and shipping, and it arrived in a few days.

Where do the parts go?

Now we got to the hard part. Parts placement on the board. It wasn’t included in the article, and the prospect of figuring it out was a little daunting. It looked like the equalizer was also sold by Heil Sound, so I wondered if I could find a picture on the web of its internals. In a stroke of luck, I found  one for sale on an auction site, and the seller had included a picture that showed the top side of the circuit board.

A few substitutions

Parts weren’t much of a problem, but the input transformer, a 2K to 10K audio-interstage type specified a Radio Shack part: 273-1378. Of course Radio Shack doesn’t sell parts any longer, so I substituted a transformer from an old transistor radio. I also used 1/8th watt resistors instead of the 1/4-watt resistors specified.

Building the board

It didn’t take long to stuff and solder the board, but I noticed some differences between the schematic and the commercial version. For one, the “audio clipping” LED circuit wasn’t included (ref. Q1, DS1, etc.). Also, some of the resistor values on the commercial board were different than those shown in the schematic.

The input transformer was not on the board at all, but appeared to be wired into the output connector. There was a large blank space on the board, so I elected to mount the transformer there and reconnect it to the input circuit.

Testing

Schematic errors

Initial testing was not promising. I puzzled over this for quite a while until I realized there were a couple of errors in the schematic as printed in QST:

• The connection between the output side of the “Low Gain” filter network and pin 1 of U1B is not shown
• The 0.002 µF capacitor that should be connected to either side of the 1 megohm “High Gain” potentiometer is not shown.

I tried both the resistor values shown in the schematic and those visible on the commercial board  in the “Low Gain” filter network. I think the commercial version values work better (eg. 6.8K instead of 10K and 22K instead of 100K).

Circuit board errors

Having followed the parts layout on the commercial board and added the missing 0.002 µF capacitor, it was starting to work, but not quite as expected. The “High Gain” pot seemed to change the gain of all frequencies instead of just the high frequencies. Comparing the schematic with the board layout, I finally realized that the board layout was also incorrect. I changed the position of a couple of resistors and added a wire to match the schematic.

Success!

The whole circuit draws 2-3 milliamps, so powering it with a 9-volt battery is reasonable. Testing was done with an audio generator set to provide a 50 mV sweep from 20 Hz to 5 KHz. The oscilloscope was set to show both  the sweep (amplitude vs frequency) and a Fast Fourier Transform (FFT). The three images below show roughly a 1) flat response, 2) low frequency boost and 3) low frequency rolled off with a high frequency boost. Other ‘scope settings can be read  with the images magnified to full screen.

The equalizer is part of a larger project for my station. It will be built into a console that also includes an audio amplifier with  EQ and a small, forward facing speaker. I plan to use it with the older radios I like to restore. In other words, a project to address both transmit and receive audio as Bob Heil suggested.

Thank you Bob, for all you did for the Amateur Radio hobby.