Microphone Compressor from 1978 lives again (ETI490)

Whilst sorting electronics junk recently I unearthed this board. My build of the Electronics Today International microphone compressor, ETI490, published in the magazine in December 1978. I remember preserving my copy of this magazine for many years, long after the pages faded and cover tattered, as it contained the construction article for this useful amateur radio station component.

Condition and styling

The board was in fairly good condition and almost complete. A couple of things struck me. One was how big the thing was. An entire transceiver would easily fit in this area today. Six or 8 ESP32 MCUs with Tensilica Xtensa LX6 microprocessor @240 MHz with 520 KB SRAM and enough features to fill the rest of this blog post could fit in this space today.

It wasn’t only the board area that stood out, but the acres of distance between individual components. So much space. Wasted space by today’s standards. Clearly, electronics had not progressed far along it’s incredible shrinking act, leading to smaller and smaller leaded components, then the first generation of surface mount parts, to today, where you can’t even see the parts without a microscope. Product size and PCB space in 1978 were without cost and not considered a negative. In fact, in so much as it made components like this serviceable, space and a wide open layout was an asset.

Another striking thing was the age of some of the carbon resistors I’d used. Resistors that looked like they’d been cut from old valve television chassis. And they probably were. That’s what I used to do in my teenage years — and I still do it today — pick up other people’s electronic junk and strip it.


Seeing this talisman from my youth brought memories flooding back from 1978, my final year of school. With Citizens Band radio at its frenzied height and C. W. McCall’s ‘Convoy’ playin’ on the AM radio hourly, speech compressors or so-called ‘Power Mikes’ were all the rage.

1970s ‘PowerMike’

Most of these were run flat chat and blasted the Good Buddy’s breathing noises S9+40dB across tens of kHz of the 27MHz spectrum. They appeared with CBs and 5/8th wave antennas every Thursday in our second hand trash’n’treasure paper, The Melbourne Trading Post, with effusive claims, such as ‘no need for a linear’ or ‘be the loudest voice on the channel’ or ‘ worked the USA three times’. A PowerMike and a 1:1 SWR were the keys to the world .

My use of a speech compressor was much more demure. At the time I was running a homebrew sideband transmitter on 80 meters called the Tucker Tin II by Fred ZL2AMJ. I say sideband and not SSB because although it was a phasing type rig, it produced almost as much upper sideband as it did lower.

Phasing was my only option in 1978. We couldn’t source nor afford crystal filters in those days, and the tsunami of cheap computer crystals was about 5 years hence. My Tucker Tin managed to somehow get an odd mix of both sidebands at 3.6MHz onto the grid of a single 12BY7, for 3 to 4 watts output. Needless to say, on a quiet band in the days before switch mode power supplies and solar inverters, and into a dipole, it worked quite well, and I made contacts all over Australia and New Zealand.

The ETI490 speech compressor gave it some much needed punch in noisy conditions. As I examined the old board, I noticed that the three trimpots, for clipping, compression and output level, were all set full on or close. I was young back then! 😉


I’d long ago lost my copy of ETI so turned to the interweb for help. Google delivered. There it was, a good quality scan of the magazine from 40 years ago. It’s worth browsing old electronics magazines to remind yourself just how far we’ve come.

We forget that the options to procure electronic components in 1978 were not what they are today. No e-commerce, eBay or Ali-express. Very limited weekend trading. Back then, I’d often find myself short of a red LED, a BC549 or a 33uF electrolytic on a Sunday, and pull them from my own projects. That’s exactly what I’d done to my compressor at some point. So first, I had to replace these parts.

Powering and Boxing

Once done, the board powered up with an electret mic and the digital scope on the output, and it worked first time. Obvious compression, and amplitude limiting was visible on the scope.

At this point I decided that the combination of sentimentality and the utility of a working mic compressor was more than enough motivation to box it up and keep it working. I remembered a nice little project box I had kept for a rainy day, and found that the ETI490 could be made to fit if I sawed about 3mm off each end. No traces were lost by doing this, such was the unused acreage of the original PCB.

The rest of the fit out was straightforward. I used an external 9v transistor battery holder, mounted on the rear panel for convenience. So often, internally mounted batteries go flat, die, then leak, damaging the electronics inside. Out of sight is out of mind, and with batteries, that’s not necessarily a good thing.


Here’s two scope waveforms with an electret microphone straight through, and clipped/compressed.


I first made this project in 1978 as a teenager and remade it in 2019. That’s 41 years. I sure hope I’ll be around in another 41 years to make it a third time. The year will be 2060. I’ll be 100 years young. Now that’s something to look forward to.

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11 thoughts on “Microphone Compressor from 1978 lives again (ETI490)

  1. Bill Meara says:

    Great stuff Paul. I wish I could find some of my creations from the 1970s! FB on building it a THIRD time. Go for it OM! 73 Bill


  2. Andrew VK1DA says:

    Just chanced on this post, Paul, and your points about space on boards are so true. I have an unbuilt compressor kit – but I think from the 90s, “much more recent”, and I think it has a roger beep option. Now it has been replaced by the chips with noise gates etc but I will eventually find time to build it. I will try to let you hear it once I’ve done that! 73 Andrew VK1DA/VK2UH


  3. HA6VSR says:

    I have reworked this old pcb with Sprint Layout and it is much smaller now, say 5×5 cm, which is not big at all.

    Liked by 1 person

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