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Old 28th Jan 2019, 2:48 pm   #1
retailer
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Default Home Brew LA2A recording/broadcast compressor

Not that long ago I built a copy of the LA2A Optical compressor for my son's home recording, his cousin asked it I would also build another unit for him he has a similar interest in recording the only difference being that I think he actually makes money out of it.

Here is the finished item, the chassis came from an old solid state PA amp salvaged from the rubble of a demolished factory, I was able to straighten out the chassis well enough to use it, as the rear panel was a heavy al extrusion cum heat sink I bent up a new rear panel, and cut a new front panel from some scrap 3mm al. The mains transformer was one I had on hand that only needed the secondaries rewound to suit, hum shields were added at the same time, the input transformer came from the salvaged PA amp.

The output transformer was wound using a old M core lamination set that had been salvaged from a junked bit of vintage Telefunken gear, 5:1 turns ratio - it turned out quite well with frequency response from 27Hz to well over 40Khz. The winding layout used was 1/2 secondary - full primary - 1/2 secondary, primary 1200 turns secondary 6000 turns .13mm wire. My homemade coil winder came in handy for this one, I did though experience some wire breakage at first - even with the static brake fully backed off. Not wanting to have any joins in the wire I just started over if the wire broke, I think my issues were caused by drag from the grease in the despooler spindle bearings, I also found that soft start for the bobbin spindle would a good idea, even with all of the slack taken up in the wire, the sudden start of the bobbin spindle would immediately break the wire, I had to resort to turning the speed control right down to zero, slowly bring it up and then start the bobbin spindle turning by hand at the right moment, once the bobbin spindle was turning I found I could increase the speed up to around 300 RPM, I may have been able to go faster but did not want to risk it.

Just about everything bar the optical module and the VU meter was salvaged from junked gear. The VU meter was sourced from the 'bay, advertised as a VU meter it turned out to be just a panel meter with a VU scale, I had to make up rectifier and shunts so it would read correctly as a VU meter.

A coat of paint, some expanded al mesh for cooling, a set of vintage instrument knobs and water slide transfers for panel graphics finished it off nicely.
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Old 30th Jan 2019, 8:13 am   #2
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Default Re: Home Brew LA2A recording/broadcast compressor

Nice job, any chance of the schematic?

Andy.
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Old 1st Feb 2019, 9:37 am   #3
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Default Re: Home Brew LA2A recording/broadcast compressor

I'm quite sure this is in the public domain and posting it is not in breach of copyright various layouts are around, including my own let me know if you need a copy.
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Old 1st Feb 2019, 10:18 am   #4
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Default Re: Home Brew LA2A recording/broadcast compressor

And AR88 Tone/Noise Limiter knobs?

David
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Old 2nd Feb 2019, 1:33 am   #5
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Default Re: Home Brew LA2A recording/broadcast compressor

Yes those knobs give it a real vintage look - I have a few shoe boxes full of knobs so I don't recall where I picked up those particular knobs, but probably from a place I did frequent - an electronics salvage store located here in Adelaide called Robbies covering around 1/2 acre, not sure when they opened but I'm guessing sometime in the 1940's and eventually closed around 5 yrs ago when the owner was too frail to continue, I made my first visit there around 1967. It was one of those Aladdin's caves type of place with stuff dating back to the 1920's, a mixture of consumer, military and laboratory surplus some complete and some partially dismantled.
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Old 2nd Feb 2019, 8:13 pm   #6
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Default Re: Home Brew LA2A recording/broadcast compressor

That diagram is missing the link between the cathodes of the gain reduction amplifier...
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Old 3rd Feb 2019, 1:27 am   #7
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Default Re: Home Brew LA2A recording/broadcast compressor

Didn't notice that error before good work for picking it up - it is only one of several circuits I have including one in a copy of the original user manual, I checked the others and they all show a link between pins 3 and 8, I checked my layout and it is correct.
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Old 3rd Feb 2019, 8:10 am   #8
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Default Re: Home Brew LA2A recording/broadcast compressor

Thanks retailer.

Andy.
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Old 3rd Feb 2019, 12:21 pm   #9
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Default Re: Home Brew LA2A recording/broadcast compressor

Hi retailer, good work. What did you use for the "electroluminescence element"?
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Old 3rd Feb 2019, 1:54 pm   #10
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Default Re: Home Brew LA2A recording/broadcast compressor

A seller on the 'bay has them - Kenetek T4B module - claims that they mimic the original as far as attack and decay go and are packaged in what looks like an octal relay case which is I believe same as the original, you can just see it on the right in 2nd photo. I'm quite sure a quick internet search would bring up a number of companies selling them.

Some do make their own I know but this was a 'love job' for my nephew and I did not want to spend time experimenting with El panels and LDR's he came to the party for optical module and the turret board but everything else was donated by me.

My other passion is model engineering and I was keen to get back to machining my Quorn Tool and Cutter Grinder, I'm 3/4 there with all of the machining on the castings and spindle finished.
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Old 4th Feb 2019, 1:51 pm   #11
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Default Re: Home Brew LA2A recording/broadcast compressor

Thanks retailer, so not a 6V lamp and an ORP12 then I like the idea of a homebrew, I wonder if LED's could be used? I'm out of the recording business now but always wanted an opto compressor, but they were too much wonga!
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