Audio compressor circuit wanted

[paulsherwin]

Does anyone know of a simple audio compressor / limiter design? I want to build something to use with my microtransmitters.

[Andrew2]

Paul, here's one I knocked up for my 80m amateur rig. It uses an NE612 double-balanced mixer, as I had run out of my stock of MC3340's. Input should be in the 100 to 200mv range and the output is adjustable to suit you transmitter. It worked first time for me.

[mickjjo]

A few fairly simple designs here:-

http://electroschematics.com/tag/aud...ssor-circuits/

Regards, Mick.

[arjoll]

Ones built around diodes can sound pretty bad. The NE571 does some stuff (if it's still available) but it's basically 2:1 compression so not really that useful for broadcast.

There is a circuit using a TL071 and a FET, but I've never managed to get good results out of it! I seem to have lost the link (I managed to trash a pile of favourites by mistake earlier this year), though it was an ESP circuit but can't see it there now.

For your FM microtransmitters you may want to look at multiband - when I was running my LPFM I was thinking of experimenting with this, using the method an old (Aussie) ETI active crossover did to get the bands - basically an op-amp based lowpass filter for the lows, then the difference between this to get the mids and highs which go to a highpass for the highs, and the difference between that and the signal you had for mids and highs are the mids (if that description makes sense, it certainly does in my head). I was going to build that to give me three bands, shove each through a compressor/limiter circuit, then mix the outputs to give me a multiband compressed signal.

I suppose you could use broadband compression and then the above technique to make a split band limiter to keep highs under control.

Of course because I never got the stupid compressor working right I didn't proceed, and at that stage Winamp compression plugins took over for LPFM.

For AM you'd need something that can do asymmetrical compression - I have a 70's BE broadcast compressor/limiter here that can have that switched in and out (AM and FM modes - FM also lowers the threshold as frequency goes up), a pity you're too far away to grab it

[arjoll]

Here's one for FM which seems to have everything you'd need - compression, limiting and pre-emphasised HF clipping. Never tried it though!

[paulsherwin]

Thanks for all suggestions so far. A multiband compressor would be OTT for what I have in mind, and I agree that a passive design would introduce too much distortion. The design in #5 is a possibility though, as it has a relatively low component count given the functionality.

Ideally I'd like to find something that I can build from junk box parts - I don't have any of the dedicated companding chips to hand at present, so I'd have to (shock horror) buy some!

I hadn't thought about doing this in software, but again it's something I'll consider.

[arjoll]

For my LPFM I used a Winamp 2 (it was a while ago before we moved to Otatara and the terrain meant legal 0.3 W LPFM was pointless) was an output plugin called Sound Solution. That, with a crossfade plugin, only just ran on an old HP Brio BA400 Celeron 466 with Win2k and 256MB - it just sat there running at about 100% CPU!

[Ian - G4JQT]

Some interesting circuits shown here, some of which I haven't come across before. Paul, presumably you are looking for an AM processor? The requirements for AM and FM audio processing are in many respects quite different.

I occasionally get involved with the technical side of Ofcom-licensed low-power AM RSLs for schools, colleges, hospitals etc. and by far the most challenging part is getting the audio processing right.

(For anyone unfamiliar with the purpose of an audio processor: it is to limit the maximum transmitted audio frequency - for AM this is about 4.5kHz. A processor reduces the dynamic range of the audio so modulation depth is more consistent and "sounds louder" especially at the edges of the service area, and stops quiet passages being lost in the noise or loud passages over modulating the the transmitter. A good one can "almost" be relied upon to disguise poor studio operating when no one is watching the audio-level meters!)

As has been said here, they can be very complicated indeed (splitting the audio spectrum into bands and handling each one differently), very expensive and when set up badly they are worse than useless!

For my own micro-round-the-home AM transmitter I don't use one; I couldn't possibly justify the time or expense of getting one that didn't actually degrade the signal. I just rely on a high local field strength and setting the modulation levels properly. In fact as many older sets are supposed to not really like modulation exceeding about 85% (non-linearity in the thermionic detector?) I keep mine on the low side anyway and any sudden loud passages won't sound too bad.

Using software is certainly an option. Perhaps I should look into that...

Regards,

Ian

[Dr-Watts]

Sorry about the picture quality, my camera's flash has died so I had to use my phone...

I built one using a lilliput lamp driven by a TDA2822M and an LDR.
You can Bias the lamp slightly if required and use the amp IC in Bridged mode (and a tranny if you need a larger lamp) and the results are superb. Basically the bigger the lamp the nicer the reaction, but I only use this on a large SUB amplifier in my PA
I use a lilliput lamp with some caps on a rotary switch to control the release time.
You can use an LED, but these tend to modulate the audio if you're not careful and make it sound nasty.
I know it's not terribly technological and most of you will be scoffing at this design but it costs next to nothing to build, is as simple as anything and works very nicely without making any difference to the sound quality as it's basically acting like an automatic pot, adjusting itself to the audio level.
The TDA2822M is available from Maplin for about 80p and the pinout's all over the internet - http://www.google.co.uk/imgres?imgur...M4a4hAfuxoWwBA

Tell you what, I'll put it in circuit with my 'Spitfire and let you know how it sounds if you like ?

Regards
Dave W

[paulsherwin]

I did build something similar about 35 years ago and it worked quite well. I used a couple of transistors rather than an IC (ICs cost a fortune back then ) I suppose I could use an op-amp with an LDR in the feedback circuit - I think I've got a bag of LM358s in the junk box.

Are LDRs easy to find now? I haven't seen any about for ages.

[cmjones01]

Originally Posted by paulsherwin

Are LDRs easy to find now? I haven't seen any about for ages.

They do still exist, but they're an endangered species. I believe this is because they contain Cadmium, which is no longer allowed in consumer electronics under RoHS.

[Dr-Watts]

Originally Posted by paulsherwin

Are LDRs easy to find now? I haven't seen any about for ages.

Again Paul, Maplin sell em

D

[paulsherwin]

I've ordered some LDRs and will have a play around. This could well become a very simple design indeed - I could derive a mono signal from both inputs and use this to drive a single transistor bulb or LED amp which would control both LDRs. For an AM transmitter I only need mono of course, but I'd like to support my Linex FM transmitter too, albeit with different compression settings.

[Dr-Watts]

I'd steer clear of the LED Paul.
I did a lot of R&D on my circuit and the lamp sounds a million times better.
As previously mentioned, just use a diode in the feed from the IC and a diode in series with the Bias voltage to keep the fillament 'just off' cold and I think you'll be pleasently surprised.
If you like I could record mine and send you an MP3 ?

[arjoll]

If you're using something that simple for FM I would look at some form of HF limiting separate to LF, otherwise you're not going to get any kind of loudness. Maybe broadband compression followed by split limiting, just knocking the tops off the HF content.

Also for AM you want to go asymmetrical if you can. Not sure on the circuits for that, I don't maintain any AM sites

[paulsherwin]

I ought to make it clear that I'm not looking for anything like broadcast quality sound processing here. I have a number of line level sources feeding my transmitters and the dynamic range varies a lot - the audio level of the BBC News Channel TV sound feed is typically over 10dB down compared to, say, BBC World Service radio from the same Freeview box. I just need some sort of automatic level control.

[Biggles]

I seem to remember an eight pin chip SL6270? that was around a few years ago which was a mic amp/compressor circuit. I used one in a hands free unit on a two way radio in my last job which worked very well.
Alan.

[Andrew2]

Hello Alan. Yes, the 6270 was an 'all in one' compressor chip and very good it was too - you only needed input and output coupling caps and a cap/resistor to set the time constant.
Sadly, they've been obsolete since Marconi were a lad and they seem to have vanished off the face of the earth!

[Ian - G4JQT]

The SL6270 is available from a number of sources on ebay. Mostly very expensive, but cheapest I can see today (1 April 11) is one for a total of £8.39.

Not sure how good they would be for "entertainment" quality - never heard of them being used in a non-communications environment for which they did work quite well.

Regards,

Ian

[Andrew2]

Ebay to the rescue again, but that price is taking the you-know-what!
I think they would be fine for general FM broadcast around the house, aspecially as in this application they'd only be required to do relatively mild 'peak catching'.