Stereo Audio Compressor/Limiter

[Mike Warren]

Can anyone suggest a cheap and readily available audio compressor IC?

Audio quality is not all that important, but price, size and minimum parts count is. I also only have a +12V supply to work with, and would prefer to keep the power supply as simple as possible.

Signals in and out are line level.

I see Silicon Chip did a project last year which is along the lines of what I want, but it looks like it has a high parts count.

http://archive.siliconchip.com.au/cms/A_112746/article.html

[Mike Warren]

Hmm, the chips I've found so far are all pretty expensive.

But I just had a thought. I wonder if something like this would work? Add a low pass filter on the output to remove the PWM signal. Using a very cheap micro the BOM would probably be just over a dollar.

[Mike Warren]

Unless someone knows of a nice cheap dedicated IC, this looks like it might work.

I built a test circuit and fed a 300kHz variable pulse width signal into the gates. Since I'd need to buffer it anyway I can make the output an active LPF.

[DrGeoff]

The NE570/571 devices were popular a few decades ago and still available for a couple of $.
It will depend on what attack time you want (ie how fast) and what level of compression you want to get (and how accurate it is channel-channel).

FET compressors can be very good, and very fast but calibrating them to track together for stereo requires some precise matching of the FET characteristics. Opto compressors are slower than FET compressors usually.

VCA compressors are good, quite fast and quite easy to do. There's several ways to implement a VCA with common parts and all you need then is a rectifier and integrator to creat a signal from the audio to feed the vca control.

[Mike Warren]

I just want an automatic volume control, so perhaps 100mS attack time and a few seconds release. Simpleness and cheapness outweigh sound quality (within reason).

[Mike Warren]

BTW, I did look at the NE570 this morning, but at $9.50 for 100 up from E14 I didn't look any further. At 2 bucks, it would be worth looking at.

[daveatol]

There's always the SSM2166 http://www.analog.com/static/imported-files/data_sheets/SSM2166.pdf
or the SSM2167 http://au.element14.com/jsp/search/browse.jsp?N=2101+203940&Ntk=gensearch&Ntt=ssm2167&Ntx=mode+matchallpartial

They're for a single channel, so you'll have independent compression on each channel... not sure if this is good or bad in your case.

[DrGeoff]

The SSM and THAT chips are for higher quality audio, and cost a bit more.
I just saw NE571 at Futurlec.com.au for $2.80 ea. That's a 2-channel IC with gain control cell and rectifier built in, so minimal parts count to implement stereo AVC or compression. That's got to be easier and cheaper than messing around with a micro.

[Mike Warren]

Thank you both. That gives me something to go on. At this stage I don't even know exactly how many will be needed. Might only be a couple of hundred.

I do need to do some other simple logic stuff in this. A micro is not necessary for it, but if I already had a micro on the board the other logic would cost almost nothing and almost no board space.

Anyway, I think I have enough of an understanding to go back the the customer with a ballpark cost to see if the project is viable.

[nctnico]

Can anyone suggest a cheap and readily available audio compressor IC?

Audio quality is not all that important, but price, size and minimum parts count is. I also only have a +12V supply to work with, and would prefer to keep the power supply as simple as possible.

Signals in and out are line level.

I see Silicon Chip did a project last year which is along the lines of what I want, but it looks like it has a high parts count.

http://archive.siliconchip.com.au/cms/A_112746/article.html

I always used an amplifier chip with a DC gain control, 2 resistors, a capacitor and an LM339 comparator to detect the peaks. The capacitor (connected to the DC gain control) is slowly charged from the power supply so the volume goes up. When the signal level is too high the comparator discharges the capacitor through a resistor. Works like a charm.

[Mike Warren]

I always used an amplifier chip with a DC gain control, 2 resistors, a capacitor and an LM339 comparator to detect the peaks.

Do you have a particular chip in mind?

[nctnico]

I used the TDA7052 as a pre-amplifier (somewhere in a circuit) and the TDA1013B for driving speakers.

[Mike Warren]

Thanks.

[C]

I like nctnico idea the best, but here are two more.

If you put your micro in the loop,
you could use the nctnico's DC to control a digital pot, but it may react slower.

you could use the nctnico's DC and control one of the digital pots with up/down inputs.

C