Compressor build

[Artspoke]

Hello all!

Great Forum!! I've just completed a build for Jack Orman's Q&D Compressor 2, based on the Analog Devices SSM2166 IC. I love it! It will take unbalanced line level, instrument level, and high impedance mic level without any modifications. The compression is musical (not invisible), and the limiter and gate are surprisingly nice as well.

Feeling lucky, I decided to add a 10-segment LED VU-scale meter to the circuit, based on the LM3915 IC. I thought I'd like to be able to switch the meter between input and output with a SPDT on-on switch, so I wired the switch to feed my little metering circuit from either the input or output jack. Since I am using a 12V 200mA wall wart for a PSU, and the Q&D only draws about 15 mA, I'm using the same supply in parallel to power the meter circuit.

The problems I'm having are:

1) When I switch the SPDT, I get a loud pop.

2) The meter starts off showing 8 LED's lit when there's no signal, and when there is, LED's 9 and 10 seem to light when the signal is strong. It's as if the meter is seeing an oscillator output set high enough to light the first 8 LED's, and then when I play music through it that's loud enough to surpass the level of the oscillator, the meter functions properly.

3) I can't meter gain reduction!

Is there some other place on the compressor circuit that I should be getting the meter feed signals from? Should I use a different IC? Is using the same PSU for both circuits illegal? If I want to run more compressor circuits from the same PSU, can I simply wire them in parralel?

For now, I've disconnected the meter portion, and am loving the compressor! The SSM2166P (DIP package) has been discontinued for several years. AD is now making the part in a surface mount SOIC package, which I was able to use thanks to a little SOIC-DIP adapter I got from Digikey, and it's working fine.

If I can work out this metering issue, I'm going to build 22 more - one for every input channel on my mixing console, one which I plan to mount inside my semi-hollowbody electric guitar, and one more inside my upright bass! I think a 5-segment LED array might be more appropriate, seeing as this whole build lives on two mini PCB's I got at Radio Shack (1.5" x 1.5" for the compressor, and 1.5" x 2" for the pot/meter card), and could easily fit in an empty cigarette pack!

Thanks in advance - for 30$ in components, this is an easy and worthwhile preamp, gate, compressor, and limiter! If anyone's interested I'll post pictures, BOM, PCBlayout, etc.

-James

[Mark Hammer]

I made one as well, and am glad I snagged a couple of those chips when they were cheaper and available. VERY nice device.

Note that the chip is predicated on the assumption that it will be fed a low level microphone signal, so the appnotes tend to show a suggested application that is not really optimized, either in terms of levels or impedances, for a guitar. keep that in mind when adapting it for various uses.

That first op-amp can be set up for a gain of 1 (the appnotes and Jack's adaptation of the appnotes has a gain of 2), and the input should be fed through some kind of JFET or similar buffer. As a standalone unit for guitar, that should improve it immensely.

Too often overlooked is the chip's ability to provide excellent noise reduction to counteract the naturally occurring hiss boost during quiet passages. I would also recommend playing with values of the averaging cap on pin 8.

And, given that the pin 8 cap is an averaging cap intended to smooth out the rectified envelope signal, it is quite possible that the envelope signal will be available to you for other uses (like metering) from pin 8. Try it out and see.

[Artspoke]

Thank you for the speedy reply!

I'm reconnecting the meter circuit right now. If I tap pin 8, what should I see, output from the compressor? Is there a way I could meter gain reduction?

I wish my soldering iron would heat up faster!!

Thanks again,
-James

[Mark Hammer]

What you should, in theory, get from pin 8 is the envelope signal. I can't confirm this. I can only surmise it from the block diagram in the SSM2166 datasheet.

It is that signal which governs the overall gain reduction amount, so you should be able to use it for a rough indication. I say "rough" because the control circuitry does something with that signal in addition. So that signal will show you what you started out with, but it won't necessarily show you how much gain reduction actually took place once the compression amount is factored in.

In its favour, what it will be able to show you is the effect of the averaging cap on the time course of whatever gain adjustment takes place.

So, perhaps not the ideal, but it seems to be easily available, not require too much additional circuitry, and serve some useful function.

[Artspoke]

I think my LM3915 is damaged, or that I'm getting some strong oscillation from the compressor to the meter. I found a link, http://sound.westhost.com/project60.htm that shows how to use one psu for both the meter and another device at once. It's not entirely clear to me what they have recommended, but perhaps the fact that they consider doing it means that this meter won't function off the same psu as the compressor, without some consideration. Is there a simpler way to decouple the 12VDC and use still use it for both?

So close! This will be an important fact if I am to build as many as 20 for my mixer. Is it simpler to use two supplies, one for meters and another for compressors?

Thanks again,
-James

[Artspoke]

Ok, I found another schematic for the LM3915, after a _lot_ of searching, that uses a couple of diodes ahead of the input:



And there are some really nice pcb layout diagrams for it too!




What will be the practical difference between a build of this schematic and the one here at aaroncake?

I'm going to extract my LM3915 from my first build, and see if it works with this one (should've used a socket. . .)

[Artspoke]

OK, with a little modification, this works great!

I changed R1 to 1M, and put a pot in in place of R3, and eliminated C2 (the response was too slow with it). And voila - a meter for my compressor!

What I'd like to do to finish this project up is:

1) Inputs - I'd like to have options to plug in
a) dynamic mic (XLR)
b) 1/4" instrument level
c) RCA unbalanced line level

2) and get out
a) 1/4" instrument level
b) RCA unbalanced line level

3) Chassis - is plastic OK?

So I'll need a few more jacks. But I'm wondering if someone can help me find the right circuits to make these options available. I have an Archer 274-017 Lo-Z to Hi Z passive transformer (XLR - 1/4") that I'm using to get a dynamic mic (Shure SM57) into the compressor, and it's working great. What's inside this thing? There's a transistor symbol, along with a pinout printed on a little sticker on it. Anyone know of a component that is equivalent?

That would take care of 1a) above, then for the other two, what should I put between the corresponding jacks and the compressor? I saw mention of a single FET unity buffer solution, are there more details about this approach to making the instrument input to the circuit correctly? Can I put a voltage divider in after a line level input to get it into right shape? And what about the outputs?

Finally, is it legal to run the meter from the same psu as the compressor by just wiring them in parallel, or do I need to decouple them some way, or use two psu's?

I'll take some pictures when I get the prototype cleaned up a bit.

Thanks for checking this out, and for the help, I really appreciate it.

-James