optical compressor idea

[EmilDK]

I just put together a schematic for an optical compressor with as few parts as possible, but then i found Craig Andertons Compressor, and discovered that they are very similar. So my question is; would I be better off building CA's comp, or should I try to get my own going?
Here's my design idea:

And here is CA's:

Thank you, and cheers, Emil

[Mark Hammer]

I'm going to say build Craig's, for 2 reasons:

1) What you have shown is actually an expander.  If I've understood the schematic properly, greater amplitude signals will result in the LED glowing brighter, and reducing the LDR resistance.  Dropping the LDR resistance, in turn, will increase the gain of the 2nd stage, resulting in exaggerating the initial transient.  Compression operates by reducing gain in response to transients and increasing gain as the input signal starts to die out.....the opposite of what you've shown.  No need for embarassment on your part.  Easy oversight, and you had the diligence to at least draw out your idea.  So kudos....AND regrets.

2) Where your design (which *could* work with some changes) uses 2 cascaded noninverting stages, CA's compressor design uses two cascaded inverting stages.  Why is the one "better" than the other?  Because the invertng configuration allows for easy treble pre-emphasis.  Compressors are infamous for the extent to which they just seem to lose bite.  Having an easy means to add a little more treble is an advantage here.  How to do it?  Easy.  An inverting op-amp has its gain set by the ratio of the feedback resistance to the input resistance.  So if feedback = 100k and input = 10k then gain = 10.  BUT, bandwidth is set by the input capacitance times the input resistance.  So, if you had the same 100k/10k arrangement and stuck a .01uf cap in series with a 4k7 resistor and put THAT in parallel with the existing 10k, you would have a separate path with a gain of 100k/4k7 = 21 and a low end rolloff at 1/[2*pi*.01uf*.0047Meg]=3.38khz.  That sort of dual-path pre-emphasis approach is used in all the Boss chorus and flanger pedals as well as many other commercial designs, so it's tried and true.

[EmilDK]

Thanks! Based on your reply, I will certainly try to build Craig's compressor. I drew up my schematic before searching for similar builds. I just wanted to make sure that Craig's was in fact better and of course, since mine actually works the wrong way, the other one would work slightly better . Btw do you have any links for some basic op-amp info? (obviously, there's something I've missed). And thanks for a quick and detailed reply.
The reason I drew my schematic was because most of the comp circuits I've seen are very complex, but Craig's really is nice and simple. There wouldn't happen to be something even simpler around, would there? anyway, thanks for the reply. Be prepared to answer many more stupid questions soon,;)

Emil

[Mark Hammer]

There are all sorts of "simple" compressor or AGC (automatic gain control) circuits floating around that are geared towards ham radio operators maintaining maximum mic signal.  While simpler, they don't generally provide the sort of tonal quality or output level that guitarists want.  Certainly the simplest guitar-oriented compressor I know of is the Orange Squeezer, which has a very pleasing sound.  Not optical, though.

[davebungo]

2) Where your design (which *could* work with some changes) uses 2 cascaded noninverting stages, CA's compressor design uses two cascaded inverting stages.  Why is the one "better" than the other?  Because the invertng configuration allows for easy treble pre-emphasis.  Compressors are infamous for the extent to which they just seem to lose bite.  Having an easy means to add a little more treble is an advantage here.  How to do it?  Easy.  An inverting op-amp has its gain set by the ratio of the feedback resistance to the input resistance.  So if feedback = 100k and input = 10k then gain = 10.  BUT, bandwidth is set by the input capacitance times the input resistance.  So, if you had the same 100k/10k arrangement and stuck a .01uf cap in series with a 4k7 resistor and put THAT in parallel with the existing 10k, you would have a separate path with a gain of 100k/4k7 = 21 and a low end rolloff at 1/[2*pi*.01uf*.0047Meg]=3.38khz.  That sort of dual-path pre-emphasis approach is used in all the Boss chorus and flanger pedals as well as many other commercial designs, so it's tried and true.

You can do pre-emphasis in a similar way with the non-inverting configuration.  I think the reasons for using the inverting configuration are elsewhere.