Johan's LA-Light 9v version build report

[MartyMart]

Johan Blomdahl recently posted a version of th LA-Light compressor simplified
for stompbox use and Lowstar made a vero layout for it.
I thought that I'd give it a try and made a vacrol from a Clairex CL7P5HL from
smallbear and a matched pair of red 3mm Led's covered in heat shrink.
After working out how to wire the vactrol ( not so clear Lowstar ! ) it fired up on
first try ( still blows me away ) and sounds VERY tasty indeed !
I could "feel" it compressing but had to look at a meter on my desk to check, as it's
not that obvious at first .. but WOW ... studio quality compression with absolutely "Zero" artifacts
like "pumping" or "ripple" on the decay.
Quite a serious volume boost too, totally clean and no high end hiss whatsoever.
Well done chaps, this is a very high quality compressor, I may be missing a little something
from not using the specified VTL 5C2, but I'm so pleased with it
Many , many thanks ....... BTW HIGHLY recommended !

Marty.

[Marcos - Munky]

How did you made the vactrol? I'm planning to build it, but I don't know how to put 2 leds in one ldr.

[MartyMart]

How did you made the vactrol? I'm planning to build it, but I don't know how to put 2 leds in one ldr.

The CL7P5HL is quite large, about 1cm across and the LED's are wired + to - , so I stacked
the LED's one atop the other, after insulating the leads from each other and putting some small
heat shrink around each one, I then took a tiny piece of perf, to sandwich between the now heatshrinked
surfaces and superglued them together, bending the remaining leads over in an " L " shape.
I had filled the LED ends flat before this, so they would stick to the LDR front, also with superglue.
Then the whole middle of this was heat shrinked with a larger piece and the LDR leads bent to an "L" too.
It was completely sealed and seems to work fine, though it only compresses from about 1/2 pot rotation.
I may try a 250K linear pot to see if this helps.

MM.

[markm]

Marty,
Any ideas on what would be the "ideal" resistance for the LDR?

[MartyMart]

Marty,
Any ideas on what would be the "ideal" resistance for the LDR?

VTL5C2 is specified, dark resistance is 1M
I used the CL7P5-HL which has a dark resistance of 14M !
... and is also fast attack, where the VTL5C2 is S L O W ...

I obviously have a much different "feel" to the compression, but I like it
Perhaps I'll order a couple of VTL5C2's and try another out ... I may also make the 2k7
a 4k7 pot.

MM.

[markm]

Thanks Marty,
Yeah, I know that VTL5C2 can be quite slow...(DOD280  ) so I'm glad you gave a bit of insight here as I don't have anymore of them so I may try and dig through my small stash of Radio Shack LDR's to try and find a suitable candidate for the job.
Problem is, most of my experience with the RS LDR's is......thery're VERY slow!!
I'll have to check 'em out though.

[MartyMart]

The way that I have it, it's acting more like just a limiter, "clamping" down the signal
( in a good way ) and it does indeed have quite a fast attack.
I'm going to follow some of Mr Hammers comp "tips" and have a further tweak, I'll
post anything interesting .

MM.

[MartyMart]

Still "tweakin" but added a GGG buffer up front and that helps is out, impedance issue I guess.
The overall sound is a nice controlled and high quality effect, I've just noticed that this "IS" a limiter
rather than a compressor anyway !
The switch is subtle, but is supposed to be a ratio switch from 1:10 to 1:3 and I can "see' that change
on meters, more than hear it .

MM.

[Johan]

thanks for the nice report Marty
if you still only get responce after halfway on the comp-pot, try decreasing the resistor just before the LEDs..it need to be a resistor there to act as a corrent source, but its value depends on the LEDs and the availeble voltageswing ( in this case rougly 3.5volt)..so try 470ohm instead of the 1k you probably have there now.
the LDR isnt critical either. if you have one that goes down to 1-2k in daylight, it will work. most optical comps/limiters can only do about 20db attenuation and with the big (68k) series resistance on the input, it only need to go to about 6k8 fast and most LDR slow down when aproatching their "end"-resistance, so if yours can get down to roughly 1k, it will be fast (insidently this also makes it possible to slow down the responce of the comp. by decreasing the series resistance..but thats an other issue..) 
the low input-Z is deliberate to be the same as LA2a ( straight out of that schematic) but you can easily increase it by just lifting the first 68k on the input ( the one that goes to ground)

..so, now Marty..since you have a studio..try plugging it into an insert and try singing throught it...I think you'll like it..

johan

[MartyMart]

Hi Johan, thanks for posting and thanks for a GREAT circuit
Yup, I had a 1k2 there, so  470 ohm solved that problem ! - having a Linear comp
pot seems to make sense also .
This is for gtr/bass and front of fX chain, so the buffer is a useful addition here anyway.

It sounds superb on vocals, so much so that I think I should build the "real"
version, which has a dual supply right ?
Or could I make one with say a single 15v/18v supply, like a tweaked higher headroom
version of this one ...thoughts ?

Thanks a bunch,
Marty.

[Johan]

they are all built for single supply, only the versions without input/output cap's expect the supply to be floating so that the circuit ground can be ground..it's just a point of reference.. I mean..how do you know your 9volt battery isnt +and- 4.5volt...you know becouse YOU decided that the -pole is gound...it's just a point of reference...
and in reallity, you really dont need much more than the 9volt powered unit can give you..the compression happens before any active circuitry, and the ALLREADY COMPRESSED 3.5volt RMS signal you can get at the output is still pretty loud, even in +4db situations
..or you could just make sure the voltage ratings on the cap's are enough and then connect two 9volt batteries in series..

johan

[Sindran]

Normal Ross/Dyna style guitar compressor works best when its first in the FX-chain.
But when recording/mixing its good to have compressor after everything else (also).

Ross/Dyna comp doesn´t sound good after overdrive, what about LA-light, would it work in the end of guitar FX-chain?
Would be great to have that studio-sound in gigs also, it would also help the soundman to do his job well...

What you think Marty?

[MartyMart]

Normal Ross/Dyna style guitar compressor works best when its first in the FX-chain.
But when recording/mixing its good to have compressor after everything else (also).

Ross/Dyna comp doesn´t sound good after overdrive, what about LA-light, would it work in the end of guitar FX-chain?
Would be great to have that studio-sound in gigs also, it would also help the soundman to do his job well...

What you think Marty?

Yes, I think it would work very well as an "end of chain" limiter
It works happily after OD's etc, so long as you're not boosting TOO much 
BUT - that may stop you from kicking the crap out of your valve amp !!

MM

[markm]

So,
The reaction of the LED and the swing isn't really that crucial.....
What about the LEDS?
Those are wired in like clipping diodes huh?

[Sindran]

Yes, I think it would work very well as an "end of chain" limiter
It works happily after OD's etc, so long as you're not boosting TOO much 
BUT - that may stop you from kicking the crap out of your valve amp !!

MM

I quess the right place for "end of chain" compressor would be after microphone. But then it's not my problem anymore.
It's just that on live situation, when using multiple distortions, modulations and filters, the output of my pedalboard can vary a lot. Specially if the output of every pedal ain't tuned really carefully. So having some mild "transparent" compression in the end of FX-chain might be good thing.
If LA-light is really a limiter it might do what you said:
"may stop you from kicking the crap out of your valve amp !!"
And nobody wants that!!! 

[MartyMart]

So,
The reaction of the LED and the swing isn't really that crucial.....
What about the LEDS?
Those are wired in like clipping diodes huh?

Not totally, Johan said that the LDR needs to be around 2K light to a few M dark
mine is about 20k to 14M and fast response, but after a few tweaks, it seems fine

MM.

[Johan]

So,
The reaction of the LED and the swing isn't really that crucial.....
What about the LEDS?
Those are wired in like clipping diodes huh?

Not totally, Johan said that the LDR needs to be around 2K light to a few M dark
mine is about 20k to 14M and fast response, but after a few tweaks, it seems fine

MM.

I personaly prefere separate LED's and LDR over comersial vactrols..that way I can have both LEDs shining on the LDR..much cleaner in the bassfrequencys...also, even thou it is a quite simple circuit, I've spent a lot of time on making it NOT to dependent on specific parts. basicly, by tweeking the resistor going to the LEDs ( Rx in Lowstar's layout ) you should get it to work with most LEDs/LDR's, even at different voltages...the VTL5c2 is certainly the one I liked most of the commersial units, but then it's only one LED inside, they cost more and still doesnt beat two LEDs and one LDR.
also, different vactrols use different LEDs and the do respond VERY differently. just using regular 3mm red LEDs gives quite consistent results...

johan

[markm]

Here's an unverified PCB layout for the LA if anyone would care to check it 

[Bagge]

I tried to build LA-light stompbox edition. I just can't seem to get compress knob working. It doesn't make any changes to the sound. The unit gives quite a boost though. I first tried my own layout, then Lowstars http://aronnelson.com/gallery/johan/LAyout_Light_stomp?full=1, but no success. I used LDR and LED's, and I figure the problem lies here. The LED's are rectangular green ones, as used in volume displays etc. Does anyone have experience with these? I currently have a 10 ohm resistor in Rx to make them light up. (Johan suggested to start from 1K) . Maybe I should try standard red ones

[markm]

I tried to build LA-light stompbox edition. I just can't seem to get compress knob working. It doesn't make any changes to the sound. The unit gives quite a boost though. I first tried my own layout, then Lowstars http://aronnelson.com/gallery/johan/LAyout_Light_stomp?full=1, but no success. I used LDR and LED's, and I figure the problem lies here. The LED's are rectangular green ones, as used in volume displays etc. Does anyone have experience with these? I currently have a 10 ohm resistor in Rx to make them light up. (Johan suggested to start from 1K) . Maybe I should try standard red ones

I gave up on mine for awhile.
Hence the reason my layout is gone.
Also, I'm not partial to optical comps but, some folks are.