Monday, 12 March 2012

Really Cheap Compressor

One of mictester's circuits from FSB, very transparent compared to Dynacomp or Squeezer's so definitely one to try for the compressor fans.

Mictester's description:

The original idea was to replace the old fashioned way of getting compression - connecting an LDR across the volume control of your amplifier, then illuminating it with an incandescent lamp driven from a tap on the output transformer. The louder the signal, the brighter the lamp, and the more the audio was shunted to earth at the volume control! It was the very simplest compressor, but along with some nice second harmonic distortion introduced by slightly changing the bias voltage to the screen grids of the output bottles, you ended up with a very "loud", very lovely sounding amplifier that would give fabulous sustain to any guitar.

The Really Cheap Compressor is a minimum parts design that was meant to be a replacement for the simple valve amplifier modification outlined above. I've built several of them (which reminds me, I need to make another one this week), and everyone who's used them, likes them. If you use a good quality dual op-amp (a TL072 is OK, but the LM833 or NE5532 are slightly quieter), you won't hear any added hiss, and no added distortion. This compressor is entirely clean, and doesn't have any part of the LEDs (or any other diodes) in the signal path, so there's no clipping (unlike with the rectifier circuit in the Dynacomp). The LDR is slightly slow to respond, so the attack of the guitar note is retained, but the squeeze is nicely applied to the decay portion of the note - just where you want it. This is one of those "is it on?" effects, but you really miss it when it's off!



and a version better suited for bass guitar:


50 comments:

  1. The squeezer parts are a little to expensive so I'll give this one a try...

    The LDR, I wrap it so light can't get to it then have it pointed at the two LED's?

    The LDR's I'm looking at state:
    Typical dark resistance = 1M Typical light resistance = 2K - 4K

    Sound ok?

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    1. Yes sounds fine. It's probably easier to make up the LED/LDR combination first, tape it up with electricians tape first so it's solid, and then solder the 6 leads in place

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    2. Ok now I'm confused, 6 leads?

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    3. Oh you mean tape up the LED's and the LDR together? lol.. that would make sense.. that leaves 6 legs to solder?

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    4. Yes, tape the LDR facing the two LEDs. I've seen these things built without the tape, but it won't work properly unless it's boxed which makes it awkward for testing. A bit of tape around them will keep them firmly ogether like a vactrol, so you don't have to think about it again.

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    5. Ok, cool. Parts ordered.. be back soon :0)

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  2. No luck :(

    I assume the onboard LED's are supposed to light up? as mine don't

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    1. If I prod the bottom of the circuit the LED's flash on and off. If I hold my thumb over the bottom of the circuit they stay on and the knobs scratch when turning.. no effect though. I have very little patience for problem solving so this one goes on the scrap yard and on to the next.

      Sorry I couldn't verify it mate...

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    2. I'm fairly confident about that circuit, I just double checked it compared to mictester's PCB layout and it certainly seems to match to me. Before you ditch it, post up a front and back pic, and havea look at the thread as others have made posts about the LED not lighting up:

      http://freestompboxes.org/viewtopic.php?f=13&t=8581

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    3. Oh it will be my error, not yours Mark. I'll snap a pic tomorrow in better light as it's in the corner along with my dummy at the moment :o)

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    4. Pics: http://s1170.photobucket.com/albums/r534/vince371/Comp/

      Looking at the back, it's a bloody mess?! Half asleep soldering. I'm betting it's bridge somewhere but really can't be bothered re doing it with the possibility of the same outcome.

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    5. re flowed solder which improved it slightly. The LED's kept flashing on/off. They also dimmed when increasing the sustain knob. Anyway it's now in the bin.. It was a mess

      Thanks anyway.

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    6. Ha ha, that sounds exactly what it should have been doing! Did you wrap the LEDs and LDR and try it out?

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    7. Yes, I also forgot the 15nf! haha! oh deary deary me.. added that but still the same though... I still didn't get any effect. Bypass was fine. Complete silence when effect was engaged.

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  3. Ok heres my 10 pence worth. I salvaged it and got it working so it is verified :)

    But, With the LED etc wrapped, I got a distorted compression. Unwrapped it was slightly better.. With one LED pointing and the LDR and the pointing away, I got the popping type of comp.. but to me it's too much of a pain in the arse fiddling with it all to get it just right. Also doesn't seem to be much volume... Distorts on higher sustain settings...

    This is just my opinion, others may love it but I'd rather spend £20 on a second hand Biyang comp.

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    1. This is often the problem with LED+LDR builds because the physical position of the components can have a dramatic effect and so it can be a lot of trial and error. This is where vactrol builds are preferable as there is no builder to builder deviation, and so a circuit that works well for one person should work equally well for someone else because there are no other outside influences like LED type, distances, wrapped or unwrapped etc. Still thanks for verifying mate, and maybe try the Orange or Rothwell?

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    2. It will be great for a compressor fan.. I rarely bother with them which explains my lack of desire to fiddle with it. but it definitely did what it says on the tin... I still have to get around to my modded Krank yet.. bass boost/switchable tranny monster :0). Quite fancy the BluesBreaker too... Oh there's so many that I have ordered parts for stuff that I have no idea what they're for!

      Still they'll come in handy I'm sure.

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  4. A made one using a TL072 (I have a NE5532 but I somehow just put a TL072 without thinkin..) and it works.. I think!

    Signal goes through, volume is good and the tone even gets a little bit fatter. However, I really struggle to hear much compression at all. I'm a studio sound engineer and I know compressors well, but as I said, hearing compression from this is really hard. So my question is, is it that transparent that you hardly even notice that there's compression going on? Even with the sustain turned all the way up? I'm trying to figure out if it's working properly because there's no reference to check it to.

    Cheers!

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    1. I have just built one - once again, many thanks IvIark (and mictester). Had quite an interesting debugging session sitting in the dark before I realised I had a solder bridge that was shorting out the feedback (LED) loop. If you set it up with the right LED/LDR combo/orientation, you will know it's on - it has a lot of compression. It sounds pretty good, is nice and clean, and is fit for purpose. I am going to put it in the same box as a slow gear clone that needs a bit of a helping hand.
      If anyone is interested, this is how I set it up: Other than a few of the caps, which are somewhere in-between the guitar and bass versions, it is stock (I used an NE5532). In the light, it will not pass any signal as the gain is nearly 0. In the dark, you get most compression with the sustain pot open (fully anticlockwise) - this I will switch before I box it up as it seems backwards to me. You can check it is working by sitting in the dark and watching the LEDs light up when you hit a note - this is a little hypnotic, so be warned. As you dial in more 'sustain' the LEDs get less bright. If you are not sure it is working, pull the LEDs out (if you used sockets) and see it is sounds different - it works without the LEDs, but you will not get any feedback/compression.
      At minimum sustain, I found 3mm yellow/orange LEDs got the brightest, so I found an LDR that seemed to play nicely with them - it measures >2M in the dark and ~4k5 under a fairly bright lamp. I moved the LDR to the edge of the board (had to move the 22p cap) to make enough room to point the LEDs at the LDR. Like Vince found, this thing can distort pretty badly. I want to get as much compression out of it as I can, so I moved the level pot to the input so it can be used to dial back the signal - seems to work pretty well like this, but you could just use the guitar volume knob.

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    2. Excellent info, thanks for that Sam

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  5. Glad to help IVImark, although in this case, I'm not sure how much of a help I am being. I had another play with the circuit tonight. I managed to get much less distortion by using 5mm green LEDs, so I wonder if this might be diode clipping. It sounds better now, but is quite fun trying to fit everything on the board.
    I then realised, that I should be listening with my ears, not my eyes and that more 'sustain' = less LED illumination = increasing gain of the second half of the op-amp and more compression, so I had that wrong in my last post - sorry about that. This all came up on the FSB forum (http://www.freestompboxes.org/viewtopic.php?f=13&t=8581&sid=d3a050e5410cee13f033307ef30acb1f&start=60).

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  6. I just built the bass version today. i made my own LDR (radioshack LDR and two 3mm diffused green leds), and it works perfect & sounds great. but i also tried a VTL5C6 vactrol in it with an additional 2m resistor in parallel with the 2M2 one and it worked perfect with that as the LDR too. IMO, this is a really nice and simple comp.

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  7. I also built the bass version, and I'm pretty impressed. It's not quite as loud as my Keeley clone, but it's a lot more transparent, making it especially good for slap playing. This one will most likely get boxed, probably with some form of preamp.

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  8. I built the guitar version and it worked first time. I mounted the leds and LDR inside an improvised tube made from a dark business card wound around a 5/16 dowel with the aid of some Pritt and insulating tape. I plugged the ends with bluetack. I did not need to adjust anything. It sounds pretty much like Pinkjimiphotons version on Youtube and thats just what I wanted.

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  9. After I soldered all the parts on board, I connected it with a battery but only found out that the two LEDs blinked quickly and then went dark. Can someone help me figure it out?

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  10. I built the bass version. I get this click if I strum it loud the first time. The pedal then remains noiseless until I strum harder than before. Just once again and then remains noiseless for that sort of attack. Like as though the compressor is changing levels. Has anyone else experienced the same?

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  11. someone way back in the thread said they built this with the LEDs and then again with a vactrol - if you were to use a vactrol - what would be the layout? I assume 2 leads would go where the LDR goes - and the other two would go where one of the LEDs is? and just get rid of the second one? He put another resistor in parallel - that makes sense. would just add another column to the board. This looks like a really interesting compressor option - especially when subtile compression is what you want.

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    1. There's a bit of a problem with that. It appears both LEDs are being used to rectify the signal. If you want to use a Vactrol, I would leave the second LED exactly where it is.

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  12. Replies
    1. a 22pf (picofarad) ceramic capacitor

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    2. Ok thanks, it's the ceramic I've don't understand

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  13. pops when i turn it on, pops when i turn the SUSTAIN pot to a certain point (when the LEDs light up) ..... anybody had anything similar? optical compressors are really tough.

    for the LEDs you guys using super brights?

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  14. Just tested this and with a NE5532 and 3mm yellow LEDs it works kind of chaotically. One of the LED flashes eratically as the notes die off, which produces a lot of popping noises in the signal. With a TL072, it's fine, but quite distorted.

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  15. Built this a couple weeks ago for my neighbor, he says it works great! I used 5mm red LEDs and 1M ldr

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  16. I built this yesterday.. The LDR is working.. So When the circuit is in the open the signal is super soft and then I cover it and the signal gets louder.. But the LEDs aren't lighting up and the Sensitivity knob does nothing.. Any ideas?

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  17. Built it, sounds nice. It's a subtle comp, but if that's what you're looking for it does the trick. Also 5mm greens and with the only LDR Tayda sells.

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  18. goddamn browser burped and ate my post... fucker...
    built one of these for my bud recently, and he smoked it... either he hit it with bad power or reverse polarity (likely) or the 47u filter cap failed... it was smoked and exploded, and when it went it popped both leds and the ne5532 chip. he needs it for a gig friday, so i told him i'd take a peek and did, and while playing with it found some intriguing results.
    no idea what the ldr was... was (as usual with me) just a random component outta the resistor drawer... i had two bright green led's in there originally that smoked, as i discovered when i cut the home rolled vactrol open.
    anyways, i replaced the filter with 100u (closest thing i had within reach) and used two different led's... one green, and one yellow/orange diffused one (both were the diffused "softer" looking ones) and i got a ridiculous change in the performance of the circuit... it's easily 4-5 x louder now than it was, enough where it can be used as a nice cleanish overdrive with some phenomenally quiet long sustain. unity used to be about 2:00, now it's about 8:30 or so, i dunno if anyone else will get these results, but i figured i'd share... i always prefer to have too much volume, and turn down, than not enough and try to make do myself..
    peace out!

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    1. That's rad. Which one is the filter cap? B)

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  19. It is a great compressor!
    I am using it with my Bass and it is very transparent.

    It was my first compressor that I built . I choose two 5mm yellow leds and a 7mm LDR.

    Once I used the CS3 boss compressor (blue pedal), but it cut off a lot of the Bass freqencies.

    I always heard about the Ross and Dyna compressors, but I do not know the main difference and how they could be applied with the Bass. I dont know when I will build them, but until there I will be using the Really Cheap Compressor.

    Thank you !!

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  20. Would the regular version or the bass version be better for use with an acoustic guitar?

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  21. Found these mods:
    Bright mod:
    470pF and 10nF caps in parallel with the 220k input resistor and the level pot, respectively. Those are controlled by a switch. I added a 2k2 resistor before the output permanently. This mod was suggested by mictester, the designer of the circuit.

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  22. Any ideia how to increase the low end for the bass version?

    I am using the layout with the capacitor value of 33nF and 4u7 for input and output, respectively.

    If I increase this values for 100nF and 10uF (eletrolytic) do I cause any significat change over the low frequencies?

    Thank you !

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  23. Is it possible to increase the sustain?

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  24. I built the bass version, but no signs of life. A schematic would be a huge help in terms of troubleshooting. Does anybody have one??

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  25. google is your friend
    https://coastaudio.files.wordpress.com/2013/10/schematic-really-cheap-compressor.png

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  26. So noob question about the LDR.. I got a big sack of them off ebay and I've been testing them in a little isolation chamber I made from like 4 layers of duct tape making an envelope and just the leads stick out.. they all seem to test well over 1 meg.. is this 'Too' dark? Or is complete black OK for the test?

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    1. I figured out I have gl55 series, 16 and 28. They are full dark 500k and 1M respectively according to the data sheet. I'm going to use some header and try a few different color LED. Just waiting on my pots of all things .. I swear I'm just going to by like 20 lots of 10 of all the types and never wait again..

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  27. I built this earlier this week and boxed it last night in one of those Hammond boxes; the same size as an MXR Phase 90. I lucked out and had most of the parts, including a pair of flat, square green LED's and a 1M dark LDR. It works exactly as described and introduces no hiss, even with the level cranked. No clicks, pops or anything troublesome. Awesome pedal.

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  28. Photos of the build process: http://www.tdpri.com/media/albums/really-cheap-compressor.1838/

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