colorsound supasustain-

[jimbob]

I built this this weekend and so far so good.  FOr the ldr/led optic. part of it i bought a 2600mcd red led and an assortmnt of ldr's and chose the one that would fit in the shrink wrap i had. To my suprise the thing sounds decent but a little noisy. I used 2n3904's for the resistors but everything else is as the schem original. The compression seems to work great so i wonder if i actually did it right ...but..it seems a little noisey if you wait a few seconds without playing a note. Is this a common thing? It does have great sustain! Anyone else have any issues with this build? Anyone ever used the original commercial one? Any idea what causes this eventual noise? Should I work on other led/ldr optic combinations?

btw- this is the one from GGG site. Also its perfed.

thanks

jim

[Ge_Whiz]

Yeah, my build is pretty noisy too, but that's not unusual for compressors like this - the gain is well up when there's no input signal. You could minimise it with low-noise transistors and metal-film resistors.

Mine distorts slightly too, and the sustain is not very flat - it rises and falls audibly but not greatly over the first second or so - but I just regard all these features as 'vintage mojo' in an otherwise perfect world.   :wink:

[jimbob]

we cant be the only ones who built this thing. Im wondering how important it is to make sure the led/ldr is correct? I just slapped the 2 togather and it seems to work ..but how good?

[Ge_Whiz]

The LED/LDR combination may well affect the linearity, but not the noise level (I'm fairly sure). Yeah, I just slapped mine together too, although I did use a high-brightness yellow LED (LDRs are more sensitive to yellow than red).

[k1enneth]

I noticed both of you have attempted an interesting pedal. I found an older
vintage Roland AS-1 Sustainor schematic. 18 volt. here is the ...http://www.geofex.com/FX_images/rolandsu.gif
I have not found the original pedal anywhere to test drive it. But I thought that you two have gotten fairly close with the Coloursound pedal. Any comments on the similiarites of the schematics?

[thehallofshields]

Is the Darlington necessary?

[thehallofshields]

So, a compressor without an envelope-follower? How was that ever going to work?

[kaycee]

I built one or two of these a few years back. Not the most subtle of devices, but then I'm not much of a fan of compressors anyway. Did have a lot of sustain, but a fair amount of hiss IIRC. I used a red led and a random LDR. Wasn't my thing so sold it/them on.

[thehallofshields]

I built one or two of these a few years back. Not the most subtle of devices, but then I'm not much of a fan of compressors anyway. Did have a lot of sustain, but a fair amount of hiss IIRC. I used a red led and a random LDR. Wasn't my thing so sold it/them on.

Well it's good to know the design works, I'm just a little skeptical considering there is no envelope follower.
The LED and LDR are fast devices right? They can keep up with low audio range frequencies right?
Isn't this thing going to do one thing to the positive peaks polarity and the opposite to the negative peaks polarity?

I'll get this breadboarded and play around with it.

[PRR]

> So, a compressor without an envelope-follower?

The LDR is an envelope follower. Light has no polarity. (That specific implementation uses just one polarity sensitive LED, so it is half-wave.)

The LDR -is- slow release. Some may run <50mS which is perhaps too fast for good sound. Others run >500mS. "Release time" is also influenced by the series resistor value (in shunt limiting; this one is trickier).

Look at studio/broadcast limiter LA-2a. Sidechain is just an audio amp, practically a small Champ. This swings straight audio onto a glow-panel. The LDRs for the LA-2 must be carefully selected for good time constants (easier back in the day when LDR makers would cooperate with any buyer; tough now.)

[PRR]

> Is the Darlington necessary?

Short answer: it is cheaper to do the Darling (integrated or discrete) than to think real hard about saving the few cents for a simple transistor.

A single transistor makes more distorted loading on hi-impedance Q2 output, and would begin to trigger at half the voltage, making output lower.

[thehallofshields]

Thanks for the responses PRR.

I guess I was imagining that the light would be pulsing at the frequency of the guitar signal, rather than being a constant flow.

[thehallofshields]

I also find it interesting that the Negative Feedback is going from Q2 Emitter to Q1 Collector.

I guess since it's two inverting stages, a connection back to the Q1 Base would have been Positive Feedback.

[thehallofshields]

I also get some noise from the Darlington. 

Sounds like a Square Wave. Frequency is loosely related to the guitar pitch. It kind of motorboats on release. Disconnect the Base, no noise.

This is on a breadboard with one of the supply rails dedicated to the Darlington. If anyone has any ideas, please share!

[PRR]

> ...light would be pulsing at the frequency of the guitar signal

It is.

> ....interesting that the Negative Feedback is going from Q2 Emitter to Q1 Collector.

I don't see that?

Hiss--- think what it is supposed to do. Turn-down loud parts and turn-up soft parts, so they all come out the same. Compressors almost always raise the gain (and hiss) in the quiet parts.

[Keppy]

I also get some noise from the Darlington. Sounds like a Square Wave. Frequency is loosely related to the guitar pitch. It kind of motorboats on release. Disconnect the Base, no noise.

Sounds like your Darlington might turning on/off too fast (needs filtering), and with too drastic of a transition (too much gain). This might be dumping too much current onto the ground rail. That current seems inherent to the design, though, so it might be an artifact of the breadboard and the high-ish resistances it involves. A build with a better ground connection might not have the same problem, though it's probably wise to solve it on the breadboard anyway.

I would try removing Q3, replacing it with a series diode and shunt cap(?) to create an R-C filter. Using a diode in place of the resistor will keep the compression threshold the same as using two transistors. Using a single NPN instead of a Darlington pair may reduce the gain of the LED driver and maybe not turn it on/off so hard.

The quick version of this experiment would be to just lift Q3 collector and let the base-emitter junction be your test diode.

Hopefully you don't need a filter, but a shunt cap to ground between the diode and Q4 base will help slow down transitions if you need to. I have no idea what value to use.

I also find it interesting that the Negative Feedback is going from Q2 Emitter to Q1 Collector.
I guess since it's two inverting stages, a connection back to the Q1 Base would have been Positive Feedback.

Q2 emitter feeds back to Q1 base, just like a Fuzz Face.

Q1 emitter feeds forward to Q2 collector. Basically when LDR is low-resistance, Q1E dominates the output, otherwise Q2C dominates the output.

This whole thing is basically an auto-panner between an input buffer and a cleaned-up fuzz face, controlled by the LED/LDR, which is pretty cool.

[thehallofshields]

> ...light would be pulsing at the frequency of the guitar signal

So doesn't that mean the gain pulsing is at the frequency of the guitar, along with the guitar's amplitude envelope?

> ....interesting that the Negative Feedback is going from Q2 Emitter to Q1 Collector.

I don't see that?

I mean the path of the LDR.

Hiss--- think what it is supposed to do. Turn-down loud parts and turn-up soft parts, so they all come out the same. Compressors almost always raise the gain (and hiss) in the quiet parts.

I've had compressors and understand the noise-floor raising. What I'm hearing in this circuit is 1.) A low level, very compressed Square Wave (just guessing) Fuzz behind the guitar signal. 2.) The Fuzz / Noise arpeggiates downwards as the note release. Yes, strangely, it starts at a frequency related to the guitar pitch, then sort of jumps down below the audio spectrum in 5-6 quick steps. I'll get a clip of it if anyone is interested.

This is all on breadboard so it isn't a deal breaker. I just thought that with how experienced some of the guys on here are, they might have experienced that kind of noise before.

[highwater]

> ...light would be pulsing at the frequency of the guitar signal

So doesn't that mean the gain pulsing is at the frequency of the guitar, along with the guitar's amplitude envelope?

The LED is fast-enough to blink at the frequency of the guitar, but the LDR is too slow to keep-up.

Some LDRs are faster/slower than others -- the original pedals could have used selected LDRs (unlikely), could have gotten lucky that the random LDRs were the just-right speed (possible), or some were better than others (most likely).

> ....interesting that the Negative Feedback is going from Q2 Emitter to Q1 Collector.

I don't see that?

I mean the path of the LDR.

Looking at it one way, there is negative feedback from Q2's collector to Q1's emitter, and the LDR controls how-much negative feedback there is.

Looking at it another way, Q1's emitter is part of what drives the output, and the LDR controls how much of that drive gets through.

Which way of looking at it is correct? Both, to some degree.

[thehallofshields]

The LED is fast-enough to blink at the frequency of the guitar, but the LDR is too slow to keep-up.

Perfect answer. Thanks.

Looking at it another way, Q1's emitter is part of what drives the output, and the LDR controls how much of that drive gets through.

Which way of looking at it is correct? Both, to some degree.

I guess that's what I was trying to get at but ask very well. I should have asked:
How much of the effect is negative-feedback controlling gain on Q1 and how much is it dumping output of Q2 to ground?

I know that's probably a unanswerable question.

[thehallofshields]

Some LDRs are faster/slower than others -- the original pedals could have used selected LDRs (unlikely), could have gotten lucky that the random LDRs were the just-right speed (possible), or some were better than others (most likely).

I assume that in contrast, an Optocoupler doesn't have this limitation. Would you call it a slew-rate?