Why 18 Volts ?

[Phend]

Hello:
Why does this circuit (as well as a few others) require 18 volts.
Is it the led needing to be super bright ? (which can't be seen)
Thanks for any edu.

[marcelomd]

What I think is happening:

Looks like a compressor of some sort. It amplifies a lot, then use the LED+LDR to reduce the volume to some level. High input level == more reduction. When input level tapers off, the volume reduction goes down, so the final level is sustained.

So you want high voltage to provide high headroom to keep the signal clean at all times.

[Mark Hammer]

It may not be a matter of "why 18V?" so much as a matter of "If it could work just as well with +9V, what component values would need to change to make that possible?".  Bear in mind that using two 9V batteries in series was something one would frequently see in pedals from the mid-70s.  The standardization to 9V was a more recent phenomenon...although "recent" is a descriptor from the perspective of a grizzled old guy.

[antonis]

Is it the led needing to be super bright ?

LED brigthness is determined by current (not voltage..)

e.g. A 2 volt forward voltage drop LED should bright the same with 9V & 700R CLR and 18V & 1k6 CLR..

[Ben N]

It may not be a matter of "why 18V?" so much as a matter of "If it could work just as well with +9V, what component values would need to change to make that possible?".  Bear in mind that using two 9V batteries in series was something one would frequently see in pedals from the mid-70s.  The standardization to 9V was a more recent phenomenon...although "recent" is a descriptor from the perspective of a grizzled old guy.

I used to have a bunch of DOD Performer series pedals, with their double batteries and 18v wall warts (which were quite a bit smaller than the usual 9v ones).

[Mark Hammer]

As someone who relied on Radio Shack's "battery of the month" for my supply, back when the Colorsound pedals were new, I would suggest that one of the problems with using two 9V batteries in series was that if one of them dropped a lot of voltage that the second one could not help, there could be issues with the pedal functioning properly.  One really had to buy and install two NEW batteries at the same time.  "Borrowing" a battery from another device to use in tandem with the 9V battery you just bought (or got for free as a "valued Radio Shack customer") would ultimately lead to unanticipated pedal misbehaviour.

Dual 9V batteries was also required of 1st and 2nd-generation BBD-based pedals.  My Pearl PH-44 phaser also requires a pair of 9Vs.

[R.G.]

Maybe not 18V required. See https://fuzzcentral.ssguitar.com/schematics/supasustainschem.gif

Same circuit, I think, and 9V listed.

Beyond that, I'd have to simulate it to see what happens with 9V instead of 18V. The first two transistors are a feedback coupled pair. Open loop you get the gain of two common-emitter stages cascaded, and with DC + AC feedback through the 220K to the first transistor's base. This sets the highest open loop gain that the pair can do.  The overall AC feedback is fed from the collector of the second transistor to the emitter of the first stage,with the emitter acting as an inverting input to what emulates an opamp in some ways. The AC feedback can reduce the gain from what the DC+AC feedback sets it to down to as little as unity. The LDR sets this, and is in turn set by the LED's average current. The LDR averages the current pulses.

There is nothing that leaps out at me about 9v versus 18V. 

[PRR]

The 18V is what makes it "supa", no?

Agree that given >2V you can make a red LED light as bright as it can stand. (This one appears to send 30mA at a 20mA LED, but audio rarely goes >50% for long enough to smoke.)

Any figuring of gain has to account for the teeny value of the cap at Q2E. This has major effects on the same plan used as a hi-fo phono preamp, here it is probably to get a "low"-fidelity top boost for "bite".

But it is 1976, quite early for home-school transistor design. It was sometimes much easier to add a battery than to work out optimizations.

[Phend]

OK, unrelated question:
What happens if you reverse the LED in that circuit ?
I did by mistake and it was a good fuzz effect.
But what else it any thing ?
Use simulator to analyze ?

[R.G.]

What happens if you reverse the LED in that circuit ?

The LED is always reverse biased. It cannot light up, and so the front-end "opamp" always runs at maximum gain.

As an aside, particularly in an 18V circuit, the LED might be damaged by reverse current. Red  LEDs are more tolerant than other colors. LEDs are different today than they were when the Supa Sustain was brought out. They make a lot more light per unit of current, so this circuit will have a lot sharper compression threshold than with less sensitive LEDs. You might be able to adjust this by increasing the value of the nominal 470 ohm in series with the LED. You might be able to increase the sharpness by using a green LED. Modern green LEDs can be very sensitive (i.e. they make a lot of light per unit of current) but they would be damaged if they go in backwards. Green is also the peak color response of the LDRs, so that would zip it up a bit.

I did by mistake and it was a good fuzz effect.
But what else it any thing ?

It was probably a good fuzz because with the LED reversed, it never reduced the gain, so it runs at high gain all the time, and clips.

[Phend]

Fortunately during my brief mistake nothing was damaged. But it sounded good. Having built this effect before, I had / have a comparison. And they both sound exactly the same since they both use the same components right down to the fish. Compressors. Easier to get running than the Orange S.