afterlife diodes

[sam_c]

Hi guys,

ive nearly finished stuffing a pair of afterlife comp pcb's and I am struggling to source the 1n270 diodes in the UK at a reasonable cost.

I know the build docs suggest 1n34a or 1n60 as alternatives and also bat41.  Has anyone tried alternative diodes and had good results? Can anyone suggest any other substitutes that are more common? 

and one final dumbass question,what do the diodes do in this circuit? 

[midwayfair]

1n60P is a perfectly reasonable alternative. I've used them multiple times. It's a Schottkey you can pick up for 4c. They're also useful in Orange Squeezers, the Bearhug, and many other applications where you need a Fv of about .25v.

Edit: The diodes rectify your signal. A rectifier is a circuit that converts AC to DC. This is a full-wave rectifier. Here's an extract specifically about the Afterlife from an article I'm working on [this is still rough, sorry]:

Next we have a bunch of diodes. We know there must be a rectifier for a compressor to work, and we know we need some semiconductors, so this must be the place. This is more diodes than we're accustomed to seeing, but breaking it down, we can see that it's actually doing something funky with the current passing through LED. So let's figure that out. How do we normally light an LED? Most of the time in stomp boxes, that's taking voltage from the 9v source, putting some resistance in series, and then connecting the LED to ground. The LED here isn't connected to ground, though. But "ground" is just a place in a circuit with the most negative potential. The only way an LED lights up is if current flows from the anode (+) to the cathode (-), so one side of the rectifier appears to be acting as a sort of faux ground as far as the LED is concerned. We've also got a gigantic capacitor there, a whopping 100uF, with its negative side pointed at ground, and no other capacitors in sight, so that must be our hold element. Its + side is oriented toward the side that connects to the anode, so from that we can see that it's storing the positive voltage supplied to the LED.

A couple bits that need clarifying without context: The "hold element" is a capacitor that stores charge. It's found in any envelope follower just after the rectifying diodes.

The charge stored by the capacitor helps keep the LED "on" at a more or less constant rate (i.e., it helps set the decay). Combined with the "light history" of the vactrol, this makes changes in the compression slower and less likely to sound unnatural.

Finally, you can also think of the diodes here as a bit like one-way street signs directing the DC where to go, so that it can only go one way: Through the LED to create light.

[sam_c]

thanks for the advice! 

[midwayfair]

Added some info about the diodes/cap.

[sam_c]

thanks for taking the time to explain that, much appreciated!