Death By Audio - Optical Compressor

1215218 · Post by **1215218** » 26 Apr 2020, 17:13

Hello all, this is my first post on this forum. Glad to join the community after lurking for a while.

So, onto the pedal. First, some context:
In the video below, at timestamp 1:21 we see Oliver flicking through a book of various DBA schematics. Only three can be properly seen as it happens very quickly. These appear to be the Armageddon V2.0, an unknown 3-transistor fuzz, and an unknown compressor. I am currently working on drawing up the schematics of the first two, but the compressor should be the simplest to test out.

(timestamp 1:21)

Of course, this is speculation, but the controls seen in the schematic there do match up with a pedal on the DBA custom page, which I believe could be the same pedal. It's about a third of the way down this page, and is called the Optical Compressor.

http://vintage.deathbyaudio.com/custom.html

I have attempted to draw up the schematic from a screenshot from the video, and it seems that this is a simple inverting amplifier with variable gain, pushing an LED+LDR combo which controls the overall output volume, which is also slightly variable. It is an unusual design, but has an even lower parts count than Mictester's Really Cheap Compressor.
(I have attached the pictures as part of a .pdf. Let me know if the attachement doesn't work)

Many of the parts values are my best guess, based on the low resolution and lack of focus in the pictures.

Due to the coronavirus, I am unable to use any of my breadboarding equipment or soldering iron for the forseeable future - therefore I must ask the community if anyone could help me verify whether this schematic works or not. I suspect it should be easier to get going than other reverse-beta DBA fuzzes.

dba_comp_info.pdf: (187.26 KiB) Downloaded 336 times

Thanks

bmxguitarsbmx · Post by **bmxguitarsbmx** » 29 Apr 2020, 00:09

That circuit looks workable. Like a very simple "Feed Forward" Compressor.

1) 741 loop gain uses B1M+ 100k against the input impedance of your guitar to determine actual gain. This could be unruly/unpredictable. Or just plain awesome!

2) I think the OPTO LED/resistor should go to the same potential that the (+) input of the Op-amp sits at. Or you could use an electrolytic capacitor to Decouple the LED from DC point of the OP-amp's output. For example: If, at rest, the (+) input is at 4.5V, then the OUTPUT pin of the 741 is at 4.5V, then you want the 12k resistor connected to 4.5V. What you don't want is the 12k sitting at ground, always drawing current at idle. (or maybe you do haha. It may not compress though). Using a cap to isolate or simply moving the LED/R string to the other side of the 4u7 may be the easiest solution. (though you still need to figure out the biasing of the 741.

Hope that is helpful. Kinda rambled there

aion · Post by **aion** » 29 Apr 2020, 17:01

Make sure to put it in the most obscenely large enclosure you can find!

1215218 · Post by **1215218** » 01 May 2020, 15:59

@bmxguitarsbmx
Thanks for your detailed reply. I agree with your idea, it certainly makes more sense to decouple the LED from the rest of signal, as it is not currently at the best place be to driven from. So, if there was a potential divider giving 4.5V, then could that be used to reference the + pin of the opamp and the LED anode?
Alos, just curious, if the LED was connected via a cap to the + input, instead of after the output 4u7 cap, then would that make this a feed-back compressor instead of a feed-forward?

I only drew the GND reference on the + pin in the schematic as I figured it would have to go somewhere. Many DBA opamp designs - including this one - just leave that pin disconnected, with no reference point for the opamp. How they still work I don't know.

@aionios
Absolutely, this will be going in a 1590J at least

bmxguitarsbmx · Post by **bmxguitarsbmx** » 02 May 2020, 00:57

Yeah your ideas are good. At least put the (+) input at 4.5v.

I do tend to not try to drive current into bias supplies. Though, there may be some magic to doing so. Given that you may end up changing the current limiting resistor to control the LED brightness, I would tend to Put a large electrolytic before the LED and then connect the LED circuit to ground. That would give you the flexibility to draw more or less current without modulating the bias supply. That will let us derive Bias from a resistor divider with Capacitor style circuit.

jrfox92 · Post by **jrfox92** » 16 May 2020, 23:40

Oh, hey, I built one of those a few years ago. It sounded absolutely awful, but in that DBA kind of way.

1215218 · Post by **1215218** » 20 May 2020, 11:43

@jrfox92 Thank you very much, that's helpful to know. Did you try any mods on it, or was it just not a circuit worth modifying much?
I know what you mean, DBA pedals can have an almost unusable sound that is still quite appealing. Is it awful in a way that is still worth a try building, or is it just better to build something else?

Also, I have some more schematics captured and redrawn from the video - can I post them here, or is it better to start a separate thread for each?

Thanks

1215218 · Post by **1215218** » 07 Nov 2020, 17:15

Hi again,

Quick update. I have now built the circuit on breadboard - and it sounds awful. I'm not sure what the original pedal is meant to sound like, but this is more like a gated/starved fuzz than a compressor. Also, rather unusually for DBA, it's very quiet.

Nevertheless, I have recorded some sound samples.
Signal chain: Jaguar -> TU3 -> DBA Comp. -> Audio interface
(I am aware of some aliasing noise in the recordings, but this is an issue with my laptop, not the pedal)
Clean first, then with the pedal on

: dba_comp_1.mp3; (666.56 KiB) Downloaded 85 times

: dba_comp_2.mp3; (707.81 KiB) Downloaded 76 times

: dba_comp_3.mp3; (686.25 KiB) Downloaded 72 times

Hope these are useful to anyone who wanted to build one of these, or just wanted to see what it would sound like.
I have two more mysterious DBA schematics that have yet to be tested, but I'll post more info on here when I can.

Thanks

Sesh · Post by **Sesh** » 18 Nov 2020, 14:47

1215218 wrote: ↑07 Nov 2020, 17:15 Hi again,

Quick update. I have now built the circuit on breadboard - and it sounds awful. I'm not sure what the original pedal is meant to sound like, but this is more like a gated/starved fuzz than a compressor. Also, rather unusually for DBA, it's very quiet.

Nevertheless, I have recorded some sound samples.
Signal chain: Jaguar -> TU3 -> DBA Comp. -> Audio interface
(I am aware of some aliasing noise in the recordings, but this is an issue with my laptop, not the pedal)
Clean first, then with the pedal on

dba_comp_1.mp3

dba_comp_2.mp3

dba_comp_3.mp3

Hope these are useful to anyone who wanted to build one of these, or just wanted to see what it would sound like.
I have two more mysterious DBA schematics that have yet to be tested, but I'll post more info on here when I can.

Thanks

That sounds awesome!
I've been wondering... there's gotta be a couple of weird/old/rare DBA schems out there. There's a guy who's made a clone of the Crash Modulator and Subsonic Bass Divider on Youtube. Wonder where he got those.

There's a couple of custom/limited ones I'd like to get my filthy hands on (i.e. build) - the A Place To Bury Strangers Pinned pedal and the Exit Index.

Death By Audio - Optical Compressor [traced]