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 Forum index » DIY Hardware and Software
Diode Filter
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Cynosure
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Joined: Dec 11, 2010
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Location: Toronto, Ontario - Canada
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PostPosted: Tue Oct 18, 2011 1:29 pm    Post subject: Diode Filter
Subject description: Square to Saw?
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I was messing around with diodes while in chat, and came up with something interesting. A schematic was demanded, so here it is.

It does work with just one diode, but the sound seemed to be very unstable (it would cut in and out and be filtered more or less). The extra diodes seemed to stabilize it (on breadboard at least).


diodefiltersamples.gif
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Examples of how the filter changes the sound.
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diodefiltersamples.gif



diodefilter.gif
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Schematic for the filter.
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diodefilter.gif


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Cynosure
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PostPosted: Tue Oct 18, 2011 1:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here is a sound sample. I start with the source sound (16' and 8' square wave with mild lowpass filtering) and then turn up the diode filter. Then I play a little solo so you can hear how it sounds being played.


diodefiltersample.mp3
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kaputtpanzer



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PostPosted: Mon Oct 24, 2011 12:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

awesome!
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Cynosure
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PostPosted: Mon Oct 24, 2011 4:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

kaputtpanzer wrote:
awesome!


Thanks. I was told that it creates an amplified feedback loop and the diodes act as highpass filters.

I also added a 0.1uF cap to ground right before the first cap near the sound in. The feedback loop was boosting the system noise when no note was on, and the cap stops that from happening.
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Cynosure
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PostPosted: Mon Oct 24, 2011 6:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

The cap at the sound in was changing my source, so I moved it to pin 2 on the opamp. It moved the spike to be at the front of the squarewaves instead of the middle. It makes some really cool sounds when combined with lowpass filtering. I'll record a demo and post later.
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Mongo1



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PostPosted: Tue Oct 25, 2011 10:13 am    Post subject: Reply with quote  Mark this post and the followings unread

The only thing adding extra diodes SHOULD do is allow more current through that path. It may be that if there is a parasitic capacitance involved, that adding the extra diodes makes it bigger, allowing some sort of high-frequency bypass to occur.

It would be interesting to change it to one diode and a small capacitance value in parallel and see what that does...

Alternately you could try a large value resistor in parallel.

Gary
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JovianPyx



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PostPosted: Thu Oct 27, 2011 6:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Under small signal conditions, when diodes are in parallel, only one will actually conduct - meaning that the other diodes are really doing nothing at all.

You can test this yourself with several LEDs. Put them in parallel (all facing the same way) and power with a 9v battery and a resistor (like 270 ohms). Only one will light. Now remove the one that lit up and a different one will light, but only one.

This happens because each diode is ever so slightly different. When a diode turns on, it presents a voltage across it's terminals (about 0.6 volts, the "diode voltage"). Let's say diode A is 0.600 volts and diode B is 0.610 volts. If they are in parallel, diode A will prevent diode B from conducting because it won't allow the voltage above 0.600 volts.

Diode parasitic capacitance is extremely small and not sufficient to cause any perceptible effect in an audio filter.

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Mongo1



Joined: Aug 11, 2011
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PostPosted: Thu Oct 27, 2011 7:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:

Under small signal conditions, when diodes are in parallel, only one will actually conduct - meaning that the other diodes are really doing nothing at all.

You can test this yourself with several LEDs. Put them in parallel (all facing the same way) and power with a 9v battery and a resistor (like 270 ohms). Only one will light. Now remove the one that lit up and a different one will light, but only one.


Thanks for the info. It makes me wonder what's going on then.
The circuit is pretty weird to begin with, so maybe the results he was seeing were just coincidence, or chaos in action????

Or perhaps he's on to something. Maybe, just maybe, he's discovered a phenomenon that no one has seen before. Perhaps with more research and a great deal of effort, this phenomenon can be nurtured and grown like an acorn into a mighty oak tree. Perhaps it will one day be the basis of a whole new technology...A technology that can lift man from his earthly worries into a new future... A bright future.... A future of hope and peace.... Where people will care about each other.... Where no child need go hungry! A future where no one will fear the darkness ever again!!!

Nahhhhh!
Gary
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Cynosure
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PostPosted: Thu Oct 27, 2011 10:00 am    Post subject: Reply with quote  Mark this post and the followings unread

lol

maybe 1 diode was getting overloaded, and having 2 gives a way for it to pass voltage once the threshold is met so that the first one doesn't overload.

I actually didn't notice any difference from 2-6. I had 6 holes in the breadboard so i filled them...

But just 1 would often be very loud at the start, and it would randomly cut in and out with some noise.
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JovianPyx



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PostPosted: Thu Oct 27, 2011 10:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Cynosure wrote:
lol

maybe 1 diode was getting overloaded, and having 2 gives a way for it to pass voltage once the threshold is met so that the first one doesn't overload.


Not sure what an "overloaded" diode is.

Cynosure wrote:

I actually didn't notice any difference from 2-6. I had 6 holes in the breadboard so i filled them...

But just 1 would often be very loud at the start, and it would randomly cut in and out with some noise.


This is not something that a diode does. I would suggest that "cutting in and out" is an intermittent connection or another problem. Same with the noise, there is something more going on than exactly what is drawn in the schematic, some bad connection, some resistive solder connection, something intermittent, something that is hooked up in a different way than what you show.

Here is a question to answer - can you make another one, just like it, that performs exactly the same way? If not, then there is something that deviates from your drawing.

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Cynosure
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PostPosted: Thu Oct 27, 2011 4:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes - my breadboard is old and often doesn't make good connections, so it is possible that was the cause of the wierdness with one diode, and having a second one gave it another path to go through when the first wasn't connecting.

If you want to use this circuit, then try it with one diode first.

I have it soldered up now and it behaves similar, but the mixing pot has lower resistance and I moved it closer in the circuit to the sound source, so a little bit of the source sound always comes through unless I add some lowpass filtering to it. Also, as I mentioned in a previous post, I added a cap to ground at pin 2 of the opamp. It lowered some of the system noise, but also moved the spike to the front of the wave instead of the middle (I have no idea why that would happen).

I will try and record a vid tonight - or this weekend at the latest. Going to try one of those screen capture apps for my scope and record from the sound in on my comp.
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Cynosure
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PostPosted: Thu Oct 27, 2011 9:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here is a video that shows the filter in action.



Sorry about the crappy framerate - new app I tried.

You can see the wave shaping capabilities that you can get from this highpass filter and a simple R/C filter.
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prgdeltablues



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PostPosted: Fri Oct 28, 2011 2:14 am    Post subject: Reply with quote  Mark this post and the followings unread

The screenshots of the signals don't give voltages, but if i'm reading the schematic right, with an amplification of about 20 on the opamp, it's likely the output from it is a rail-to-rail square wave. That's then being high-pass filtered by the 1u cap/R combination, and then only the positive-going part of the wave will get through the diodes.
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