Tag Archives: audio

sqrt() reverb…


this was the final build before it shipped out. circuit is a simple MOSFET (BS170) driver stage followed by two JFET (J201) recovery stages. i’ve included a sound sample of a similar reverb i built later without the second recovery stage and clipping diodes. it makes for a much more subtle reverb, but also tames the noise floor from the Belton module.

[audio:http://abrammorphew.com/notes/wp-content/uploads/2013/02/sqrt_reverb_demo_02.mp3|titles=sqrt() reverb demo #01]

this is the actual first build. the “dirty verb” comes in towards the end which just switching on a pair of germanium clipping diodes. you get more reverb for your buck with the second JFET stage, but i haven’t found a way to cool down the noise just yet. that’ll be revision 5… maybe 7. [audio:http://abrammorphew.com/notes/wp-content/uploads/2013/02/reverb-test02.mp3|titles=sqrt reverb demo #02]

Mellotronium revised…

here’s an update on the new additions/approach to the Mellotronium. i’m attempting to redo the SD card routines once i get the functionality added. using the SD library just doesn’t work right when reading byte values at 8kHz. i’ve looked into the WaveHC library with the most success, but had to modify not to use an external DAC. a new wav file playing solution from the SD card will have to be found.

without the clunky SD lib, program space has opened up… a lot of it in fact. now i’ve started using a wavetable. this video just has a single sine wave, but it’s modulate with the LFO and its seven different waveforms not to mention an amplitude modulating ADSR filter.

the breadboard to the side contains an experimental active 2-band EQ (TLO82-based) which needs some work. if anyone has any experience with this, i would love to know why the schematic in the datasheet doesn’t work at all. i wound up having to recall the usage from a different schematic where you make a voltage divider from and peer it into the positive terminal on both opamps. it works… in a way. i think i’ve inverted the wave form or something strange. it also sometimes works better as a radio than an EQ which i think i like. it made for some interesting heterodyning.

Arduino: the 8-bit Mellotronium prototype

i’ve been pretty Arduino obsessed over the past month. i got in my head this idea about building a midi-controlled digital sampler that uses SD cards for storage after thumbing through the Arduino Cookbook and have finally started to make some headway on the project.

there were some major obstacle to overcome, unfortunately. the first came about when i had some trouble loading the larger libraries (e.g. MIDI.h, SD.h). i spent days trying to figure out what the problem was and even went so far as to update the bootloader to use optiboot. it turned out to be the version of GCC that i was using to compile my sketches. the toolchain setup on Gentoo is no easy task, so i went ahead and just compiled it manually. for those of you tempting to use develop AVR software in a Linux environment, i’d recommend the avr-libc install guide as your path to unbridled success. i myself could never get cross-dev to work with out failing and it needs certain USE flags which it just always overrode when i specified them.

from then on, things were pretty standard. i was able to load SD.h and begin reading files from the card. i used simple voltage dividers to convert the ATMega328’s 5v logic to the SD’s 3.3v like the standard schematic shows and then hacked up the PCMAudio Library to work as i’ve wanted. i borrow some of the techniques from Max’s article on generating real-time audio using PCM. much different than my overall goal, but extremely educational. if you’re baffled by the ATMega328’s use of PWM as i was, Ken Sherif’s article on PWM will clear all that up.

the code’s not worth posting at the moment. it’s a commented out mess of gray. i’ll most likely post it (for my own sake) when i’ve got more of the kinks worked out.

LM386 Square-wave Oscillator

Here’s a small LM386-based square-wave oscillator built from the following schematic. I replaced the 30k resistor with a 50k Potentiometer from my stash of parts which then, of course, acts as a pitch controller. The following audio is some track recordings made in Ableton 8.0 with a little filter on one track and some reverb the air-raid siren sounding noise.


External audio with USB 1.1 and Gentoo Linux PPC…

In the event that anyone decides to use a junked iBook in a Linux environment, I’m hoping that this article might save someone a lot of experimentation.

The Problem:
Started having trouble getting packet information through Soundmodem after a system upgrade to Kernel version 2.6.36-gentoo-r8. I began to investigate the sound card using Audacity then arecord/aplay to see if audio was even getting through ALSA into the external USB audio interface (snd-usb-audio). The audio was almost unintelligible through the distortion and crackling. After some playing around, I realized that audio recorded through the internal card (snd-powermac) was fine but USB audio was crap. The iBook conveniently has no TRS inputs. Therefore, I’m required to use USB.

The Solution:
I opted to upgrade the kernel from scratch. I disabled the EHCI (USB 2.0) support in the kernel since the iBook has no USB 2.0 support. Secondly (and this is where I think the problem really stemmed from), I enabled both the Big Endian/Little Endian option for the OHCI (USB 1.1) driver after reading about Endianness.

My /etc/modprobe.d/alsa.conf file looks something like this:

alias /dev/mixer snd-mixer-oss
alias /dev/dsp snd-pcm-oss
alias /dev/midi snd-seq-oss
alias char-major-116 snd
alias char-major-14 soundcore

options snd cards_limit=1

# ALSA portion
alias snd-card-0 snd-usb-audio
alias sound-slot-0 snd-card-0

# card #1
options snd-usb-audio nrpacks=1 index=0
alias sound-service-0-0 snd-mixer-oss
alias sound-service-0-1 snd-seq-oss
alias sound-service-0-3 snd-pcm-oss
alias sound-service-0-8 snd-seq-oss
alias sound-service-0-12 snd-pcm-oss
alias /dev/dsp snd-usb-oss

LightBox prototype

here’s a small battery-powered guitar amp that i’ve constructed from reused parts. the housing is a busted computer power supply (some of which is in the No. 5) as well. i’ve got several options for this little fellow in mind, but, for now, it’s a tiny amp with a mean distortion. lo-fi enthusiasts should contact me if you’d like one custom built.

LightBox Prototype: a battery-powered guitar amp w/ distortion from abram on Vimeo.

the first of four…


This was my first attempt at building the circuit on the PC board. Unfortunately, I think I’m going to have to get a different board since this one isn’t quite big enough with two opamps. Soldering it is also a bitch considering it’s tiny and my iron tip isn’t as small as it should be.

The 1/8″ plug is the output soon to have a pot attached to it for volume control. I’ve been thinking about attaching an light meter to each channel, but I still need to know what the dimensions of the chassis will be before I jump that far into it.

exploring the LM386…


In my attempts to recently mod a Peavey Classic 30, I’ve been researching audio amplification like a demented mad scientist scheming to destroy the world. I’ve understood the conceptual model of amplification for a while, but I’ve never actually plunged into the crafting part of it before now. After changing out some tone caps, I couldn’t help but wonder how something like capacitance could actually affect the way the electrical signals are converted back into mechanical sound even though it’s pretty obvious.

A while ago I came across a schematic that included a simple 8-pin IC as the workhorse of the circuit. It’s called the LM386, and it’s got to have near a thousand uses judging by the amount of information about it. My cohorts and I have been toying with the idea of headphone sessions, a improvisational mixing session where the four of us write and play music while being conscious of the stereo field. This of course requires the purchase of a headphone amp. Why buy one when you can just build one.

I played with the LM386 for about five hours. It was mostly blind experimentation since I was just using parts I could pull off other dead devices. I had also purchased a shitload of resistors earlier in the day which I was also dying to play around with. I could only get it to sound clear enough at lower volumes through a .5 Watt speaker with a tear in the cone. It seems ample enough for headphones. I figure four dual PCBs with essentially 8 tiny amps attached to a pot on the jack should just very well do the trick. I’ll make a better effort to document as well.

lm386 amp made with random caps from abram on Vimeo.