I built a couple of different synth circuits, schematics are here:
http://www.geocities.com/usenet_daughter/tone_generators.htm
They work - they basically play a tone through a speaker. However, can
someone explain (or point me in the right direction) how I can
1. add an output for a guitar amp. Right now they play a sound through
an 8-ohm speaker. If I disconnect the wires from the speaker and
attach them to a guitar cable into an amp, I don't hear anything
(evidently doing this hasn't yet fried my guitar amp or the circuits -
yet)
If you disconnect the speaker there is no load on the circuit.
The 2-transistor tone generator needs that to function.
But you can easily find a resistor value to replace the
speaker (and the 10 ohm resistor). To get started,
connect the input of the guitar amp to the top lead
of the speaker, and the amp connector shield/ground
to the bottom of the 10 ohm resistor. I'm assuming here
that the guitar amp is AC-coupled, which is pretty
standard. (If not, you will need a series capacitor.)
You should get plenty of sound from the amp.
Now (turn down the amp first to avoid a "pop")
replace the speaker with a resistor. A very low value
(like 10 ohms) will probably work, but you might want
to see what happens if you go higher... you should
expect that this will change the operation of the
circuit, so you might need to tweak other values.
2. alter the waveform to something a little more pleasant sounding
(such as a sine or square wave)
In the Olde Tyme days, synth circuits almost always started out
with a sawtooth (ramp, not triangle) that sounded pretty
harsh, then used filters to mellow it out. They used voltage
controlled filters (VCFs) so they could change the timbre of the note
dynamically. That may be a bit much to start with, but you can
easily make simple low-pass filters to alter the tone.
After you get the circuit modified to drive the guitar amp,
you can add a series resistor between the tone generator
and the amp, and a parallel capacitor at the amp input
to ground. The "cutoff" frequency of the filter is given
by f = 1 / (2 * pi * R * C), so pick some appropriate
R and C values. Say, if you want the filter to start
cutting frequencies above 500 Hz, and you want to
use 0.1 uF for C, then rearrange the above to get
R = 1 / (2 * pi * f * C) and plug in 0.1*10^-6 for C
and 500 for f. R comes out to be 3183 ohms.
Pick the closest value you have on hand... this
is just to get you in the ballpark. You might want
to use a 10K control pot for R, to make playing
around easier.
As it turns out, the real key to synthesizing different
sounds is surprisingly not the waveform or harmonic
content, so much as the envelope - the way the sound
builds up (attack) and dies away (decay) as well as any
sustain portion. (If you've ever seen an old-fashioned
organ with voice tabs, you may have wondered why
none of them sounded much like their names. That's
because the envelope was always the same, not because
the voices didn't have the proper harmonics.)
So, if you want to proceed beyond tone generation,
you have to look up envelope generators.
Best regards,
Bob Masta
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!