I want to play around with guitar effects circuits on a breadboard, but I don't have a guitar. How can I test my circuits without a guitar?
Electrical – How to test guitar effects box without a guitar
guitar-pedal
Related Solutions
First a schematic diagram rather than pictorial.
This is a very basic single amplifier stage with a voltage gain of approx R3/R4.
Its more of a bass cut than a treble boost (you're cutting out the bass notes) so you lose the bass below the cut off frequency allowing more of the treble to come through.
C3 and R1//R2 form a high pass filter (CR). With C3 = 100nF the sound is OK.
When you switch in the 10nF your are raising the lower cut off by a factor of 10 and the 2n2 by a factor of 45 (ish).
Let's assume that the 100nF gives a lower cut off of 500Hz. Then 10nF gives a cut off of 5kHz and the 2n2 gives about 22.5kHz (i.e. above the threshold of human hearing). Little wonder you can hardly hear anything.
A further complication is the value of C2. If this is too low it also acts as a high pass filter.
Some possible solutions:
(1) Make sure C2 large enough to give a full frequency range output and not acting as a second high pass filter.
(2) Choose values for C3 in a much closer range (2:1). Try a slightly larger value than 100nF as well (say 220nF (more bass), 100nF (clean) and 47nF (trebely))
First thing: if directly feeding headphones make sure you don't induce acoustic shock by a signal overload - you don't want to damage someone's hearing so make sure the output voltage is suitably limited in level to prevent this.
For the stereo output of the iPod you need to mix left and right via 1 k resistors forming a potential divider with the centre tap a mono signal. Take that mono signal and feed it via a 10k pot into pin 2 of the 386 chip. Use exactly the same technique as you have done with the volume pot in your circuit; one end grounded and the wiper connected to the original wiper on pin 2. Use a 47 nF capacitor like in the original design and feed that from your 1k potential divider that converts stereo to mono.
That should work after a fashion and be reasonable.
I also need the stereo input from the iPod to go into the headphones, but without any drop in volume for either source
If you really want this level of functionality then use an op-amp mixer circuit like this: -
Best Answer
As Andy said, the easiest way is to use a line out from your PC. However, there is an important caveat here:
The output impedance of the line out is drastically different from a guitar pickup output. Guitar pickups (and the associated tone/volume circuit) have a very high output impedance, often at least 10kΩ. With the volume and tone controls at midrange, the output impedance may be much higher, in the realm of 100kΩ. This is why most guitar effects are designed with a very high impedance input stage, typ. 250k-1MΩ.
The consequence of this is that you might have a situation where you test a circuit with a PC line out and it works fine, but when using a guitar, it's almost inaudible or has massively reduced high end. This is because your circuit may present much to low of a load for the guitar to adequately drive, while the line out has no trouble.
So, as long as you make sure that you have a sufficiently high input impedance in your guitar effect circuits, a PC line output will be reasonably close. You will want to attenuate it; a guitar output might swing +/-1V or +/-2V with hot pickups, but the line out might be able to drive much more than that, so you should keep the volume pretty low on the PC for a fair representation. What's best is if you can record yourself playing a guitar directly into an audio interface without any post-processing, and play that back (yes, I know you don't have a guitar so this might not be an option), because most stuff you download will have at least some post-processing that changes the dynamics.