First attempt at circuit design. Could I get some feedback on it?

[ejendres]

This is based on a simple 1 transistor distortion. I added a switchable passive bandpass filter. The idea is to be able to have a sweet lofi/filtered sound in a pedal. I made it switchable just so I also have another distortion pedal kicking around.



This is my first time trying my hand at anything like this so be kind 

[kurtlives]

Won't work

Might want to read up on common emitter amplifiers.

[ejendres]

Could you be more specific? It could be I just copied some stuff down wrong, its my first time using CircuitMaker.

The actual distortion part of the circuit it based on this from GGG:
http://www.generalguitargadgets.com/richardo/distortion/onetrans.pdf

[CynicalMan]

You seem to have made some mistakes in copying the transistor stage; the GGG circuit has a 47k resistor going from the 9V line to the transistor's collector, and the resistor going from the transistor's emitter to ground is only 680 ohms. As your design is set up now, you have C4 connected to the 9V power, so no audio will pass.

Also, your input filter has a passband of around 20kHz to 70kHz, meaning that it rolls off any frequencies outside of that. This is outside of the audible range of around 20Hz to 20kHz, so you'll want to increase the resistors' values and/or decrease the capacitors' values  to get the frequency response you want. I find this to be a useful tool for quick and dirty calculations of filters like that: http://www.muzique.com/schem/filter.htm

[Keppy]

A couple other notes:

1) The filter as designed will work at different frequencies depending on the impedance of the guitar pickups (or the effect in front of this one), so you may want to add a buffer ahead of it so the filter will work the same all the time.

2) Adding a pulldown resistor to ground ahead of the input cap will help prevent popping when you switch the effect in or out (assuming true bypass with a mechanical switch). Try a 1M.

[R.G.]

The circuit as shown has a collector resistor of zero ohms to the 9V power supply. So no matter what current the transistor pulls, C4 can only pass whatever signal - which is ideally zero - is on the power supply to the output.

The emitter resistor is much larger than it should be. As it sits, the 470K resistor lets current flow into the base of the transistor until the base voltage rises from current flowing through the collector-emitter path and stops more base current coming in. In this case, the base will be up near 9V, the emitter will be a diode-drop lower.

No signal will come through at all.

Before a transistor will allow any signal through at all, it must be set up so the proper voltages and currents flow through it.  If this circuit had a resistor between the collector and +9V, and both the 470K and C4 were still connected to the collector, not +9V, and the emitter resistor was much smaller, like perhaps 680 ohms, not 680K, it would probably amplify some.

[ejendres]

A couple other notes:

1) The filter as designed will work at different frequencies depending on the impedance of the guitar pickups (or the effect in front of this one), so you may want to add a buffer ahead of it so the filter will work the same all the time.

2) Adding a pulldown resistor to ground ahead of the input cap will help prevent popping when you switch the effect in or out (assuming true bypass with a mechanical switch). Try a 1M.

Do you mean making it an active filter? I was debating that but I didn't want to complicate the circuit, I'm trying for something really basic. Would it be that much better to have an active filter?

I had a pull down resister on paper, forgot to put it in the circuit. I'm gonna revise it real quick.

The circuit as shown has a collector resistor of zero ohms to the 9V power supply. So no matter what current the transistor pulls, C4 can only pass whatever signal - which is ideally zero - is on the power supply to the output.

The emitter resistor is much larger than it should be. As it sits, the 470K resistor lets current flow into the base of the transistor until the base voltage rises from current flowing through the collector-emitter path and stops more base current coming in. In this case, the base will be up near 9V, the emitter will be a diode-drop lower.

No signal will come through at all.

Before a transistor will allow any signal through at all, it must be set up so the proper voltages and currents flow through it.  If this circuit had a resistor between the collector and +9V, and both the 470K and C4 were still connected to the collector, not +9V, and the emitter resistor was much smaller, like perhaps 680 ohms, not 680K, it would probably amplify some.

I had forgotten a component. Revised:

[PRR]

> No signal will come through at all.

Another way to say it: C4 passes signal to the output. C4 is directly connected to the battery plus. The other side of the output circuit connects to battery minus. You are "listening to the battery".

Which is usually quite boring. (A small crackle if the battery is weak.) (Some hum or whine with wall-wart power.)

EDIT

> I had forgotten a component.

OK, now you can listen to R6. R6 has similar signal to R4 (emitter), but different by the ratio R6/R4.

R6/R4 is 0.07. Or: output is 14 times weaker than input. Probably not what you want.

Re-steal the amplifier stage more carefully.

[ejendres]

oi. stupid typos.

[armdnrdy]

Re-steal the amplifier stage more carefully.

Got a chuckle out of me! and I don't chuckle!

[ejendres]

heres a version with an active filter section. Thoughts on this? V2 should be 9 volts not 10.

[ejendres]

So I spent the day tweaking the circuit in CircuitLab. Heres what I have:







What do y'all think? Theres a significant drop in output, but that's to be expected when you're cutting out a huge range of frequencies.

[timd]

Re-steal the amplifier stage more carefully.

Got a chuckle out of me! and I don't chuckle!

I was going to quote the same line! Classic.

[R.G.]

What do y'all think? Theres a significant drop in output, but that's to be expected when you're cutting out a huge range of frequencies.

I think it's probably misbiased and only conducting signal on the very topmost pips of the input signal.

Have your sim program tell you the voltages on the three pins of both transistors. You may want to change this later, but you'd like to start with the collectors near half the power supply voltage. From there you can tweak it. Right now, you're not getting much.

You might like the way this sounds, as sometimes distortion preferences are unusual, but those things get discovered by lucky accidents, not a part of the design process.

For an NPN to amplify at all, the collector needs to be the most positive pin at idle, the emitter is the lowest voltage, and the base must be one diode drop (about 0.4 to 0.7V for silicon) higher than the emitter. Unless these are true, it's not amplifying.

As another part of the design process, you need to realize that guitar signals aren't sine waves, and are not fixed size. A guitar pickup puts out something like 100mV to 1V peaks depending on whether it's a single coil or a humbucker, and which example at that. And it declines from that peak to  nearly zero as the note decays.

A misbiased stage will usually produce no sound at all until a big signal pushes it into conduction. This is usually described as "gated" or "farty".

[Keppy]

The filter as designed will work at different frequencies depending on the impedance of the guitar pickups (or the effect in front of this one), so you may want to add a buffer ahead of it so the filter will work the same all the time.

Do you mean making it an active filter? I was debating that but I didn't want to complicate the circuit, I'm trying for something really basic. Would it be that much better to have an active filter?

I wasn't suggesting an active filter, just a buffer in front of the passive one. Probably an emitter follower since you're using discrete transistors rather than opamps. The buffer section from the Tube Screamer would work fine. An NPN, cap & 3 resistors would do the trick.

The active stage you posted appears to have relatively low input impedance, which does not fix the issue. The important thing is to present a consistent impedance to the filter stage, which has an input impedance of ~10k. Otherwise, a naked guitar signal (high output impedance) would sound one way through it, while a buffered signal (low output impedance, as from most effects or active pickups) would sound different. Possibly drastically different, like what happens with a Fuzz Face. An input buffer would allow you to design the response of the filter and then have it STAY that way.