First Post, first build

[hesamadman]

Hey guys. Im new to pedals. Been building tube amps for a while. Im trying to learn the similarities and differences in using IC/transistors. I designed a small simple circuit and having some issues. heres what I got.

I made a very simple overdrive pedal on my breadboard. This is the first one ive done. Once I plugged in guitar. It had an amazing overdrive tone but LOTS OF NOISE. Now I got rid of the noise two ways. One way got rid of all the noise except a very low volume high pitch scream. I put an electrolytic cap from positive to negative on my power supply. Like a filter cap. Like I mentioned....most noise gone....not all. Second way I added various values of resistors between the pedal and the 9 volt power supply. This eliminated the noise all together but also acted as a noise suppressor on steroids. If i plucked the strings hard it played the notes but if I barely hit the strings no noise came out.


My signal goes as follows.


input -> .047 cap -> pin 3 lm386 (in) -> out of pin 5 lm386 -> 220uf cap (ive used many values) -> output


LM386- pin 2 and 4 ground, pin 6 9v+


I do not have any diode clipping after pin 5 of lm386. I put them on and doesnt even seam to be altering sound. Nor do I have a resistor to ground after pin 5. Same scenario. No real change. My goal was to add more to this circuit once I get this cleaned up.

[armdnrdy]

See reply #5

[hesamadman]

See reply #5

Im not seeing anything else in this topic

[knutolai]

Whats the value of the cap between positive and negative on your supply? Depending on the PSU you are using you might need more powerfiltering. Try a 47u electrolytic and 100n ceramic cap in parallel.
Also a schematic with labeled components illustrating your circuit would be nice. That way its easier to help out.  

[hesamadman]

Whats the value of the cap between positive and negative on your supply? Depending on the PSU you are using you might need more powerfiltering. Try a 47u electrolytic and 100n ceramic cap in parallel.
Also a schematic with labeled components illustrating your circuit would be nice. That way its easier to help out.  

Thanks for your response.
Honestly my power supply is a cheap radio shack adjustable adapter. I use it with my rat and tube screamer so I assumed it was ok. The value of the cap used as a filter cap on the supply was a 100uf.

My schematic is here ---> http://rdkelectricalservice.com/pedal%20design.pdf

[antonis]

The 100nF ceramic cap that is mentioned by Knutolai should be connected as close as possible to the Vcc pin..
(and another same cap to the Vee pin - if you use bipolar power supply..)

Place a small resistor (10-100Ω) in series with the power supply and Vcc pin and another (about 1k) in series with the output of the OpAmp (if there isn't such a resistor in your circuit..)

[knutolai]

Also you could probably lower the 220uF output cap to 10u. Is this design based on some other circuit? what are your resources?

[antonis]

My schematic is here ---> http://rdkelectricalservice.com/pedal%20design.pdf

Is this the COMPLETE circuit or some rough description..??

[armdnrdy]

See reply #5

Im not seeing anything else in this topic

As you shouldn't have! I forgot to post the link! Too much coffee this morning! 

I'll try it again. Reply #5
http://www.diystompboxes.com/smfforum/index.php?topic=109041.0

[hesamadman]

Wow thanks for all the replies guys. Im going to try the suggested (and read the link provided) and I will report back

[hesamadman]

The 100nF ceramic cap that is mentioned by Knutolai should be connected as close as possible to the Vcc pin..
(and another same cap to the Vee pin - if you use bipolar power supply..)

Place a small resistor (10-100Ω) in series with the power supply and Vcc pin and another (about 1k) in series with the output of the OpAmp (if there isn't such a resistor in your circuit..)

Ok...so that worked fantastic. This is a nice light overdrive. Kind of gainy but I put a pot on the output and its fantastic. Ok so is there any info you can give me as to how you came with these conclusions? I mean, filtering was my first idea, but my values were wrong. I did your suggested filtering first. And it was great. Then I added the 1k resistor in series with the output then it made it even better!!

[knutolai]

on the topic of the filtering caps. The reason to use a big and a small cap is that the big cap doesn't eliminate higher frequency noise very well. A smaller value cap i parallel fixes this.

[hesamadman]

on the topic of the filtering caps. The reason to use a big and a small cap is that the big cap doesn't eliminate higher frequency noise very well. A smaller value cap i parallel fixes this.

that makes perfect sense

[hesamadman]

Whats the point of the 4.5 vdc thats applied to the input of the IC of some of these similar pedal designs?

[knutolai]

The resistor to 4.5V at the input of semiconductors (transistor, opamp..) has (at least) two purposes. They are part of setting the input impedance of the circuit, and they are biasing the input signal to a voltagerange "comfortable" for the semiconductors, distancing your signal from the clipping-bounds near 0V and 9V. You will also see "pulldown" resistors going to ground on both the input and output of effects.

I suggest you read these short (amazing) articles:
http://www.muzique.com/lab/imp.htm     on impedance
http://www.muzique.com/lab/buffers.htm    on buffers, impedance and biasing
http://www.muzique.com/news/pulldown-resistors/   pulldown resistors
http://www.muzique.com/news/pulldown-resistor-vs-input-impedance/   more impedance stuff

[antonis]

is there any info you can give me as to how you came with these conclusions?

Just proposed general "hints" about practical OpAmp handling..

The links proposed by Knutolai will give you a "in depth" knowledge for some more complicated buildings in the future..

[PRR]

> cheap radio shack adjustable adapter

Probably sags with any significant load.

Many pedals are not much load.

A LM386 is a "POWER" amplifier, can suck real power, may sag that poor wart.

> diode clipping after pin 5 of lm386. I put them on and doesnt even seam to be altering sound

Again... '386 is a POWER amplifier. Its direct output can deliver 500mA (on a good day, downhill). If you used litle 100mA diodes, they may have popped on the first whang, and are now open. (Opamps only deliver 30mA.)

Also-- if you use 1A power-diodes, those half-amp clip surges make your power demand VERY high. Enough to sag even non-cheapo warts. And for what point? You never need more than 0.1mA into a guitar-chain or line input. Flogging thousands of times more current than you need burns parts, sags supplies, is generally poor form.

MOST diode clippers include a series diode, which you have not mentioned. Grabbing a raw output is like grabbing a bucking bronco. You can't control him. Some series resistance is like a leash on that bronco... now you have some hope of control, at least at your end.

Also hanging a diode down from the 4.5V DC (see below) on the output of this chip means BIG current even without signal, and no output.

> Whats the point of the 4.5 vdc thats applied to the input of the IC of some of...

You know tube circuits. A typical preamp stage has a 300 Volt supply and the Plate sits around 100V to 200V, "halfway up the supply". We have to swing a one-way device both ways, this is how we fake it. In tubes "halfway" is a loose goal and there may be reasons to go a bit high or low. In much chip-work we can nail "halfway" pretty close, easily, and with good result. 4.5V is of course half of a 9V battery.

The LM386 does this *internally*. (Look at the gut-shot. There's one 15K back from the output to the groundy input stage, and two 15K series from B+ to the groundy input stage. The amp can only be happy with the output near half the B+ voltage.)

(Yes, transformer-loaded tubes are different. The OT can "kick" above B+, and the idle point is essentially B+.)