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After following along with a hobbyist's video, I breadboarded a phaser. Unfortunately it seems my circuit is not achieving anything close to what the hobbyist achieved with his.

I believe the issue is related to my method of achieving an audio in signal as well potential problems with my audio out. For my audio out, I have a cheap, unbranded speaker that is only powered via the input signal (3.5 mm audio jack.) For my audio input, I was attempting to use a 3.5 mm audio jack from my cellphone that branches into three wires (brown and red for left and right stereo channels, black is ground.)

The video I was following comes from Barbarach. He has a blogpost corresponding to the video.

From what I understand, a 3.5 mm audio jack really only puts out a couple of milliwatts. When I analyze the signal directly with an oscilliscope, I get a signal that never seems to go past a peak voltage of 100mV. Analyzing the signal after each stage reveals more or less the same signal, with significantly more noise and slightly lower peak voltages.

Could I possibly rectify this issue by raising the gain of the first stage of this circuit? Does anyone notice any glaring issues with this circuit, or my attempt at it on the breadboard? When I hook up the phone's output directly to the speaker, it has no issue playing the sound (despite not being all that loud.) I understand the impedance of the circuit could very well be lowering the power of the signal to a value that makes it impossible to hear. Perhaps my only issue is that I need match my JFETs, since I know low quality control means even the same JFET can have very different values.

Phaser Schematic

Breadboarded Phaser

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  • \$\begingroup\$ Further to JRE's answer below, what is the resistance of your speaker? How does that compare with R13? \$\endgroup\$
    – Transistor
    Dec 7, 2023 at 9:12
  • \$\begingroup\$ If you'd like, I can give you a direct measurement for resistance once I have access to a DMM on Monday. May I ask why the resistance of the speaker as well as the resistance of R13 are correlated? Thanks for this additional information to consider. \$\endgroup\$ Dec 8, 2023 at 22:14
  • \$\begingroup\$ I think you will find that while the phaser's output transistor is supposed to drive a 47k resistor (which will draw maybe 0.1 mÀ) your speaker's resistance will be in the order of 16 ohms or less and would require 2500 times as much and the poor little transistor couldn't provide that. \$\endgroup\$
    – Transistor
    Dec 8, 2023 at 23:59

1 Answer 1

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You can't expect to drive a speaker directly from this circuit. It is intended to be connected to a high input impedance amplifier.

This part of the output circuit will severely limit the signal to a speaker:

enter image description here

C4 together with your low impedance speaker will form a high pass filter with a cutoff frequency far above audio frequencies.

Along with that, the output transistor circuit is not set up for low impedance. Even if C4 weren't filtering away the output, the output of the transistor circuit would fall flat under the load.

You must use an amplifier with this circuit. It is an effects circuit, not a power amplifier. It is made to be used between the guitar and your regular guitar amplifier.

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  • \$\begingroup\$ I figured that would be the issue, hence why I sort of intimated as such. I'm glad to have confirmation from someone much more experienced and knowledgeable than me. Next step, off to buy an amplifier! Thank you sincerely for answering my question, and also a thank you to whoever edited my question for readability. \$\endgroup\$ Dec 8, 2023 at 22:09

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