Hi
I have just completed a guitar amp build with built-in digital effects and it's gone fairly well. This has been a combined electronics, woodwork and software project. I'm pretty pleased with it but I am struggling to debug a noise issue and was wondering whether anyone on this forum might be able to suggest any ideas.
Bascially I can hear an annoying warbly tone, more so when I increase the amp gain to get distortion. So I guess the noise is getting in right at the input. I can provide more details of the design but essentially it's as follows:
- enclosed switch mode power supply module adjusted to give 24V DC from 240V mains. 0v clamped to earth on PSU.
- Opamp buffer stage to give high impedance input for guitar 1Mohm.
- Teensyduino 3.6 + teensy audio module.
- approx 10W Class D power amp module (loud enough for home use) running from 24V into 8 ohm.
- Switch mode step down module to give 5V for tensyduino from the 24V
- 3.3V LDO for analogue stuff.
Any help appreciated, thanks.
I have just completed a guitar amp build with built-in digital effects and it's gone fairly well. This has been a combined electronics, woodwork and software project. I'm pretty pleased with it but I am struggling to debug a noise issue and was wondering whether anyone on this forum might be able to suggest any ideas.
Bascially I can hear an annoying warbly tone, more so when I increase the amp gain to get distortion. So I guess the noise is getting in right at the input. I can provide more details of the design but essentially it's as follows:
- enclosed switch mode power supply module adjusted to give 24V DC from 240V mains. 0v clamped to earth on PSU.
- Opamp buffer stage to give high impedance input for guitar 1Mohm.
- Teensyduino 3.6 + teensy audio module.
- approx 10W Class D power amp module (loud enough for home use) running from 24V into 8 ohm.
- Switch mode step down module to give 5V for tensyduino from the 24V
- 3.3V LDO for analogue stuff.
Any help appreciated, thanks.
What happens if you short the input to ground and turn up the volume, is the noise still there?
Hi Mark
Thanks for the suggestion, with the input shorted and gain craked up I still get the warbling tone plus some white noise as well.
Richard.
Thanks for the suggestion, with the input shorted and gain craked up I still get the warbling tone plus some white noise as well.
Richard.
Sounds like it might be microphonic or possibly oscillating given that there is sort of a whistling type sound mixed in there, if so tapping on the case or internal parts should produce some audible effect you could hear. If its not that, then its probably just the stuff inside interacting through stray coupling, power supply, and or common ground currents. Could be as simple as needing to move some wiring around, or could be more complicated. You could try nudging wires around while listening to see if any effect.
Other than that, to get more specific help it would probably be a good idea to open a new thread in the guitar/instrument amp part of the forum and post pictures of the inside of the amp and a schematic or wiring diagram, whatever you have or can make, so that members have as much information available to them as possible. Lots of good people here can probably help you get to the bottom of the whatever is causing the sound and help you decide on a fix.
Hi richardlane
You have several switching supplies. The main PS is what I would guess is the warble tone as they change the ON & OFF switching time. You could try adding a bigger filter and see if the warble changes tone? Another thing is to use a 24VDC bench PS and see if the warble tone is still present. I typically run an FFT to find what the switching frequencies are as part of the debug process.
Duke
You have several switching supplies. The main PS is what I would guess is the warble tone as they change the ON & OFF switching time. You could try adding a bigger filter and see if the warble changes tone? Another thing is to use a 24VDC bench PS and see if the warble tone is still present. I typically run an FFT to find what the switching frequencies are as part of the debug process.
Duke
Hi again guys
thanks for your suggestions. I have resolved the issue of the tone now. The teensy audio module I am using has a thread describing an issue whereby a high pass filter in the ADC causing this tone. It's not clear to me why the SGTL5000 device would have an obvious issue but it does. Anyway the thread I found describes how to disable the HPF with a register write. This has removed the tone at about 1.8kHz. What remains is a kind of broadband noise which is kind of ok for a guitar amp.
Regarding comment switch mode supplies. I was concerned about them being a noise source so I chose modules that switch at 500kHz hoping that would resolve most of the issues.
thanks for your suggestions. I have resolved the issue of the tone now. The teensy audio module I am using has a thread describing an issue whereby a high pass filter in the ADC causing this tone. It's not clear to me why the SGTL5000 device would have an obvious issue but it does. Anyway the thread I found describes how to disable the HPF with a register write. This has removed the tone at about 1.8kHz. What remains is a kind of broadband noise which is kind of ok for a guitar amp.
Regarding comment switch mode supplies. I was concerned about them being a noise source so I chose modules that switch at 500kHz hoping that would resolve most of the issues.
Following on from my previous post. Here is the URL which provided the eventual solution.
Open Audio: Teensy Audio Board Self-Noise
Open Audio: Teensy Audio Board Self-Noise