Hello, I am building an amplifier for an electric guitar, and I am encountering an annoying issue when playing the higher frets of all 6 strings of the guitar (Stratocaster) with more force: when playing a string after fret ~19 with greater intensity, the sound distorts, as if it were an overdrive, accompanied by a parallel drop in voltage at the current peak.

I summarize the structure of my circuit, currently implemented on a PCB development board:
- Battery power supply at 3.7V
- Step-Up that brings the supply voltage to ~9.4V, followed by a π LC filter. TLV61048 (with mt3608 I get worse performance)
- Non-inverting preamplifier with TL072, with variable gain (on the development board).
- 10W class D power amplifier
- 4 ohm 5W and 8 ohm 5W speakers. The 8 ohm one provides better audio output.
The gain of the amplifier is involved in the problem. With low gain, there is no distortion, but the guitar volume is too weak to be appreciated. Increasing the gain even slightly brings back the problem.
Adding capacitors to the amplifier to compensate for the voltage drop does not seem to improve the situation.
I have tested with an external 9V 3A power supply and the phenomenon does not occur, or at most it is barely noticeable.
What could be the cause of my problem? A simple power issue? Or a issue with high frequencies? I appreciate any ideas, suggestions, or requests for details.
Thank you.

 

The first and most glaring problem is trying to power this setup from a single Li-Po-Cell.
It will start clipping if You simply look at it the wrong way.

Why are You trying to do this as a first project ?, it's not simple.
.
.
.

 

Check to make sure that it is not caused by the guitar setup, i.e. the strings are buzzing against the frets.
The truss rod and bridge may need adjusting.

 

A 3.7V battery is probably a Lithium rechargeable type. 3.0V is its minimum charge, 3.7V is its storage and selling fairly weak voltage and 4.2V is its fully charged voltage. It must produce about 2.8A of continuous current.
A TPA3116 amplifier IC produces about 8W into 4 ohms with a 9V supply or about 4W into 8 ohms.
That is not much power. You probably need a higher battery voltage and a better speaker.

More than one speaker? Two 4 ohms in parallel overload the battery and the amplifier. Two 8 ohms in series produce only 1W each.

Maybe your weak and small speaker is overdriven?
Maybe your boost inductor has a low maximum current rating?
Have you measured the supply voltage at the amplifier when it is playing hard?

Please attach the amplifier schematic.

 

Thank you all for your opinions and suggestions.
The battery is an NCR18650, it should provide up to 5A of discharge current according to the specifications.

More than one speaker? Two 4 ohms in parallel overload the battery and the amplifier. Two 8 ohms in series produce only 1W each.

I tested the speakers individually, but I should also try with 2 of 8ohm 5W.

In fact, my power amplifier is the little brother of the TPA3116, the TPA3138D2, which with a 9V power supply and an 8 ohm load should reach 5W. The circuit used is the one in the data sheet. Yet it holds up very well with the high volume Android mp3 input that bypasses the preamplifier, and even connecting the mp3 output to the preamplifier it held up well beyond what was necessary.
The current limits for my step-up are 3.7A, and for the inductor 3.9A.

I should try to emulate the battery with the power supply set to 3.7V and misuse consumption and voltage drop, I should be able to exclude that the problem is with the battery. If so, the issue lies in the step-up.

I wonder why this distortion only occurs with the guitar.

The schematic of my preamplifier is as follows, with the values of the two highlighted gain resistors changed.

 

I wonder why this distortion only occurs with the guitar.

The plucked sound likely has high peak levels which are clipped by the amp.

 

Your preamp circuit has a highpass filter that filters away all low frequencies.
When you turn up the gain so that low frequencies are loud enough then the gain is too high for the high frequencies then the amplifier produces high frequencies with severe clipping distortion.

Fix the lack of low frequency gain by increasing the capacitance of C11:

 

Hello, I am building an amplifier for an electric guitar, and I am encountering an annoying issue when playing the higher frets of all 6 strings of the guitar (Stratocaster) with more force: when playing a string after fret ~19 with greater intensity, the sound distorts, as if it were an overdrive, accompanied by a parallel drop in voltage at the current peak.

I summarize the structure of my circuit, currently implemented on a PCB development board:
- Battery power supply at 3.7V
- Step-Up that brings the supply voltage to ~9.4V, followed by a π LC filter. TLV61048 (with mt3608 I get worse performance)
- Non-inverting preamplifier with TL072, with variable gain (on the development board).
- 10W class D power amplifier
- 4 ohm 5W and 8 ohm 5W speakers. The 8 ohm one provides better audio output.
The gain of the amplifier is involved in the problem. With low gain, there is no distortion, but the guitar volume is too weak to be appreciated. Increasing the gain even slightly brings back the problem.
Adding capacitors to the amplifier to compensate for the voltage drop does not seem to improve the situation.
I have tested with an external 9V 3A power supply and the phenomenon does not occur, or at most it is barely noticeable.
What could be the cause of my problem? A simple power issue? Or a issue with high frequencies? I appreciate any ideas, suggestions, or requests for details.
Thank you.

You mention that the problem almost disappears when you use a 9v 3A supply. That means that the problem is caused by the boost converter. What is its voltage output when the problem is there? It is probably going into current limit.

 

The post #5 schematic has some issues.

C11 and R12 form a high-pass filter with a corner freq of 1.6 kHz. Signal frequencies below this do not get the full gain of the circuit. The fundamental frequency of the low E string is 83 Hz. At that freq, the C11-R12 combined impedance is around 20K, not the expected 1K. If the gain pot is in the middle of its rotation, the circuit gain is 13.5x, not 251x.

R18-C13 form another high-pass filter. By themselves, the corner freq is 590 Hz. The actual filter effect is determined by the input impedance of whatever is not shown in the schematic.

ak

 

An amplifier operating at a low voltage is easily driven into distortion, and as the frequency is higher the capacitors couple more signal. So it is a fuzz circuit.

 

Thank you all for your opinions and suggestions.
The battery is an NCR18650, it should provide up to 5A of discharge current according to the specifications.

I tested the speakers individually, but I should also try with 2 of 8ohm 5W.

In fact, my power amplifier is the little brother of the TPA3116, the TPA3138D2, which with a 9V power supply and an 8 ohm load should reach 5W. The circuit used is the one in the data sheet. Yet it holds up very well with the high volume Android mp3 input that bypasses the preamplifier, and even connecting the mp3 output to the preamplifier it held up well beyond what was necessary.
The current limits for my step-up are 3.7A, and for the inductor 3.9A.

I should try to emulate the battery with the power supply set to 3.7V and misuse consumption and voltage drop, I should be able to exclude that the problem is with the battery. If so, the issue lies in the step-up.

I wonder why this distortion only occurs with the guitar.

The schematic of my preamplifier is as follows, with the values of the two highlighted gain resistors changed.

.
If You would be more specific about exactly what You expect as an end-result
You will receive more specific suggestions.

I think that what You are proposing will be very disappointing.

Is this an amplified Acoustic-Guitar ?,
or an Electric-Guitar ?

ANY, even accidental, "Transistor-Amplifier-Clipping", ( without special Filtering ),
is going to sound like pure garbage.

You need lots of "Head-Room", this means Power,
and an adequate Speaker and Cabinet to go with it,
in order to reproduce "Clean-Tones".
The same goes for "Distorted-Tones", they must be created "on-purpose",
not by accident caused by inadequate Power.

It's hard to beat a genuine "Fender-Champ" for a practice-Amp., or even small Gigs.
.
.
.

 

.
If You would be more specific about exactly what You expect as an end-result
You will receive more specific suggestions.

I think that what You are proposing will be very disappointing.

Is this an amplified Acoustic-Guitar ?,
or an Electric-Guitar ?

ANY, even accidental, "Transistor-Amplifier-Clipping", ( without special Filtering ),
is going to sound like pure garbage.

You need lots of "Head-Room", this means Power,
and an adequate Speaker and Cabinet to go with it,
in order to reproduce "Clean-Tones".
The same goes for "Distorted-Tones", they must be created "on-purpose",
not by accident caused by inadequate Power.

It's hard to beat a genuine "Fender-Champ" for a practice-Amp., or even small Gigs.
.
.
.

The TS has a Fender "stratocaster", so do I. It is very much an electric guitar.

 

Thank you all for your advice and opinions. Tomorrow I will have time to test your advice and share the results, and I will also do some additional testing. I also want to be able to see what happens to the signal with an oscilloscope and will share the screenshots, even though it is a hobbyist model.

 

The oscilloscope will show distorted clipping at the output. Clipping causes signals to look like squarewaves.
Here is what it looks like on a 'scope:

 

I have performed several tests and trials, and the problem is undoubtedly related to the power supply rather than a filter issue.

The issue, as mentioned, occurs in the higher frets, around the 18th fret, regardless of which string is played. This is because the signal in those frets comes out much more amplified compared to the open strings, for physical reasons related to the guitar; in fact, the current consumption of a fretted string compared to the 20th fret is ~10 times smaller. At the 21st fret, the 3 pickups of a stratocaster cover about 90% of the vibrating string right above them, while on the open string they only see about 30% of the string, I believe this is the issue of the different intensity.

Some tests and considerations:

1. The signal comes out clean from the TL072, no signal clipping occurs at this stage, in clean mode. Observed from the oscilloscope.
2. 2 8ohm 5W speakers in parallel do not solve the problem, they worsen it due to higher current consumption. Using headphones, there are no clipping issues even at very high gain.
3. Increasing the output voltage of the step-up to 13V from 9V has greatly mitigated the voltage drop, or at least prevented it from dropping below ~7V/8V, below which the amplifier cannot function with a 5W output.
4. An external 13V power supply placed after the step-up minimizes clipping that does not occur during normal use.
5. An external power supply replacing the battery in its voltage range does not cause many changes: at 4.2V (charged battery) no clipping; below 3.2V, clipping starts, probably caused by the strain on the step-up.

This is what happens in the battery during clipping.

Vbat = 3.1V, disregard the incorrectly calibrated vertical scale.
At the entrance of the step-up, I added a 470μF capacitor to the existing 10μF one, which improved the performance. At this point, how can I intervene to prevent voltage drops?

 

What is producing the 383.7Hz signal in your test? The guitar or the voltage step-up converter?

A single 18650 size Lithium-Ion battery cell is DEAD at only 3.1V, it provides a low maximum current which causes its voltage to vary up and down with the current as shown in your sawtooth waveform. The battery cell produces its rated maximum current only when it is fully charged and produces less maximum current at 3.7V. At about 3V to 3.2V it should have its load removed to avoid battery damage. A capacitor parallel with the battery reduces voltage fluctuations caused by the signal. Use at least 2000uF for audio.

The voltage drops are caused by a weak battery that has its current overloaded. Maybe the weak battery is old or is a cheap one that was purchased online.

 

The frequency response of a guitar single coil pickup like the Strat is anything but level. The output increases considerably with frequency and there is also a fairly high frequency resonance. When designing a guitar pre-amp, one should take these characteristics into account compensate the circuit to give a more level frequency response at the output. Here is a site that will explain the details of how to measure the response and how to build a pre-amp for it:

https://guitarnuts2.proboards.com/thread/7723/measuring-electrical-properties-guitar-pickups

I hope this helps.

 

What is producing the 383.7Hz signal in your test? The guitar or the voltage step-up converter?

That's the battery voltage. You are right, I didn't take into account the drop in charge level during its use, but the same issue was occurring when it was at 3.7V. I haven't tried recharging the battery since I increased the output of the step-up, maybe it could completely remove the clipping.

 

I mentioned low supply voltage as a cause for distortion in post #10. It seems that the explanation for why was in another thread with an amplifier running low voltage.
To make it clear, an op-amp can not deliver an output beyond the span of it's supply voltage, usually 20% less than the span of supply voltage. THAT is why we have +/- 15VDC as the op-amp legacy supply for when the normal output range was limited to +/-10 volts. That is why low distortion amplifiers use higher voltages. And why Phantom Power is often 48 volts.

 

I think that nobody has asked the question of what is the overall concept of this project ?
I'm starting to guess that it's something along the lines of .......
"wouldn't it be cool to have a Practice-Amp that You could put in your Pocket ?"

Well, that ain't gonna happen, for a variety of reasons.

boromyr
Please state the overall concept of this project,
in as much detail as possible,
so that less time and effort will be wasted trying to
figure-out why it may not be coming together quite like You imagined.
.
.
.