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I have been trying to make a simple guitar amplifier and I've been having a lot of trouble trying to get it to work properly. It keeps on motorboating at a very low frequency which is driving me nuts. I'm still kind of a beginner so I really don't know what I've done wrong but I'm sure there's something.

I've tried it with and without the two bypass capacitors (the 1000uF & 100nF), played around with IC1's gain and tried a few different coupling capacitor values (the other 100nF & 220nF.)

The 741 Op Amp is supposed to be more of a preamp rather than a buffer. Its gain is very low because of a long, unsuccessful debugging session. I will replace the 50 ohm with something much higher.

IC1: UA741CP

IC2: Phillips TDA1519A

Power Supply: 12V, 3A

To me it sounds like IC2 is the culprit as changing the volume control does not simply change the loudness of the motorboating but changes its behaviour.

To describe the noise the amp is making better: signal is getting through, just not how it's supposed to. I can hear the guitar, though it sounds more like it's oscillating at the same frequency as the guitar signal rather than actually going through and being amplified. It is unusable as the amp only makes noise if the signal is loud enough (I have to hit the guitar really hard and single notes just choke out). Sounds neat though.

I have also tried with the volume control bypassed to no avail.

The other thing is that the speakers are behaving as if DC is going through them.

When I turn on the power, they suddenly push to one side and stay there (they still vibrate and all but not in the middle) which is really confusing because I'm not sure how that could even be happening.

I've also gone through an LM833 kit preamp with the TDA1519A set up in an identical way to this which also didn't work.

schematic

simulate this circuit – Schematic created using CircuitLab

I thought I'd ask here before I consider IC2 a dud and try a replacement.

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    \$\begingroup\$ See Reasons not to use a 741 op-amp?, in particular, the minimum supply voltage. Why do you need a unity-gain buffer anyway? The input impedance of the TDA1519 is already on the order of 60 kohms. \$\endgroup\$
    – Dave Tweed
    Mar 20, 2022 at 11:01
  • \$\begingroup\$ Alright. Do you think that would cause stability issues? I decided to go with the 741 because I thought it might sound 'cool' even though lots of people say not to but maybe I will try a different one if I can't fix the issue. IC1s gain is very low at the moment because I originally thought that too high gain may be the problem but I will probably be increasing it again. \$\endgroup\$
    – Will Paul
    Mar 20, 2022 at 21:59
  • \$\begingroup\$ Note: C4=220nF as shown in the datasheet is a mistake. It should be twice that. That datasheet sucks, by the way... lots of important details are omitted. \$\endgroup\$ Mar 21, 2022 at 19:28

6 Answers 6

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Here's what causes the motorboating: The resistive divider that biases your input amplifier is coupling power supply fluctuations into the amplifier. The output amplifier draws heavy currents from the power supply, creating voltage fluctuations on the power supply, and so you have a positive feedback loop at some low frequency.

The cure is to power that voltage divider with something that will isolate the input. One way is to use an RC filter with a very low corner frequency:

  1. Disconnect the top of the upper 100k.
  2. Connect a 10k resistor to the 12V supply.
  3. Connect a big capacitor, say 100 uF from the other end of that 10k to ground, then connect the top of that upper 100k to the top of that new capacitor. If it still motorboats increase the RC time constant of your new filter.
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  • \$\begingroup\$ Thank you for this. I have tried it and it has helped but not quite fixed it. now I think that the root of the problem has to do with the supply itself as it may not be adequate. It all makes sense though. \$\endgroup\$
    – Will Paul
    Mar 21, 2022 at 0:19
  • \$\begingroup\$ You can do better than using the RC filter approach - you can use a zener based or single chip regulator based circuit to provide a stable bias for the op-amp input. \$\endgroup\$
    – electrogas
    Mar 21, 2022 at 4:56
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The other thing, and usually the only thing, that causes this is grounding. Audio power amplifier chips, from the lowly LM386 to Sanken 50 W blasters, are notoriously picky about power supply decoupling and grounding.

The capacitor on pin 3 is 10x smaller that in the datasheet. Change it.

It must be as close as possible to the IC pins.

Use the exact decoupling values shown in the datasheet. Again, as close as possible to the IC pins.

Separate the power connections to the 741 buffer and the 1519 into two wires so the power amp currents do not modulate power to the buffer.

Keep the buffer and ignore all of the 741 blabber. It is not the problem. Other parts have better signal fidelity, but that was not your question.

Disconnect R1 and R2 from the buffer input, reduce thwir values to 10 K, decouple the pants off of them, and run a 100K resistor from that node to the buffer input.

The image is pretty small, but it looks like you are driving both amplifiers from one signal, and expecting the outputs to be out of phase for a bridge-tied load. This will not work, and there is no way to use this chip that way because neither amp has an inverting input brought out to a pin. You can work around this by using another opamp as a unity-gain inverter, but I recommend against this.

BTL circuits are even more finicky that regular power amps, and some parts just will not behave. The big clue is that the circuit is not in the 1519 datasheet. If there were any way for the part to be used that way, you can bet the designers would have it in the datasheet. There are other TDA parts that have the app circuit, a clear indication from the designers that it will work in that configuration. If possible, change to one of those other parts.

UPDATE: This is interesting . . .

The 1519 datasheet I pulled down yesterday had an app circuit as in the OP's first post, with both inputs shown as non-inverting and no mention of BTL operation. But the one I pulled down today (1519A) is a different part, with one input called inverting, and BTL right there on page one. However, it does not have the app schematic in the OP's post. SO - which part does the OP have, a 1519 or 1519A?

https://pdf1.alldatasheet.com/datasheet-pdf/view/19203/PHILIPS/TDA1519A.html

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  • \$\begingroup\$ Thank you. I'm not sure I understand what you mean about separating the power connections of the two ICs, sadly I only have a single 12v rail available. I'll correct the component values according to the datasheet. Originally the 1519 chip I'm using came from a kit, I have it set up the same way it was. Maybe it was a bad kit? though it definitely produced audio just very badly that's all. \$\endgroup\$
    – Will Paul
    Mar 21, 2022 at 4:24
  • \$\begingroup\$ I have changed the capacitor on pin 3 from 10uF to 100uF which has made a turn on delay (I don't mind) in case that's useful. \$\endgroup\$
    – Will Paul
    Mar 21, 2022 at 6:24
  • \$\begingroup\$ See the update. \$\endgroup\$
    – AnalogKid
    Mar 21, 2022 at 12:38
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Could it be the volume potentiometer? How is it wired? Is one of its pins connected to ground? If so that would that would be something to fix, as the 220nf capacitor is there to remove DCoffset. One fix could be to connect the wiper to the capacitor, one end to pin 3, and the other to pin 1. Now it is referenced to its own internal voltage.

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  • \$\begingroup\$ I have it set up so that it either directs IC1s output straight to ground or to IC2s input and anywhere in between. I think I will change it to how you've suggested though so thank you. \$\endgroup\$
    – Will Paul
    Mar 20, 2022 at 22:04
  • \$\begingroup\$ if the input signal is from a grounded guitar pickup then the volume control needs to be at least 1M log and have its slider output fed through a 33nF coupling capacitor to the biased input of a TL071 opamp. The opamp must be biased with two 2M ohms resistors in series (like the 100k resistors on your 741 opamp) and the top 2M resistor must be fed from the RC filter in the comment by JRE. \$\endgroup\$
    – Audioguru
    Mar 21, 2022 at 0:37
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I would ditch the UA741P and switch to a more modern op-amp. The OPA1671 would work but be careful it is fast and decoupling etc. is important. You could use an old school one such as 1/2 of a LM358. Also in all the reference drawings I have it shows capacitors in each of the output legs. That decouples the speaker from the FV.

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  • \$\begingroup\$ I don't know what an FV is. \$\endgroup\$ Mar 20, 2022 at 15:01
  • \$\begingroup\$ I think I understand what you mean. If you mean to use capacitors along the speaker wires to block DC to the speaker then I will give that a go. \$\endgroup\$
    – Will Paul
    Mar 21, 2022 at 0:12
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    \$\begingroup\$ OOPS my fingers were over a key, it should read DC. \$\endgroup\$
    – Gil
    Mar 21, 2022 at 0:39
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    \$\begingroup\$ @Gil Edit your answer, don’t correct it in the comments. \$\endgroup\$
    – winny
    Mar 21, 2022 at 9:01
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Why do you need the 54 years old lousy 741 opamp? The obsolete TDA1519A BTL amplifier datasheet does not have it. With a high current 12V power supply the amplifier output into your 3 ohms speaker will be 15W RMS or 30W peak and its heating will be another 30W so it draws peak currents of 60W/12V= 5A. Can your power supply produce 12V at 5A without its voltage dropping much?

TDA1519A

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  • \$\begingroup\$ Wow ok I did not expect it to need that much power. If that really is the case then that must be the problem because I am only using a 3A supply. I thought that would be more than enough as my Vox Pathfinder 10 (10W) appears to require only 15W according to the sticker on the back (~15W * 1.5 = 22.5W < 36W, this was really just an assumption). I used the 741 specifically because it was lousy, I thought it might sound interesting. I won't be able to test right away sorry but I will come back to this. Thank you \$\endgroup\$
    – Will Paul
    Mar 21, 2022 at 0:10
  • \$\begingroup\$ You changed the circuit and now it is missing an important 220nF series coupling capacitor between the slider of the volume control and the TDA1519A amplifier so that the amplifier DC inputs are not shorted to ground by the volume control Look at the 220nF input capacitor on the amplifier datasheet. \$\endgroup\$
    – Audioguru
    Mar 21, 2022 at 2:08
  • \$\begingroup\$ You still have the input biasing of the 741 opamp bouncing up and down with the 12V because the RC filter feeding the upper 100k resistor is missing. The old 741 opamp is doing nothing except adding hiss and distortion, remove it. If a guitar pickup is the signal source then the two 100k resistors biasing the opamp are loading it down and killing high frequencies from the pickup which is why I said to use a high input impedance TL071 opamp and two 2M biasing resistors. \$\endgroup\$
    – Audioguru
    Mar 21, 2022 at 2:14
  • \$\begingroup\$ My apologies I have added all missing components now I think. I will have a go without the 741. \$\endgroup\$
    – Will Paul
    Mar 21, 2022 at 6:22
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As shown in the question, the input circuit that feeds IC2:1, IC2:9 is equivalent to:

schematic

simulate this circuit – Schematic created using CircuitLab

V1 is the R1||R2||C6 buffered by IC1.

RA4/RB4 is the potentiometer.

VIN is the effective input voltage. As you can easily see, there'll always be a DC offset, since a current flows from the internal IC2V2 through input impedance and via RB5. The lower the volume setting - the smaller RB4 - the larger the offset, since there's always DC current flowing from PIN1||PIN9 to ground via RB4.

Recall that the amplifier is a DC-coupled amplifier, so the input voltages are subtracted from 6V, amplified by 100x, added to voltage on pin 3, and then output.

So the input circuit cannot look like that: the only thing you can connect to PIN1 and PIN9 is a capacitor. There cannot be a DC path.

Furthermore, R4 is much too large: 1MOhm vs 30kOhm effective input impedance. For most of its range, the output will be inaudible. Furthermore, R4 in series with 30kOhm input impedance acts as a low-pass filter. So you don't want to make it any larger than it needs to be. A value of 10kOhm would be much better: it's 1/3rd of the input impedance, and is well within the capability of LM741's output driver.

Thus, the circuit should be modified as follows:

schematic

simulate this circuit

Changes:

  1. Feed the supply directly to C1||C2, and from there to pins 7/5 of IC2. The positive and common rails to the rest of the circuit must be connected directly to pins 7/5, as shown.
  2. R7+C7 is a low-pass supply filter for IC1.
  3. R4 is down from 1M to 10k.
  4. C4 is moved after R4, and couples R4 to IC2's input. The value shown in the datasheet is wrong, it should be 440nF ideally, since it drives 30kOhm instead of 60kOhm. The nearest standard value of 470nF is fine.

At mid-volume C4 is connected to 5kOhm voltage source on the left, and 30kOhm source on the right. Thus, its time constant is \$470\cdot 10^{-9}\text{F} \times 35\cdot 10^3\Omega = 16.5\text{ms}\$, maximum, or less at other volume settings. That's acceptable.

Now, generally speaking IC1 is somewhat pointless, since the input impedance as seen into IC1 is 50kOhm, vs. 30kOhm into IC2 directly. You could remove R1, R2, R3, R6, C6, IC1, and feed the input directly into top side of R4, for an input impedance of 10kOhm. That's quite acceptable for a line input impedance.

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