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Motorboating: What exactly is it?

Raven Luni

Oct 15, 2011
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Oct 15, 2011
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I've come across this term when reading about amplifiers, never experienced it until recently, but cant find any in depth information about the phenomenon (or at least something that doesnt involve a de-railed conversation about b00bs - seriously...)

What causes this?
What the usual remedy?

What I have ruled out:
- Power supply noise
- Damaged power transistors

What I experience:
Very low frequency oscillations (from a few Hz to less than 1 Hz), square in shape, directly connected to the gain control (and this is where it gets weird / why I'm asking here). When the gain control reaches a certain point (going up), the oscillations start suddenly at their fastest but get slower as it continues to go up.

I dont have a recent diagram for you just yet but its basically a later version of this project. The main differences to note are that all the coupling capacitors are 22uF (low ESR), the 100uF across the drivers has been removed (made no difference)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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Jan 21, 2010
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25,510
Motorboating is a low frequency oscillation.

There are numerous causes. Many are related to capacitors or high impedance power supplies (flat batteries).

You need to look at the supply rails to see if they are fluctuating in time to with the motorboating because one cause is coupling between the amplifier and the power supply -- this can sometimes be improved by having a separately filtered rail for the preamp (it can be as simple as a series resistor and a capacitor forming a low pass filter.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
25,510
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25,510
It is quite likely that the volume control is part of the feedback loop for the oscillations.

Try placing a 100 ohm resistor in series with the power supply to the quad op-amp and a 1000uF capacitor to ground. A series diode in addition to this may also help. (actually this should power everything to the left of "power" on your circuit diagram)

If that improves things, but does not eliminate the problem, you need to consider better regulation for the op-amp. That will necessitate operating the op-amp from a lower supply voltage and since you're using a single ended supply, it might introduce other problems.

One of the issues with a simple RC filter is that it can cause a phase change in the signals on the power supply which might cause the amplifier to oscillate at a new (lower) frequency.

A discrete stage after the final op-amp stage might help as it could be used to buffer the op-amp's output (thus reducing the power requirement of the op-amp and increasing the effect of filtering the supply.) A common collector buffer would be appropriate, especially since it does not invert the signal.

It may also help to take a separate power and ground to your output stage.

In fact I think the signal may be being injected through the ground connection from your volume control. I wonder if you could take it back to the junction of those 68k resistors? It also might help to bypass the 68k resistors with capacitors, say 1uF.
 

Raven Luni

Oct 15, 2011
798
Joined
Oct 15, 2011
Messages
798
Problem solved. I had forgotten to cut one of the tracks on the strip board! The non inverting inputs of the 2 pre amp stages were connected together. It works perfectly now. Weeks of hair pulling for that one tiny thing!
 
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