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Re: Pocket Radio Amplification Factor

So I fed a few sine waves of various individual frequencies in to see

>what came out the speaker end. To my surprise, there is a series of >jagged pulses contained within rather sketchy sinusoidal envelopes >matching the input frequency. > >I then tried a music track and saw much the same effect upon the audio >signal. I have never seen this before. > >Can anyone explain why this occurs? A way to reduce output power and >still have some kind of recognisable sound? Or is it just a cheap audio amp?

That sounds like a cheap-and-awful Class D (or similar) audio amplifier at work. The speaker is driven by a high-frequency pulse-width- or pulse-density-modulation signal, which (when low-pass-filtered) reproduces the desired sinusoid.

Class D amps can produce very good fidelity, if well implemented (high switching frequency, proper low-pass filtering _before_ the signal is fed to the output, and the use of a good modulator to create the right pulse patterns). They can also be rather awful (e.g the PWM or PDM signal isn't low-pass-filtered in the amplifier, just fed to a speaker whose mass and voice-coil inductance act as somewhat of a low-pass filter).

The advantage of these amplifers is electrical efficiency. The output stage operates in a "bang-bang" mode - the output devices are either off, or driven hard into saturation, rather than being driven linearly. As a result there's very little power lost/dissipated in the output devices; the amp runs cooler than with a Class AB amplifer circuit and the battery lasts longer (a big commercial advantage in small battery-powered devices).

Reply to
Dave Platt
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If the speaker is driven differentially from a class D integrated power amp designed to run without an output LC filter then the two output signals will probably be approximately in-phase with no signal is applied, the differential voltage across the speaker will just be very narrow pulses. You lose a lot of efficiency at no-signal 50% duty cycle by running a regular PWM class D without the LC filter, the speaker has a large resistive component.

Reply to
bitrex

Assuming a class-D amplifier, one would expect a pulse train of uniform voltage,

But the output of this radio, as viewed across the speaker, has variable width pulses plus an amplitude variation that corresponds roughly to the input waveform.

So I assume some filtering is taking place, but not so much as to remove the switching frequency. I guess that is the effect of the speaker as some have described.

Just for fun, I will replace it with a resistor and see how the output looks then.

Robert Martin

Reply to
Robert Martin

There's something wrong there, then. It can't have been designed that way! Perhaps your filtering has broken down. :-|

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Reply to
Cursitor Doom

The radio is brand new.

Could be some kind of a Chinese innovation. Maybe having amplitude modulation as well as PWM improves the sound at the speaker.Don't know how they do it.

This morning I tried replacing the 4 ohm speaker with a 12 ohm resistor and the output looks just the same.

Robert Martin

Reply to
Robert Martin

Can you get a scope probe in there to check the filter section before and after to see if it's actually doing what it should?

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Reply to
Cursitor Doom

It is multi-layer surface mount. Not my forte. Since the output is not audibly distorted, I have to assume it is working as designed.

Probably built with American IP.

Robert Martin

Reply to
Robert Martin

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