Why won't this drive a speaker?

[rea5245]

Hi,

I'm using an FM radio module (the Mikroe FM Click) that includes LM4864 amplifiers. It drives headphones fine, but not an 8 speaker. I'm a little surprised, since I assumed the LM4864s would be able to do that.

I've attached the module's schematic. Are the 20K resistors the problem (R6 and R8)? What would I need to replace them with?

Thank you.

[bob91343]

How much signal input do you have?  The output amplifiers should put out close to one Watt so they need to be driven hard enough.  I presume the 20k resistors set the gain, which should be around unity, so to get 2V into 8 Ohms (one half Watt) you need 2 V input.

[Benta]

The problem is the output caps. They're fine for 600-ohm headphones, but should be more like 470 uF for 8-ohm speakers. Replace them and it's fine.

[bsfeechannel]

Moreover, the headphone cable ground is the antenna. How are those speakers connected to this module?

[Ian.M]

Its wired wrong for driving speakers.  As-is it could only deliver about 60mW per channel, due to the 3.3V Vcc to the LM4864, and the single-ended load connection, and as Benta pointed out, with a low impedance load at lower audio frequencies there is an additional voltage drop across the 10uF output coupling capacitors, further limiting its output power.

Even if you wired in 8 ohm speakers direct to each LM4864's Vo1 and Vo2 pins, so they operate with a bridge-tied load as intended, following the datasheet's Figure 1. Typical Audio Amplifier Application Circuit, it would only deliver about 250mW, so would be pretty quiet even assuming you use a reasonably large high efficiency speaker.   Raising Vcc to 5V would help significantly, but either you'd need to feed the Si4703 tuner with 3.3V separately, or at least separate out its VIO pin and feed that 3.3V, so depending on the FM Click's track layout, that may not be possible.

[rea5245]

Moreover, the headphone cable ground is the antenna. How are those speakers connected to this module?

I wired an audio plug with GND/Antenna and Left to a speaker and GND/Antenna to an antenna wire.

[rea5245]

Raising Vcc to 5V would help significantly, but either you'd need to feed the Si4703 tuner with 3.3V separately, or at least separate out its VIO pin and feed that 3.3V, so depending on the FM Click's track layout, that may not be possible.

I might be able to replace the capacitors, though it would be tiny work. There's no way I'd attempt to rewire Vcc.

If replacing the capacitors is not going to do the trick, I'll leave the module alone and add another amp circuit.

[Ian.M]

Did you look at the Figure 1. Typical Audio Amplifier Application Circuit from the LM4864 datasheet?

All you need to do to get a *LOT* more volume out of it by wiring the speakers as bridge-tied loads, is connect the four separate speaker wires direct to the audio amp pins 5 and 8.
 
That will give you about 250mW per channel with 3.3V Vcc and 8 ohm speakers.  I'd suggest using 30AWG Kynar wirewrap wire, and gluing a four pin header or other suitable connector to the board at the edge over the words 'FM Click' to transition to stranded speaker wires.  Keep the extra wiring well away from the Si4703 tuner chip and the headphone jack.  Do not attempt to connect headphones and speakers at the same time - just plug in an antenna (quarter wave for your national FM band) connected to only the ground of a stereo jack plug.

[DavidAlfa]

Seem the circuit is wrong? The speakers are not supposed to be connected to ground, but between VO1 and VO2, and no output capacitors:
They hacked it a little bit so they could use common ground for the earphones. Might work for small loads.

[rea5245]

All you need to do to get a *LOT* more volume out of it by wiring the speakers as bridge-tied loads, is connect the four separate speaker wires direct to the audio amp pins 5 and 8.

This worked! Thank you! It's now audible coming out of a 4 speaker.

Alas, it's still not loud enough for my application (a clock radio). Would changing R6 and R8 help, our should I add another amp to the circuit?

[Ian.M]

You may be able to squeeze a little more out of it by changing the input to feedback resistor ratio, but the supply voltage and amp chips used set a hard limit to the maximum output power, which I doubt will be enough for a clock radio.   

Therefore your best bet would be to add a external amplifier.  A class D bridge-tied load one could deliver up to 3W per channel into 4 ohm speakers with a 5V supply - if you need more power or want to use a class B amplifier (which generally cant drive as close to their supply rails without distortion), you'll need an even higher supply voltage for the external amp.