The LM386 and PWM (schematic included) what sort of filter is on the input?

Here's the schematic for a Soundgin sound generator. My question is how does R1 , R2, C12 & C13 affect the upper frequency. Is it a good general purpose design for a PICs PWM output? Will it run ok at 5V?
**broken link removed**

It's 3dB down at 800Hz on the high end.
No problem on the low end. In fact, for the low end, you could replace the 10uF cap (C3) with 100nF and still have the LF corner at 25Hz. What is your PWM frequency?

Well I wanted to develop a general amp for PWM based sound. Does it have a sharp cutoff above 800Hz? Telephone audio quality 2KHz should be fine.

It's for the Junebug & Firefly kits as a user built option.

The LM386 is rated down to 4-5 volts but will only give maybe 200mw at that Vcc driving a 8 ohm speaker. Should be enough for experimental level quality audio.

Lefty

It'll be a pretty weedy wee speaker, only about 2" as it has to fit on a 57mm PCB. Maybe the 800Hz isn't so bad.

As I'm sure you know, your filter requirements depend on the audio bandwidth you want, the pwm frequency, and probably some other stuff (I'm no expert on PWM).

On another thread I posted a link to a PIC that sings. It is this Soundgin circuit.
It makes some noises that resemble somebody singing but I can't understand the "robot" lyrics. It is almost 2008 and I though it would sound better than that.

The simple lowpass filter is very droopy. A Sallen and Key Butterworth lowpass filter is flat up to its cutoff frequency which might be able to be higher if used in this circuit.

The LM386 produces a whopping 78mW into an 8 ohm speaker at clipping with a 5V supply. Like loud headphones.
With the 9V as shown its output is 450mW, nearly 6 times more power, like a cheap clock radio.

R3 is wrong at 10k ohms. It is supposed to be 10 ohms.
The polarity of C4 and C8 are backwards.

A tiny 2" speaker without a proper enclosure will try to make a squeak but there are no high frequencies so it will just beep. (Hee, hee. I am a poet)

My 486 Compaq Presario pc had 1" x 2" oval speakers in small ported bass-reflex enclosures. They actually sounded pretty good, about 1W each.

audioguru said:

The polarity of C4 and C8 are backwards.

Click to expand...

According to the MAX232 datasheet, pin 2 is +10V, so the polarity of C8 is OK.

basic filter info

See 'Series Lowpass' in figure 1.

http://www.qrp.pops.net/filter1.asp


Christopher

cnetherton said:

See 'Series Lowpass' in figure 1.

http://www.qrp.pops.net/filter1.asp


Christopher

Click to expand...

Also note this quote from the same site:

Unfortunately, interactions between the two series filter sections will change the cutoff frequency from the design frequency somewhat.

Click to expand...

The calculated cutoff frequency for the circuit in question is 595Hz. The measured cutoff frequency (in simulation) is about 800Hz. As Audioguru noted, an active 2-pole lowpass might be adequate if this filter isn't.