The LM386 and PWM (schematic included) what sort of filter is on the input? - Electronic Circuits Projects Diagrams Free

blueroomelectronics · 18th November 2007, 09:58 PM

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?

Roff · 18th November 2007, 10:13 PM

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?

blueroomelectronics · 18th November 2007, 10:18 PM

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.

Leftyretro · 18th November 2007, 10:31 PM

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

blueroomelectronics · 18th November 2007, 10:33 PM

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.

Roff · 18th November 2007, 11:12 PM

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).

audioguru · 19th November 2007, 12:16 AM

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.

Roff · 19th November 2007, 12:47 AM

Quote:

Originally Posted by audioguru

The polarity of C4 and C8 are backwards.

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

cnetherton · 19th November 2007, 01:31 AM

See 'Series Lowpass' in figure 1.

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

Christopher

Roff · 19th November 2007, 03:45 PM

Quote:

Originally Posted by cnetherton

See 'Series Lowpass' in figure 1.

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

Christopher

Also note this quote from the same site:

Quote:

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

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.

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18th November 2007, 09:58 PM	(permalink)
blueroomelectronics	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? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

18th November 2007, 10:13 PM	(permalink)
Roff	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? __________________ Ron

18th November 2007, 10:18 PM	(permalink)
blueroomelectronics	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. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

18th November 2007, 10:31 PM	(permalink)
Leftyretro	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 __________________ Measurement changes behavior

18th November 2007, 10:33 PM	(permalink)
blueroomelectronics	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. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

18th November 2007, 11:12 PM	(permalink)
Roff	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). __________________ Ron

19th November 2007, 12:16 AM	(permalink)
audioguru	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. __________________ Uncle $crooge

19th November 2007, 01:31 AM	(permalink)
cnetherton	basic filter info See 'Series Lowpass' in figure 1. http://www.qrp.pops.net/filter1.asp Christopher