In the interest of putting some numbers out for debate, I just ran a SPICE simulation of this simple amplifier circuit. I modeled the speaker as 4 ohms in series with 170uH and a 1 mV AC source. My database does not have BC109C transistors, so I used two BC109B transistors instead. The B's have a lower hFE (typical value about one-half the C spec). I used a 4.7 k resistor for the output load. The Vcc is 12 volts. Other parameters are as shown in the original schematic. Anyway, the result shows the 3 dB bandwidth is 70 Hz - 9.6 KHz. Total harmonic distortion at 1 KHz is 0.57% (through 9th harmonic). 2nd harmonic is about 45 dB below the reference, 3rd is about 55 db below the reference. A transient analysis shows no significant start-up distortion (looked at for the first 5 mSec.) Overall gain into the 4.7K load is 103.5, or a hair over 40 dB if you use a sloppy definition of dB by ignoring the different impedances. The DC operating point analysis shows Q1 collector = 7.2 volts, Q2 emitter = 6.5 volts. Assuming that SPICE reflects reality and my speaker model isn't too far out, the circuit appears to function reasonably well. Jack -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Olin Lathrop Sent: Wednesday, June 04, 2003 9:06 AM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: Can a speaker be Input Devices Mike Singer wrote: > "AC (the sound) feedback" you were talking about is getting > by emitter resistor. Not really. The 8.2ohm is so low compared to everything else in this circuit that the speaker, series capacitor and 8.2ohm resistor are essentially a voltage source from the rest of the circuit's point of view. If there was a resistor in series with the speaker and the emitter, it would provide some negative feedback, but this one is in parallel with the speaker (for AC analisys). ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body --- Incoming mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.486 / Virus Database: 284 - Release Date: 5/29/2003 --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.487 / Virus Database: 286 - Release Date: 6/1/2003 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body