Re: [Repeater-Builder] Audio mixer circuit

[Kevin Custer]

Steaven Rogers, W4YI wrote: Kevin,     This is perfect.  I have only one more question.  What is the component labeled 386.  I would expect it to be an op-amp or something along that line.  What is the actual part number that I would need to look for. Hi Steaven, The part is a LM-386, available at Radio Shack.  While this part resembles an op-amp, it's actually a power amplifier intended to run loads down to 16 ohms.  Why do I use it?  Several reasons, including available locally cheap, low support components count, and highly RF immune because of having a poor slew rate.  The slew rate limitation allows signals below 10,000 cps to be amplified without distortion/roll-off, and those above that will be attenuated to a greater degree as frequencies increase.  Since most FM communications we deal with here have bandwidths that are somewhat below 10K, it makes a great amplifier for many 2-way instances.  Also, realize that this amplifier *does not* need to be loaded to 16 ohms; it's stable into just about anything, including nothing when the 10 ohm and .05 uF network is used. There are two things I have found that are a must when using this amplifier IC.  You must use a 100 uF or larger filter capacitor on the power supply rails placed near the IC, and you must use the 10 ohm - .05 uF network.  These components suppress parasitic oscillations, and keep the amplifier stable. Many common op-amps cannot source a load below 2K, the LM-741 and common TLO series are an example.   These op-amps become unstable when loaded below 2K, and proper design requires a build-out resistor to ensure the op-amp's stability.  The build-out resistor is lossy when driving low impedance loads and the use of this chip eliminates the need for a build-out resistor, and is stable down to 16 ohms. More on the 386: http://www.repeater-builder.com/rbtip/audioamp.html Hope this helps... Kevin Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/   To unsubscribe from this group, send an email to: [EMAIL PROTECTED]   Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.