[Elecraft] OT: modifying your amp for lower drive requirement

[Jim Brown]

On 12/20/2010 5:28 PM, Phil & Debbie Salas wrote:
> "It's very unlikely any solid-state amplifier uses an input attenuator:
> Transistors and FET's just don't have that much gain."
>
> The Ameritron ALS-600 has about a 15 ohm input impedance.  Then they put a
> series 35 ohm resistor for matching and reducing gain.

Actually, input attenuators are fairly common in power amps. It is one 
of two good ways to reduce power amp gain to meet the FCC requirement. 
Another good method is to use a bit more negative feedback, which has 
the added benefit of improving linearity (that is, reducing 
distortion/splatter/harmonics) and reducing noise.

As others have noted, most output stages are cleaner if run a bit below 
their maximum possible output.  While one could safely pick up several 
dB of gain by removing the input attenuator, I'd still be careful not to 
overdo that, I wouldn't try to reduce the feedback, and I'd run the amp 
pretty close to it's published rating.

73, Jim Brown K9YC