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Monday, August 16, 2010

Introduction to Amplifiers

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Comments: 3
an amplifier is an electrical circuit that takes an alternating current (AC) as input signal and usually increases the amplitude/voltage of the AC signal.  amplifiers are used for MANY applications, but the type of amplifier we will be focusing on is a guitar amplifier.  this is important to note mainly because:

-there are many principles of amplifiers that are important for an electrical engineering student to understand but will be beyond what the average guitarist will need to know.

-narrowing the application of the amplifier will allow the talk to be more specific.

a guitar amplifier has to be able to accept a very weak passive AC signal that is produced by a guitar pickup and needs to be able amplify that signal many times to an AC signal powerful enough to drive a loudspeaker cabinet.  a guitar amplifier does this by dividing the amplification process into 2 main sections: the preamp and the power amp (aka power section).

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PREAMP:

-the preamplifier section specializes in amplifying low-level passive signals that are generated by pickups or microphones.

-preamps are usually run in a 'class a' operation with 'dual triode' tubes like 12AX7's or 12AT7's or transistors/FETS/opamps performing the amplification.  as a side note, it is possible to see pentode tubes used in a preamp section (think EF86's).

-the signal is actually sent through a series of amplifying stages to amplify the signal in small sequential steps, this allows for particular tubes/transistors to perform particular roles in processing the input signal (for example, the second preamp tube is often used as a 'driver' tube, or a tube can be used to buffer send/receives to stuff like reverb tanks or effects loops).

-the 'volume' or 'gain' knob is most often wired in after the first gain stage of an amplifier (this is why it sounds different when running you amp at low volume with the volume knob on the guitar all the way up; as compared to turning up the amp and turning the volume knob on your guitar down). 

an important section in the preamp is known as the 'Tone Stack'; you may recognize the interface for the tone stack as the EQ knobs (bass, mids, treble).  the job of the tone stack is easy: to rid the signal of unwanted frequencies while being able to adjust frequency response in a pertinent ranges.  the reason a tone stack is desirable or needed is quite a bit more complicated, but it has much to do with undesirable frequencies/noise being amplified with the input signal and how your ear/brain hears sounds differently at low volumes and high volumes.  the actual implementation of a tone stack consists of a web of capacitors and resistors wired to potentiometers or rheostats.

 
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POWER AMP
-the roll of a power amp is to take a line level signal (usually provided by a preamplifier) and to amplify that signal to a level that can be sent to a loudspeaker for conversion to an audio signal

-power amplifiers in guitar amps can come in many different modes of operation, but the most common is some type of class A/B push/pull configuration.  when an amp is called 'class a' or 'class A/B', then this is the part of the circuit they are referring to.

-the signal from the preamp may need to be prepared for the power section, so at times a dual triode tube must be used as a phase inverter (PI) for class A/B push/pull amps.

-depending on the mode of operation, the power section may amplify the signal many different ways; but power sections will generally provide amplification in 'one stage' as opposed to the multiple stages you see in preamps.  in tubes amps, much more powerful beam pentodes and kinkless tetrodes are commonly used for power amplification (but a triode in the power section can be used, like the 12BH7).

-the 'master volume' is normally wired into the circuit after the preamp and before the power amp.  this is why you can't get power tube saturation with the master down.

after the power section of a tube amp the signal is usually sent to a transformer to convert the final signal from the amplification into something appropriate to drive a loudspeaker (though there are transformer-less tube amp designs out there).  solid state amps usually lack this 'output transformer' because the impedance of the output AC signal from the transistors in the power section is low enough impedance to be properly 'loaded' with a speaker cabinet.  most guitar amplifiers have certain impedance requirements from the speaker attached to the amplifier:

-tube amplifiers usually require a 'matching impedance', meaning the outputs of the amplifier are label and a speaker cabinet with corresponding impedance must be matched to the speaker jack.

-solid state (SS) amplifiers use impedance bridging instead of impedance matching (because of the lack of the output transformer), so usually they require a minimum impedance from a speaker cabinet, this means that any speaker cabinet above the minimum setting may be used.  solid state amplifier output is usually dependent upon the impedance of the cabinet (example: 100 watts @ 16 ohm, 200 watts @ 8 ohm).

sometimes speakers are included with a guitar amplifier (a combo) and sometimes the speaker and head components are separated (piggy back or stack).  the benefits and merits of these setups will be covered in the 'speakers and cabinets' section.
10:17 am - 3 comments - 6 Kudos - Report!
Comments
VintageAmigo wrote on Apr 26th, 2011 12:32am

Thanks for the post man, it actually helped me out quite a bit

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gumbiliciousa wrote on Sep 4th, 2011 2:18am

oh... thx. i didn't realize people actually read this. i guess i'll keep it up.

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Blktiger0 wrote on Nov 7th, 2012 10:01am

Good article! I really enjoyed it, and I've officially learned MANY things today, thanks to you! If I had the money and I wasn't already in debt to a college, I would head off to become an EE tomorrow. This stuff is extremely interesting!

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