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Old 7th February 2003, 06:00 PM   #1
TomJ is offline TomJ  United Kingdom
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Default what is the theory behind amplifiers

This probably sounds very stupid to any one who builds amps but why dont poeple use a simple transistor. could you explaine why. I would like to know the theory behind amps. At the moment I keep hitting this wall when tring to understand amplifiers. Are they as complicated as they first appear when you are a beginner
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Old 7th February 2003, 06:08 PM   #2
halojoy is offline halojoy  Sweden
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Default Re: what is the theory behind amplifiers

Quote:
Originally posted by TomJ
This probably sounds very stupid to any one who builds amps but why dont poeple use a simple transistor. could you explaine why. I would like to know the theory behind amps. At the moment I keep hitting this wall when tring to understand amplifiers. Are they as complicated as they first appear when you are a beginner
You put a good question.
And you are right that we can use only one transistor.
And 2 resistors.

And what is "an amplifier".

It is simply one or several transistors
grouped together to do the work as good as needed:
To put some more current into the speaker coil.
And according to the signal.

Several transistors and resistors can provide
more a more linear operation.
Transistors are not linear to their nature.
This causes distortion.

Well, keep on studying. And ask good questions!

/halo
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Old 7th February 2003, 08:36 PM   #3
Jeff R is offline Jeff R  United States
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Audio amplifiers can be fairly simple or very complex. Interestingly, decent amplifiers can be designed by those not having an electrical engineering degree or other advanced technical training if they simply study other designs and maybe reads a couple of books on the subject.

While a single transistor can certainly amplify, it would be totally unsuited for an audio amplifier for a host of reasons. I would think that about the minimum number one could get away with for a semi-reasonable audio amplifier would be about 5 - 2 for the input diff amp, 1 for the voltage amplifier stage (VAS) and 2 for the output drivers. We can add more transistors to the output stages to increase the power levels, for example, and more to the VAS and diff amp to achieve different things.

There are arguements pro and con to using complex designs. Some like the minimalist approach while others feel that a large numbers of properly designed-in transistors can achieve a remarkably high level of performance.

Your question is not stupid, but to fully answer it does require you to do some reading and studying on your own. It would take many pages here to teach you want you need to know.
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Old 7th February 2003, 09:32 PM   #4
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Like many other complex things, once you start looking at an ampflier you will start to notice that it is compoised of several indivdual sections.
There is the input amp section, a treble/bass section, and an output section, for example and don't forget the power supply section. Like speaking a new language, the more you look, the easier it seems to get.
And dont feel bad about asking questions, we all started out with no knowledge about this stuff at one time.
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Old 7th February 2003, 09:36 PM   #5
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I wanted to add:

Using current technology, we can't make a one transister amp that does everything and sounds good.

But questions like yours may one day lead someone to make one that does that.
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Old 7th February 2003, 09:47 PM   #6
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A transistor is a transistor, modern or not.

For one-device amplifiers, you must take a step backwards and investigate tube amplifiers.

Tim
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Old 7th February 2003, 09:59 PM   #7
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Quote:
For one-device amplifiers, you must take a step backwards and investigate tube amplifiers
Hi Sch3mat1c:

hey! tubes? a step backwards?

For theory I like to remember the old Marshall advertisement which went:

"It comes out bigger and upside-down."


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Old 7th February 2003, 10:08 PM   #8
UrSv is offline UrSv  Sweden
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Quote:
Originally posted by Sch3mat1c
A transistor is a transistor, modern or not.

For one-device amplifiers, you must take a step backwards and investigate tube amplifiers.

Tim
I remember an old project using one, thus running a single-ended stage, of the FETs in the 2SK135/2SJ50 pair biased quite high and using a Tango output transformer normally used for tubes. Quite like a Zen philosophy amp really. Sounded really nice and used only ONE transistor. Output was 15-20 W I seem to recall (don't ask me how that was possible with one of those FETs, maybe it was some other).
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Old 7th February 2003, 10:17 PM   #9
JoeBob is offline JoeBob  Canada
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The BOZ uses a single FET, and I do believe the ZEN uses a single FET (not sure, haven't read the article in a while and never built it). So it is possible to build a good sounding amp with a single transistor. As for tubes, it's even easier to build a good sounding amp with a single tube. I like the minimalist aproach because it's easier to build something simple that sounds good, I'm sure it's possible to build something complex that sounds good but I can't.
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Old 8th February 2003, 05:59 PM   #10
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"why dont poeple use a simple transistor"
Hi Tom,
This simple question needs a long answer. But let me add to what's been said with some basics.

An "audio amplifier" as you know is a short for for "audio voltage amplifier". A transistor is a current amplifier, not a voltage amplifier. So part of the complexity of the circuits is to try to achieve control of voltage using current devices. The simplest way to turn a current into a voltage is with a resistance but this effect can also be achieved using feedback.

Another factor is that a loudspeaker demands several amps of current at normal room listening levels. A typical CD player will provide 100s of microamps. So a current gain of some 100,000 is required. A single transistor will have a current gain in the 100s. So you can see that you need 3 transistors just to provide the gain.

The current gain of transistors is not very linear. Often resistors and other methods are used to trade current gain for linearity. This means more transistors are needed.

A simple, feedback amp needs 5 to 7 transistors to amplify current and 2 or more extras to provide current biasing and voltage biasing.
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