Amplifier?

Thread Starter

iliev986

Joined Oct 29, 2020
19
Hello, I have a question about a basic electrical circuit, what is its idea and how exactly does it work? I will be happy if someone can explain to me in more detail. I am also interested in why all power supplies are reverse polarity (including some electrolytic capacitors)?
Thank you!
 

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crutschow

Joined Mar 14, 2008
27,704
That's a simple two stage common-emitter amplifier.
R4 provides negative feedback for both DC bias level and to reduce AC distortion.
Pot UR1 adjusts the amplifier AC gain.
Pot UR2 adjusts the output level.

The capacitors are not reverse polarity.
Note the polarity of the power supply, which is needed for proper bias of the PNP transistors.
 

Thread Starter

iliev986

Joined Oct 29, 2020
19
I'm still a beginner in electronics. I'm starting to like radio electronics more and more, but I just don't know where to start studying it and with what schemes to start doing it, I need advice honestly, and again sorry for those stupid questions
Thank you!
 

Dodgydave

Joined Jun 22, 2012
10,050
Most electronic magazines have a Teach In series, like Elektor, Practical Electronics, etc,

This is a great way to get going.

The circuit in question uses Positive as Gnd which is the Old way of drawing, hence PnPs had their Emitters on the bottom.
 

Audioguru again

Joined Oct 21, 2019
3,509
You found a very, very, very old circuit that used transistors that were called "germanium" and the first ones were only PNP.
Those old transistors stopped being made about 53 years ago. Then NPN and PNP "silicon" transistors were invented and used.
 

ScottWang

Joined Aug 23, 2012
7,065
You found a very, very, very old circuit that used transistors that were called "germanium" and the first ones were only PNP.
Those old transistors stopped being made about 53 years ago. Then NPN and PNP "silicon" transistors were invented and used.
When you said that then you also shown your age?

It seems that I also have some similar transistors, they like as 2SBxxx, does it means that we have the similar age? ...
 

Audioguru again

Joined Oct 21, 2019
3,509
Yes, it is an awful-sounding Fuzz Face circuit that produces plenty of distinctive distortion.

I still have some original AC128 germanium transistors from my first lob as an electronics tech.
The AC127 and AC128 were used as a class-AB low distortion 1/2W amplifier.
 

Ian0

Joined Aug 7, 2020
3,198
It's a really poor design for an amplifier, because the end result is highly dependent on the parameters of the components (but that is almost mandatory for guitar equipment)
I built two of them the other day - they sound completely different, because the bias points for the input transistor are in two different places to due to different Vbe and hfe values. The transistors came from the same batch!
 

Audioguru again

Joined Oct 21, 2019
3,509
Isn't the Fuzz Face circuit a "pedal", and is not actually an amplifier for a guitar because its input impedance is much too low. I think it is connected between the high input impedance preamp and the power amplifier.
 

Ian0

Joined Aug 7, 2020
3,198
Yes. definitely a pedal. An amplifier in that it makes a signal bigger.
A guitar output impedance is about 6k+1.5H, and an amplifier input impedance is generally 1M (though 220k would avoid a lot of the pickup-cable resonances)
But the fuzzface DOES connect straight to the guitar, even though the impedance match is dreadful. The effect is to attenuate all the harmonics, leaving the fundamental which it distorts into a trapezium shaped waveform.
Apparently, that sounds best.
Remember that we are in the "guitarist" domain where quality does not equate to low distortion and flat frequency response!
My friend, for whom I made the fuzzface, has a guitar with a switchable built-in preamp, and it really does not sound as good when connected via the preamp with its low output impedance.
 
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