Hi all,

I just got interested in electrinics few days ago. Trying to focus on simple analog audio circuits for now and done quite alot of reading on this subject, but there are still a few questions that bugs me..

As an exaple lets take this tone control circuit (attached).
1. How do I count current I should use for the circuit? (If I had one resistor in circuit, I would use I = V / R, I gues but what if I had one capacitor, diode or transistor or when it gets comlex like in pic ?)
2. How do I count max safe input? Is that linked somehow to "input inpendance" (take a look at notes at the end of the post)?
3. What is a "buffer" (notes)? I find this in almost all audio circuits, but can't find info about it. ;(


Thanks very much and sorry for noobyness, it's just I can't find this info anywhere and thought you guys could help a little

Notes for the circuit:
"The first BC109C transistor (left hand side) is acting as a buffer. It provides the circuit with a high input impedance, around 250k has a voltage gain of slightly less than unity. As the Baxendall tone control circuit is a passive design, all audio frequencies are attenuated. The position of the controls and reactance of the capacitors alters the audio response. The last transistor provides a slight boost of about 3x. The output is designed to feed an amplifier with input impedance of 10k to 250k. Both tone controls should be linear type potentiometers."

Circuit and notes taken from: http://www.zen22142.zen.co.uk/Circuits/Audio/t-ctrl.htm

Attached Images

t-contr.gif (5.3 KB, 17 views)

 

If you wanted to, you may add the current that each branch of the circuit takes to find the total current...

But, in practice, this type of circuits take very little current and you just build it and then if you need to know the current measure it with a (mili)ammeter.

This type of circuits are usually used with "line level" audio - arround 1Vpp.

To damage it, the input level should be much bigger - the probable "danger points" are:
>> The input capacitor's rated voltage (usually at least 25 or 50 V), and
>> The first transistor reverse base-emiter voltage (when the input is negative) - You may look for it in the transistor's datasheet.


A buffer is an amplifier, usually with a voltage gain of 1, that has a high imput impedance and a low output impedance.

Filters (like your tone control) have to be fed from a low impedance source

__________________
E Cerfoglio
Buenos Aires
Argentina

 

Thank you for the answers

Can anyone explain me what is impendance, cause I get a stupid word in a dictionary and google defines it to something I still don't get :P

 

Impedance is similar to resistance, but the term is commonly used in AC circuits. Think of it as the opposition that the circuit exhibits to the flow of current, or the "resistance" of resistors, inductors, and capacitors. When analyzing AC circuits in the frequency domain, imaginary parts are often involved which can be explained easiest using impedence principles. Sorry if that is confusing. Heres an example.

Impedances:

Resister = R
Capacitor = 1/(jwC)
Inductor = jwL

j = imaginary part
w = angular frequency = 2(pi)f

So the impedance of a circuit with a resistor and an inductor in series then becomes R + jwL

 

I don't want to annoy anyone here by giving 'wrong' non-academic info but impedance can be a toughy to explain.

As mojoman pointed out, it is generally thought of as the resistance of a circuit to AC signals. 'DC resistance' when thinking of resistors, will behave the same with AC and DC signals (in an ideal world). So, say a voltage divider of 1/2 would reduce 10v to 5v. And an AC signal from 0-10v (say a sine wave off set, centre at 5v) to 0-5V.

But when dealing with components like inductors and capacitors, these 'resist' AC signals differently depending on the frequency of said signal, and the components value. using the above voltage divider example, if one replaces one of the resistors with a reactive component (cap, or inductor) then the voltage division is no longer fixed, it becomes dependant on the frequency of the singal. Google 'RC low pass filter' and see what you get.

As I said, I'm not giving you the full story here, just basic practical stuff.

In terms of 'high/low impedance inputs/outputs... in lay terms, a high impedance input will not 'use' the input signal as power...as that could change the signal itself. Puting a high impedance output, to a low impedance input would degrade the signal. In audio it would reduce the high frequency components.

With outputs, a low impedance output has the ability to 'drive' inputs. That is not to say necessarilly that it has a lot of power, but the signal is not too weak for something its put into to affect it. As a rule of thumb with basic audio, outputs should be low impedance, inputs should be high impedance.
A buffer, or 'voltage follower', with a gain of 1 does nothing except prepare a signal for another part of a circuit, or to an output. It 'looks' at a high impedance output from something, and copies it to its output, without using much of that input signal (which would change the signal itself).

So you can see why buffers are used everywhere as they convey information about a signal, without affecting it. All components require 'power' but we don't want to use signal inputs as power (sometimes we do ), we are just interested in the information they contain.

If you have an audio circuit that modifies the signal in some way and its 'active' (ie: powered) its probably a good idea to have a buffer at the input, and one at the output. Neither changes the signal in any way but it allows various impedance sources to be connected to the input and output. Otherwise, the circuit *could* have a different effect depending on what devices you connect to it...Prime example is a guitar amp.

Hope that helps. As I said, its not academic info, my analogue is terrible and ecerfoglio's explaination was spot on. But it sohuld help o clear some things up.

Blueteeth

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Inconsistency is the key to flexibility!