Questions about electric current and audio circuits.
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 2nd November 2021, 11:12 AM #1 BasicHIFI1   diyAudio Member   Join Date: Jan 2015 Location: Colombo Questions about electric current and audio circuits. 1) What is electric current? Electrons flow or electron drift is very slow, to the order of 2.3 x 10^-5 m/s. According to the Wikipedia article: "In physics, a drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field. In general, an electron in a conductor will propagate randomly at the Fermi velocity, resulting in an average velocity of zero. Applying an electric field adds to this random motion a small net flow in one direction; this is the drift." Drift velocity - Wikipedia 1 Ampere of current is 1 C /s . How many electrons need to flow past a point per second to generate this current? This depends on how many Coulombs are contained in copper wire and what the charge on each electron is. Number of electrons per second = 1C / (charge of an electron * number of electrons) The calculation is given in the article. Is this an oversimplification?
 2nd November 2021, 11:19 AM #2 BasicHIFI1   diyAudio Member   Join Date: Jan 2015 Location: Colombo 2) What are the electrical characteristics of the output from audio playback devices, and how to can it be safely measured? In audio amplification, the signal from an audio player: mobile phone, DVD player, tape deck, turntable or preamplifier is what gets amplified. I have connected the output from a computer audio out directly to a small speaker, and and could hear music. In a simple class A amplifier, it is this signal that goes into the base of the transistor, is it less than the current needed to turn on the transistor? Is this why the transistor is 'biased', that is a small current is applied to the transistor to move it into the working range?
Mooly
diyAudio Moderator

Join Date: Sep 2007
Quote:
 Originally Posted by BasicHIFI1 [B]2) In audio amplification, the signal from an audio player: mobile phone, DVD player, tape deck, turntable or preamplifier is what gets amplified. I have connected the output from a computer audio out directly to a small speaker, and and could hear music. In a simple class A amplifier, it is this signal that goes into the base of the transistor, is it less than the current needed to turn on the transistor? Is this why the transistor is 'biased', that is a small current is applied to the transistor to move it into the working range?
You might be confusing voltage and current here.

A signal of for example 100 millivolts peak amplitude may be delivered from a source of high current ability.

What does that mean... it means that lets say you placed a 0.1 ohm load across the 100 millivolts (silly I know but...) then the current flow iwould be 0.1/1 which is 0.1 amps.

That signal is easily audible if applied to a speaker.

If you apply it to an unbiased transistor then it will not cause the transistor to conduct because the transistor needs about 0.6 volts between its base and emitter before any current can flow anywhere.

So as you correctly say, the transistor is pre biased to move the operating point to a more linear part of the transfer curve. When pre biased the small change in signal voltage can linearly shift the operating point by modulating that preset bias we have applied.

As well as needing voltage (signal voltage) we also need current (signal current) for the transistor to work.

Our 100mv peak voltage source if fed for example via a 10 meg resistor is still a 100mv peak source but now we can draw virtually no current. It would be inaudible in a speaker because 99.99% of the voltage is lost across the 10meg.

There would now be insufficient current to operate a transistor even though in basic terms we still have a 100mv source.

BasicHIFI1
diyAudio Member

Join Date: Jan 2015
Location: Colombo
Computer, mobile phone and DVD player output voltages

Quote:
 You might be confusing voltage and current here.
Yes I think I am, and that is what I want to sort out.

In the meantime I have made measurements to the outputs from my laptop, mobile phone and DVD players: (1 kHz sine wave, online tone generator, 50% volume)

Source zero volume to max volume (Multi-meter in AC Voltage mode)

Laptop: 0.00 mV to 100 mV

Mobile Phone: 0.00 mV to 70 mV

DVD Player: 2 Volts to 9 Volts (Multi-meter in AC Voltage mode)
DC mode - a few millivolts.

I can even sense the current in my fingers when I touch the RCA jack running from the DVD player. Is there a short internally? Is this how it works normally? The player plays music fine, though it skips sometimes.

The next step will be to connect the output to a high impedance headphone or speaker and measure current in Amps.

Last edited by BasicHIFI1; 2nd November 2021 at 05:35 PM.

 2nd November 2021, 07:14 PM #5 Mooly   diyAudio Moderator     Join Date: Sep 2007 How well a DVM works at those low levels is an unknown... some are super accurate and others not. Also you need to be sure 1kHz is within the range of AC frequencies allowable. It probably is but some DVM's might only be good for 50/60 Hz and perhaps up to a few hundred Hertz. A scope is best The high DVD player reading is almost certainly caused by you in some way picking up the voltage that the chassis might be floating to. That is caused by the switching power supply and leakage currents... all normal and safe... but can cause weird results. I doubt you would record any meaningfull current on a DVM at the low values involved. 100mv (lets assume that is an rms value) across an 8 ohm speaker is just 1.25 milliwatts. The current would be 12.5 milliamps. If the speaker was higher impedance then the current and power would be even lower. __________________ Installing and using LTspice IV. From beginner to advanced.
 3rd November 2021, 02:42 AM #6 BasicHIFI1   diyAudio Member   Join Date: Jan 2015 Location: Colombo Those were weird results all right. I found an article on current leakage here: What is Leakage Current? - Sunpower UK I am seriously looking at this scope , the built version, not DIY. https://www.amazon.com/DSO-Shell-Osc.../dp/B06Y1T6WZ9 KKmoon 2.4" TFT Digital Oscilloscope Kit with Power Adapter and BNC-Clip Cable Probe DS0150 (Assembled Finished Machine) US Plug or a sound card based scope for my laptop. I think it is about time I got a scope.
 3rd November 2021, 05:48 AM #7 BasicHIFI1   diyAudio Member   Join Date: Jan 2015 Location: Colombo Output from laptop audio jack. (Headphones connected). Why is it uneven? Do the headphone drivers cause this or is the response not smooth? 50 Hz : 427 mA 100 Hz : 419 mA 1,000 Hz : 424 mA 10,000 Hz: 365 mA
 3rd November 2021, 07:24 AM #8 Mooly   diyAudio Moderator     Join Date: Sep 2007 I think those numbers are very suspect tbh That is nearly half an amp. No laptop/phone/source component would deliver those kind of currents. Something is going wrong with your measurement technique here. No decimal point after the first number perhaps? To get the feel for it all first listen to your 1kHz tone through a normal amp and speaker at very low volume. Now set the AC voltage across the speaker to just 0.1 volt. It will be quite loud with even that low voltage. Now put your meter on AC current and insert the meter in series with one of the speaker leads. The level of sound should seem the same and the meter will show current. The exact reading depends on the speaker impedance but it should be in the 10 to 20 milliamp region. If it is not then something is wrong with the meter and or your measurement set up. __________________ Installing and using LTspice IV. From beginner to advanced.
BasicHIFI1
diyAudio Member

Join Date: Jan 2015
Location: Colombo
The ability to provide a current.

Current, Voltage and resistance.

This simple circuit in Falstad sim shows a resistor and a current source. It is confusing to me to talk about voltage. Current consists of the flow of electrons, or charged particles. For example, a charged capacitor connected as the current source will discharge through the resistor. A battery will also run down after a time and will eventually not be able to produce a current.

So in effect what we have is current, and 5V means that the current source will provide 5 amps of current across a 1 Ohm resistor, or more realistically, 1 A across a 5 Ohm resistor. The voltage then refers to the capacity to provide a current across a certain resistance. The effect may not be linear.

I look at it this way: when the circuit is closed, the current flows through the circuit. If the current source is a battery, or mains current converted to DC, the current flow will increase quickly (at the speed of light?) and then stabilize at a certain current value, depending on the current source and the resistance that the current encounters in the circuit. What I am trying to do is understand electric current flow in terms of current only, I am not sure if this will add clarity or be inadequate.
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Last edited by BasicHIFI1; 3rd November 2021 at 07:45 AM.

BasicHIFI1
diyAudio Member

Join Date: Jan 2015
Location: Colombo
Quote:
 Originally Posted by Mooly I think those numbers are very suspect tbh That is nearly half an amp. ... If it is not then something is wrong with the meter and or your measurement set up.

Yes will try it this way: was using the multimeter in series and set at AC Milliamps.

For reference typical output from laptops and phones is about 10 mA as far as I can find out. Can I calculate the current if I measure the resistance of the headphones (impedance is different and depends on frequency.) Can I call impedance 'apparent resistance' at 1 kHz for example? There is a circuit to measure impedance I think I can find it.

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