Question does the DC in a hifi have AC in to carry the sound signal ?

Divad123

Member
2019-09-08 12:57 pm
Hi all,


Was watching fault finding on YT on an 80's Sony hifi amp and the repair man had music running in the rear phono inputs, the hifi amp volume was turned down. He was using a probe connected to a small speaker and on touching tracks and components, the music would come out of the small speaker - he said that there is AC in the DC current running through the amp and it's the AC which carries the sound/signal and it's the AC which the probe is picking up, hence getting sound from the small speaker attached to the probe


I'm a newb to electrical stuff, learning it for several months but this is the first time I have heard this - Is this correct ?


Any info on this would be much appreciated
 
Last edited:

egellings

Member
2010-08-21 7:59 pm
CA
It's pretty much correct. Audio signals can ride on a DC voltage. An example would be an amplifier that has just one power supply voltage, instead of 2 voltages, one positive and the other negative. The positive speaker output sits at 1/2 the supply voltage in a single supply setup. A musical signal causes this centered voltage to swing above and below the center set point. This swing is the AC signal superimposed on the DC center voltage. A capacitor connects just the AC (music) signal to the speaker terminals while blocking the DC voltage, which a speaker cannot tolerate.
 

mbz

Member
2013-07-05 1:12 am
Wondering if he's talking about the DC biasing and the audio signal running on top of that.
I prefer to not call the audio AC. I haven't seen the vid but is sounds like he is using a
simple audio probe, a useful device for tracking faults when an oscilloscope is not
available. Useful tool for DIYers consisting of a probe (or bare wire) a dc blocking cap
(0.1uf-1uf/250/400/xxxV) connected to a spkr, other side of spkr connected to chassis/GND. Alternatively you could buy a cheap (USD25?) "oscilloscope" from china
on the bay, useful for binary yes/no audio signal tracing
 
Yes. AC means Alternating Current, it goes in one direction then the other, which ultimately causes the speaker cone to go one way and then the other so producing sound waves in the air.

The language is somewhat abused as what we call an AC signal riding on a DC signal is not alternating direction, just varying in time. Only if the AC amplitude is greater than the DC offset does the current actually reverse.

And it is further abused in that we talk of voltages as AC just as much as currents!

We think of any signal as having an AC component (varying with average of zero), and a DC component (the average of the total signal over time).
 
I'm a newb to electrical stuff, learning it for several months but this is the first time I have heard this - Is this correct ?

Any info on this would be much appreciated

To describe it as AC on top of DC is a common, but somewhat inaccurate parlance. It is most accurately a DC voltage that is altered (modulated) by the audio waveform.

Yes, you will find extractable audio almost everywhere you go inside an amplifier or receiver.... occasionally including places it should not be.

The probe amp he was using is a very quick and efficient way of finding where a signal stops, as a hint to more comprehensive trouble shooting. They are very simple devices, often built for under $10.
 

Divad123

Member
2019-09-08 12:57 pm
Hi all,


Thank you very much for the prompt responses. I'n pretty amazed - I have watched 40 or 50 hours of audio hifi repair, refurbs, upgrade, teaching videos (including instrument amps), read posts and pdf's on hifi and faultfinding/repair and the one I watched a few days ago was the first one to mention that there's AC in the DC which carries the signal.



I every video I've seen, instructor/repairer correctly says the bridge rectifier turns the incoming AC to DC - not yet have I heard someone say 'converts AC to DC - but some of the AC remains to carry the sig/sound to the speakers'...


Maybe it's a fact so obvious that they don't think that anyone interested in the subject wouldn't know this...maybe that's it...



MBZ - that's it excactly - an audio probe. I have an oscilloscope and a sig generator but built the audio probe as it seemed to do the same job as a scope, is easier to use and easier to get my head around eg : play music though hifi amp - probe from the phono inputs over the boards till you don't hear the music any more - examine that area and it's components. Simple



MarsBravo - Electronic Demonstrations will be my next port of call. Thank you.


egellings : 'This swing is the AC signal superimposed on the DC center voltage. A capacitor connects just the AC (music) signal to the speaker terminals while blocking the DC voltage, which a speaker cannot tolerate' - Perfect - I can read and watch vids for hours but till someone puts it in understandable laguage like that, I have a hard time getting it. (I'm an old newb, so maybe that explains it lol)


Please, everyone, accept my thanks in helping a newb clear up a question which I'm sure you folks find is pretty elementary and has been asked here a million times.



Merry Christmas and a happy New Year to all
 
Hi all,


<snip>


I every video I've seen, instructor/repairer correctly says the bridge rectifier turns the incoming AC to DC - not yet have I heard someone say 'converts AC to DC - but some of the AC remains to carry the sig/sound to the speakers'...


<snip>

Merry Christmas and a happy New Year to all


The bridge rectifier in conjunction with one or more filter capacitors converts AC to DC to power the amplifier electronics.

Bit of a misunderstanding in your comment above, you don't want residual line frequency or its harmonics in the DC used to power your amplifier. Any residual AC in the DC power may be heard as buzz. (multiples of the rectified AC's line frequency)

As others have pointed out the audio signal is superimposed on the DC circuit voltage, and if you remove the DC component by using a coupling capacitor or audio transformer you have an AC signal which is the audio signal previously discussed.

Think about a simple battery powered device, the power source in the simplest case involves no AC at all, but audio from whatever source (mp3, cassette, FM, etc) is superimposed on the circuit DC and this fluctuating DC voltage is eventually converted to AC in order to drive your headphones typically using a capacitor which blocks just the DC component of the signal.

Hope I have clarified things a bit and hope I've not just added to the confusion here..

Merry Christmas
 
I'n pretty amazed - I have watched 40 or 50 hours of audio hifi repair, refurbs, upgrade, teaching videos

Ummmm ... that's probably not the way to learn about audio and electronics in general. What you need is a sound basis in electronic theory, an understanding of how stuff works and why it's done as it is, before you ever start messing with repairs... and especially before diving into modifications.

Moreover; I've seen so much painful incompetence in YouTube videos, I gave up trying to keep track of it years ago.

Start your journey here ... All About Circuits ... and get some REAL lessons in electronics theory.

I every video I've seen, instructor/repairer correctly says the bridge rectifier turns the incoming AC to DC - not yet have I heard someone say 'converts AC to DC - but some of the AC remains to carry the sig/sound to the speakers'...

Don't mistake the DC power supply for the amplifier's signal path. They both exist in the same amplifier and in a sense they do interact, but they are two different things. The audio signal does not come from the power supply, it comes from an input specifically designed to receive it. From there the various amplifier stages magnify and manage the signal, using power from the supply.
 
I every video I've seen, instructor/repairer correctly says the bridge rectifier turns the incoming AC to DC - not yet have I heard someone say 'converts AC to DC - but some of the AC remains to carry the sig/sound to the speakers'...
No, it's the amplifier that uses the input signal to turn the DC from the power supply into AC to drive the speakers.
 

Divad123

Member
2019-09-08 12:57 pm
Hi all again,


Thank you for your further responses to my question. I've learned much. Douglas, thank you for the 'All about circuits' link, I'll get studying that after the festivities have eased off.



Douglas, you stated - 'Don't mistake the DC power supply for the amplifier's signal path etc' - so, for instance, mains power comes in from one source/direction and the signal/music comes in from another - is there a nexus or junction point or special component in hifi amps in which the AC and DC are channeled towards eachother and are combined in readiness to carry on their journey together through the pcb's circuitry ?



Thank you again, all who have given me valuable information, I promise, there won't be a whole list of supplementary questions after this