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Audio amplifier - how difficult?

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giftiger_wunsch

New Member
Hi,

I have a server set up at home, and one of its major functions at the moment is to play media which has been downloaded and stored on it, via a VGA interface with my plasma TV and phono cables fed through the DVD player into the surround sound. However, the audio volume is very quiet, and requires maximum volume on the DVD player to be heard, despite the fact that my sky box can usually be heard perfectly well at 25% of the maximum volume.

I have now measured voltage output on the phono cables from both devices prior to amplification by the DVD player, and detected an average of ~6mV from the server and ~200mV from the sky box, demonstrating this difference.


Either way, I'm an absolute beginner with electronics but I'd like to start pursuing some projects, so this seems like a good thing to try. How difficult is it likely to be to achieve this, and does anyone have any tips or resources which may help?

Thanks in advance.
 

crutschow

Well-Known Member
Most Helpful Member
I confused about how everything is hooked up. Can you supply a diagram?

If I understand, the output of the PC server is low. What output are you using from the PC? There is usually a line out (constant volume) and an adjustable output. If you are using the adjustable output you need to connect the the line out instead, or go to the volume control (Sounds and Audio Devices in the Control Panel) on the PC and increase it.
 

RadioRon

Well-Known Member
Hi,

I have a server set up at home, and one of its major functions at the moment is to play media which has been downloaded and stored on it, via a VGA interface with my plasma TV and phono cables fed through the DVD player into the surround sound. However, the audio volume is very quiet, and requires maximum volume on the DVD player to be heard, despite the fact that my sky box can usually be heard perfectly well at 25% of the maximum volume.

I have now measured voltage output on the phono cables from both devices prior to amplification by the DVD player, and detected an average of ~6mV from the server and ~200mV from the sky box, demonstrating this difference.


Either way, I'm an absolute beginner with electronics but I'd like to start pursuing some projects, so this seems like a good thing to try. How difficult is it likely to be to achieve this, and does anyone have any tips or resources which may help?

Thanks in advance.
Building an audio amplifier is a reasonable first project, but I agree with Crutschow that such a thing should not be needed. A soundcard should be easily capable of delivering 200mV, so something is mis-adjusted I think.
 

giftiger_wunsch

New Member
I've already exhausted virtually all other possibilities before arriving at the conclusion that it'll require an audio amp. All volume controls on the server are, of course, set to maximum. I replaced the original phono cables with gold-contact cables, but had no improvement. I've also tried other devices such as an ipod as the output device, but didn't receive a stronger signal than I received from the server.

As a result, I came to the conclusion that the sky box was pre-amplified and produced a stronger signal than most devices, rather than my server output being low. Perhaps I will check the output voltages of various other devices, including my other computers. The server wasn't originally going to be used for this purpose, so its sound card is actually onboard sound, so perhaps that is responsible for the poor signal.


I'll get back to you with the results, feel free to make suggestions.
 

giftiger_wunsch

New Member
Hmm... when my ipod was used as the output device to feed into the DVD player, it didn't seem any louder than my server's output. I suppose it's deceiving; I measured the ipod as having a 400mV peak signal when at maximum volume.

I also played the exact same movie file used on the server, on my desktop PC this time. And I measured an average of approximately 100mV, about 15x more than my server outputs. Perhaps I should simply buy a sound card.


Regardless though, an audio amplifier sounds like a worthwhile project to attempt, so if anyone could give me any tips or point me to any resources which may help me, that would be appreciated.

Thanks.
 

Techie7

Member
Usually onboard soundcard has three outputs: Pink: Mic in; Blue: Line in; Green: Front speakers out. Sometimes Line in can have a low output for rear speakers and will deliver full volume when 4 channel output is enabled. So you've to connect the dvd player audio in to the green socket. Should be a strong output mostly you dont need more than 25% of max volume. I had this problem before when I connected to line in socket.
Else audio amp is not necessary, either troubleshooting soundcard or dvd audio should work out.
 

giftiger_wunsch

New Member
The line to the DVD player is already connected to the green speaker socket; it would be immediately obvious if this was not the case, as neither of the other two produce an output at all.

As mentioned above, I'm getting an average of around 6-8mV output at max volume, and when that reaches the DVD player it requires the DVD player's maximum volume to produce a reasonable volume to hear conversations, etc., in the programmes I'm trying to watch; some programmes are too quiet even at maximum volume. I believe the DVD player has a 40W amplifier.
 

flat5

Member
I'm interested to know the audio voltage at the cable plug before and after connecting to the DVD player. Is the DVD player loading the cable too much? If so, is the DVD player defective?
 

giftiger_wunsch

New Member
The DVD player has always worked correctly before; the audio voltage at the cable plug averages 6-8mV unloaded, it would be difficult to test the voltage with the cables plugged into the DVD player. However, the sky box plugs into the video 1 port of the same DVD player and has no audio issues, nor are the volume issues resolved by connecting the server to the video 1 port instead of video 2, where it is currently connected.

The DVD player doesn't seem to be responsible for the issue.
 

Techie7

Member
I would give some practical debugging rather than theoretical measurements and assumptions.
1. Connect an ordinary computer speaker to the server and check for acceptable loudness.
2. If fails, try re-installing the onboard soundcard driver - usually within the MB CD.
3. If not solved, buy a cheap(according to your pocket) PCI soundcard and you are done!
This is applicable only if your dvd audio is OK.
 

giftiger_wunsch

New Member
Thanks for the advice techie, but I have a reasonable amount of experience and had already gone through all of these options before deciding to test the output voltages. I'm going to buy a cheap soundcard since all the tests I've done appear to suggest that the onboard sound's output is very weak.

Anyway, whether I need one in this situation or not, I'm currently extremely bored inbetween years at university and I'm trying to find some worthwhile electronics projects to attempt, so if someone could point me to some information I might need to know when attempting to build an audio amplifier, that would be helpful.

Thanks.
 

audioguru

Well-Known Member
Most Helpful Member
What is needed is called a preamplifier, not a power amplifier which most people call an "audio amplifier".
A low noise audio opamp makes a good preamplifier and a dual opamp is used for stereo.
 

audioguru

Well-Known Member
Most Helpful Member
An opamp is an electronic "chip" that is used to make many different things.
Look for it in Google.
 

giftiger_wunsch

New Member
Thanks, I looked up op-amp on wikipedia but had a hard time understanding it, especially the difference between impedance and resistance. I guess I'd better do some research before considering this project.
 

confounded

New Member
To really explain an op amp you need to know a little about transistors, and a few clever ways of using them first.

To explain without knowing about transistors, an op amp is a chip.
It can come with 1 op amp on a chip or a few.
An op amp requires a power supply.
An op amp has 2 inputs called inverting input and non inverting input.
An op amp has one output

An op amp will mutltiply the difference in voltage across its inputs by around 200,000 and the result will be its output.
Obviously unless your input difference is very small the output will be saturated and go to either fully positive or negative. (ie nearly 15V or - 15V in my sketch)
This kind of gain is pretty useless as it is, due to distortion

Something called negative feedback is used to reduce the gain to a much smaller amount and this makes the op amp have extremly low distortion.

Negative feedback is obtained using resistors (see diagram)
Note the power supplies are not usually shown

Typically you keep gain to less than a few hundred.
Notice inverting amp gives gain and inverts polarity
non inverting amp just gives gain
gain = vout/vin ie 10 gain = 10x voltage in is outputed. But op amp can only output to close to its power supply.

An op amp has extremmly high impedance in the noninverting amp and reduced impedance in inverting amp due to the feedback resistors. Impedance can be seen as if you supply a voltage, what current is conducted. High impedence = low current

I used this site that has cool little models you can input variables in and see the result to learn about op amps.
ElectroSim: Electronic Circuit Simulations
 

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giftiger_wunsch

New Member
Thanks for the explanation. I'm still trying to get my head around what 'inverting' and 'non-inverting' inputs are though. Maybe I just need to learn a bit more about electronics in general before I can attempt electronics projects such as this.
 

audioguru

Well-Known Member
Most Helpful Member
The output of an opamp is "in-phase" with the non-inverting input and is "out-of-phase" with the inverting input. Therefore the inverting input is used for negative feedback.
 

giftiger_wunsch

New Member
Right... so combining the output with the inverting input will essentially dampen the signal, hence the term negative feedback? Now I just need to get my head around how the circuit diagrams posted by confounded work...
 
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