Audio Attenuator circuit

[Tianfeng]

Hi,

I am new to building stuff and I am trying to attenuate an audio signal used to drive an 8ohm 5w speaker for an arcade cab.  So far I have found that the audio out has a 0-6v swing and I used a 5.62k and 2.2k resistor as a voltage divider to bring the voltage down to 1.66v which is about where I want it incase some boards are quieter. I think this is the proper way to do this but if I am worng please let me know.

 After the voltage divider I am putting it through a capicitor but I can't figure out how to properly calculate the value. I have tried 1uf and 47uf.  The 47 seemed more muddy but I can't be sure. 

When I asked in an arcade forum they just pointed me to pre-made ebay stuff but I want to learn why and how this works as much as I want to get it working right. 

 

[Richard Crowley]

Please clarify WHERE you are trying to attenuate the audio signal.  Are you trying to attenuate it at the INPUT to the power amplifier, or at the OUTPUT?  It is not clear why the specs of the speaker are significant here if you are asking about attenuating the "line-level" audio signal going into the power amp.

Why wouldn't you simply use a potentiometer as a variable "volume control"?  Depending on how you are deploying your gadget, you could put  knob on the back of the console for adjusting the volume, or you could put it on the front where it is a user-control.

The larger capacitor will better pass low frequencies (along with the higher frequencies).  When you say that it sounded "muddy" it seems possible that is more a function of the speaker and whatever enclosure you have it in.  There are ways of calculating the value of the capacitor, but you wold need to know the "input impedance" of the next stage (the input of the power amplifier).  It would help if you revealed what you are using for the power amplifier.

[Tianfeng]

Ok I can try to provide more details. I am using it as part of a supergun the the there is an onboard amplifier that is set up to drive a an 8ohm speaker and supposedly peaks at 6v. I am trying to bring down the voltage to line level audio after it is amplified.  I read that line level audio was generally 500mv before put into an amplifier or tv but many devices are actually closer to the 2v level.  I don't want to have an adjustable volume but just a consistent output.

EDIT*  The audio amp to drive the signal to the jamma edge is a TDA7240A.

[Richard Crowley]

So simply install a potentiometer ("volume control").
Isn't the solution that simple?  Are we missing something here?
A potentiometer is simply an adjustable voltage divider or attenuator.

[JPM_AEIC]

I don’t have much to add to this conversation, but I can tell you definitively that Line level audio is 1.23V. This is the voltage level you should be sending to the power amplifier.


Sent from my iPhone using Tapatalk

[Tianfeng]

So simply install a potentiometer ("volume control").
Isn't the solution that simple?  Are we missing something here?
A potentiometer is simply an adjustable voltage divider or attenuator.

There are pots for volume on the main board.  The issues is the device is designed to be set up with multiple different arcade boards and for safety you want to have it set to line level. I am not so much worried about the resistance, the circuit I am using is fine.  I am trying to calculate what value should be used for a capacitor.   This was one of the things I was reading about it and I was trying to figure out how to do those calculations.  Amy I completely off base and I should just chuck a 47uf cap in there and be done with it?  Does it not really matter that much?

[Tianfeng]

I don’t have much to add to this conversation, but I can tell you definitively that Line level audio is 1.23V. This is the voltage level you should be sending to the power amplifier.


Sent from my iPhone using Tapatalk

Ok, so using a 1.4k would give me 1.197v.  That makes it easy because that is what is being used on the video lines to attenuate the signal before the video amplifier. Thanks.

[Richard Crowley]

... I am trying to calculate what value should be used for a capacitor. 

The data sheet for the TDA7240A clearly shows that they recommend a 1uF capacitor.

Ref: http://www.mantech.co.za/datasheets/products/TDA7240A.pdf
Page 1  Fig. 1
Page 4 "APPLICATION SUGGESTION"

1.23V. will likely over-drive the amplifier.  That is a "nominal" voltage and not necessarily representative of your circuit.  The reason we use potentiometers is because the Real World often does not measure up to the theoretical values.  You may be over-thinking this.

[Tianfeng]

I forgot to link the article. https://sciencing.com/calculate-capacitance-ac-coupling-8735810.html

The specs for the amp would be when it is taking in power from the 12v line and designed to drive speakers without an amplifier.  I thought because I am adding additional resistance to the signal through attenuation I would need ac coupling caps on the output.

 I get the potentiometer suggestion but I thought that line level audio outputs were fixed for a reason?  The more I keep reading the more confused I get.
Thanks for your help though. I appreciate it.

[Richard Crowley]

I forgot to link the article. https://sciencing.com/calculate-capacitance-ac-coupling-8735810.html

That is great if you are designing a circuit from scratch.  But you are using a commercial commodity power amplifier chip with a known input impedance.  The people who make the power  amplifier chip (ST Microelectronics) clearly recommend a 1uF input capacitor.  Furthermore, on page 4 (see my reference above) they tell you the effect of using a smaller or larger value for the input coupling capacitor (C1)  You couldn't ask for a more clear and straightforward answer to your question.

The specs for the amp would be when it is taking in power from the 12v line and designed to drive speakers without an amplifier.

What does "without an amplifier" mean?  The TDA7240A is the amplifier.

I thought because I am adding additional resistance to the signal through attenuation I would need ac coupling caps on the output.

No.  You need an AC coupling capacitor to isolate the source circuit (your gaming gadget) from the destination circuit (the TDA7240A amplifier).  It has nothing to do with using "additional resistance" or "attenuator" or "volume control" pot or anything else.  The source of the signal (the gaming gadget) may (or may not) have some DC coming out of the circuit. If you don't decouple the DC, then it will get amplified in the power amplifier and that is bad. So you simply use a DC blocking / AC coupling capacitor.  And ST tells you exactly what value to use.

I get the potentiometer suggestion but I thought that line level audio outputs were fixed for a reason? 

Audio levels are NOT FIXED!  Those numbers (like "1.23V" are NOMINAL values.  That means that they are the "names" of the signal levels.  The actual level will vary widely, mostly LOWER than the "nominal value".  Don't get so hung up on theory without actually getting into how circuits function in the Real World.

[Tianfeng]

I just wanted to finish this off with a quick photo of what of was talking about. You can ignore the video section.   Is this still a voltage divider with the placement of the capacitor between the two resistors?

[Richard Crowley]

Yes. The audio signals are going through a fixed attenuator where roughly 20% of the voltage goes to the TV.  Apparently the source puts out a pretty "hot" audio signal and the circuit knocks it down to a more conventional audio level for the TV.