[Q] Potentiometer for controlling audio volume

[1234568]

Hey. I am not sure if this is the best site to post this question in but I figured that some bright spark might be able to help.

I am making a little box that will contain a switch to allow me to direct an audio input to one of two outputs. For this I have purchased a 3PDT switch and I am comfortable with how that will work.

I also want to be able to control the volume outputted. For this I believe my best option is a potentiometer, as opposed to a rheostat. However, I am unsure what resistance and power rating I need. I would appreciate it if anyone with experience on this kind of project could help.

Many thanks
Ed

 

[Marvo]

There are a few guys with an electronic background around the forum. Can you give more details about what the audio input is for and why you want to split it?

 

[1234568]

Of course. All connections will be made in standard stereo audio cable carrying line level audio. The source is my PC and the outputs are some speakers and some wireless headphones.

To achieve the switching I have ordered the following 3PDT switch. http://www.amazon.co.uk/gp/product/B008AGVBWM/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

As mentioned I also want to control the volume. I was hoping that I could place the potentiometer in between the input and the switch, thus enabling volume control of both outputs.

I know that for volume control I will need a log POT. I believe line level audio is <2V. I am unsure what resistance is needed though. After reading about opinions seem to range from 10 ohm to 100 ohm.

Thanks in advance for any pointers!

 

[tazz]

Volume control pots are normally install before the amplifier, and not after. Will need more info on what you are trying to install, how many speakers, over what area...etc

 

[1234568]

With this setup the POT will be pre-amp, carrying a line level audio feed.

 

[tazz]

So I assume you are then feeding the amplifier(s) from the audio line......so again how many amps speakers.....as we have two methods here....4 speakers than ok, any more we need to look at 100v line....the more info we have the better we can help

 

[1234568]

Exactly... feeding the amplifier(s) from the audio line.

One of the amps is built into some stereo speakers.

The other is in some wireless headphones.

As far as I understand things though placing the POT in the audio line before these devices and the switch will negate their affect. This is all new to me so please correct me if I am wrong.

 

[tazz]

Again.....how many amplifier speakers are you using, over what areas...?
And what are you exactly trying to install..?

 

[1234568]

Apologies if I am being dense and misunderstanding the question. As linked to the speakers are a 2.1 setup (2 monitors and 1 sub), that are either side of a PC screen so very small area. The headphones are also 2 speakers.

 

[tazz]

Again very brief info......do you want audio and video to be distributed to 2 other monitors from a computer, for teaching or entertainment....?

 

[amlu]

double gang 10k ohm log pot should do the job...
Potentiometers (Beginners' Guide to Pots)

 

[tazz]

double gang 10k ohm log pot should do the job...
Potentiometers (Beginners' Guide to Pots)

Without knowing the installation, or the equipment used....how can you presume this..?

 

[Lucien Nunes]

For controlling ordinary line level audio, 10kΩ log is typical. It is a compromise - too low and you form an attenuator with the source impedance, too high and your output side has such a high source impedance that it picks up interference and forms an attenuator with the load impedance at midway settings.

If you are driving it from a headphone output, then I would go for 1kΩ as the output is intended to drive loads as low as 32Ω. Remember to ground the shell of the pot if it is metal. Power rating is unimportant. If it is going to get heavy use, go for conductive plastic track rather than carbon.

 

[tazz]

Again presuming without info.....If were talking about one line output to multi inputs we need to balance mix......unless we get the full info here, you are all guessing

 

[1234568]

Again very brief info......do you want audio and video to be distributed to 2 other monitors from a computer, for teaching or entertainment....?

Video? As mentioned it's a stereo audio feed, at line level (i.e. what your headphone out is from a computer). Going from PC to speakers or headphones.

double gang 10k ohm log pot should do the job...
Potentiometers (Beginners' Guide to Pots)

Thank you. My research suggests 10k to 100k ohm log POT's. I am unsure of exactly what to try in this range. What makes you say 10k out of interest.

For controlling ordinary line level audio, 10kΩ log is typical. It is a compromise - too low and you form an attenuator with the source impedance, too high and your output side has such a high source impedance that it picks up interference and forms an attenuator with the load impedance at midway settings.

If you are driving it from a headphone output, then I would go for 1kΩ as the output is intended to drive loads as low as 32Ω. Remember to ground the shell of the pot if it is metal. Power rating is unimportant. If it is going to get heavy use, go for conductive plastic track rather than carbon.

Is a headphone output not also a line level output? Why the change from 10kΩ to 1kΩ? Thank you for the tip about grounding the shell of the pot.

Again presuming without info.....If were talking about one line output to multi inputs we need to balance mix......unless we get the full info here, you are all guessing

Please let me know what further info you require. This is all new to me and I appreciate the help. My reading about line level (mainly the wiki article) leans me to believe that it is usually relatively similar regardless of device and so the balance should be ok. Is this incorrect?

 

[Lucien Nunes]

A headphone output is similar to a line level output but it is optimised for driving a lower load impedance, as typical headphones offer 32-120Ω load compared to a line input's 10-100kΩ. The signal level (voltage, not power) of a headphone output is also typically lower. However, a headphone output will serve quite well as a line output and they are often used as such. Some equipment offers you a mode selection for the output socket, with line mode delivering a higher level.

Any value from say 470Ω to 47kΩ will work with a line output, with varying degrees of excess signal attenuation. Given that you expect to drive it from an output capable of driving a 32Ω load, I would tend towards the lower end of that range for best results. The lower the pot end-end value, the lower source resistance it will present to the amplifier and hence the less attenuation will occur there and the less sensitive it will be to noise pickup.

I note you mention a 3PDT switch. Do not switch the grounds, join them all together, otherwise if the ground connection breaks first as you switch, you might get a loud thump.

 

[tazz]

ok audio line only, but feeding how many speaker amps, from one line output...?

 

[1234568]

A headphone output is similar to a line level output but it is optimised for driving a lower load impedance, as typical headphones offer 32-120Ω load compared to a line input's 10-100kΩ. The signal level (voltage, not power) of a headphone output is also typically lower. However, a headphone output will serve quite well as a line output and they are often used as such. Some equipment offers you a mode selection for the output socket, with line mode delivering a higher level.

Any value from say 470Ω to 47kΩ will work with a line output, with varying degrees of excess signal attenuation. Given that you expect to drive it from an output capable of driving a 32Ω load, I would tend towards the lower end of that range for best results. The lower the pot end-end value, the lower source resistance it will present to the amplifier and hence the less attenuation will occur there and the less sensitive it will be to noise pickup.

I note you mention a 3PDT switch. Do not switch the grounds, join them all together, otherwise if the ground connection breaks first as you switch, you might get a loud thump.

Thank you very much! This is starting to make more sense in my head. I didn't realise there was a difference between headphone output and line level. I will read up more on these differences and think how they relate specifically to me.

Also, great tip about the grounds. If I am joining them all together would a DPDT switch have done the job?

 

[Lucien Nunes]

One at a time, he said. In practice with average consumer equipment you can get away with dozens or even hundreds of loads from one source before the attenuation becomes a problem. Only if the input impedance is unusually low or the output impedance unusually high, would you have to limit the fan-out to one or two loads.

Obviously it is better to buffer outputs to a low impedance and this is done in all pro gear. Ideally, there would be a buffer after this volume control, but because it is not driving a long cable or a low load impedance, one can manage without. The lower the pot end-end resistance, the stiffer the source it provides, hence my choice of a lower value where possible.

If I am joining them all together would a DPDT switch have done the job?

Yes.

Now there is another factor to consider with switching, and that is that an open switch has capacitance across its contacts and when the load impedance is very high, this can pass enough current at high audio frequencies to be audible, and a floating input is very prone to picking up hum and static. Therefore, the correct way to switch audio is to switch unused inputs to ground, not merely disconnect them from the source. That would require a 4PDT switch.

 

[tazz]

19 posts and still no information, on what you are trying to install.....all guess work