Choosing Trimpot for volume control

Kian · Sep 12, 2019

Hi all,

I have a 4 ohm 6W audio output and I like to control the volume using a trimpot or potentiomenter. My question is, how do I choose the resistance of the trimpot? Also, does the physical size and type of trimpot matters? I need something really small to be soldered onto the PCB.

Something like this:

100K Ohm Trimpot Breadboard Trim Potentiometer P104 with Knob for Arduino (Pack of 5): Amazon.com: Industrial & Scientific

www.amazon.com

Thank you.

dr pepper · Sep 12, 2019

Assuming you want to control the amp output strictly speaking for that kind of thing you need an L pad, you can get such things designed for speakers, to control the level of tweeters, or you used to be able to get them for use as faders in cars.

alec_t · Sep 12, 2019

Volume is normally controlled at the pre-amplifier stage, not at the power amplifier output. Where were you thinking of connecting the potentiometer?

Kian · Sep 12, 2019

I am connecting it just before the speakers. The audio is coming out from an AV board that plays a video file stored in SDCard. The audio connector has 2 pins (+ and -) this is originally connected to a speaker. I want to put a trimpot between the AV board and speaker and I hope to use a small trimpot if possible. Isn't this how those cheap computer speakers with a volume control knob work? They are using a bigger potentiometer.

alec_t said:
Volume is normally controlled at the pre-amplifier stage, not at the power amplifier output. Where were you thinking of connecting the potentiometer?

alec_t · Sep 12, 2019

Kian said:
I like to control the volume

If the control is going to be used frequently then a trimpot is unsuitable. Trimmers are intended for occasional use only.
Is the 'speaker' a powered unit with its own built-in amplifier, or is the 'speaker' simply the transducer?

Nigel Goodwin · Sep 12, 2019

Kian said:
I am connecting it just before the speakers. The audio is coming out from an AV board that plays a video file stored in SDCard. The audio connector has 2 pins (+ and -) this is originally connected to a speaker. I want to put a trimpot between the AV board and speaker and I hope to use a small trimpot if possible. Isn't this how those cheap computer speakers with a volume control knob work? They are using a bigger potentiometer.

No it's not, they use a potentiometer before the amplifier, a conventional low level volume control.

To do what you're looking for is possible, but you need a low resistance and high wattage (expensive and large) potentiometer, and you are unlikely to be able to find a log one, only linear. Something like 10 ohms, 10 watts should do.

Some thing like this:

Arcol Ohmite 12.5W 10Ω Rheostat 1.12A, RES10RE | RS

uk.rs-online.com

AnalogKid · Sep 12, 2019

Kian said:
I want to put a trimpot between the AV board and speaker and I hope to use a small trimpot if possible.

While this is possible technically, it is a bad idea. AND, a trimpot will not work. You need a pot rated for at least 10 W, something that is large and expensive.

Way back when, this was called an L-Pad.

L pad - Wikipedia

en.wikipedia.org

ak

crutschow · Sep 12, 2019

Autotransformer speaker volume controls are also available, which are more efficient than a resistive pot, but they are still large..

Kian · Sep 12, 2019

Basically the volume come out from the AV board is too loud. And I want to reduce the volume coming out from the speakers. I don't need to adjust it often and I don't need the volume to change linearly. As long as I can make the volume softer and leave it there, that will work for me.

I saw many youtube videos and online instructables and they all just use a simple potentiometer just before the speaker. Does it mean that all of them are doing it wrong?

I have some space constraints hence I want to use something as small as possible to adjust the volume. I don't want to use a big potentiometer with a huge knob.

crutschow · Sep 12, 2019

Kian said:
I saw many youtube videos and online instructables and they all just use a simple potentiometer just before the speaker. Does it mean that all of them are doing it wrong?

Generally I's have to say yes, they are doing it wrong.
Can you post a link for any of these videos/circuits?

Nigel Goodwin · Sep 13, 2019

Kian said:
Basically the volume come out from the AV board is too loud. And I want to reduce the volume coming out from the speakers. I don't need to adjust it often and I don't need the volume to change linearly. As long as I can make the volume softer and leave it there, that will work for me.

I saw many youtube videos and online instructables and they all just use a simple potentiometer just before the speaker. Does it mean that all of them are doing it wrong?

Most probably - unless it's a LARGE pot as I posted the link to.

If you're not wanting it adjustable, them simply add a wirewound resistor of a suitable value in series with one of the speaker wires - you will probably have to try a fair number before you find a value you like the results from. Depending how quiet you want it, you may be able to use a low wattage resistor anyway, it's quite incredible how little adding a series resistor drops the volume - I would sugest tryind 47 ohms or so for a start, and see how loud that is.

I have some space constraints hence I want to use something as small as possible to adjust the volume. I don't want to use a big potentiometer with a huge knob.

Well you could use a large pot with a small knob

Les Jones · Sep 13, 2019

Kian,
When you say "speaker" do you really mean a unit that is just a speaker (Or number of speakers. ) in a box with no built in amplifier or do you mean a unit that contains a speaker AND AN AMPLIFIER ? The difference between these two situations makes a big difference to the way you control the volume. Some information on the AV board and speakers would be helpful. It seems strange to have a system with no means of controlling the volume.

Les.

alec_t · Sep 13, 2019

Agreed. I asked the same question in post #5 but the TS hasn't replied.

Nigel Goodwin · Sep 13, 2019

alec_t said:
Agreed. I asked the same question in post #5 but the TS hasn't replied.

Well he did say:

I have a 4 ohm 6W audio output

In his original post - which leads you to believe it's a standard speaker.

But as always details are vague, and not much help.

Kian · Sep 13, 2019

Hi everyone, my apologies for the late reply. I am using a standalone single speaker without no amplifiers. I think the amplifier is built into the AV board. The AV board is something like this:

**broken link removed**

Also, the audio signal coming out from the AV board is + and -, and the - is not connected to ground. There is a remote control which allows me to control the volume on the AV board. But I need to be able to control it without the remote control hence I wanted to add a volume control knob between the AV board and the speaker.

Les Jones · Sep 14, 2019

Why can't you use the volume control buttons on the remote control ? (The two buttons to the right of the input select button.) If you insist on attenuation between the output and the speaker use switch selected attenuation values with the attenuation done with fixed resistors.

Les.

audioguru · Sep 14, 2019

His info comes from an "Instructable". Some Instructables are admitted to being written by a 10 years old kid who knows nothing about what he wrote about.
A volume control is supposed to be logarithmic and connected at the input of an audio power amplifier so that the extremely low output impedance of the amplifier damps speaker resonances.

Kian · Sep 15, 2019

I can't use the remote in my design. I have no other choice, the volume control must be placed at the output of the AV board and before the speaker. I am willing to compromise and go for a bigger potentiometer if needed. I need a cost effective solution, something less than one or two dollars. The L-pad is too expensive. I also don't need the volume control be logarithmic. As long as I can adjust the volume (even in fixed steps that is fine too, eg soft, medium, loud etc). If I was to go for a standard potentiometer, what resistance value should I use and what wattage of should the potentiometer be? The output of the AV board is rated 6Watts.

Les Jones said:
Why can't you use the volume control buttons on the remote control ? (The two buttons to the right of the input select button.) If you insist on attenuation between the output and the speaker use switch selected attenuation values with the attenuation done with fixed resistors.

Les.

Nigel Goodwin · Sep 15, 2019

Kian said:
If I was to go for a standard potentiometer, what resistance value should I use and what wattage of should the potentiometer be?

I already told you, way back in post #6!

If you only wanted a few fixed levels, you could use a multiway switch, and a wirewound resistor in series for each position. You would need to find the values you like by experimentation.

Your problem is you're doing this in completely the wrong way, it should have been sorted out at the original design stage, not bodged on when it's far too late.

Kian · Sep 15, 2019

Thank you Nigel for the information. I must have missed your suggestion in post #6.

However, I need to clarify that I am just working within the constraints that were given to me. Its not a problem with my original design. I am given an AV board that already gives a 6W audio outputand this is connected to a speaker. My task is to find a way for the end user to control the output volume with a knob (without using the remote control). Of course, we learn in school about using a voltage divider for volume control. But it occurred to me that a potentiometer at the output stage would have challenges because of impedance mismatch problems and also the high current.

Nigel Goodwin said:
I already told you, way back in post #6!

If you only wanted a few fixed levels, you could use a multiway switch, and a wirewound resistor in series for each position. You would need to find the values you like by experimentation.

Your problem is you're doing this in completely the wrong way, it should have been sorted out at the original design stage, not bodged on when it's far too late.

Choosing Trimpot for volume control

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