Control external speaker volume

GreenGiant83 · 2006-10-04 20:02

I want to be able to change the volume a speaker via my basic stamp.· The speaker will have its input from a seperate CD player, I need to be able to change the volume from completely off(no sound)· gradually all the way to full blast.· Is this something feasible, and if so what should i do?·

metron9 · 2006-10-04 20:56

Ahhh I found it,
http://www.electronic-circuits-diagrams.com/audioimages/audiockt11.shtml

I searched "circuit volume control" on google. It was tough finding anything using the word speaker in the search parameter, this is one of 15 million hits but it is number 1 in the 15 million.

After looking at the circuit it simply shorts the speaker output to ground to turn it off. I don't think an power amplifier would like that very much so better do some more digging. Speakers are rated in ohms, to low of resistance can blow an amplifier so I guess I would like to see how to do it right as well.

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Post Edited (metron9) : 10/4/2006 9:06:35 PM GMT

Mike Green · 2006-10-04 21:26

Any of the digitally controlled pots from Dallas/Maxim could work. Some are serial interface (SPI or I2C) and some are Up/Down with a reset pin and and Up and Down pin. They may have anywhere from 256 to 1024 or more discrete positions. I would try to control the volume between the CD player and the speaker amplifier. It's easier that way. Typically, you ground one end of the pot, connect the other end to the CD player, and the tap to the power amplifier. A 5K or 10K pot would probably work better than a higher value.

GreenGiant83 · 2006-10-04 23:47

I was thinking that was how I would have to go.· I was hoping for a cheaper solution.· I want to control a 4x4 speaker array, I dont know for sure but I was thinking that digital pots costs around $5, so that would be around $80.· Is there any way I could send on and off pulses to a resistor and a capicitor pair linked to a npn transitor on the speakers line to vary the output?

metron9 · 2006-10-05 00:16

$8.41 for a QUAD 24 pin at digikey.com

http://www.intersil.com/data/fn/fn8189.pdf

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Mike Green · 2006-10-05 01:05

How many actual channels do you have? Are you using all 16 speakers simultaneously?

It's usually not the best idea to try to control the low impedance, relatively high power signals at the speakers themselves, but control the levels upstream in the amplifier chain. It's also easier to switch signal sources there. Usually when you control speaker levels at the speaker, there's a low resistance pot (8-16 ohm) with multiple tracks/windings that's designed to maintain a constant load on the amplifier as you decrease the signal to the speaker (sometimes called an L-Pad or T-Pad depending on the design).

GreenGiant83 · 2006-10-05 01:20

I am not exactly sure what you just said, but I appreciate the information.· My setup would at best employ 4 speakers at the same time.· Basically I am trying to move sound around a room by raising the volume on one speaker and lowering on the previous one to give the impression that the sound is coming from a place inbetween the two, or rather four.· So at any given time there may be four speakers on, but each only at 25% power. Or two on with one at something like 70% and the other at 30%.· I hope I am being clear.· And if you can spare the time, could someone explain the difference between resistance and impedance?· I know discussions always talk about impedance, but I am unclear on the difference.

Thanks.

El Paisa · 2006-10-05 02:52

Are you looking just for a plain volume control or volume control with AGC?

metron9 · 2006-10-05 03:01

Here is what you are looking for
L-PAD
http://amps.zugster.net/articles/attenuation
T-PAD
http://www.uneeda-audio.com/pads/

The Thread I got that from
http://www.hawthorneaudio.com/forum/viewtopic.php?t=243&sid=59396f543251326675c471f732cd440e

Also on the impedance/resistance question
http://en.wikipedia.org/wiki/Electrical_impedance

My son plays guitar with a very loud amplifier, he constantly says he has to have it loud for the tone to be good, I am glad I researched this question maby I can get him to turn it down.

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Post Edited (metron9) : 10/5/2006 3:06:49 AM GMT

Mike Green · 2006-10-05 03:24

You could get 16 L-Pads from an electronics/audio supplier and control them with servos. A Parallax servo controller would handle all 16 and could be controlled from a Stamp or from a PC. This would be noisy because of the servo motors and would have to be in some kind of box with sound deadening material. Alternatively, you would need a small amplifier for each speaker and control the input to the amplifier with a digital pot set from a Stamp.

There are some nice little stereo amplifiers in kit form with a power output of several watts, enough for a small speaker. You could get 8 of them and do the digital pot thing on each of 16 channels.

I did a sound installation a couple of months ago with two independent channels switched among 4 speakers. The speakers were changed between sound bites (so there were no pops or clicks). It's not quite what you have in mind, but it was effective with the voices changing every 15 to 60 seconds or so and moving around the gallery.

metron9 · 2006-10-05 03:59

After doing some reading. i wonder if any real EE's have a comment as to if this design would work for a simple volume control. Using 2 speakers one with the cone removed or muffeled. Simply running the one side of the speaker wire through a pot you can adjust the volume by diverting the signal to the muffled speaker.

Comments?

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Think outside the BOX!

Mike Green · 2006-10-05 05:36

A pot is a potentiometer which is a variable resistor. Do you mean pot or a relay (switch)?

Rather than using a modified speaker, you could just switch in a fixed resistor in parallel or series with the speaker to convert some of the energy into heat rather than sound. This would be smaller and cheaper than using a modified speaker.

The problem with controlling the sound at the speaker is that there's a significant amount of power involved, at least several watts. You could use 2 relays at each speaker to add more resistors in parallel with the speaker or in series with the speaker to reduce the sound volume. These relays would have to handle the power level at the speaker. With 8 ohm speakers and several watts of power, you may be talking about 1 amp of current with peaks higher than that. That's not huge, but a small reed relay may not handle it. More substantial relays typically need more than 20ma to drive, so you'd need a transistor between the Stamp and the relay or an I/O expander like the 74HC595 and the relay. You're now talking about 32 relays plus the I/O expander plus the transistors.

Mike Green · 2006-10-05 05:45

Resistance is literally the resistance to electrical flow and is independent of the rate of change of the electrical flow so resistance is not affected by the frequency of any signals involved or the shortness or length of pulses.

Reactance is similar to resistance, but is dependent on the rate of change of the electrical flow. There's capacitive reactance where there's little opposition to electrical flow initially, but this increases as time goes on. There's inductive reactance where there's huge opposition to electrical flow at first, but this becomes less with time. You can have both in the same circuit along with resistance and their effects combine. A capacitor impedes the flow of current at low frequencies, but passes high frequency current relatively easily. The opposite is true of an inductor.

metron9 · 2006-10-05 06:20

I mean potentiometer. I was thinking he was using just small cheap speakers like 2 inch transistor radio speakers, the idea here was the load would not be static like a resistor so the amplified signal would not change (from discussion on guitar amplifier link)

If the OP is using 16 speakers and a higher power amplifier, he better check the impediance minimum of the amplifier, an array of 16, 8 ohm speakers, 8 on one side would only be 1 ohm impediance, not many amplifiers can drive that load. I have a 2000 watt crown that drives 2ohms I use 4 dual12 bass reflex speakers with horns for my outdoor sound system.

Hmmm I wonder if you could just cascade the speakers with potentiometers like picture attached. (Its 1AM and this is a quick idea sketch)

So the pots just bypass each speaker sending the voltage through the least resistive path.

Or just use transistors in linear mode!

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Think outside the BOX!