Impedance Matching Speaker level switch

I'm wondering if anyone has ever seen a circuit for such a device or knows what type of relays or transistors to use? This is amplified audio, not line-level so I don't believe I can use standard relays. I'd like to control it via serial and, optionally, TCP. That part I can handle, it's the switching that I am lost on. Any thoughts to get me started?

Exactly what do you want to achieve?
Impedance matching is something you do at the design stage, or you use transformers to do.

I have a 2 channel amp. That will be my input. My output will be 8 speakers that are 8 ohms a piece. I need to be able to select which speakers are on. I also need to make the impedance stay above 4 so that I don't blow the amp. I'd like to keep the final impedance fairly constant regardles of the number of speakers that are on in order to avoid volume level issues. Here is a similar product but it is no longer in production. There is a chart towared the bottom that shows the impedance relative to the number of speakers that are turned on: http://hacs.com/nab8ssspec.php

For the price it will probably be better for me to buy one of these: http://www.monoprice.com/products/product.asp?c_id=109&cp_id=10903&cs_id=1090305&p_id=8230&seq=1&format=1#largeimage then hack it to be controlled by the Arduino. Agreed?

Relays are fine. Just make sure they can handle the current. Current = square root of (Power/Resistance.)

You don’t need a constant impedance load on the amp. Solid state amplifiers are low-impedance “constant voltage” devices… The output voltage doesn’t vary with the load, so when you switch-on additional speakers (in parallel) , the signal to the 1st one doesn’t decrease.

I have a 2 channel amp. That will be my input. My output will be 8 speakers that are 8 ohms a piece. I need to be able to select which speakers are on. I also need to make the impedance stay above 4

Put an 8 Ohm resistor in series with each speaker.

Each one of your speaker-resistor combinations will be 16 Ohms. If you put 4 of those resistor-speaker circuits in parallel, the total impedance will be 1/4th, or 4 Ohms on each channel.

Now, the total power for each stereo-channel is going to be divided among 4 resistors and 4 speakers. So, if your amp is rated at 8 Watts per channel at 4 ohms, each speaker will get 1 Watt.

FYI - A more-professional set-up would use a separate amp for each pair of speakers, or an 8-channel distribution amp ([u]example[/u]), or a [u]70.7V system[/u] with a transformer on each speaker. The downside to transformers is you generally loose some frequency response. (i.e. Most gocery stores use 70V systems.)

I think I understand the constant voltage part but why does the power seem non-linear in the specs? 6 ohms 2 x 125 watts/channel 8 ohms 2 x 100 watts/channel It seems like if it were constant voltage, considering 100 watts @ 8ohms, the power @ 6 ohms would be 75 watts, no?

Sadly, after taking a closer look at my amp, it is only good down to 6 ohms :( So to use your method, I believe I would have to up the resistance of each speaker to 24ohms by using the 8 ohm speaker and a 16 ohm resistor. This would give me a range from 6 ohms with all speakers on and 24 ohms with just one on. Is there any harm in running an amp at 24 ohms? Since it seems non-linear, can I approximate the power at that resistance? With all on, if I did the calcs correctly, I believe that at 6 ohms, I'm only going to be driving the speakers with 4.17 watts? My speakers have a 5 watt min. so I'm kinda out of luck there. Is there any other methods to accomplish this without adding resistance? Some sort of multi-plexing scheme or something? The device I posted kept the impedance pretty constant around 4 or 5 ohms. I wonder how they accomplished this.

A friend of mine had a sugestion that's sounded good. He suggested using 4 relays with NO and NC contacts. Put an 8 ohm resistor on the NC contact and connect all the relays in a parralel series such that I end up with 8 ohms total impedance. Then on the other contact, connect the speaker. This way, when I de-energize the relay it cancels out the speaker and flows through the resistor. Energize the relay and the resistor is removed from the circuit. That way I will always have 8 ohms total impedance.

n8huntsman:
I think I understand the constant voltage part but why does the power seem non-linear in the specs?
6 ohms 2 x 125 watts/channel
8 ohms 2 x 100 watts/channel
It seems like if it were constant voltage, considering 100 watts @ 8ohms, the power @ 6 ohms would be 75 watts, no?

Sadly, after taking a closer look at my amp, it is only good down to 6 ohms :frowning: So to use your method, I believe I would have to up the resistance of each speaker to 24ohms by using the 8 ohm speaker and a 16 ohm resistor. This would give me a range from 6 ohms with all speakers on and 24 ohms with just one on. Is there any harm in running an amp at 24 ohms? Since it seems non-linear, can I approximate the power at that resistance? With all on, if I did the calcs correctly, I believe that at 6 ohms, I’m only going to be driving the speakers with 4.17 watts? My speakers have a 5 watt min. so I’m kinda out of luck there. Is there any other methods to accomplish this without adding resistance? Some sort of multi-plexing scheme or something? The device I posted kept the impedance pretty constant around 4 or 5 ohms. I wonder how they accomplished this.

I suspect that the reason you see non-linearity in the specs is that the specific amplifier is limited by it’s heatsink capacity. Do a thermo analysis and see if that is not what is driving/limiting the specification.

No harm running a semiconductor amplifier with higher impedance loads (even into an open circuit), where it could be a problem in the old tube output amps where transformer impedance matching was often used.

Lefty

I suspect that the reason you see non-linearity in the specs is that the specific amplifier is limited by it’s heatsink capacity

I was wondering if it had anything to do with power transfer under impedance miss match. You get the maximum power transfer between devices when the output impedance matches the input impedance. When there is a difference you get an impedance mismatch and as a result you do not transfer the maximum power. But is is just speculation because we don’t know how these figures were arrived at.

Grumpy_Mike:

I suspect that the reason you see non-linearity in the specs is that the specific amplifier is limited by it's heatsink capacity

I was wondering if it had anything to do with power transfer under impedance miss match. You get the maximum power transfer between devices when the output impedance matches the input impedance. When there is a difference you get an impedance mismatch and as a result you do not transfer the maximum power. But is is just speculation because we don't know how these figures were arrived at.

But modern class AB semiconductor amps utilizing voltage feedback (constant voltage output, irrespective of load impedance) represents a very low source impedance, the load is not 'matched', the maximum power output is limited only by maximum current availability from the power rail suppies, device max current ratings and of course keeping the output devices in their SOA from a heat dissipation view.

Lefty

But modern class AB semiconductor amps

Yes but do we know the OP has one of these?

Grumpy_Mike:

But modern class AB semiconductor amps

Yes but do we know the OP has one of these?

It would be most rare at those power levels that he not have a push-pull class AB amp, in fact I would bet money on it. ;)

It's a fairly modern Onkyo 2 channel amp. I'm sure it it.

Got my speaker selector from monoprice today. After opening it up and looking at it, I'm even more confident that I can build my own. This one would be too hard to hack. They use DPDT switches instead of DPDT relays. This would make it hard to tap into. The way this one works is simple. There are two circuits. Each one has the grounds ties together and places 5 ohms across the + line in, into a bus connected to 4 switches which will then go out to the speakers, essentially putting all 4 speakers in parallel. This will give you 11 ohms with one speaker on, all the way down to 7 ohms with all 4 on. There is also a switch to bypass the resistors if you don't wish to use the impedance protection.

Which method sounds better, the one above except relays instead of switches or the following... Either way I'll connect the relays to some transistors to be controlled by my Arduino and a max232 chip.

After some additional thought, I don't think that I want to use resistors for this because of the power that they consume and the voltage drop into the speaker. Looking at some other commercially available devices that do things similar, it appears that the higher end devices use transformers rather than resistors. I have a general idea how this might work. Does anyone have any insight into this?

Grumpy_Mike:

I suspect that the reason you see non-linearity in the specs is that the specific amplifier is limited by it's heatsink capacity

I was wondering if it had anything to do with power transfer under impedance miss match. You get the maximum power transfer between devices when the output impedance matches the input impedance. When there is a difference you get an impedance mismatch and as a result you do not transfer the maximum power. But is is just speculation because we don't know how these figures were arrived at.

I didn't notice this at first but P=(V^2)/R, therefore, 100=(V^2)/8 => V=28. Similarly, 125=(V^2)/6 => V=28

Yes, all modern amplifiers have a (relatively very) low output impedance. This is a good thing, in that it results in better speaker performance and lessened resonance effects. The ratio of output impedance to speaker impedance is called "Damping Factor" (W)http://en.wikipedia.org/wiki/Damping_factor

What this means is, IF you don't need the loudness of the speakers to approach the maximum loudness the amplifier is capable of, you can get away with paralleling the speakers, most commonly with relays.

Can you say more about the location of the speakers, the purpose, the background noise levels?

When I first worked as a Broadcast Engineer, impedance matching was highly important, and power amplifiers needed to have a load attached at all times. Relays were used to cut off the monitor speakers in a studio when a microphone was turned on, to avoid feedback and room echoes. The relays switched in a power resistor of 8 ohms in place of the speaker. But that was a while ago, before modern low-output-impedance audio amplifiers. Um.. OMG, 56 years ago this coming March 1.

Fortunately, Physics is still the same.

Can you say more about the location of the speakers, the purpose, the background noise levels?

I have 4 sets of these :http://www.nilesaudio.com/product.php?prodID=OS6.5&recordID=OS%20Indoor/Outdoor%20Loudspeakers&categoryID=Speakers&catcdID=1&prdcdID=FG00995 throughout my backyard. I live in a typical so-cal housing tract so keeping all the speakers on at all times is not desirable, to the neighbors anyways. If I go in the spa, I'd like the ability to only turn on that pair of speakers. Occasional, during parties and such, I'll turn on all the speakers, and chances are, that's when I will want the volume to be the highest. This is why I want to avoid a resistor in series before the 4 speakers in parallel. As stated earlier the amps is rated for 100w@8ohms or 125w@6ohms. For the last year I've only been running two pairs, in parallel (4 ohms total impedance) and havent had any issues with the amp. The volume is controlled via the zone 2 pre-amp out of my AVR. On the amp, there is an input level knob which I have turned down to about 25%. So I don't think that I'm pushing the amp very hard and even though I'm running it at 4 ohms, I haven't noticed any clipping.

Another method just using complex, but passive, switching contacts is to arrange to be able to have the speakers work in various series and parallel configurations so that they can present a relatively constant impedance to the amp, but divide the total power equally among themselves.

For instance if wanting to drive four 8 ohm speakers at the same time from a single amp channel, wiring two speakers in series and then other two is series, and the two series strings in parallel will present a 8 ohm load to the amp and equal power to each speaker. To run two, I would just wire them is series as the amp will have no problem driving a 16 ohm load. The switching can get a little complex but using either rotary switches or relays you should be able to come up with an arrangement to satisfy the amp but allow equal power to each speaker with no losses that resistors would cause by using various series/parallel speaker wiring.

There was a famous mono speaker construction project in the 50s that ran in Popular Electronics called the Sweet Sixteen, it was an array of 16 4" speakers wired in series and parallel arrangement such that the amp saw the array as a single 8 ohm speaker, but of course it was just the wiring arrangement that allowed that ( I will leave that as an exercise for the student). It was said to be quite a good sounding speaker, but of course the real challenge was the carpentry skills needed rather then electronics skills.

Lefty

That crossed my mind but the problem arises with three speaker. There are three combos for this. You could have all in parallel, which would give 2.6 ohms which is far too low. All in series would be 24 which, IMHO, is too high. Or could have two in series, with the third parallel. The problem with that one is that the third speaker in parallel will see higher voltage than the other two, and thus, have a different volume level. One option to accommodate that is to have an 8 ohm resistor in series with the parallel speaker, such that it is only there when there are 3 speakers enabled. This would have to be coded on the arduino side as I don't see how to do that with the circuit. I'd use less resistors but more relays. I still may choose this route if the transformer method doesn't pan out.