Instructables

Surround sound for free (it could save your marriage!)

If you've got four speakers and don't want to lash out on a new surround sound system (or 'er indoors refuses to have any more AV equipment in her living room!), try this Hafler circuit.

I first found this in Everyday Electronics in 1975 (thanks guys!), and have been using various systems (once in the car) ever since.

It's not a 7.1 system, but it will add hugely to your enjoyment of stereo - and especially to DVD film soundtracks. Plus it doesn't matter where you sit in the room - you always hear stereo.
 
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Step 1: Warning

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I have heard that this system can overload your amp.

I have been using this circuit on a range of hifi equipment for thirty years, and I've never had a problem. But continue at your own risk.

Step 2: Wiring

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The diagram shows what you have to do.

Stick two new speakers in the corners of the room OPPOSITE your existing ones.

Identify the + outputs from the back of your amp (your existing speakers will already be connected to them).

Now add an extra wire to each, without disconnecting the existing speakers, and connect those two wires to the + terminals of your two new speakers.

Now connect the two negative (-) terminals on your new speakers together.

Now wind up Guns n Roses - or whatever it is you like.

Hurrah! Instant surround sound!

Step 3: How it works

The two extra speakers actually produce exactly the same sounds as each other. They play the difference between the original stereo speakers.

In other words, anything that is in the middle of the stereo image is not reproduced by the extra speakers: this gives the impression of surround sound.

Try a DVD: the swoops and whooshes of an action film do actually sound as if they are moving around the room.

Extraordinary!

Respect to the great David Hafler who invented this system.

Happy listening.
iahaleem1 year ago
Somthing seems wrong here. By connecting the rear speakers to each other, wouldn't you end up crossing the Left an Right channels to all speakers? This happened to me once while I was connecting Stereo sound a 4 speaker setup similar to this.

What I ended up doing was running 2 sets of wires to the rear, one from the Left channel and one from the Right. I'm no expert, so please correct me if I'm wrong.
Adding an L-Pad to this set up also allows for controlling the volume of two extra speakers. Helpful in balancing the overall effect. I've used this set up successfully since the early 70's.
Could you elaborate on that a little? I'm using this setup and my rear speakers are quiet.
Front are 8 ohms
Rear are 4 ohms
My receiver says "A , B : 8 ~ 16 (ohms)"
See link for general principals.

http://www.bcae1.com/lpad.htm

for an example of what one looks like

http://www.minute-man.com/acatalog/Mono_L_Pad_Volume_Control.html

I found mine at radio shack. Only usefull if rear speakers are louder than needed, cannot boost volume but can dampen it if rear speakers are too loud.
LDW (author)  sc lightning3 years ago
I've always used the same impedance front and back. The back speakers are supposed to be quiet in comparison to the front ones, but any audio with a good - or wide - stereo image should give you the effect you're looking for.
i'm kinda confused about the overall resistance you would get from this. i have an amplifier that says not to go below 8 ohms, but i have two 8 ohm speakers per channel in parallel (which is 4 ohms per channel). apparently this is a huge no-no, but i dont blast the volume, and i've read that as long as you dont hear distortion, you should be fine. anyways, if i used four 8 ohm speakers in this system, what would the overall resistance be per channel? Also, what is this business with "common negative loudspeaker outputs"? my receiver has the standard speaker jacks that are on the backs of speakers and receivers and stuff. (i'm pretty sure this is a stupid question, but i have no clue.)
LDW (author)  zvillesurfer3 years ago
OK well I'm no expert here. But the general feeling seems to be that using higher impedance speakers is not going to cause damage. Check this out: http://community.whathifi.com/forums/post/18641.aspx "Higher impedance speakers are "easier" for the amplifier to drive because they require less current, but ultimately it's the sensitivity and the consistency of the impedance for speaker across the frequency range that really matters. Speakers with less than 8 (certainly less than 4) ohms are generally aimed at higher end amplifiers and will often be harder to drive. Driving lower impedance speakers can tend to make smaller amplifiers overheat and shut down so caution is advised..." Your speaker connections - at the speaker end, and/or at the amp end - should be identifiably negative or positive. This could be as simple as a red and a black connector - or even the way the connections are laid out. Hope this helps!
Keba4 years ago
About overloading your amp.

The way I see it, the only way you could overload your amp, would be to use speakers with too low a resistance, forcing your amp to deliver much more power than what is was intended for. Using this speaker configuration, your system would (assuming the ground on each output are connected internally) see a smaller resistance than the one speaker it was designed to drive. In the worst case scenario, your system would see each output as one speaker in parallel with 3 more speakers.

That means your system would see each output:
With 8 Ohm speakers, as a single 6 Ohm speaker
With 6 Ohm speakers, as a single 4.5 Ohm speaker
With 4 Ohm speakers, as a single 3 Ohm speaker

My suggestion: Look in the datasheet for your amp, for a minimum speaker resistance. Divide that by 3 and multiply that by 4 and you have the absolute minimum resistance your speakers can have, so that you don't overload your amp with this speaker configuration.

Sincerely, Keba

RRC5 years ago
Compare "Surround sound for free" with my "ABC surround sound" and you will be impressed. Four speakers is good configuration for surround sound since it creates three dimensions for semi-sphere up front of listener. No subwoofer is needed, three way speakers will shake liver like at rock concert. 'Surround sound for free" employs sound delay between speakers and listener, that is why it works well for fast changing sound effects. In contrast my system employs ABC decoding matrix which works for all stereo and digitally encoded signals.
HarleyDK RRC4 years ago
According to Dolby® the rear fill is the difference between the front channels, of course they use a delay circuit, but the idea to take the signal from the positive side of the front-channels is just up their alley.
LDW (author)  RRC5 years ago
Yes - your system is clearly better. But free? As I understand it, you are selling your decoder for $30 from www.igor777.com. So is yours an instructable? Or a sales pitch? No offence intended - but just want to clarify.
RRC LDW5 years ago
I realized that and come up with "Surround sound from 3.5mm stereo" which is "true" instructable describing decoder in details. The difference between two of them is handling power. However low power version can be upgraded to high power by increasing resistors dissipating power. It can be build for "free" if someone has NiCr wire or high power resistors. I was thinking to introduce my "Surround sound from stereo" as a "true" instructable, however soldering with acid requirement stop me. In case of great interest I will write instructable on my "ABC-100-8-25" decoder". I also do not want to limit inexperienced persons or someone without extra hobby time from enjoying very good system. I'm using it for long time (before 5.1 etc.). Recently (it took couple years) I demonstrated system to about 100 listeners with very different experience and come up with following statistics: 80% were not been able to distinguish between 5.1, 7.1 and my system; 10% did not like it, 9% switched from 5.1, 7.1 to my SYSTEM immediately; 1% agreed that it can be used for Hi End systems.
marc925 years ago
Cool idea. Would it be possible to add in a subwoofer to the circuit? And if so, where would you connect it?
LDW (author)  marc925 years ago
Cripes! I haven't got an earthly. My understanding of subs (can be written on the back of a postage stamp) is that you really need a feed from a .1 amplifier. But I expect someone clever here will know.
marc92 LDW5 years ago
I think that you should be able to use the sub without the .1 amplifier because it is the nature of the speaker to produce low frequencies better than high ones. I was just wondering where it should go within the circuit.
With a POWERED sub, you could get away with it. You would feed either +L+R, and -L-R into the + and - inputs(for a "full sub" sound) or feed -L and -R into the + and - inputs(for a "center channel" sub). You'll have to try your particular sub to see for sure. Some subs have internal clipping, to filter out the higher frequencies. You'll be able to clearly hear the massive distortion if the sub passes the full signal frequency through to the speaker. This would only really be useful if the base frequency response on your woofers sucked. An unpowered sub is gonna COOK your non .1 receiver. You have the main speakers. Double the load by adding the faux-surround. Not a problem yet, as long as you don't crank the volume too high. But then you add a Sub to those two channels? Unless your speakers are hacked together from old headphones... you're gonna be way over the amps rating even on very low volume. Expect the magic smoke to escape.
PKM5 years ago
Hmm.. intriguing. I wouldn't worry about it overloading your amp too much because even if the left channel is at full positive and the right channel at full negative, they are still connected by two speakers in series, so theoretically double the resistance. I know that's not quite how speakers work, but it's a decent approximation to show we're not doing anything nuts like connecting the outputs together with no load. My question is, surely if there is something on just one side channel, it will still come from both rear speakers so "water down" the stereo effect? I guess having it from front left, rear left and rear right will "average out" to sounding like rear left, which will provide more subjective "depth" than just front left. If I had my quad speakers I'd give it a try... but then I have 5.1 on my computer anyway :)
LDW (author)  PKM5 years ago
Yes, you're right: anything that is in just one speaker will come out of both 'rear' speakers. In practice, actually, you don't notice that, because one 'rear' speaker is always further away from you than the other. So you tend to hear the front left and the 'rear' right. Maybe it's psychological? Anyway, if you have a stereo system, try this hookup for a few days. then try it without. I bet you'll have it reconnected within the hour! I only put in that stuff about overloading the amp to silence all those nice folk who like to come along and tell you how rubbish your 'ible is. Look what they did to my paella recipe! :-)
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