Thread: HAFLER CIRCUIT: passive surround sound

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Post by Beagle January 14, 2007 (1 of 29)
This little note will doubtless make MCH people ask why I don't jump into multichannel itself, and it will make 2-channel people ask why I bothered, but...

I finally gave in to my curiosity (and a bit of encouragement from 'fafnir') and hauled the unused Castle Pembrook speakers downstairs and set them behind the Listening Couch/Divan/Settee.

Then (1) I ran a single wire from the right-positive post on the power-amp to the positive post on the right rear speaker. Then (2) a single wire from its negative post to the negative post on the left rear speaker. Then (3) I completed the circuit with a single wire from the left rear's postive post back to the left-positive post on the power-amp. (Oh yes, an on-off switch stuck anywhere in the circuit; alternatively a volume control.)

In shorthand: R+ to R+, R- to L-, L+ to L+.

That's the famous Hafler Circuit, given to the world by the famous designer David Hafler (and picked up and made into millions of dollars by Mr Dolby). The effect is subtle; it "warms up" the acoustic feel of the room. It isn't time-delay reverb, just an out-of-phase 'difference' between left and right, which itself is neither left nor right.

The cellist wife approves, so the Pembrooks are downstairs to stay.

Post by mandel January 14, 2007 (2 of 29)
Grr the forum ate my post.

I think you may find in time that what you gain in warmth you lose in clarity. The Hafler Matrix sounds pretty good for such a simple 'hack' but a decent decoder like DTS:Neo is way better (and discrete surround is way better again). If you have any QS quadraphonic vinyl or Dolby Prologic encoded CDs you should give them a try, they should decode quite nicely. Might be worth trying any SQ encoded records you have too, though the Hafler Matrix will only partially decode them.

Post by Claude January 15, 2007 (3 of 29)
Here's the description of the Hafler circuit from Wikipedia:

"Passive pseudo quad" can be much more realistic than would appear from the name. It has been observed that ambient sounds in a concert, such as applause or even coughs from the audience, are generally received in "opposite phase" by the stereophonic microphones, while sound from the musicians is generally in "synchronous phase". Thus, if rear speakers are fed with the difference between the stereo channels, audience noises and echoes from the auditorium can be heard from behind the listener. This can be most easily achieved by wiring two similar additional rear speakers in series (typically 8+8=16 ohms) between the live feeds to the front speakers. This arrangement was colloquially known as the "Hafler circuit," after audio engineer David Hafler, an early proponent of the idea. The "crosstalk" or loss of stereo separation in the front speakers is less than 2dB while the rear sound level in a typical stereo-recorded live performance is about 7dB below the front, but clearly audible. This "passive" method is arguably as good as any of the expensive "active matrix" electronic decoders which attempt to reconstruct ambient sound from a stereo recording.

http://en.wikipedia.org/wiki/Quadraphonic

Post by amatala January 15, 2007 (4 of 29)
mandel said:

Grr the forum ate my post.

I think you may find in time that what you gain in warmth you lose in clarity. The Hafler Matrix sounds pretty good for such a simple 'hack' but a decent decoder like DTS:Neo is way better (and discrete surround is way better again). If you have any QS quadraphonic vinyl or Dolby Prologic encoded CDs you should give them a try, they should decode quite nicely. Might be worth trying any SQ encoded records you have too, though the Hafler Matrix will only partially decode them.

I don't really think that using DSPs like DTS:Neo6 or DPLII is a good idea.
First of all in order to do so you have to use an A/V receiver or processor and these machines are not really the best choice for listening to music.
Moreover, from my own experiences it is quite clear that this kind of DSPs seem to sound better than stereo only on low-end entry-level systems because these systems are quite bad in stereo. Once you go to up in range to better systems it is usually the plain stereo (pure direct, DSP off) mode which sound better than any sound processing mode because higher end machines are better in stereo and are also capable of showing all the faults of these DSPs (DSP modes just sound 'processed' and less natural than plain stereo).
Basically, if your system sounds better in Neo6, PLII, Logic7, SRS (and so on) than in stereo, it means that it is not capable of good stereo playback.

Post by fafnir January 15, 2007 (5 of 29)
amatala said:

I don't really think that using DSPs like DTS:Neo6 or DPLII is a good idea.
First of all in order to do so you have to use an A/V receiver or processor and these machines are not really the best choice for listening to music.
Moreover, from my own experiences it is quite clear that this kind of DSPs seem to sound better than stereo only on low-end entry-level systems because these systems are quite bad in stereo. Once you go to up in range to better systems it is usually the plain stereo (pure direct, DSP off) mode which sound better than any sound processing mode because higher end machines are better in stereo and are also capable of showing all the faults of these DSPs (DSP modes just sound 'processed' and less natural than plain stereo).
Basically, if your system sounds better in Neo6, PLII, Logic7, SRS (and so on) than in stereo, it means that it is not capable of good stereo playback.

This post IMHO is absolute nonsence. Any claim that excellent sound cannot be obtained from A/V machines is typical of the mindset of the ultra high end elitists. It defies testing, both listening (blind to make it real) and with instrumentation; in short it is jaw-droppingly ridiculous.

For many of us, once you have experienced mch, then plain jane stereo, no matter how well-done, is simply a non-starter. (I know that there are some who can't stand mch, but lets not go there for now - it's been well covered in other threads). Therefore, there is continued interest in dressing up stereo to provide an approximation of the mch experience. Some of these attempts at artificial mch are more successful than others, but depending on the sound source many stereo recordings can be revitalized.

After a few years, many years ago, I stopped using the Hafler circuit not because it gave poor results but because better circuitry came along. Currently I use DTS Neo 6, Logic 7, or Lexicon Panorama to simulate mch. I have found very few stereo recordings that are not significantly improved by at least one of these approaches.

Post by amatala January 15, 2007 (6 of 29)
fafnir said:

This post IMHO is absolute nonsence. Any claim that excellent sound cannot be obtained from A/V machines is typical of the mindset of the ultra high end elitists. It defies testing, both listening (blind to make it real) and with instrumentation; in short it is jaw-droppingly ridiculous.

For many of us, once you have experienced mch, then plain jane stereo, no matter how well-done, is simply a non-starter. (I know that there are some who can't stand mch, but lets not go there for now - it's been well covered in other threads). Therefore, there is continued interest in dressing up stereo to provide an approximation of the mch experience. Some of these attempts at artificial mch are more successful than others, but depending on the sound source many stereo recordings can be revitalized.

After a few years, many years ago, I stopped using the Hafler circuit not because it gave poor results but because better circuitry came along. Currently I use DTS Neo 6, Logic 7, or Lexicon Panorama to simulate mch. I have found very few stereo recordings that are not significantly improved by at least one of these approaches.

Hello,

First of all I am a great fan of multi channel sound. I absolutely LOVE recordings which fully use all channels in order to create a stunning multichannel experience.
On the other hand I also think that if a recording is stereo only, it should be left alone and not processed with all these DSPs which only break the sound. Like I said, on low end systems this can create an illusion of better sound, but on better systems these DSPs cannot fool anyone.
The reason I'm saying that AV Receivers don't perform that well with music is specifically because I've made lots of tests and comparred many configuration.
My conclusion is quite obvious: not only the AV receivers perform rather badly in stereo, their analogue multichannel performance (via the 6/8 channel direct inputs) is also not quite satisfactory.
This is why today I use a separate (analogue) preamplifier in stereo, another fully analogue multichannel preamplifier for MCH SACD et DVD-A and, finally, I only use my AV Receiver for movies. In my opinion this is a configuration that works quite well and does not compromise sound quality.

Post by mandel January 15, 2007 (7 of 29)
amatala said:

This is why today I use a separate (analogue) preamplifier in stereo, another fully analogue multichannel preamplifier for MCH SACD et DVD-A and, finally, I only use my AV Receiver for movies. In my opinion this is a configuration that works quite well and does not compromise sound quality.

Some of us can't afford that much gear unfortunately and have no choice except to compromise. I've only tried DTS:Neo6 and Dolby Prologic 2, PL2 destroys the soundstage for classical music but DTS:Neo6 seems to preserve it very well. On some CDs/vinyl DTS:Neo6 produces a better soundstage than some multichannel SACDs (though you're missing the advantages of hi-res and real acoustics in the rear of course). I agree it sounds a bit processed but listening through two speakers only just sounds flat to me now.

As for saying my system is not capable of good stereo playback, unfortunately that is fairly true. It's impossible to position the front speakers correctly here, they have to be spaced by 80 degrees rather than the recommended 60. This setup works very well for multichannel but inevitably leaves a bit of a 'hole in the middle' effect in stereo. A fair number of people with far better systems than mine have reported pleasing results using DTS:Neo6 though.

Post by amatala January 15, 2007 (8 of 29)
mandel said:

Some of us can't afford that much gear unfortunately and have no choice except to compromise. I've only tried DTS:Neo6 and Dolby Prologic 2, PL2 destroys the soundstage for classical music but DTS:Neo6 seems to preserve it very well. On some CDs/vinyl DTS:Neo6 produces a better soundstage than some multichannel SACDs (though you're missing the advantages of hi-res and real acoustics in the rear of course). I agree it sounds a bit processed but listening through two speakers only just sounds flat to me now.

As for saying my system is not capable of good stereo playback, unfortunately that is fairly true. It's impossible to position the front speakers correctly here, they have to be spaced by 80 degrees rather than the recommended 60. This setup works very well for multichannel but inevitably leaves a bit of a 'hole in the middle' effect in stereo. A fair number of people with far better systems than mine have reported pleasing results using DTS:Neo6 though.

One of the biggest show stoppers for using DSPs is the very nature of these processors: they are all digital.
On the other hand the signal that comes out of your CD/SACD player is analogue (you can of course send the CD signal directly in digital, but this is not normally a good idea because a good CD player will be much better at performing the conversion than any AV receiver).
This means that your receiver will have to convert the signal back to digital, apply the DSP and then convert back to analogue for amplification.
Each conversion means quality loss and on good systems this can be easilly heard.

When I first bought my AV receiver I was also using a digital connection from my CD player and I loved playing around with DSP modes: sometimes PLII was better, other times Neo6 was better, and sometimes Logic 7. But in all cases all these modes were much better than stereo. For a long time I thought that stereo was just bad and it was normal than DSPs sounded better. But then I did some system upgrades (better CD player, better front speakers, external power amps) and suddenly stereo became very good and DSPs became processed and fake in comparison. The final step was to move away from the AV receiver to dedicated preamplifiers and another huge imporivement could be heared!
Today I am a big MCH fan but I also know stereo can also sound EXTREMELY good when done properly!

Post by pgmdir January 15, 2007 (9 of 29)
I started using the Hafler cicuit in the late 70s, and it served me well with my 4 B&O speakers for years until I got my first Dolby Pro Logic receiver. It worked surprisingly well. Now, of course I enjoy multi channel SACD and movies. But I dislike music with most DSP and decoding settings The true test is-- does it change the front channels at all. And in my experience DTS and propriatary DSP fields do. I don't want to hear it working--- I just want to notice when it's gone. Ambience recovery is all I ever want for two channel.

My living room Denon has a "matrix" setting which sounds good. My Den system does not, and I tend to listen to two channel recordings front only, or occasionally pro logic II music with the rears turned way down.

I recently set up a hafler effect circuit for a friend who can't afford a new receiver.

Post by krisjan January 15, 2007 (10 of 29)
I have been using the Hafler circuit for about 25 years now. I could not go back to conventional stereo after hearing what this simple arrangement can do. Furthermore, the Hafler set-up does such a good job of expanding the sound stage and creating a more 3-D experience that I also don't have a strong desire to move to current generation m-ch although I am pretty confident such a change would result in even better realism. When I began using the Halfer circuit, I did the simple routing from the stereo speakers back to a single rear speaker with an L-pad.

Today, I use a passive box called Phasearound which was made for a short while by Fine Line Audio. In this arrangement, I send an extra pre-amp out to the Phasearound box which extracts the out-of-phase info from the stereo channels and then I route that output to a separate power amp and to a pair of rear speakers. The Phasearound unit has a volume control so I can adjust the effect for best results. The only thing lacking is a delay circuit which would make the effect more realistic.
Mark

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