[goodguys]

Just been reading about the famous hafler circuit to get 4-channel surround sound from a 2-channel amp. This is the description:

Then 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.

What I want to know is if it is safe for the amp as there is a mention of low impedance dips and  dangers

Any thoughts?

[abuhannibal]

I used this for years with quite a number of different amps, and never had any problems. It merely sends a difference signal to the rear speakers; I don't believe it has a substantial effect on the load seen by the amp, but I don't know that for certain.  One can also do it with various generations of the Dynaco Quadaptors (or QD-1) which work well and give you somewhat more control. 

[Speedskater]

While it never worked well with vinyl, because a] the difference signals were not as loud as the front & b] much of the record surface noise ended up in with the difference signals. So the rear channel noise was way loud.
But with CD or higher that surface noise doesn't exist and other uncorrelated noise should be tolerable.
I don't know what perceptual coders do with uncorrelated noise.

[WGH]

In 1971, the Dynaco QD-1 Quadaptor put the circuit in a box along with a potentiometer and a balance slider. They are still available on eBay:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Dynaco+QD-1+Quadaptor&_sacat=0



More info: http://home.indy.net/~gregdunn/dynaco/components/QD1/index.html

[dB Cooper]

I ran a single rear speaker conected across the amp's hot terminals for years. It worked great. My room was deep so it added a lot of spaciousness, especially on 'live' material. Your amp must be reasonably tolerant of low impedance loads as the rear speaker is in parallel with the mains when signal is passing through it. My Dyna amp tolerated it fine but a friend's Pioneer blew its output stage almost immediately.

Speedskater, I don't remember having much of a noise problem but I was using a small single driver RadioShack speaker which probably didn't go all that high so it wasn't obtrusive  (I never put my ear up to the speaker to check it). The speaker was well back from the listening position whic probably also helped.I also wasn't listening to music at the time that had much in the way of 'quiet parts'  . At the time, vinyl, FM, and analog tape were the only options, so there wasn't another way to go.

I'll post Dyna hookup diagrams later (gotta leave for work).

[dB Cooper]

Here is a scan from my well-worn SCA-80 manual describing the Dynaco passive 'Dynaquad' setup.. Imperfect condition but everything I was able to find online was from the 80Q manual which reflects Dynaco's capitulation to the horrible four-corners speaker placement that was customary at the time for 'quad'. In-corner placement is the worst possible choice for most speakers not specifically designed for it. The rear speakers in that scheme are connected in series but out of phase electrically which (due to the difference signal) results in them being in phase acoustically.

I was lucky; my room was fairly large and long and I was able to place the rear speaker well behind the listening position (fronts along the short wall) and high on the wall just as suggested. With the right source material (especially live albums) the effect was fairly convincing. Studio material was more hit-and-miss and sometimes there I switched the rear speaker out. I ran the speaker outs into a switch and made the depicted connections there so I was able to switch the rear speaker in or out at will. I also used an L-pad as suggested to 'tame' the level of the rear as needed (the rear plays L minus R and R minus L so can sometime sound obtrusive if not potted down). The Quadaptor does all this but is designed for the horrible four-in-the-corner scheme. The balancing procedure described in the text was often necessary to get the best results given the el cheapo wirewound pots used by Dynaco and can maybe be dispensed with if your equipment uses something better like Noble or Alps pots.

[dB Cooper]

I think the upload process downsized my scan. If anyone needs/wants a ful size version, PM me with your email address.

Note that it said Dyna had more complete literature 'upon request'. Dyna obviously isn't around to request from anymore... Used to be someone here who said they had a bunch of old Dyna literature and would look for it for me but I never heard anything further and don't remember who or whether they're still around. If they are, and see this, reach out.

BTW the Quadaptor didn't have its own balance control. That's just a graphic. It had a three way switch with 'Normal Operation', 'Front Only', and 'Balance Check' positions, but the balance 'tuning' was done using the amp or preamp's volume control. Budget equipment often had tracking errors in the Vol and/or Bal controls, hence the 'fine tuning'. Dyna was good equipment but definitely built to a price point.

[Mark Korda]

Hi DB and Goodguy,
  I have the Dyna Quadapter manual. Under Operating Instructions/Amplifier Suitability it says the Quadaptor can be used with nearly all amps and receivers. All Dyna amps are recommended.
   A very few amps( notably those with a ( floating output circuit) cannot be used with the Quadaptor. The Dynaquad requires that a common ground reference exsist between the 2 channels.
   Goodguys, I don't know what a floating output circuit is....maybe DB could help on that.
I had my SCA-80Q going in 1988 with Boston A-40's for my rear speakers. I'll always remember Predator on VHS as a favorite memory when Jesse Ventura said he was dug in like an Alabama tick!....Mark K.
.

[JerryM]

I think the upload process downsized my scan. If anyone needs/wants a ful size version, PM me with your email address.

If one checks the picture in your Gallery, then selects 'Huge' as the image size, it shows the full-size image. 

https://www.audiocircle.com/index.php?action=gallery;area=browse;image=180787;size=huge

[goodguys]

In the article it states that hafler surround sound is only possible if the amplifier has the left and right negative speaker terminals tied together, a common ground. Is there some way to make this hafler sound work with amplifiers which do not have this common ground.

[WGH]

In the article it states that hafler surround sound is only possible if the amplifier has the left and right negative speaker terminals tied together, a common ground. Is there some way to make this hafler sound work with amplifiers which do not have this common ground.

Yes, a NEC PLD-910 will work nicely, in fact a lot better than the passive Hafler setup. I had one back in the Hi-Fi VCR days before the DVD was invented. Besides decoding movies with Dolby soundtracks you could make your living room sound like a small jazz club or as big as a stadium.

And you are in luck, there is one on eBay for only $41
https://www.ebay.com/itm/RARE-Vintage-NEC-PLD-910BU-AV-Component-Surround-Sound-Processor-Home-Theater/222968970018?epid=1024593149&hash=item33e9fc7b22:g:G5gAAOSwCxdarYJz

Review: http://www.hifi-classic.net/review/nec-pld-910-543.html

[dB Cooper]

How satisfactory the result will be probably depends to a large extent on how satisfactorily the 1989-vintage DSP and D/A-A/D circuits work. More versatile than the Hafler passive setup maybe, but more electronics, more amps, more signal manipulation (the passive setup does none.) If you have an extra amp collecting dust though, it's kind of a WTF purchase for $41. Some of the 'Quadapters' are going for more than that.

On the other hand, if you have the rear speaker(s) already (I never liked the 'two-rears' setup), you can implement the Hafler scheme for literally the cost of the hookup wire (this was before the days of $1K speaker cables, remember) and maybe the cost of an L-pad if needed.

[dB Cooper]

If one checks the picture in your Gallery, then selects 'Huge' as the image size, it shows the full-size image. 

https://www.audiocircle.com/index.php?action=gallery;area=browse;image=180787;size=huge

Thanks Jerry

[dB Cooper]

   Goodguys, I don't know what a floating output circuit is....maybe DB could help on that.
I had my SCA-80Q going in 1988 with Boston A-40's for my rear speakers. I'll always remember Predator on VHS as a favorite memory when Jesse Ventura said he was dug in like an Alabama tick!....Mark K.
.

Not sure what a 'floating' output circuit is, maybe one of the EE's here can shed some light on that, but I do remember that both channels had to refer to a common ground (ie, no dual-mono setups need apply).

[goodguys]

Thanks!

I am sure you are right and that a processor would be better, but it is just for a cheap set up and i do not want to spend the money.
I am certainly liking the cleanliness of the hafler audio, versus the coloured feel of a processor and all its extra circuitry.
Would love to be able to find a way do this to an amp which does not have this common ground thing.

[dB Cooper]

This setup will work with anything that is not dual mono or balanced (or intolerant of low impedance loads, assuming all 8-ohm speakers), i.e. 98% of all amps. Unless your intended amp is one or more of those three things, you're good to go.

[dB Cooper]

In the article it states that hafler surround sound is only possible if the amplifier has the left and right negative speaker terminals tied together, a common ground. Is there some way to make this hafler sound work with amplifiers which do not have this common ground.

To repeat: probably around 98% of all amps have one ground point internally for both speakers. One giveaway: Does the amp in question have a headphone jack? If so, unless it is a dedicated discrete circuit, your amp has a 'common ground reference' (one 'hot' for each channel and one 'ground' shared by both.) 

[Saturn94]

I enjoyed using the Hafler Hookup method for some time many years ago.  I had no issues using my Hafler amps.