A duophonic headphone amp?

[samhay]

I have been wondering recently if a 'duophonic' or similar circuit might make for an interesting headphone amp, as it can, in principle, be used to fake a stereo sound stage.
http://en.wikipedia.org/wiki/Duophonic
The idea is to cut bass in one channel, cut treble in the other, and delay one channel with respect to the other.
I haven't found a lot of concrete details about what works (frequencies, and especially the delay time) - I know a lot of dupohonic records sound terrible, but it seems this is often due to the heavy handed use of reverb rather than a fatal floor in the idea.

So - has anybody tried it and/or does anybody have a feel for how much delay should be introduced in order to make it convincing - I am guessing 10s ms?

Thanks.

[tca]

Found this: http://www.youtube.com/watch?v=gJ4xNbVaa6g#t

There should be a patent somewhere!

P.S.
- http://www.google.com/patents/US3219757
- http://www.google.com/patents/US2852604

- http://www.google.com/patents/US3790711 Sterophony - simulating earphone

- http://www.google.com/patents/US3560656 This might be the simplest one

[samhay]

Thanks tca - had heard some of the demos before, but had not thought to look into the patents of yesteryear.

[Mark Hammer]

There were a variety of methods for faking stereo from a mono source.  Philips even produced a chip for a while to do just that.  http://www.nxp.com/documents/data_sheet/TDA3810_CNV_2.pdf

Philips had an earlier chip that was basically a sort of fixed phase shifter that introduced a half-dozen or so notches in the spectrum for one channel, so that left and right would sound different.  I forget whether it also introduced notches in the alternate channel, located in different parts of the spectrum than the first channel.

There are also the "spatial imaging" circuits that enhance an already stereo image, which sound great, but are more complex than what you're imagining.

The challenge lies in producing something where the two ears hear something different, but which is not dominant in one ear due to any sort of level difference or frequency emphasis.

[samhay]

Thanks Mark - hadn't seen that chip before. It still seems to be fairly easy to get hold of, and will run off a 9V supply, but I think I will see how far I can initially get with HP/LP filtering and some delay from all-pass filters.


^The challenge lies in producing something where the two ears hear something different, but which is not dominant in one ear due to any sort of level difference or frequency emphasis.
That indeed does seem to be the case.

[Mark Hammer]

Which is why the complementary comb-filtering approach worked kinda sorta ok-er than some others.  The trouble is that one can end up with two channels that each lack any punch whatsoever.

I suspect it was the consistent abject failure of most turn-the-one-into-two approaches that resulted in the "back to mono" craze of the 70's.  People eventually decided that if it wasn't originally engineered for stereo, it was best to simply leave it in mono form.

[samhay]

I suspect that a big part of the problem is/was that it can work fairly well in a subtly way, but if you try to have too much of 'it' then it doesn't sound very good any more.

To get comb filtering of the output, you have to mix the delayed signal with the dry. If you don't do this and simply delay one chanel, will our head comb filter the two channels as we listen to it?

[Mark Hammer]

The complementary comb filtering really ought to be called "fork" filtering, since it does not assume more than a handful of spaced notches.  certainly nothing so complex that a BBD would be called for.  Six fixed allpass/phase-shift stages, with the 3 notches produced by the one side sitting in between the 3 notches produced on the other, would pretty much do it.

Or, like I suggested to Bill in this thread - http://www.diystompboxes.com/smfforum/index.php?topic=107004.0 - regarding tunable notch filters, you could make 3 multiple feedback bandpass filters per side.  The mono input gets split up 8 ways.  Two go to their respect output mixing stages (left and right).  Three go to tunable bandpass stages and subtracted from the dry signal on one side.  And the other three get routed to the mixing/output stage on the other side, and subtracted from that.

Note that the three bandpass stages on each channel are in parallel with each other, not in series.  Note, as well that since the outputs of the bandpass sections are mixed in with the dry signal, one can adjust the subtlety of the effect by the amount of opposite-phase filtered signal one subtracts from the dry signal.

Nine op-amp stages overall.  A single and a pair of quads.

[samhay]

Hmm - 9 op-amps in an Altoids tin will be an interesting build.

[tca]

^ Why not try something simpler: http://www.google.com/patents/US3560656 ? 1, 2 or 3 BJT?

[samhay]

^ Why not try something simpler: http://www.google.com/patents/US3560656 ? 1, 2 or 3 BJT?

That was essentially the intial plan, but I was thinking of using NE5532s, which should drive headphones a little better.
When I have some breadboard time, I will see how far I can get with this approach.

[Nasse]

I believe I have seen some electronics magazine construction article perhaps some 20 or more or less years ago, it was some US hobby magazine. With luck I might have that magazine somewhere "archived". But it was not for phones but stereo. And it was huge circuit. Lots of parts, graphic eq type bandpass thing or something. Dunno if it is worth the trouble when you can find free plugins

[Nasse]

Just googled this http://zpostbox.ru/how_to_create_stereo_from_mono_signal.html

[samhay]

Just googled this http://zpostbox.ru/how_to_create_stereo_from_mono_signal.html

Very interesting - thanks.

[garcho]

I have an old Orban stereo synth rack unit, I'll take it apart and see what's inside, just for sheets and giggles.

[tubegeek]

I suspect it was the consistent abject failure of most turn-the-one-into-two approaches that resulted in the "back to mono" craze of the 70's.  People eventually decided that if it wasn't originally engineered for stereo, it was best to simply leave it in mono form.

When I was in high school (mid-'70's) I was VERY excited by the first few Rolling Stones albums and subsequently by Chuck Berry, Bo Diddley, Muddy Waters and Howlin' Wolf. But there was something really wrong with the sound. It got better when I summed the fake stereo into mono. Much Better. Still something strange though.

I went so far as to seek out original mono Chess label 45's of the Chuck Berry tracks I liked, and of course mono Stones records.

Oh yes, there was something wrong with the fake stereo effects - something very wrong. Don't forget that the early Stones records weren't exactly brilliantly recorded to begin with. With fake stereo on them, the results were just godawful.

"Back To Mono" indeed.

[Mark Hammer]

When I was in high school (mid-'70's) I was VERY excited by the first few Rolling Stones albums and subsequently by Chuck Berry, Bo Diddley, Muddy Waters and Howlin' Wolf. But there was something really wrong with the sound. It got better when I summed the fake stereo into mono. Much Better. Still something strange though.

One of the most disappointing music experiences of my life was stepping into one of the better record shops in my city in the mid-70's and finally hearing "Brown Sugar" on a decent stereo, after having only heard it through a mono transistor radio.  It was awful.  Sometimes, details are better left undifferentiated.

I might note that there is a Phil Spector box set called "Back to Mono", and if there was anything to distinguish the Spector "wall of sound" sound, it was the presence of a LOT of undifferentiated details.

[samhay]

To clarify - the idea is/was a fake stereo-esque headphone amp specifically for guitar monitoring. I have no intention of this being used for HiFi listening - as the experiments of yesteryear have shown, this is simply not a good idea.

I had a a quick play on the breadboard with a simple circuit that cut a significant amount of bass in one channel and treble in the other, with some phase shift relative to each other. The effect was not very pleasing, and was almost nauseating, but did present something like a stereo field.
Will try some comb filtering next.