Project: Stereo line level X-feed & EQ for Headphones

Discussion in 'The Completed Projects Collection' started by PeteHL, Apr 26, 2017.

  1. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    Most stereo recordings are not directly suitable for listening with headphones. This circuit modifies the stereo recording in a way that compensates for what listening with headphones doesn't allow. The justification for cross-feed and equalization is fully explained in the attached article.

    Below is a photo of the circuit constructed on strip board. As you can see, the circuit is not overly complex and building it on strip board was not too time consuming. Layout had to be compact as I intend to add two output stages with output current capability to drive a headphone.

    ACTIVE-STRPBD.jpg
    Parts List (see the schematic diagram, Fig. 7, in the attached article)

    Op Amps
    U1, U3,U4, U6: RC4558 dual op amp
    U2, U5: LM301

    Resistors, 1/4W 5% carbon film
    R1-10k
    R2-4k7
    R3-1k8
    R4-5k6
    R5-15k
    R6-15k
    R7-4k7
    R8-4k7
    R9-15k
    R10-10k
    R11-330R
    R12-15k
    R13-33k
    R14-15k
    R15-10k
    R16-4k7
    R17-1k8
    R18-5k6
    R19-15k
    R20-4k7
    R21-4k7
    R22-15k
    R23-10k
    R24-330R
    R25-15k
    R26-33k

    CAPACITORS, polyester film 10% unless noted otherwise
    C1-18nF
    C2-150pF polystyrene
    C3-18nF
    C4-390nF
    C5-390nF
    C6-6n8
    C7-18nF
    C8-150pF polystyrene
    C9-18nF
    C10-390nF
    C11-390nF
    C12-6n8

    This is intended to be connected between (in cascade) a line level stereo source such as a CD player and an amplifier capable of driving headphones. Please see the attached article for the schematic diagram of the circuit (Fig. 7) and a fairly extensive description of the derivation (justification) of the design.

    -Pete
     
    Last edited: Apr 26, 2017
  2. wayneh

    Expert

    Sep 9, 2010
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    Clever stuff, but the whole scheme presumes that the record producer intended the recording to be listened to over speakers in some sort of acoustic chamber with know properties that the circuit tries to reproduce for headphone listening. What if the mix was prepared by a producer wearing headphones, and is thus intended to be best heard wearing headphones? Then this circuit just introduces unintended artifacts.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    They actually do that?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Most music today is listened to using earbuds, so I would expect them to mix for that just as they used to mix for car radios.
     
  5. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    My understanding is that the great majority of recording engineers check what they are doing by listening to the recording with speakers.

    There are some recordings done specifically for listening with headphones, and then they are identified as such. But even then as far as I know, they don't do equalization to mimic outer ear gain. -Or use special microphones with reflectors designed to do what the outer ear does, which would be better than equalization.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    The purpose of this circuit is to make earphones sound like you are listening to speakers in front of you and minimize the "sound-in-the-head" effect from headphones.
    I don't think you can readily adjust the mix in the studio to achieve that.
    You would need to use a circuit, such as this.
     
  7. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    I disagree with the crossfeed and equalization premise.

    I believe that the reason people like headphones.....is to prevent crossover and increase separation. That's the advantage. It takes real good separation to individually fill the many rooms between the ears. Crossfeed fills too many rooms with the same thing. One should use phase to fill the room....not crossfeed. Don't believe me? Put a Q signal in one ear and an I signal in the other. Adjust phase.

    Tune in a cw pile-up on one of the ham bands. Put on a pair of I/Q headphones (from an I/Q receiver). Every beat of that pile-up.....will have a separation (in a room) between your ears. AND it's very easy to listen to that one room, even when other rooms are louder. Phase gives a location or a spacial gradient between the ears.

    The physical instrument location in an orchestra.......is not natural. It's for separation. And since when does a modern audio engineer worry about natural? When's the last time that you used a class A amp? And even if you did......somewhere down the line....someone will chop the poop out of it.

    If there is a problem to be solved with your circuit.......one would need gain and response adjustments for each channel. Every ear is different. And there is a certain amount of inherent crossfeed in the sound powered side tone circuit. What sound powered side tone circuit? The Eustachian tube. It connects right behind the eardrum. This also is variable, both individually and collectively.

    What we really need is a circuit to solve the number one complaint of media consumers today.

    A working volume control.

    We use to have them on everything and they worked very well. One use to set the volume when you turned the set on........never had to adjust it all nite long. Can't do that today. Modern tech? Only old people have experience with such a device and the relaxing pleasure it brings. You don't know what your missing. To watch media today......one hand has to remain on the volume control. One has to stare at the tv....watch the lips.....turn it up for dialog (because all the male actors whisper today....what's that all about?).....then turn it down real quick for the background sound effects.....which will deafen you. A broken window will knock you out of your chair.

    I know of no such circuit that works today. The circuits are there......I see them on the prints. I adjust them in the settings.......but they have no effect. The old analog circuits worked great. But I can find no working AVC on a modern device. Scanning the channels on any digital tv will show you this.

    Now the working volume control is only a temporary fix while you work on the main root problem.

    And that is an universal UN-processor. You put process audio in.......and get the original UN-adulterated audio out.
    Yes......we want something back from nothing......we deserve it.

    The best audio I ever listened to........was analog quadraphonic in the 70's.
    Audio has went down hill ever since.

    This is not personal against you Pete............it was for me........the more I read the more I bit my tongue.

    If I hadn't of commented.........I'd bit it clean off.

    Music is not natural. We make it un-natural on purpose. We use space, distance, frequency and time......to occupy different areas of the brain with different tones.

    The joy of music is called filling the head(all the rooms) with music, for a reason. Music was considered holy and/or magic for most of our history.

    Today......music is defined as power and volume. No joy or pleasure. Just stimulus.
     
    Ramu Pujari likes this.
  8. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    Granted that there are those people who like the great deal of separation that listening to most stereo recordings with headphones produces. But I don't, and I think that there are many others who also don't care for it.

    Here is what Siegfried Linkwitz had to say about stereo music listening with headphones (1971!):

    "Finally there is the problem due to unnatural spaciousness of the sound reproduction and the complete separation between channels which does not correspond to to our normal listening experience. Thus this "super stereo" effect, while very impressive at first, becomes very tiring after a while."

    I agree that there is a huge difference between live performance and stereo reproduction, and stereo reproduction doesn't need to be completely like a live performance to be enjoyable.

    The goal of the processing (cross-feed and EQ) is to make listening with headphones closer to the experience of listening to 2 channel reproduction with loudspeaker systems. The cross-feed results in the reproduction of each channel signal heard at both ears, wearing headphones, that corresponds to what would occur listening to a pair of speakers arranged in front of the listener in the usual way. The EQ results in boosting sound intensity at each ear for frequencies in the vicinity of 5 kHz that headphones defeat but is heard when listening to a pair of speakers in the triangular arrangement.

    My experience is that this processing does make listening with headphones closer to what occurs when listening to a pair of stereo speakers. Except for what I hear as greater fidelity of reproduction by headphones, without modifying the customary stereo signals, I prefer stereo speakers.
     
    BR-549 likes this.
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    A lot of 1970s recording were severely separated and you could hear the drums on one side, bass guitar on the other, rather guitars on each side and vocals evenly split. Now, there is more blending in the studio and severe channel separation is considered a rookie move.
     
  10. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    Listening to a pair of stereo loudspeaker systems, all of the reproduction of the left channel speaker is heard (mostly at a lower level) by the right ear as well as the left. All of the reproduction of the right channel speaker is heard (mostly at a lower level) by the left ear as well as the right ear.

    Listening with a pair of headphones, normally all of the reproduction of the left headphone is unheard by the right ear. Normally all of the reproduction of the right headphone is unheard by the left ear.
     
  11. GopherT

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    Nov 23, 2012
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    But. A well-mixed recording will already have some of the energy from each instrument on both (or more) channels. Your device is just making more cross-over to the point of making a 2-speaker mono recording. A gimmick that has interesting effects but (obviously) cannot push back to more separation. Good luck and enjoy your toy.
     
  12. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    My device is not intended for reproduction with loudspeaker systems, but rather when listening with headphones. When listening with headphones in the normal (usual) way, no matter how well-mixed the recording is, the left channel signal is not reproduced by the right headphone, and vice versa.

    If I experienced the circuit converting stereo recordings to mono, I wouldn't post it here.

    -Pete
     
  13. wayneh

    Expert

    Sep 9, 2010
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    I think it was earlier than that. My favorite was the Beatles' Rubber Soul (1965), one of my favorite albums of all time. The stereo separation is extreme. If a bad mono hookup only reproduces one channel, it sounds ridiculous.

    I think we agree that this device introduces an artifact, just like cranking the bass and treble knobs. Some people might enjoy the effect, some will not. I'm probably in the latter camp. Yeah, most recordings sound a little different on headphones than from speakers. I consider that one advantage of wearing headphones, and that it helps compensate for the disadvantages, like forgetting the cord as you walk away.
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Then what other goal does your device accomplish? If you want the left channel to include more right channel (and vice versa) you are reducing the stereo-ness and increasing the mono-ness. At some point, both channels will make the same sound and you will have 2-channel mono.

    Is there something about your goal that I am misunderstanding?
     
  15. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    The goal is to perceive the reproduction or "sound stage" in front, similar to what happens when listening to a pair of left channel and right channel loudspeaker systems. Many people who listen to music with headphones complain of the reproduction as apparently coming from 90 degrees to the left and right side, or something like localized "in" the listener's head.
     
  16. PeteHL

    Thread Starter Member

    Dec 17, 2014
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    Let's say we have one of your well-mixed recordings in hand. This well-mixed recording has just the right amount of separation when reproduced with a pair of stereo loudspeaker systems.

    Next we listen to this well-mixed recording with headphones. One of the key differences is that the left channel signal is reproduced for the left ear only, and the right channel signal is reproduced for the right ear only. How does this affect separation? It makes channel separation greater.

    What is the solution to this problem? The solution is to electronically mix some of the right channel signal with the left channel signal going to the left headphone in a way analogous to how the reproduction of the left channel speaker is heard at the right ear. And vice versa.

    Cross-feeding for headphones makes that reproduction closer to that which is had listening with stereo speakers.

    By the way, cross-feeding for headphones was first proposed more than 50 years ago.
     
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