Bottlehead Forum
General Category => General Discussion => Topic started by: rlyach on June 11, 2014, 06:15:43 PM
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I may be getting in over my head but I was wondering if it would be possible to incorporate a 3D imager into a pre-amp circuit. I have been enjoying my Crack headphone amp, but I have noticed that because my right ear only hears the right channel and my left ear only hears the left channel, the stereo image is between my ears. My setup (Crack with RCA clear top 12AU7 and a tungsol 5998, and a pair of Tesla T1s) gives a nice large soundstage but I want the image in front of me. After a little thought I discovered the reason. So... I wanted to try an experiment. I wanted to build a preamp using a 12AU7 for each channel. One half would be for the inputting the main signal, the other half for sending the signal to a cross-feed circuit with a Bridged Transformer delay circuit tuned to about 60 degrees. Then sum the delayed right channel into the left signal by way of an attenuator and coupling capacitor. The attenuator would be stereo, one half for each cross-feed signal. The output stage would be a capacitively coupled cathode follower. With the attenuator controlling the cross-feed signal, I should be able to move the sound stage close or far away, simulating speakers. Is this a waste of time. I know there are several DSP options out there but they are all set up for surround sound processing. I simply wanted an analog cross feed solution for simple stereo.
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Start with the inexpensive experiments.
http://headwize.com/?page_id=790 (http://headwize.com/?page_id=790)
http://yrzorg.uk.cloudlogin.co/amp/headphones/acoussim/Acoustic_Simulator_for_Headphone.htm (http://yrzorg.uk.cloudlogin.co/amp/headphones/acoussim/Acoustic_Simulator_for_Headphone.htm)
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PB,
Thanks so much... This is exactly what I was looking for. I knew what I wanted to do but wasn't sure where to begin. I like the passive approach since my DAC can drive pro audio levels. This will give me something fun to try.
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Well... I did some pretty heavy research. I analyzed the classic cross-feed circuit (below) and found some fundamental issues. For instance, in order to get the proper delay (0.5ms to 1.0ms) you need to use a resistor / capacitor filter which also acts as a low pass filter with a cutoff frequency of 585 Hz. This translates to more than a 30db drop at 20Khz. Consequently, only the low to mid frequencies are cross-fed. To compensate, the designer of this particular circuit put a high pass filter on each side, boosting the primary signal. This is an artificial solution since these frequencies are not cross-fed into the opposite channel. I discovered that the real difficulty in designing this type of circuit is how to generate a long and accurate analog delay. I experimented with the shunted transformer circuit but there is a limit there as well. The longest delay you can get with a max frequency of 20Khz is 25 microseconds. This is not enough delay to even simulate the difference in sound across the separation of your ears. Rather than trying to find a circuit solution, I evaluated a digital plugin. I used HiJack Pro and Redline Monitor to simulate near field monitors on my headphones. While the sound was a little more forward, it sound more focused and less open. It also seemed that I lost some fullness and dynamic range in the low end. After all this I decided to just enjoy my Crack and Tesla T1s just as they are. If anyone has another solution for a full and open forward soundstage please let me know. I do like to experiment.
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Well... I had some time on my hands so I actually built the Soundfiled Simulator from PB's link. I made some slight modifications to the circuit based upon recommendations I found. I also moved the POT to allow the actual soundstage to be adjusted. I also made use of my Bottlehead experience in designing the chassis. I was not expecting much but when I plugged it in and adjusted the soundstage, I was very surprised that to my ears the filter actually opened up the sound. It did not move the sound very far forward at all but it definitely was more pleasing. There is a 4.5db loss with this circuit so it also pads the source slightly. At this point I will be using this little box and giving it a more critical listen. Thanks PB for the link. In case anyone else wants to try this little circuit, I have posted a few picts on how to build it. I also have top and bottom plate layout files and the circuit board layout as well if anyone is interested. Enjoy!
Update: I updated the schematic to the actual values I used. I found an error in the original post.
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Someday I will be able to do something like this!!! Very cool.
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OK, Perhaps someone can enlighten me. My engineering analysis says this thing should be marginal at best, but it sounds fantastic. Now that I have had more time to play with it, I can honestly say I love the sound. It is indeed more forward. Not only that but there seems to be added clarity, which is exactly opposite of what I would expect. According to simple analysis and verified through simulation, this thigh should be a little darker but that is definitely not the case. I even tried it on a different amp with similar results. I had two colleagues give it a listen and now they both want one. One has already asked for the circuit and plans on building it. The engineer in me keeps looking for the reasons why. If anyone has a little time, perhaps you could reproduce this circuit and let me know what you think.
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I decided not to question success. When Carver Corp. came out with the Sonic Hologram I knew what it was immediately. It feeds some L out of phase to R. It feeds some R out of phase to L.
Years later I bought a C9 cheap on ebay. It does all sorts of things within the soundstage. A few live recordings have ambient sounds from way behind me. It is not usually in my system but I drag it out when a recording strikes me that it would benefit from it.
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Thanks for the info Granger. I seem to recall several times where PJ would mention the apparent discrepancy between engineering judgement and sonic perception. I am pretty much a skeptic on some things but I can't argue with direct experience. Your comment reminded me of one thing I forgot to mention. You stated that some recordings benefit from this little trick more than others. I have found the same thing, but there is a slight difference. I modified the circuit slightly and inserted a POT in the phase delay portion of the circuit. With this change you can adjust the delay for maximum effect. I found that some recordings like a large delay while other a smaller one. All-in-all, I like the result. Cheers.
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I seem to recall several times where PJ would mention the apparent discrepancy between engineering judgement and sonic perception. I am pretty much a skeptic on some things but I can't argue with direct experience.
Yeah and this is not really an indictment of engineering as much as it is another example that we need to keep looking for new ways to measure these things that we are hearing. Like why different wires sound different. It makes no sense based on the way we measure things. But we hear the difference, so we need to figure out what measurement we are missing that will help us to understand what is going on.
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Great explanation DOC. I once heard a lecture by Dr. David Menton in which he said that the human ear is so sensitive that it can detect the pressure change from simply standing on a piece of paper. Apparently our ears are more sensitive than some of the crude measurements we make, or like you say, we have yet to measure the correct thing. One more thought. I have had the pleasure of visiting a few recording studios and I was amazed at what was used to cobble together signal paths from microphones to the actual recording equipment. Not to mention the patch bays.
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I'd like to experiment with this, too, for my SEX amp, but I can't read a schematic. How cheap is it for parts, overall?
EDIT: I noticed that there's a feature in DSP output for J.River Music console called Crossfeed that I think does exactly this. It's pretty remarkable how effective it is.
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The cost of the parts is about $25, not including the enclosure. I used all 1% metal film resistors and Mallory Coupling caps. As always, you can upgrade the parts. Here is the BOM
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At low frequencies, sound goes around the head to both ears, with a time delay difference that is perceived and with little difference in loudness. The RC lowpass filter creates the delayed cross feed.
At higher frequencies, the ear/brain becomes less sensitive to phase differences, and the head blocks more of the sound from the far side. The ear/brain at high frequencies thus relies more on loudness differences to detect location.
Different recording microphone techniques have a huge effect on how and whether this works.
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Thanks Paul, Now I know why I was confused. I assumed that since low frequencies are omnidirectional that delaying the high frequencies is more important. With your explanation on the psychoacoustics of the stereo image, the circuit makes perfect sense. So when they say that bass is omnidirectional, does it mean that the brain does not use loudness to locate the sound like it does with higher frequencies? It is rather convenient that delayed low frequencies make this work. Hence you can use the simple RC low pass filter.
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Yeah, you have the concept correctly.
I think it's evolution, not a coincidence. Since loudness does not work well at low frequencies, there is a survival advantage to developing phase sensitivity in that range. It's actually quite remarkable that we can detect phase effects up to 1500Hz or so - most neurons are not usually anywhere near fast enough for the task. I believe the phase/timing information is retained all the way into the brain, not decoded at the first layer or two. But I read about this a long time ago so there may be more knowledge, ort my memory may have gotten some of it wrong.
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I'd love to try this as well. Could you post the layout file for the circuit board and the top/bottom plates?
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Here is the board layout, a picture of the placed components and the backside of the board. I inserted the components from the top, soldered them in place on the bottom, bent the leads together to form traces, and inserted one lead back through the board for a top connection on all nodes that need to connect to the RCA jacks, switch and POT. The node names match the schematic. There is also a simple instruction file. Enjoy! I will be away for a couple of weeks.
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Thanks!
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Finished mine today!
Caps still to burn-in (pio matched russian ones...lot of time ahead!), but hey, the effect is incredible!!
I put this little wonder in the same case of my passive BSC circuit (Paul Joppa's open source circuit found here http://www.t-linespeakers.org/tech/bafflestep/passive-BSC.html (http://www.t-linespeakers.org/tech/bafflestep/passive-BSC.html) ).
A killer add-on to every headphone setup!
Bottlehead crew, what about throwing a kit for the Quickie+Quicksand users with both those simple circuits? ;)
Thank you again for the scheme rlyach!
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This looks like a fun little project! Is this totally a passive device (no power needed?) What is the insertion loss on this? I have too much gain in my system, so it looks like this might help with that problem too 8) Is anybody using this in their main system? I'm guessing this would go between the preamp and amp? Thanks,
Dave
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Is this totally a passive device (no power needed?)
yes, no power needed!
What is the insertion loss on this?
I don't know, but I think it's a bit high, better have a preamp with my cmoy headphone amp (which is quite strange given cmoy driving capability!)
I'm guessing this would go between the preamp and amp?
Yes, better!
Ciao!
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How do the controls work?
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The only controls are a pot to adjust crossfeed effect to taste (purple one) and a switch to disable the circuit; the other pots and controls are for the BSC filter for luodspeakers use only.
I must sa the build is really straightforward, thanks to the excellent instruction published by rlyach.
Ciao!
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What do the two items circled in red mean?
Thanks, Dave
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The point where the musical signal (conventionally a sine wave) get in and out the circuit.
You can substitute it with RCA jack symbol!
Ciao!
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I'm shuffling through my parts boxes. I'm thinking that if I don't have a 1nF I might just keep the same RC product and use a different value. It looks like one of my RS 100k Audio taper pots will do well, great value for $3.
I have a torn plantar fascia so I need to play with something while healing.
Oh, thanks for the pictures I have some of that perfboard or what ever it is called.
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Thanks Paul, Now I know why I was confused. I assumed that since low frequencies are omnidirectional that delaying the high frequencies is more important. With your explanation on the psychoacoustics of the stereo image, the circuit makes perfect sense. So when they say that bass is omnidirectional, does it mean that the brain does not use loudness to locate the sound like it does with higher frequencies? It is rather convenient that delayed low frequencies make this work. Hence you can use the simple RC low pass filter.
There is a common misconception in audio circles that goes along with the phrase "bass is omnidirectional." What does that phrase really mean?
If you take it to mean, "most speakers approach omnidirectional radiation as you go lower in frequency" that's entirely correct.
But a lot of people have, without thinking it through carefully, translated it to mean, "we can't hear what direction low frequency sounds are coming from." This is not correct at all. You often see this assumption come up in discussions of single subwoofers, or the point one channel in 5.1 surround systems.
Think for a moment of hearing the sound of an airplane from a large distance. This is an example of low frequency sound without a high frequency component. The transmission loss in air has erased all high and even mid frequencies, leaving nothing but the lows. We have no difficulty locating the source of the sound.
In fact the common experience of hearing the sound emanate from a different point than the point where our eyes place the airplane is a further illustration of just how strongly we are able to localize the origination point of the sound.
It's also true that low frequency sound wraps around objects in its path, like our heads, in a different way than high frequency sound. This has many ramifications for us, but I don't think "omnidirectional" is the right word to describe this effect.
Just trying to bring some clarity.
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Would this do anything positive for a near field desktop system?
Thanks, Jess
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Would this do anything positive for a near field desktop system?
Thanks, Jess
I don't know, but you gave me a good idea! ;D
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Bonzo,
Please post your impressions. I don't use headphones much with my SEX amp, but if this has good affect on near too field I just might build one.
Jess
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Bonzo,
Yes, please do ;D What are the Russian PIO caps called/who sells them? I'm looking for the parts to build this thing now, that 1 nf cap = .001 uf, right? Thanks!
Dave
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Russian K40Y-9 PIO's. They're all over Fleabay, other places too.
1 nF = 0.001uF = 1000pF
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Are they similar sounding to the Russian K75-10's? I have some of those as coupling caps in my quickie.
Dave
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Not compared them directly, but this article has reviews/thoughts on the Russian capacitors (as well as many others!).http://www.enjoythemusic.com/diy/0708/capacitor1.htm (http://www.enjoythemusic.com/diy/0708/capacitor1.htm)
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thanks.. just one more question, how critical are the voltage/wattage/tolerance ratings on the components? I have some Vishay Dale 1/8 watt 100k resistors already, but might need some help figuring out what voltage/wattage/tolerances to buy.
Dave
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@aragorn723
I bought caps on fleabay, searching for "___uF matched" (fill the correct value). The search will return you almost only russian PIO caps. Please take note that russian 1nf caps have big lead legs, you'll need to enlarge pcb holes and trim a bit the legs to accomodate them. I don't know about resistors, I used 1/4w but I think 1/8 will be ok too
@jake111
I'll try with a simple setup: quickie, t-amp and fostex fe103 on my desk. Test disc "Misterioso"... 8). Will report later.
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might need some help figuring out what voltage/wattage/tolerances to buy.
I forgot to add thath it's better to match components between L and R channels (as usual) and that for capacitors the higher the voltage the better.
But given the small capacitance required, you won't have a lot of voltages to choose from anyway.
Ciao!
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Sounds like the voltage and wattage don't matter much. So tolerance is probably the same? Thanks,
Dave
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Ok,
some quick impressions when using in nearfield setup: no quickie, only ipod touch with direct output > 3D preamp > Fenice 2024 amp > Fostex fe103.
The first thing I noticed was the apparent lack of low frequencies, voices and trumpets were thinner than before, but after a moment the effect of broadening the sounstage was there! And made me forget the harshness
My ears were at 1m from the speakers, so after a while it was very fadiguing to keep on listening anyway.
I wish I had my Yamaha small monitor at home, just to try a more bass friendly speakers.
The "3D effect" added, however, is really interesting!
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Hi Bonzo,
Have you put a lot of time on the components? To me that sounds like they aren't burned in yet (I had a similar experience with the Quickie before burn-in).
Dave
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Thanks, Bonzo
I think I'll build one.
Jess
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Have you put a lot of time on the components? To me that sounds like they aren't burned in yet (I had a similar experience with the Quickie before burn-in).
Not at all aragorn...maybe is too early to judge...
I'll give it a burn-in period connected to my tv, thank you for the suggestion!
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Please let us know your thoughts when it's burned in!
Dave
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ATM it's still hooked to my bedroom tv setup.
Speakers are close to each other (no stereo image needed), but this preamp in some way made the sound more "open", it seems it's acting like this software: http://www.ambiophonics.org/index.html (http://www.ambiophonics.org/index.html)
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Bonzo,
Do you notice a difference by putting the speakers further apart? Sounds like this gizmo is pretty handy for less than optimal setups.
Dave
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No, I didn't!
This is a bedroom setup, I cannot change speaker setup due to small space :'(
But this circuit is really surprising! Bass seems to have increased a bit after 20 hours of use.
Ciao!
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cool! Looking for some Russian caps now, gonna build one of these babies to play with :)
Dave
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Hi,
Would it be possible to switch the output on this 3d preamp box? One side of the switch would be for rca outputs, and the other would be to a 1/4" headphone output (this would allow you plug an ipod's output to the 3d preamp input, then connect headphones to its output). Thanks,
Dave
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So I built one following the drawings and the shopping list. (The shopping list has 240k and the drawings show 249k) I notice 2 things on first play:
1- while it doesn't give the illusion that the music is coming from somewhere out in front of me, it definitely gives a fuller depth presentation to the music playing in my head.
2- the potentiometer works backwards, what would normally be all the way down gives the most effect, turning it "up" lessens the effect.
I checked several times and my build wiring matches the drawings and photos.
Similar/different experiences?
Thanks for any input.
Karl
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So I built one following the drawings and the shopping list. (The shopping list has 240k and the drawings show 249k) I notice 2 things on first play:
1- while it doesn't give the illusion that the music is coming from somewhere out in front of me, it definitely gives a fuller depth presentation to the music playing in my head.
2- the potentiometer works backwards, what would normally be all the way down gives the most effect, turning it "up" lessens the effect.
I checked several times and my build wiring matches the drawings and photos.
Similar/different experiences?
Thanks for any input.
Karl
Karl, it mirrors my experience exactly.
My first thought were the sound was slightly "above" the center of my head...
I forgot: the pot works backwards too!
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Is there any way to make the pot go the right way?
Dave
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I would need to check the schematic to be sure, but I would think that you could reveres the 2 end connections at the pot...John
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Is there any way to make the pot go the right way?
Dave
For me it's not a real problem: it's a bit counterintuitive, I admit, but it's not "plain wrong"...we're dealing with an "effect", not with a volume control ;)
Ciao!
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This does have quite an insertion loss. Using my SEX 2.1 at work to burn it in, it is taking a significant twist of the volume pots to get it up to normal listening levels. On the plus side, I notice the "fuller" effect it has on the sound, albeit with a little loss of the laser sharp imaging the system usually has.
Karl
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This does have quite an insertion loss. Using my SEX 2.1 at work to burn it in, it is taking a significant twist of the volume pots to get it up to normal listening levels. On the plus side, I notice the "fuller" effect it has on the sound, albeit with a little loss of the laser sharp imaging the system usually has.
Karl
You'd better buy a quickie... ;D ;D ;D
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Funny, I have the reverse problem, way too much gain (with a solid state amp):) The Quickie is pretty nice though, it beats the NAD and Cary ones I used to have (with pjccs).
Dave
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Got a newbee question on the parts list from the first page. The part number listed for the switch doesn't look like the right one ( i pulled it up on parts express and a SPST switch came up). Can someone help me find the right one? Searching by DPST at PE gives me a switch with spade connectors, or one with screw connectors (I would prefer solder tab). Thanks in advance!
Dave
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Has anyone tried to bypass this pre-amp? I built one and it is definitely nice. Like others, i definitely agree that it has a LOT of insertion loss. There is an on/off switch for the circuit which turns off the 3d part, but the insertion loss is still there regardless of switch position. Is there a way to add a bypass/defeat switch into the circuit that would eliminate the effect of the insertion loss? Sometimes I just want more volume, and would definitely be a fun toy to play with! (and nice not to have to disconnect everything to do that).
Dave