Better than SET? The radically different 535SE Headphone Amplifier/Preamplifier

Discussion in 'Product Announcements' started by liamstrain, Jun 2, 2016.

  1. liamstrain

    liamstrain MOT: The Audio Guild

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    NOTICE: An error has been discovered in the 535SE schematic in Post #30 (PDF and graphic are now updated).

    From Steve:

    The value of R105/205 is incorrect. The 4.3 ohms that was shown was a holdover from a previous biasing scheme (resistor/red LED). When I decided to switch to the current CRD/Zener scheme, I updated the schematic to the CRD/Zener, but left the 4.3 ohm resistor designation until I could establish the new resistor value. This was nearly two years ago when I decided to put the amplifier on the back burner and didn't breadboard it. I subsequently forgot that 4.3 ohms was not the correct value.

    My apologies.

    The schematic has been updated and R105/205 given a "TBD" until I can breadboard it and dial in the final value. It will likely be in the range of 10-15 ohms. When that has been done (we're still waiting on the heatsink samples) I will update the schematic with the proper value and power rating. Hat tip to @dsavitsk for catching this. Thanks, Doug!
     
  2. liamstrain

    liamstrain MOT: The Audio Guild

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    FIrst parts delivery! We're on our way!

    [​IMG]
     
  3. bazelio

    bazelio Friend

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    Cool, and big props to Doug, indeed!
     
  4. TwoEars

    TwoEars Friend

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    I'm not an EE so please correct me if I'm wrong here... but you've got a JFET in the signal path. The way I see it everything that happens before the JFET (input transformer specifically) is just there to feed the JFET with an appropriate voltage. Then the signal is created anew on the right hand side of the JFET using the +32 volt part of the circuit.

    The 16 volt part of the circuit and the 32 volt part of the circuit never meet. So the signal on the left hand side of the drawing is a different signal than the one of the right hand side of the JFET. They might be mirrors (or inverse mirrors) of eachother but none of the electrons circulating in the input tranformer ever make it to the right hand side of the drawing.

    So to say that "all of the signal amplification takes place in the transformer".... I don't know. In my mind it's kind of strange way of looking at it. The input tranformer to my mind does nothing but provide an appropriate voltage for this specific model of JFET. Had you picked another JFET maybe you would have needed another input transformer, or maybe none at all?

    Furthermore - from what little I know of JFET's I know that they tend to sound better the more voltage you put through them. So I suspect you upped the voltage on the right hand side of the drawing (+32 volt) and then you needed a input transformer to up the signal going to the JFET to control all that voltage. Again - had you picked a lower voltage on the right hand side of the JFET maybe you wouldn't have needed an input transformer. Or maybe just a 1:1 input transformer.

    I don't know what I'm really adding to the discussion here... it's just that saying that "the input transformer is where all of the signal amplification takes place" is kind of a strange way of looking at it to me. For me the JFET is the key component here and it creates ALL the signal amplification depending on what voltage and amps you feed it. Just my way of looking at it....

    Nevertheless it's an intriguing design and I suspect it should do a pretty good job of simulating tubes with some distortion here and there. It's definitely not going to sound like solid state, I can almost guarantee that.
     
    Last edited: Jun 16, 2016
  5. liamstrain

    liamstrain MOT: The Audio Guild

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    Thanks @TwoEars - I got a reply from Steve on your post:

    --

    I think your confusion may be coming from the +16 volt line.

    This is a wholly single-ended circuit, including the power supply which just has a V+ and ground as opposed to V+, V- and ground. As such, if it were not for the +16 volts, it would not be able to swing any signal to speak of. So the +16 volts is not providing power, it is simply to bias the output stage to half of the supply voltage, so that it can swing close to +/- 16 volts.

    The input transformer isn't there to simply supply a specific JFET with a specific voltage. That +16 volts comes from the power supply and the transformer has nothing to do with how much voltage is supplied.

    As has been said, the transformer provides all of the signal amplification. It is a 1:5 step up transformer meaning if you feed it a 1 volt signal, it will amplify that signal by a factor of five so that at its output you will have a 5 volt signal instead of a 1 volt signal.

    The output stage does not amplify the signal at all. It outputs the same voltage that is fed into it. So with 5 volts in (from the transformer), you get 5 volts out. What the output stage does is provide a low output impedance and sufficient current to drive the load.
     
  6. HAL9000

    HAL9000 Almost "Made"

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    @liamstrain what is the timeline and pricing for the preorder amps looking like?
     
  7. liamstrain

    liamstrain MOT: The Audio Guild

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    Hey there @HAL9000 - we are still working on the preproduction units which will inform our final timelines and pricing. Currently we are still expecting it to hit the $1,750 - 1,850 range, and a few more weeks before we have testing units out, and preorders accepted.

    I will update once we learn more from some of our key suppliers.
     
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  8. liamstrain

    liamstrain MOT: The Audio Guild

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    Lots going on. We've gotten faceplates and sent them to the chassis builder to get everything lined up. First transformers have arrived, small knobs, and the long frame phono jacks.

    We are quite impressed with these jacks. One of the features we wanted the 535 to have was for the preamp outputs to be muted when you plug in your headphones. While this feature has been a mainstay on mass market consumer gear for decades, high end headphone amps with preamp outputs require that the user do this manually using a separate front panel switch. In some cases, there is no switch at all and the headphone output is hard wired in parallel with the preamp outputs.

    Headphone jacks with integrated switching are readily available, but typically the switch contacts are nothing more than dimples pressed into the main contact springs. We found this unacceptable. So we went back to the roots of the 1/4" "headphone" jack which is the telecommunications industry. Or as it was affectionately known back in the day, Ma Bell.

    It was Bell Telephone that developed the 1/4" plug and jack combo that would later become the de facto standard for headphone plugs and jacks on consumer audio gear and why they are referred to even today as "phone plugs" and "phone jacks." If you have ever seen old film footage of telephone switchboard operators plugging and unplugging cables into the switchboard, those were 1/4" plugs and jacks.

    The plugs and jacks used by Ma Bell were made to incredibly high standards compared to consumer audio plugs and jacks. There was no cutting corners to save a nickel here or a dime there. Shit had to work and work reliably under constant, long term use.

    To that end we are proud to say that the 535 is the only headphone amp that we are aware of to use a long frame telecommunications-grade phone jack for the headphone output. A deconstructed jack is shown below.

    [​IMG]

    The jack is made up of a number of contact leaves that are alternately stacked between phenolic insulating spacers. The assembled stack is then screwed securely to a rigid, stamped steel frame.

    Note the little rectangular bits near the ends of the contact leaves. Those are the switch contacts for the integrated switching of the preamp outputs. No cheap stamped dimples here. Each of those rectangular bits is a miniature bar of solid palladium that is resistance welded to its respective contact leaf in a crossbar fashion (i.e. each contact is oriented 90° with respect to its mating contact). As we said, Ma Bell didn't cut any corners and allows us to offer integrated switching of the preamp outputs without sacrificing quality.

    [​IMG]

    Below shows an assembled jack mated with the telecommunications-grade plug that was designed for it. Of note is that the leaf contacts are substantially thicker than those found on cheap commercial jacks. Also note how the inserted plug disengages the switch contacts.

    [​IMG]

    The plug also differs from commercial offerings. It uses a ball tip as opposed to the more bulbous heart-shaped tip of commercial plugs. Also note the insulator between the tip and ring contacts. Commercial plugs use a much thinner insulator between the tip and ring contacts which allows the jack's ring contact to short between the plug's tip and ring contacts, which has caused some headphone amplifiers to famously blow their output stages. This should not be an issue with the 535 regardless of which plug is on your phones, but we did want to show this one in use.

    We will provide additional information on the timeline as we hear from more of our suppliers. And, of course, more pictures soon.
     
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  9. jelt2359

    jelt2359 Friend

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    That last picture looks a bit.... erm.... phallic? You even use the word "mated"! This thread is really becoming NSFW
     
  10. liamstrain

    liamstrain MOT: The Audio Guild

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    Sex sells? ;)
     
  11. Armaegis

    Armaegis Friend

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    Hey, I think I actually have some of those jacks (or something very very similar) in my collection somewhere...
     
  12. bazelio

    bazelio Friend

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    Who knew Ma Bell was an audiophile, eh?

    BTW, have you guys tested these with plugs other than the PJ-051?

    @Armaegis Switchcraft MT334BX ?
     
  13. Armaegis

    Armaegis Friend

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    It's a Switchcraft, but not that one. The plug goes in perpendicularly rather than axially, and the switching contacts are bigger and squarish. I'm guessing it's a discontinued model because I can't find it on their site.
     
  14. liamstrain

    liamstrain MOT: The Audio Guild

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    @bazelio - the plug shown is a 483NCP, not a PJ-051. And that yes, the jacks work fine with every commercial plug we've tried them with (lots).
     
  15. liamstrain

    liamstrain MOT: The Audio Guild

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    It's alive! First pre-production units are alive and kicking (and sounding great). Time to get them in chassis, and out for testing.

    [​IMG]

    FYI - These circuit boards lack the solder mask and silkscreen and are ONLY for these pre-production units.
     
    Last edited: Jul 19, 2016
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  16. Kattefjaes

    Kattefjaes Mostly Harmless

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    Oh goodness me, those look a lot like what are called "Post Office Jacks" here in the UK. They used to be used in broadcast applications here, as well as in telecomms, and even though you'd expect them to be all but extinct, I am always finding them when looking for 1/4 inch TRS leads in the store cupboards. Great to see someone using them in anger!
     
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  17. pedalhead

    pedalhead Friend

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    Ignore me, wrong thread.
     
  18. liamstrain

    liamstrain MOT: The Audio Guild

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    You are welcome regardless. ;)

    Chassis are completed, we are waiting for powdercoating and silkscreening. We are hoping they'll be done this week so we can get the wood cabinets built, and get these out for reviews.

    I hate waiting.

    Your patience and interest is always appreciated.
     
  19. liamstrain

    liamstrain MOT: The Audio Guild

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    Getting there, slowly but surely. Powdercoat and silkscreen are done, and they are sent in for wood. No ETA yet, but will update as soon as we know.
     
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  20. Kattefjaes

    Kattefjaes Mostly Harmless

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    It's nice to hear it's going smoothly. It looks like a really interesting amp.
     
    Last edited: Dec 19, 2016

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