New Portable Amp - "Pocket Class A" by xrk971 - now available as complete PCB

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  1. xrk971
    The way to do it is a DC step up from 3.7v and the cap multiplier for soft start then CRCRC for filter before going to amp. This method produces a very low noise flat noise floor FFT. Plus 3.7v batteries in huge 10,000mAhr or more can be found.
     
    Last edited: Nov 23, 2017
    DBaldock9 likes this.
  2. DBaldock9
    Are there already assembled 3.7V -> 18V step-up circuits available, or is it something I would have to design and assemble on a breadboard?
     
  3. xrk971
    Last edited: Nov 23, 2017
    DBaldock9 likes this.
  4. DBaldock9
    So the MT3608 DC step-up circuit, with its 2A rating, is enough for a NHB Amp, driving high & low impedance earphones & headphones?
     
  5. xrk971
    Yes, you can see it powering amps in the DIYA thread. You need the cap multiplier circuit or it won’t work. That circuit provides a soft start so that batteries and DC step up do not shutdown.
     
  6. DBaldock9
    Is the cap multiplier circuit something that can be purchased, or does it have to be assembled from scratch?
     
  7. stellarelephant
    Right now the only way to get the CapMx is to have your own PCBs printed using the files provided by RaptorLightning and then build it yourself.
     
  8. xrk971
    I might be able to make you a cap Mx Dbaldock9. Send me a PM if interested.
     
  9. krunchcrispy
    HEy good call -- I checked, and they are NOT rechargeable, I thought they were. . by bad!
     
  10. xrk971
    I just got word that Brooko received my NHB safely in the mail. He will listen and provide a review in the near future. Stay tuned!
     
    DBaldock9 likes this.
  11. xrk971
    I have a new public-domain DIY preamp designed by Hugh Dean of Aspen Amplifier that was inspired by the Lender topology. It is capable of 40vpp drive into 7kohm load with low distortion. The distortion it has is all SE Class A in character (mostly H2, a little H3 and nothing else.) It has 0.002%THD driving 20v peak-to-peak into a 7kohm load (more than enough to drive any power amplifier). The dynamic range and headroom are enormous. Gain is adjustable from 12dB to 22dB with change of feedback resistor,

    More info here:
    http://www.diyaudio.com/forums/analog-line-level/314563-aksas-lender-preamp-40vpp-output.html

    There is a group buy for some nice PCBs designed by a pro (JPS64). A second round GB may be possible if there are more interested parties:
    http://www.diyaudio.com/forums/group-buys/315521-aksas-lender-preamp-40vpp-ouput-gb.html

    Briefly, here is the very elegant circuit by Hugh Dean, using only 5 actives (1 is a CCS):
    [​IMG]

    Here was my SMT prototype (there are nice SMT versions of all the TH components in the above schematic, and they actually work even better):
    [​IMG]

    And here is measured FFT for 20vpp into 7kohm load with 0.002% THD, note the absolutely clean flat silent noise floor at -130dB:
    [​IMG]

    Here is the proposed finished PCB with two core amp module daughter boards and input selector, cap multiplier based PSU and onboard CLC and CRCRC filter:
    [​IMG]

    Amp core module daughterboards are available in SMT, TH, or metal core SMT:
    [​IMG]

    [​IMG]
    [​IMG]

    This preamp was designed to be able to drive power amps with unity gain output stages (Pass F4 for example) or a simple single MOSFET common collector output type SE Class A amp. But I have found that it makes any amplifer sound better by giving it the proper driving headroom and clean low noise floor that it needs. PSU will be external SMPS walwart from 12v to as high as 35v. A 19v laptop SMPS is perfect.

    Oh, and if you have no need to drive 20vpp into an amp, here is what it can do driving typical Line-Level inputs at 4vpp - 0.0011% THD, and this is at 22dB gain. If you only need 12dB gain, the distortion will be much lower, about 0.0007%THD 0 that's 7ppm!:
    [​IMG]

    Finally, what does this have to do with headphones? Well, I use it to drive my Silicon Harmony SE Class A and the sound is glorious. I have also used it to drive a unity gain SE Class A, the MoFo amp as a headphone amp. Finally, with a simple swap out of 3 resistors to the above circuit (to increase bias current from 12mA to 90mA), this preamp becomes a bonafide low distortion SE Class A headphone amp!

    Here is 1vpp into 50ohms at 90mA bias current and achieves 0.0056% THD: This low level of THD is hardly ever seen in a SE Class A headphone amp:
    [​IMG]

    Sounds very very nice.
     
    Last edited: Dec 2, 2017
    Slater and DBaldock9 like this.
  12. xrk971
    I had some time to test the Aksa Lender Preamp driving the single-transistor MoFo amp. I found a 67mH 0.47R choke from a dead microwave oven.
    [​IMG]

    Using that as a reactive current sink (double the output swing for free). Here is data for 2.83vrms into 8ohms.
    [​IMG]

    Here are various distortion components for 2.83vrms into 8ohms:
    [​IMG]

    Here is max output voltage before clipping, 26.8vpp (from 19v Vcc - extra power free as I said). That’s 11Wrms into 8ohms. Normally a 19v Vcc and MOSFET out limits max swing to 14vpp.
    [​IMG]

    And here is a 10vpp square wave:
    [​IMG]

    If this amp we’re used to drive say 50ohm headphones, distortion would go down due to lower output current.

    Anyhow, this is a good demo of what is capable with this pre.

    For those not able to make a DIY version of this amp, a fully built XRKaudio version called the Silicon Ephiphany will be available soon.
     
    Last edited: Dec 3, 2017
    DBaldock9 likes this.
  13. xrk971
    8464E437-5CD7-434B-9806-708CDC345373.jpeg 1CD8EBD6-2226-4673-A4B0-F0E589F39742.jpeg Happy New Year folks!

    I have some cool new tricks for this Pocket Amp. I just wired up a full-fledged rechargeable LiPo battery solution last night using a DC step up boost regulator that maintains constant 18.0vdc Vcc for constant sound quality (THD doesn’t rise as batteries deplete). The boost goes to a MOSFET cap multiplier that smooths the output of the DC boost, but more importantly, it forces a gradual ramp up of voltage over several seconds. This has the benefit of zero turn on/off pop. The cap multiplier feeds a CRCRC filter to clean up any residual noise. So the system is absolutely silent even with 106dB IEMs plugged in. The measured noise floor is also just as low as a plain battery. That is flat down to -130dB so very quiet as you have come to expect from this amp. I have also optimized the resistor network to provide better performance for low impedance and high impedance sources. Lower THD and lower H3 relative to H2.

    So here is he new amp configuration. Can be charged from USB even when playing. LED changes from red to blue when charged as usual.

    Testing it now on my commute on the metro. Works very well with my Cayin N3 and 5ohm ZS5 gen1.

    Here is FFT for 1.0vpp into 270ohms:
    [​IMG]

    Here is FFT for 1.0vpp into 50ohms (note profile is about the same with just greater THD):
    [​IMG]
     
    Last edited: Jan 3, 2018
    stellarelephant and DBaldock9 like this.
  14. DBaldock9
    I haven't been keeping up with the thread over on DIY Audio, so I was wondering if anyone is working on a compact circuit board layout, that includes all those different power supply sections?
     
  15. xrk971
    Yes, I am planning at some point on making a new one with charging and battery capability. Not sure about the schedule though.
     
    Last edited: Jan 3, 2018
    stellarelephant and DBaldock9 like this.
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