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

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  1. DBaldock9
    Is the power resistor in series with the output?
    What is the output resistance of this amp model?
  2. xrk971
    IMG_7336.JPG This amp has a topology similar to its big brother - a 50w/ch into 8ohms Class AB Quasi Complementary amp shown here. The headphone amp is different in that all the transistors are different along with DC setpoints, and it uses a single rail supply with capacitive output for headphone safety.

    The output Thiele network consists of a 10ohm power resistor in parallel with a 1.5uH inductor and those are in series with the output. They serve to suppreas an oscillation should an instability arise if the outputs are connected to a nasty reactive load.

    I will have to run some simulations to see what the output impedance is. I suspect very low as it can drive a 16ohm load no problem.
    Last edited: Jul 23, 2017
  3. xrk971
    I just ran the LTSpice simulation to calculate the output impedance of the HyQu and got Zout=0.183ohms. Pretty low so ability to drive low impedance phones ois definitely there.
  4. xrk971
  5. DBaldock9
    The 18V 3400mAh Li-Ion Battery Pack that I ordered from China, isn't going to be delivered (due to new shipping restrictions for Li-Ion batteries coming into the USA) - the vendor refunded my purchase price.

    So, now I'm looking at Li-Ion Battery Packs from vendors in the USA.
    I've found an 18.5V, 2600mAh, 48Wh pack, that can supply 7A peak current, and 5.2A of continuous current.
    It has a Low Voltage Cutoff of 12.5V.

    What is the series voltage of your two EBL 9V Li-ion batteries at the time that a Pocket Class A NHB Amp shuts off?
  6. xrk971
    That's too bad. You could do the trick I suggested which is to use a DC-DC step up converter to go from 3.6v to 18v. I never actually measured the voltage at switch off. It follows guidelines set accepted standards for LiPo batteries to prevent damage. If we assume it is a 2s protection circuit, the cutoff is programmed at 2.5v or 5v per battery. So two in series would be 10v cutoff. Fully charged they are 8.4v ea or 16.8v. So you get 16.4v to 10v drain.

    Example of 2s self protect circuit (look at specs):
    DBaldock9 likes this.
  7. xrk971
    Some new tweaks after long listening sessions and much experimentation to get the tuning just right, I think I have finally struck Yukon Gold. The HyQu amp now has the harmonic distortion profile of a SE Class A amp, but the power reserves of a discrete Class AB amp. It is small, compact, and powerful. Driving 270ohm load to 1,41vrms it achieves remarkably low distortion levels of 0.007%THD with -82dB H2 and H3 at -12dB below that, and nothing else. The noise floor is flat. So you say 270ohms is no big deal? Ok, for 51ohm load - similar to typical demanding planar magnetic offerings, it can drive 22.6v peak-peak or 8.0v rms for 1250mW with 0.06%THD and a harmonic profile that still resemble a nice SE Class A amp. Intermodulation distortion is also excellent at only 0.05% for 1kHz and 10kHz. Stereo crosstalk is only -82dB without resorting to independent PSU's. For 51ohm load at 5.05vrms (500mW power), the amp remains in fully Class A operation with very low distortion. This is a level that is generally threshold of pain on most 96dB or more sensitive headphones.

    So how does it sound? Magnificent! Superb clarity, bass authority and articulation - even with 16ohm headphones no problem. Beautiful airy highs and wonderful glass smooth mids. The best sound stage and spatialization I have heard without a doubt. The classic sound signature of a little warmth and body as if it were a SE Class amp. The bass is unbelievable.

    HyQu-33pF-FFT-Left-1.41v-270ohms.png HyQu-33pF-FFT-Right-8v-51ohms.png HyQu-33pF-FFT-Right-5.05v-51ohms.png

    Last edited: Aug 1, 2017
    Sopranino and DBaldock9 like this.
  8. DBaldock9
    Can you describe the 4 or 5 assemblies that we're seeing in the photo?
    Looks like two, single-channel amplifiers in the foreground.
    Is the item on the left, a DC-DC power converter? What is input voltage? How much current supply is needed? What voltage(s) & current requirements do the amplifiers have?
    Is the board, that's full of capacitors, part of the power supply?
    What is that single large capacitor, that's in the center, behind the amp boards?

    Is there an estimate for what an unbalanced, or a balanced, version of this amp would cost?

  9. gikigill
    That looks pretty good, if it can be done for 500, it would be great.
  10. xrk971
    The amp uses a small 19v SMPS to power a DC to DC step up converter, this feeds a MOSFET capacitance multiplier (underneath the big cap in between the two amp modules which has a large IRFP240 bolted to the heatsink), this then feeds the CRCRC filter which has qnty 6x470uF 50v low ESR caps in parallel for providing a low impedance path for deep bass transients. The clean and plentiful 28v power from these 3 components then feeds the amp modules. The bias is about 110mA quiescent so it burns about 3w per channel. Since this is Class AB, it stays Class A below 110mA and digs into Class B when the huge transients or excessive volume is desired. It will be Class A up to 5.05vrms 14.24v peak-peak. That's good enough for 500mW into 50ohm cans. The amp can supply up to 1.25watts rms sustained power with excellent low distortion that sounds and measures like a SE Class A amp. The function of the cap multiplier (besides reducing any AC mains ripple by -50dB) is to provide a smooth graceful turn on and turn off ramp to eliminate any possibility of thump/pop. DC protection for headphones is inherent in the cap-coupled output stage. I am using modest caps right now and it sounds great. Final version will use several ultra-low ESR caps in a array for even better bass articulation.

    This is strictly a proof of concept prototype - and despite being a prototype that is hand made and in separate parts-pieces, sounds fantastic and measures exceedingly well. In fact, it's the best measuring amp I have built, ever.

    Production version will be a single integrated PCB for both channels, cap array, CRCRC filter, and capacitance multiplier. The DC step up will will be separate but that gives flexibility for even a large toroidal trafo linear regulated supply to feed all this.

    The final PCB, cost of components, and how much effort it takes to make it fit into the casework etc really determines the final cost and pricing. I think given then amount of componentry, the large size of the board and final case - this is very preliminary, but I am guessing a $700 to $1000 price. Looking at the performance and listening to the sound - this is a bargain. I predict it will beat just about any other amp out there, regardless of price.
    Last edited: Aug 2, 2017
    DBaldock9 likes this.
  11. xrk971
    DBaldock9 likes this.
  12. xrk971
    Above: a table full of Schiit :L3000:

    The Head-Fi organized SchiitStorm in the DC area was a great event. Lots of very high end cool gear. I feel like more than 10 people brought their Focal Utopia headphones, and several members brought theirs over to try with the Silicon Harmony. So now I know what those sound like on the Silicon Harmony. Sublime. HiFiMan and Audeze were also there in force. I got to listen to a lot of SE Class A tube amps. Also heard some very nice DACs. A great environment to check out what stuff sounds like and everyone is so nice and easy going with you testing their multi thousand dollar headphones or amps.

    Lots of feedback on my amps and by far, everyone agrees that Silicon Harmony is still king when compared to HyQu despite HyQu having an edge in power. I was really impressed by the sound that some of the $1000+ headphones sound like. And they all sound very good on the SH, and the PCA was also very well received. The HyQu was also well liked but in the end, could not hold up to the SH when people listened with resolving headphones like the Utopia.

    Here are some photos of the event, which Schiit sponsored by providing the venue (a very nice clubhouse in a beautiful neighborhood in Gaithersburg, Maryland). I listened to several of the top Schiit tube amps and they sounded very nice. They ran very hot - the case felt about as hot as my SH (about 55 Celcius). I enjoyed all the amps. There was a young DIY'er there (sprigs) who had some impressive sounding P2P SE Class fleawatt tube amps. Overall it was very nice.

    Here was a really nice sounding tube amp - the Zana Deux:

    A bunch of cool DIY amps:

    The XRKAudio table:

    It was a nice way to spend a Saturday afternoon. Btw, for those of you who saw something you liked from my table, feel free to PM me or visit my shop on Etsy.

    A big thanks to Pjabber and others from Head-Fi for organizing this.
    Last edited: Aug 13, 2017
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  13. xrk971
    There is a 10% off Labor Day sale going on now for the PCA's in my shop.
  14. daid1
    How do you see this amp with a shozy alien gold?
  15. xrk971
    This amp is perfectly suited for the most resolving and highest quality digital sources you can find as it will give the device the best possible sound. Users have paired it with their ultimate DAPs and DACs including Yggdrasil and found it works even better. The noise floor is so low and use of only two transistors in signal path ensure the ultimate in signal transparency and realism.
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