[Slater]

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:

Here is FFT for 1.0vpp into 50ohms (note profile is about the same with just greater THD):

What is the battery capacity/runtime?

 

[xrk971]

What is the battery capacity/runtime?

Using a 3000mAhr 3.7v LiPo cell and boosting to 16v should give about 4hrs. I need to test duration of run.

 

[xrk971]

In addition to a LiPo and DC step up and Cap Mx, would having BT 4.0 or a built in DAC be more of a desired feature?

 

[BucketInABucket]

In addition to a LiPo and DC step up and Cap Mx, would having BT 4.0 or a built in DAC be more of a desired feature?

Not for me, it's a dedicated amp and should stay one imo.

 

[DBaldock9]

In addition to a LiPo and DC step up and Cap Mx, would having BT 4.0 or a built in DAC be more of a desired feature?

If you're designing the circuit board, considering future upgrades, maybe you could add a header, for a "Digital Audio" daughter-board?

 

[xrk971]

I’m designing a new desktop preamp/headphone amp that will have a DAC and BT. But a header for future expansion is certainly a good feature. I currently use headers for core amplifier daughter boards on the group but Aksa Lender Preamp. It lets one swap boards for different gain, bias, even output devices. Daughter boards are inexpensive and allow ultimate flexibility of main amp.

More info here:
http://www.diyaudio.com/forums/group-buys/315521-aksas-lender-preamp-40vpp-ouput-gb.html

Here is one of the daughter boards:

This is the sort of distortion this amp achieves for 20vpp:

The new Pocket amp will be Class A of course but with topology similar to Aksa Lender for lower distortion. Will have LiPo, DC step up, cap multiplier, BT, and am working on adding a DAC even. Won’t really have room for expansion cards - as it will be pretty compact and still pocket sized with custom CNC case.

 

[xrk971]

Hi Folks,
I just wanted to let you know that member Brooko just reviewed this amp. It’s here:

https://www.head-fi.org/showcase/xrk-audio-nhb-portable-class-a.22873/reviews#review-19723

Thank you, Brooko for a thoughtful and well written review. You hit all the key features of his amp nicely and I agree that the one that keeps me listening to it is the sound quality. It is magical.

Cheers,
X

 

[DBaldock9]

Will you be offering a USB rechargeable Li-Po battery pack, for the Class A NHB Amplifier?

 

[xrk971]

There is a DIY solution you can follow just like I did using a small cap Mx board, a DC boost board, and a LiPo pack. It requires cutting a trace on the PCB and some wiring.
The details can be found here:
http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-144.html

Looks like this laid out flat:

And like this all packaged up:

If there is enough demand, maybe I can make an integrated board that has the DC step up, cap Mx, CRCRC, and battery holder all on one board. But that still requires surgery to the main PCB and I don't know how many people are willing to do that.

I am working on an all new design with a CNC case and integrated LiPo USB power charge system though - but that is kind of down the road.

 

[Slater]

Will you be offering a USB rechargeable Li-Po battery pack, for the Class A NHB Amplifier?

Or better yet, let this accept a USB battery power pack (all output 5v 1A, and they even have them with 5v 2.4A nowdays). You can buy the power packs in almost infinite capacities and case configurations, from single 18650 "lipstick" tube style (usually around 1500-2000mAh), all the way to 20,000mAh (and even larger). That way, it's up to the user to decide what amplifier runtime they want.

You'd just need to have a female microusb port on the amp as the power input (as well as the built-in boost and power filtering circuitry obviously).

 

[MyPants]

Welp, I'm extremely interested in one of these, but I'm torn on whether or not to take the NHB plunge. Decisions decisions...

 

[xrk971]

Welp, I'm extremely interested in one of these, but I'm torn on whether or not to take the NHB plunge. Decisions decisions...

You can be certain that the amp will sound excellent and is sure to please. Everyone who has listened to one or reviewed it has loved it. Even the last review by Brooko, he liked it so much he bought it despite the fact that I gave him a free sample to keep.

What cans or IEMs are you using? I can custom tune the amp for best performance with your headphone or earphone.

 

[MyPants]

What cans or IEMs are you using? I can custom tune the amp for best performance with your headphone or earphone.

I just read Brooko's review before my post, and I tend to trust his impressions, so I'm confident that either version will indeed satisfy. I'd be using the amp primarily with the HE400i, Fostex T20RP MK3, and LZ A4 iems.

 

[xrk971]

The LZ A4 will be an excellent match with high sensitivity and 16ohms. The Fostex can work but sensitivity is on low side so may not get as loud as you want. Given that it is 93dB and 50ohms which is about the borderline of what can be driven by this amp. The HE400i being 32ohms and 91dB is too low of sensitivity and low impedance and will probably not sound it’s best. I have not personally listened but will guess that it’s going to be a struggle. I use 55ohm and 96dB OB-1’s and that works great. 91dB and 250ohms would be fine for example.

Anyhow, that’s just a limitation of the amp in not being a high current amp.

 

[MyPants]

The LZ A4 will be an excellent match with high sensitivity and 16ohms. The Fostex can work but sensitivity is on low side so may not get as loud as you want. Given that it is 93dB and 50ohms which is about the borderline of what can be driven by this amp. The HE400i being 32ohms and 91dB is too low of sensitivity and low impedance and will probably not sound it’s best. I have not personally listened but will guess that it’s going to be a struggle. I use 55ohm and 96dB OB-1’s and that works great. 91dB and 250ohms would be fine for example.

Anyhow, that’s just a limitation of the amp in not being a high current amp.

Hmm, two questions then;

1. Considering that my current mobile amp is a 9v cMoyBB kit from JDS that I put together, would the NHB version of your amp perform at least as well or better than the cMoy with my sub-optimally paired planar cans?

and

2. I also have a 600ohm set of DT880s, and the extremely sensitive NuForce Primo8. Any better luck with these two as far as pairing goes?