[xrk971]

Hi Carmantom,
Thank you for the detailed listening impressions. So glad the amp worked out well and was able to drive your LCD-2's! Your review and impressions are not unlike others that I have heard from others regarding the musicality, bass articulation, to the sound stage and imaging. One really important technical detail about this amp is that it uses only two transistors! There is something to be said for having the audio signal pass through as few as possible to keep the noise low and the purity of the signal clean. If you have very sensitive IEMs, you will find that the self noise of this amp is truly the blackest of blacks - inaudible.
Cheers,
xrk971

 

[alpha421]

Damn it! Waiting for the batteries to charge to test out the amp (low gain version, purchased from Funch).

 

[xrk971]

If you are in a bind and want to hear it, plain old 9v from drugstore works. Just cheap carbon cells from dollar store works for a few hrs while you wait. Alkalines work nicely too.

When using Li-ion 9v's or Lipo 9v's note that they have a smart charger chip inside that sometimes cuts off power when first turned on as the in rush current to charge the two large 2200uF caps is seen as a short circuit. Click power on off several times until LED stays lit brightly.

The gain is all the same so not sure by what you mean by "low gain version" from Funch. Maybe lower value capacitors?

 

[Carmantom]

Hi Carmantom,
Thank you for the detailed listening impressions. So glad the amp worked out well and was able to drive your LCD-2's! Your review and impressions are not unlike others that I have heard from others regarding the musicality, bass articulation, to the sound stage and imaging. One really important technical detail about this amp is that it uses only two transistors! There is something to be said for having the audio signal pass through as few as possible to keep the noise low and the purity of the signal clean. If you have very sensitive IEMs, you will find that the self noise of this amp is truly the blackest of blacks - inaudible.
Cheers,
xrk971

Absolutely true about the black. Sorry I failed to mention how quiet it is. I am truly impressed with this amp. I also made the changes you asked for.

 

[alpha421]

If you are in a bind and want to hear it, plain old 9v from drugstore works. Just cheap carbon cells from dollar store works for a few hrs while you wait. Alkalines work nicely too.

When using Li-ion 9v's or Lipo 9v's note that they have a smart charger chip inside that sometimes cuts off power when first turned on as the in rush current to charge the two large 2200uF caps is seen as a short circuit. Click power on off several times until LED stays lit brightly.

The gain is all the same so not sure by what you mean by "low gain version" from Funch. Maybe lower value capacitors?

Possibly. Funch was selling a high gain and this low gain. Yes, he relayed me the info of turning on/off the pot quickly a few times with the Li-ion 9Vs'

 

[xrk971]

Possibly. Funch was selling a high gain and this low gain.

I have competition from Funch it seems

Let me know if you are interested in a hand-tuned NHB model. The tuning uses a resistor network that is different than run of the mill standard amps and makes a difference. Anyone can roll caps and match FETs, but the tuning is key to the upgraded sound.

If you read the review by Michael on Headfonics.com - he noticed a substantial improvement with the NHB.

http://headfonics.com/tag/xrk-audio/

 

[alpha421]

Temping. Other than the loud initial pop, this is an outstanding sound. Is the pop inherent to the design? I've own desktop amps that produced a loud pop. I made a dummy resistor plug to absorb the pop before I connect the head/earphones.

Kudos to your design. I listened with my Grado and although it's been a while since I had a Melos amp, the dynamics and rich tone had me with a flashback.

 

[xrk971]

Hi Alpha421,
Turn on pop is something I avoid by turning on before inserting phones. It can be avoided electronically with a delay timer and relay circuit. A lot of added complexity and cost. Moreover, it takes up valuable real estate that is used for larger caps. I have a version with turn on pop suppression - it works wonderfully but with 2 extra TO92 MOSFETs, a TO92 7812 regulator, two small caps, a resistor, added 4mA current Drain, a gold/iridium contact low current relay, so the board space is limited to a single output capacitor due to space limitations (so max is 470uF). In my opinion, not worth the added complexity and loss of real estate.

What caps are you using now?

 

[alpha421]

I tried waiting for a minute or two before inserting my headphones, and still got a pop. No biggie as the dummy plug works like a charm. The caps installed on the board, assuming, they are the set on top of the board facing up are brown colored Elna (about the size of a tic tac).

 

[xrk971]

There shouldn't be any pop after 2 seconds. Strange that you get it after such a long time. Basically it is the output cap charging up.

It sounds like Funch may have opted to not install any dummy load resistors on the board. There is provision for two and I usually put 270R there. If you open board you will see two labeled RL near the output jack. Install two 0805 270R there. Or at least 680R or 1k if you are afraid of it pulling down impedance any lower. That will discharge the caps on power up.

The tic tac Elna is on the SMT (visible side) is just a 10uF bypass. The big one are hidden underneath.

 

[DBaldock9]

I can report that the Hitech 9V @ 720mAh Li-Ion batteries DO NOT work in the Pocket Class A NHB amp.
When the knob is turned from Off to On, there's a soft "pop" in the earphones, and the LED flashes on, then fades off.
It's probably the in-rush current causing a battery protection circuit to activate.
If the earphones are disconnected, and the power is switched On / Off / On rapidly, the LED lights up brighter, but the amp doesn't stay on.

So for now, I'll just have to buy a box of Alkaline batteries.

Just for fun today, I attempted once again to see if I could get the Hitech 9V @ 720mAh Li-Ion batteries to work in my XRK NHB Amp:

First - With the earbuds disconnected, I tried turning the power switch On / Off / On rapidly - many, many times - but I'm still seeing getting the same result - the LED lights up brighter, but the amp doesn't stay on.

Second - I tried leaving the power switch On, and unplugging / re-plugging one of the batteries - and the amp came on and stayed on!
So, then I plugged in my TY Hi-Z HP-650 earbuds, and I'm listening to my Roku 4 Net Streamer, playing ThistleRadio through the XRK NHB amp, running on the Hitech 9V @ 720mAh Li-Ion batteries.

The analog audio is being decoded from the Roku HDMI output, using one of these - http://www.ebay.com/itm/HDMI-Dolby-...Digital-Audio-Converter-Decoder-/230840436904 - but I'm changing the setup of my audio system, by adding a 4x4 Matrix SPDIF (Toslink) switch, with inputs from my PC, Roku 4, VCR/DVD, and a stand-alone ADC; and outputs going to my Micro iDSD (feeding my powered monitor speakers & sub-woofer), a Bluetooth transmitter, and a FiiO D3 DAC (to connect to amps, like the XRK NHB, Walnut v2, and Zishan Z1).

Since the amp powered On when the battery was unplugged / re-plugged, I'm wondering if there might be a inrush current carrying issue with the power switch part of the Volume Pot?

 

[xrk971]

The little power switch on the Alps pot is not designed to carry huge continuous current but a turn on transient shouldn't be a problem. It's rated for 10,000 cycles and 3amps at 16v. Proof tested at 300vac for 1min.

The part specs are here:
http://www.alps.com/prod/info/E/HTML/Potentiometer/RotaryPotentiometers/RK097/RK0971221Z05.html

 

[DBaldock9]

The little power switch on the Alps pot is not designed to carry huge continuous current but a turn on transient shouldn't be a problem. It's rated for 10,000 cycles and 3amps at 16v. Proof tested at 300vac for 1min.

The part specs are here:
http://www.alps.com/prod/info/E/HTML/Potentiometer/RotaryPotentiometers/RK097/RK0971221Z05.html

I had read the specs, and was a bit concerned about using one switch to control power for a pair of amps, and the THAT 1646 circuitry, as I'm assembling a balanced output amp model.
But for single amp operation - I can't figure out what's causing these batteries to work fine when one is unplugged / re-plugged vs. leaving the batteries connected, and switching the power On with the switch?

 

[DBaldock9]

I wish I had noted the exact time that I got the batteries to work with the amp this afternoon, but they just stopped - after 4 or 5 hours of operation, driving my TY Hi-Z HP-650 earbuds.
The way these particular batteries "run down", is to cause the amp to make what to me sounds like someone hitting an Irish Bodhran drum (a percussive thumping sound, almost in-time to the music), for about 10-15 seconds - and then silence.

 

[xrk971]

My amps simply stop making sound suddenly. No warning and it happens because the internal autoshutoff circuitry on the batteries kick in to prevent over depletion. This is with EBL batteries.

Why yours work when one is in first and then add the second one could be that the current in series when suddenly turned on is too much. But half as much when one is in? But batteries are in series so no current flows with one disconnected. So very strange.