[hakushondaimao]

  He's also considering adding a volume pot if enough people request it.  I've advised that it might be worth looking into - especially if people would prefer using it from a DAP with a line-out.
 
More info later.

Yeah, that was my question... if this is designed to run from HO. I'd certainly like volume control.

 

[Brooko]

Just going to post a few pics for now.  I have to talk to Martin about this as soon as he is free.
 

 

 

 
Thoughts (tested with iPhone 5S, X3ii and X1) - volume matched with calibrated SPL meter and test tones (@ 1kHz), and utilising a splitter so I could fast switch.

• Red amp shown is the low impedance one.  Blue high impedance one arrived as well - unfortunately one channel is almost out.  It happens.  I've let Martin know and we'll work through it.
• No volume pot - pretty sure he's going to add one.
• The low impedance amp is actually quieter than the source, so at this stage it is less an amp than a tube buffer.
• It sounds pretty good - clean - not going to comment more on the sound until I've had more time with it (and a chance to talk further with Martin).
• Actual build quality is extremely good - really nice.   Surprised as well - virtually no heat.
• There is some tube ringing if it gets a hard knock - but at this stage, I'm thinking you could pocket and walk with this. 

 
More once I've had a chance to actually talk to the maker.

 

[hakushondaimao]

  Just going to post a few pics for now.  I have to talk to Martin about this as soon as he is free.
 

 

 

 

 
It LOOKS gorgeous. Looking forward to further impressions as they come.

 

[Brooko]

I spent a bit of time on the phone to Martin tonight and we discussed a few things already.
 
He's already amending the design to introduce the following:
 

• An adjustable volume pot
• A gain switch for different headphone impedances
• A charging indicator (there is one but its obscured at the moment)

 
I've had the red amp burning in all day (to give the tubes a chance to settle).  The surprising thing is the heat control, and also the genuine clarity.  I'm now using it with the Fiio X1 and absolutely enjoying it (even if its largely being used as a tube buffer).
 
I'll be sending both units back to Martin on Friday - and he's hoping to have a new prototype with the changes we've covered so far implemented.
 
Anyway - pretty positive about this - and looking forward to seeing the progress he makes.

 

[CJG888]

I already have two portable tube hybrid amps for use with cans, but it would be nice to have one which is both genuinely portable and quiet enough for use with sensitive IEMs (particularly FAD Heaven V).

 

[billybob_jcv]

I don't really know anything about tubes, but here's a datasheet on the 6418 (this is a Tungsol, not a Raytheon, does that matter?):
http://frank.pocnet.net/sheets/127/6/6418.pdf

And here is the 6088 that I think is similar to what is used in the Vali:
http://frank.pocnet.net/sheets/138/6/6088.pdf

The 6418 seems to have a lower output and the plate voltage is also lower. I'm guessing that makes sense for a portable vs a desktop.

 

[CJG888]

The 6418s used are the same as those in the TU-HP01. They use little power and run very cool (actually need no extra ventilation). They are, however, fairly microphonic (but can be damped). Incidentally, they have cool-running heaters and do not glow. In the Elekit amp, tube glow is simulated by a pair of orange LEDs!

 

[hakushondaimao]

Been a while since we've seen any updates on the project. Anything to report, @Brooko?

 

[Brooko]

Waiting for the updated boards and new pots.  As soon as he gets a prototype, I'll be able to test it properly and then report back.

 

[Brooko]

Hi all - just a small update.  Martin has been busy with the new board, and the pots aren't too far away.  In the meantime he's been running a series of tests which is what I wanted to update you with.
 
 
He's going to get me the graphs so I can post them when I review everything.  I love that we're going to get a lot of performance details - and that they'll be properly measured.
 

• He's run a series of sine waves through the amp and taken measurements with an oscilloscope to see how they look.  In Martin's words:

   I'm very happy with how they look. Nice clean waveforms all the way from 10Hz up to over 20kHz with no sign at all of any clipping or distortion- just text book looking sine waves.  I'll include dual trace so you can see input vs output and the amount of amplification. I'll run the amp into a few different loads as well. I'll also show the frequency response over various sample frequencies.

• I've asked Martin to measure maximum output and associated distortion into 16, 32, and 300 ohm loads - so we should get those figures as well.
   
• He's also measured output impedance of the headphone out.  I think he's going to post a video of this on the KS site at some stage - but his method was:

  First test was with a 200Hz sine wave into the amp and measured no load voltage at 1.25 volts and with a 33 Ohm resistor 1.22 volts. If I plug these values into the impedance calculator that gives 0.81 Ohms output impedance.  
  For test number 2, I ran a 2 kHz sine wave into the amp at a louder volume (just to vary the mix a bit) and measured with the oscilloscope 3.01 Volts no load and 2.95 Volts with the 33 Ohm resistor as the load. This gives an output impedance of 0.67 Ohms which is close to the first result allowing for experimental error.

  
   
  So output impedance of under 1 ohm - excellent result.

 
More info as I get it.  If the output power and distortion results are equally as good - then this is definitely looking like being a very good amp.

 

[CJG888]

The other big KPI is noise, of course.

It would be great if this amp is quiet enough to use with sensitive IEMs. The Elekit isn't. The BL-2 is, but it also sets fire to your pocket...

 

[Brooko]

Another update:
 
Martin reran the output impedance test again - this time with soldered connectors.  He used multiple combinations of frequency and volume - and the average across all the tests run was 0.265 ohms at the headphone out.  High was 0.369 ohms.  Low was 0.160 ohms.
 
He also ran a frequency response test - using a signal generator and measuring the peak to peak output voltage over many different frequencies. The output was dead flat from 60 Hz to 17 kHz not even a millivolt of change. There is a slow fall off under about 50 HZ and also over 17600 Hz. He'll plug these into a graphing app and produce the frequency response curve and post it on KS.
 
Next he's testing maximum output under load - and with any luck the actual pots should be due very soon, and then I'll have a working model I can quickly write a review on.

 

[CJG888]

Any idea if it's quiet enough for use with IEMs?

 

[Brooko]

I want to wait until I get the new prototype (with the volume pot) - but according to the measurements - definitely yes.  It's supposed to also have a gain switch - so that should help as well. I tried his original low impedance prototype with my A83 and it was very good as far as noise (hiss) was concerned.  Dead quiet.

 

[mochill]

any plans on adding mic input and does it have glass in front of the tubes or can you touch it with your fingers.