[xrk971]

 


How about a bass boost and a gain switch?

 
There is a volume knob you know
 
In all seriousness, since this is a zero global feedback design, one can't just change the "gain" by setting resistors for Rf/Ri, etc. as would be done on a conventional amp with large amounts of negative feedback.
 
So the gain is fixed by the topology at 3x so all we can do is attnuate it - and the way that is done is by the volume pot.  I suppose a switch could be added to attenuate by an additional amount but that would require rework of the PCB and finding room for a switch on the ever so small knob/jack real estate of the mint tin.
 
I don't have bass boost as it is not needed - superb bass that follows the music as intended.  Bass response is absolutely flat measured -0.25dB at 20Hz, extrapolating to 10Hz I estimate -0.5dB at 10Hz.  Upper end falloff is due to 96khz sampling, the inherent bandwidth is in the 100's of kHz (approaching 1MHz) according to sims.  
 
I listened to some intense sub-bass tracks and it rocks my head - literally feel the air pressure pulsating behind the cans on my my eardrums (not a freqency one can hear but just feel).

 

[gikigill]

What headphones did you use?
 
I,m thinking of a 18v power supply for desktop usage too.

 

[xrk971]

I use DT880-250 special edition chromes.

Desktop PSU is a good idea and I can provide that. Here I demonstrated the amp working on 120vac mains with my own PSU filtering to provide clean noise free power. Notice lack of 60Hz hum peak.

 

[Slater]

 
I,m thinking of a 18v power supply for desktop usage too.

 
That's my intended plan as well.

 

[gikigill]

I use DT880-250 special edition chromes.

Desktop PSU is a good idea and I can provide that. Here I demonstrated the amp working on 120vac mains with my own PSU filtering to provide clean noise free power. Notice lack of 60Hz hum peak.

Cant see the picture, I would need a 240V power supply though.

 

[xrk971]

This is really annoying how my images from DIYA cannot simply be linked with a url.  They all seem to fail, so I have to re-upload the images here.
 
Ok, here is the FFT from a desktop version of this amp I was working on -  a prototype.  But it shows how clean of a power supply I can make without resorting to heroic efforts like a shunt regulator or even a cap multiplier.  Just using a basic SMPS then cleaning that up.
 

 
Noise floor went up a bit from -126dB I can get from batteries, but still pretty low. Main thing is lack of any 60Hz bump at the AC line frequency. Hardly any PSU shunt or otherwise can achieve this in practice. Almost any other wall-plug PSU will have some sort of 60Hz and oftentimes 120Hz (or 50Hz and 100Hz outside of USA) bumps or peaks.
 
240VAC supply is not a problem as the main power source is a universal 110vac/240vac 50Hz/60Hz SMPS.

 

[gikigill]

  This is really annoying how my images from DIYA cannot simply be linked with a url.  They all seem to fail, so I have to re-upload the images here.
 
Ok, here is the FFT from a desktop version of this amp I was working on -  a prototype.  But it shows how clean of a power supply I can make without resorting to heroic efforts like a shunt regulator or even a cap multiplier.  Just using a basic SMPS then cleaning that up.
 

 
Noise floor went up a bit from -126dB I can get from batteries, but still pretty low. Main thing is lack of any 60Hz bump at the AC line frequency. Hardly any PSU shunt or otherwise can achieve this in practice. Almost any other wall-plug PSU will have some sort of 60Hz and oftentimes 120Hz (or 50Hz and 100Hz outside of USA) bumps or peaks.
 
240VAC supply is not a problem as the main power source is a universal 110vac/240vac 50Hz/60Hz SMPS.

 
I can also obtain a linear supply though of the same rating.

 

[xrk971]

Congratulations to gikigill as the new owner of the NHB (No Holds Barred) Convertible Ediition (desktop capable) amp!

Truly a one of a kind amp - and I must admit that I am saddened to see it go to Australia tomorrow. Reassuring to know it is going to a great home though.

 

[xrk971]

Some kind words by member Funch regarding how this amp sounds here:
 
http://www.head-fi.org/t/833106/new-diy-altoids-tin-amp-the-class-a-wintergreen-handwarmer-by-xrk971/90#post_13344874
 

 

[gikigill]

Can hardly wait, managed to source a power supply locally for desktop usage.

So when is the bigger brother coming? First dibs on a proper desktop setup.

 

[xrk971]

Can hardly wait, managed to source a power supply locally for desktop usage.

So when is the bigger brother coming? First dibs on a proper desktop setup.

You mean this?  I am waiting for BabyDontHertzMe to help me with the new layout.  The prototype using a pocket PCB seems to show that it works well though.  93mA bias current should drive lower impedance cans much better.  Harmonic profile is the same.
 

 
What power supply did you get?  What regulator chip is it using, if any?  A cap multiplier can work very well too - regulation is not needed more ripple removing is the order of the day and aim for 15v to 16.5v supply as 18v is pushing it with the heat generation.

 

[Slater]

  93mA bias current should drive lower impedance cans much better.  Harmonic profile is the same.
 

 
What is the optimal impedance range of each of the amps (the original and the big brother)?

 

[xrk971]

The original pocket Class A was designed to be used with headphones as low as 60ohms. The bigger desktop one can go to 30ohms. Although I have used the original on 16ohm IEM's and earbuds and they work quite well - perhaps with just a bit less bass extension. 250ohm cans have theoretical reach of 8Hz -0.2dB point with 390uF output caps. So 60ohm would be about 32Hz for -0.2dB. -3dB would probably still be in the tens of Hz.

Now that you have your amp kit, let us know how the build goes.

Testing amps must go on. AB box used to check for QC vs the reference amp (bone stock one) with blue cord.

 

[Slater]

Great, thanks for the clarification.

 

[xrk971]

The good stuff just arrived! Elna Silmic II's 4.7uF 25v and 100uF 16v RFS's