New DIY Altoids Tin Amp - The Pocket Class A "Wintergreen Handwarmer" by xrk971

Edit 7/30/17: PCBs are now available at the XRKAudio Etsy Store!

For real!
Up until now there have been hardly any portable Class A head amp designs, commercial or DIY.
Here's a brand-new CMOY-sized amp project with the goods! I'll be building a couple of these in the coming weeks and will post my build pics and listening impressions here.
Until then, here's some info on the little beast:



An experienced amp builder on the DIYAudio forums, xrk971, with the input of the ever-helpful community over there, is currently finalizing his design for this amp. In the works are a detailed bill of materials for the build, a circuit schematic, and hopefully a GB (on DIYA, not here!) for printed circuit boards (design courtesy of user BDHM)...all to be found in this thread:

http://www.diyaudio.com/forums/head...2-based-se-class-headamp-without-heat-11.html

"X" has run extensive simulations of this circuit as well as real-world measurements for THD and FR, and the results look stellar. Low noise, textbook-perfect harmonic distortion profile, and a razor flat frequency response. Solid bass. Plenty of juice, even for 250 Ohm cans (possibly higher). And most importantly... according to him, it sounds amazing. It runs slightly warm to the touch, and gets 4-5 hours of battery life.



What makes it most exciting to me is that it's noob-friendly, with a low parts count, and all through-hole components except for the JFETs. My solder-slinging experience is limited to much simpler things like speaker crossovers and volume pots, etc., but I think even I'll be able to pull this off. I'm excited about this little amp project!

The amp in the photos above is X's current prototype, built with his hand-drawn circuit board and all SMT components.  DIYA member BabyDontHertzMe currently has two DIY PCB designs under development and soon to be available for printing.  One will use SMT components like the original, while the other will use all through-hole components to facilitate easier soldering for beginners.

 
 
Those look like rechargeables there, is there a charging circuit built in?  A slightly better enclosure would probably eliminate any heat issues
 
I will be extremely interested to see this in action. thanks for posting this up.

well, I'm down with a couple of boards.

hutnicks said:

 
Those look like rechargeables there, is there a charging circuit built in?  A slightly better enclosure would probably eliminate any heat issues
 
I will be extremely interested to see this in action. thanks for posting this up.

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X has reported 4 hours of battery life using that pair of 600 mAh rechargeable Li-Ion batteries you see in the pic (there are now 800 mAh cells on the market, BTW, which I will probably use).  There is also the possibility of reducing bias in the amp circuit and potentially getting upwards of 6 hours.
 
There was some talk of a charging circuit at one point (it was me who brought this up, in fact) and I think the conclusion was that a charging circuit should indeed be possible to fit in the tin, and that this might appear eventually as an add-on.
 
Also according to X, the amp never became uncomfortably warm while in use for hours at a time.

stellarelephant said:

   
X has reported 4 hours of battery life using that pair of 600 mAh rechargeable Li-Ion batteries you see in the pic (there are now 800 mAh cells on the market, BTW, which I will probably use).  There is also the possibility of reducing bias in the amp circuit and potentially getting upwards of 6 hours.
 
There was some talk of a charging circuit at one point (it was me who brought this up, in fact) and I think the conclusion was that a charging circuit should indeed be possible to fit in the tin, and that this might appear eventually as an add-on.
 
Also according to X, the amp never became uncomfortably warm while in use for hours at a time.

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Thanks. The add in board would be a neat idea. It's winter here so a handwarmer is not a bad idea

nick n said:

well, I'm down with a couple of boards.

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Great! I must reiterate...please express all interest in acquiring these PCBs over at the original DIYA thread. http://www.diyaudio.com/forums/headphone-systems/300862-bf862-based-se-class-headamp-without-heat-11.html
Head-fi no longer allows group buys.

I'll continue to post updates on the development of the amp here, and I certainly welcome all manner of discussion about this project.

Of course. Will do thanks.

Thanks 

This amp is exceptional in yet another way: Single-Ended (SE) circuit topology!  This is absolutely unheard of in portable amps, and until now has been the domain of pricey desktop units such as the higher reaches of the Schiit lineup, all the way up to the $4000 Woo Audio tube amp offerings.  So what's so special about a Single-Ended Class A Design?
 
To be honest, I had no clue until I did some research today.  Come to find out, Audio God Nelson Pass says Class A SE gives the BEST SOUND, bar none.  Here's a quote from him:
 
"The promise of the transconductance characteristic in power amplifiers in providing the most realistic amplified representation of music is best fulfilled by Mosfets in single-ended Class A circuitry where they can be used very simply and biased to very high currents. [...] In systems where the utmost in natural reproduction is the goal, simple single-ended Class A circuits are the topologies of choice."
 
The whole article is pretty awesome, so if you enjoy the challenge of trying to understand a genius' musings, here it is:
https://passlabs.com/press/single-ended-class
 
So, SE sounds the best.  The downside?  It's also the least efficient of ANY amp circuit topology...which of course is why we haven't ever seen it in a portable amp before.  4-5 hours of battery life is a bit less than most folks expect from their portable listening setup, although I'd argue that it isn't unreasonably short either...I rarely listen for more than 2 hours a day, myself.  Regardless, if sound quality comes first, this amp is truly something to be excited about.  Can't wait.

stellarelephant said:

  This amp is exceptional in yet another way: it uses a Single-Ended (SE) circuit topology!  This is absolutely unheard of in portable amps, and until now has been the domain of pricey desktop units such as the higher reaches of the Schiit lineup, all the way up to the $4000 Woo Audio tube amp offerings.  So what's so special about a Single-Ended Class A Design?
 
To be honest, I had no clue until I did some research today.  Come to find out, Audio God Nelson Pass says Class A SE gives the BEST SOUND, bar none.  Here's a quote from him:
 
"The promise of the transconductance characteristic in power amplifiers in providing the most realistic amplified representation of music is best fulfilled by Mosfets in single-ended Class A circuitry where they can be used very simply and biased to very high currents. [...] In systems where the utmost in natural reproduction is the goal, simple single-ended Class A circuits are the topologies of choice."
 
The whole article is pretty awesome, so if you enjoy the challenge of trying to understand a genius' musings, here it is:
https://passlabs.com/press/single-ended-class
 
So, SE sounds the best.  The downside?  It's also the least efficient of ANY amp circuit topology...which of course is why we haven't ever seen it in a portable amp before.  4-5 hours of battery life is a bit less than most folks expect from their portable listening setup, although I'd argue that it isn't unreasonably short either...I rarely listen for more than 2 hours a day, myself.  Regardless, if sound quality comes first, this amp is truly something to be excited about.  Can't wait.

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Pass has a great take on audio circuits he is well worth listening to. The heat and inefficiency you mention are the big drawbacks (Why the Krells used to have cooling fans). On something this small you can heat sink the poo out of it to solve the issue there so that just leaves power consumption. 4 or 5 hours is not bad and really if you needed more a spare pair of batteries or perhaps looking for a larger capacity battery might be in order. All depends on how small you want it.
 
Waiting for pricing to get finalized over there on DIY for the PCB's.

X's Pocket Class A GB has begun on DIYA...only 2 boards left for the first round!  
http://www.diyaudio.com/forums/headphone-systems/300862-bf862-based-se-class-headamp-without-heat-25.html

More are available for a few more days!
 
"It's a superb sounding single ended (SE) Class A topology that has a predominantly a second harmonic distortion profile with just a tad bit third harmonic and not much else. The noise floor is about -125dB and dynamic range is about 96dB with THD measured to be about 0.029% at 1kHz and driving a 270ohm load at 700mV RMS. Note that most of that HD is H2 so the sound is very much like a nicest SE tube amps. Frequency response with a 47uF output coupling cap was measured at -1.5dB at 20Hz and 45kHz (probably limit of measurement as ADC sampling was 96kHz). The sims show that it can go beyond several MHz on the high end. Best part is that it fits in an Altoids tin and looks like a CMOY headamp - but it's no op amp. It relies on just two FETs. The ever popular BF862 JFET for the input and a ZVN4306 MOSFET for the output stage. The bias runs between 50mA and 60mA which allows it to last about 4 to 5 hours on a pair of 600mAhr Li-ion rechargeable 9v batteries. The amp dissipates about 2w so never gets hot but makes a nice hand warmer for your pocket in the winter. "
 
X posted the above summary of his amp today as part of this new thread:
http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb.html#post4966596
 
The board being ordered will use both through-hole and SMT components in the circuit, and X says it will be easy to solder.  (I hope he's right for my sake!)

 
Bonus!  The components are affordable--about $20..  Here is the Mouser BOM::
 
---------------------------------------------------------------
 
Description Part No Qnty
n-channel power MOSFETs ZVN4306GTA 2
n junction JFET BF862 771-BF862-T/R 2
Panasonic OSCON 390uF 20v output caps 20SEPF390M 2
Output cap bypass WIMA 1uF MKS caps MKS2C041001F00KSSD 2
Input bypass Wima 2.2uF MKS caps MKS2B042201F00JSSD 2
Panasonic OSCON 100uF 25v rail caps 20SEP100MX 2
Switchcraft 3.5mm stereo jacks 35RAPC4BV4 2
Alps stereo pot with power switch RK0971221Z05 1

Plus you will need the following 0805 1% metal thin film SMT resistors (qnty):
33R(2), 47R(2), 270R(12), 1k(2), 15k(1), 47k(2), 100k(2), and qnty 4 x 100nF X7R 0812 MLCC bypass caps, and basic 3mm red or green LED.
 
---------------------------------------------------------------
 
In case you aren't excited yet...here are some listening impressions from the prototype:
 
"I have been listening to the Pocket Class A for an extended period now and have compared it to some world class head amps designed on this forum. I am happy to report that my listening impressions have not changed regarding how good it sounds. Being battery powered, it is utterly quiet and able to resolve some amazing low level detail when listening from a good DAC. My latest measuremts show an absolute flat noise floor devoid of 60Hz hum at -125dB and a dynamic range of 92dB - well matched to CD quality sound. I am using 47uF output caps at present so at 20Hz there is about a -2dB falloff but I plan on upgrading to a 390uF oscon output cap in the future. This should take the -3dB point to about 5Hz and give this amp the bass extension to meet any audio need. The one area where my "big amps" such as the Salas DCG3 and BF862 preamp with DAO buffer amp have an advantage is bass "slam" due to the fact that they have huge 18v to 20v dual rail multi-ampere power supplies. But for the rest of the music - this little amp surprises me all the time with how great it sounds. Natural, musical, easy on the ears, lively, not sterile, resolving, quiet when needed, expressive, accurate (due to flat response), most of all - enjoyable for hours on end. It beats all my other amps on noise. It is the quietest amp of all with only two transistors in a tight layout that doesn't pick up noise, and ultra quiet batteries to provide juice."
 
and to sum it up:
 
"Natural and very musical sound that is engaging, lively, never sterile."

I would be interested in doing a review of one of these if someone has a completed one they wouldn't mind sending. I'll pay insured shipping back as well.  I've experience with the ALO CDM, Chord Mojo and soon to be Hugo. 

ike1985 said:

I would be interested in doing a review of one of these if someone has a completed one they wouldn't mind sending. I'll pay insured shipping back as well.  I've experience with the ALO CDM, Chord Mojo and soon to be Hugo. 

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I've signed up for two boards. Assuming that I'm successful with both builds, I'll have a loaner on hand!