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portable headphone amp build with sub mini pentodes!

post #1 of 12
Thread Starter 

Hi all! with this i make my first post, I'm planning on building a headphone amp for outdoor use and had always loved the idea of tube amps and their cool retro styling, however as many of us know said tube amps are generally heavy massive pieces of desktop equipment. Enter the sub mini pentode and the Oatley electronics portable tube amp kit. the original idea was to build said kit and enjoy this tube amp bliss that many audiophiles swear by. I consider myself a budding audiophile and already own several fairly average headphones, my current workhorses being the Sony XB600's and even though my iPod can drive them pretty dang well, i plan on eventually making that jump into the true audiophile grade headphone world, so why not build something I can use now and then in the future. and then there was the day i came up with the idea to combine the ideas of the crystal CMOY amplifier and use the same construction methods with this kit. the purpose of this thread is to monitor my progress on this project and also to gain information on building amplifiers,since even though this is not my first shot at electronics, it is definitely my first attempt at an amplifier. the tubes this project uses are raytheon 6418 sub-miniature pentodes, but since they are solder style tubes and not socket i was wondering if there were any other tubes in the sub mini range that would be an equivalent or an upgrade from these? thanks for reading about my project and i hope you have some suggestions for components! K272C STEREO TUBE HEADPHONE AMPLIFIER

the photo is a "little" big but this is the kit i hope to recreate in Crystal cmoy form (copyright oatly electronics, i do not own this image)

post #2 of 12

Maybe the 6088?  The filament voltage is the same and chances are, the batteries in there are never going to exceed the plate voltage.

 

FYI, there's a reason all those huge rubber tube dampers are there. ;)

post #3 of 12
Thread Starter 
Yeah, read about the interference and vibration dampening that they need, any resources on converting a PCB to point to point like the crystal cmoy? I can read schematics for connections here and there, but how to turn it into something like the crystal cmoy......
post #4 of 12
Quote:
Originally Posted by Alaska Skier View Post

Yeah, read about the interference and vibration dampening that they need, any resources on converting a PCB to point to point like the crystal cmoy? I can read schematics for connections here and there, but how to turn it into something like the crystal cmoy......

Yes.  You need to draw:

  1. Start by drawing every component on the PCB and its connection points.  Pay attention to the physical requirements that you need.  For instance, where do you want the inputs, where do you want the outputs, and where do you want the volume pot?  An exact location is not important - simply which end of the drawing (or your eventual box that connects to whatever).
  2. Draw lines to connect to every component's connections according to the PCB traces.
  3. Try to re-draw what you've drawn without crossing lines or loops around all of the components.  This may take several iterations.  Pay attention to the common connection points - those can be moved together can be used to re-arrange your drawing at any time.  In particular, all of the grounds (typically the negative poles) can be moved to a single point.  In a PCB, this might correspond to the ground plane and you needn't worry too much about location.  In point-to-point, try to think of it as a "star" that collects all of the connection points (that's easier to produce physically in point-to-point than a ground plane).
  4. Once you think you've got all the inputs/outputs/volume pot on the ends that you need, see if there's anyway you can further reduce the lines that cross.
  5. When you think you've got it as good as you can get it, you're done.  See how it maps out physically.  You should be able to place the components far enough apart to allow easy wiring, but close enough that extra space and wire crossovers are minimized.

 

There's no special secret, especially with tubes - only recognizing common connection points in the schematic/PCB and using those to simplify the layout.

post #5 of 12
Thread Starter 
Wow , nice to have someone around who really knows about this stuff, never thought of that but it really makes sense, by the way I was originally going to use digikey for most of the components, but I don't know if they ship to Finland, any good European based equivalents to them where I could get fairly high grade components?
A.S
post #6 of 12
Thread Starter 


Here is the schematic, where should I start?
post #7 of 12
Quote:
Originally Posted by Alaska Skier View Post



Here is the schematic, where should I start?

 

Typical.  It's a bait and switch.  You need an 18VDC power supply.  The onboard battery pack (2 x AA) is used to supply the heaters for the two tubes.  18VDC, supplied externally, is needed to power the amplifier circuit.  Even in a section of their notes, they title it, "18V BATTERY INDICATOR:," but there isn't one.  Later on they state, "Note that 18V supply must be present to effectively switch on the filament [heater] supply.  Yet, there is no battery supply or separate wall-adapter supply - it could be either one, they just don't supply it, period.  Neither would it be portable unless you packaged together 2 x 9V batteries into a separate case.

 

Sorry about that - but about turning the schematic into a simple layout ... see my 2nd post again up there.  I can't do it for you.

 

Notes for the Oatley Electronics K272C Stereo Tube Headphone Amplifier: http://secure.oatleyelectronics.com/files/K272C.pdf


Edited by tomb - 10/27/13 at 1:25pm
post #8 of 12
Thread Starter 


It's not done, but it's getting there, just missing the heater supply circut, but where to put it?
post #9 of 12
Thread Starter 
Ok, i know its been a long time, but about the amp and everything, in the schematic you see there is an op amp that also does some of the amplification, I wanted to make a tube amp, not a hybrid amp, I know a little about tubes, and still wanted to use said sub mini pentodes, can anyone direct me to schematics or documents for how one might design a amp around 2 jan6418's? I would rather design it myself as it would make shipping cheaper and i could choose my own components, (sorry, serious DIY'er here) smily_headphones1.gif
post #10 of 12
Datasheet says you can get .002 watts of power at 12% distortion from these tubes with an output impedance of around 100,000 Ohms. If you put 50 of them in parallel you might get somewhere.
post #11 of 12

The op amps do not provide any Voltage gain. They are buffers. It is interesting to note that they are set up as inverting buffers, which is not normally done in headphone amps, but solves some problems that only exist on paper quite nicely. 

 

So, um, what is wrong with a hybrid amp using a SS buffer after the small pentode? 

Top down design? 

post #12 of 12

The outley kit is a good place to start because once you listen to the amp for a while your ears will be bleeding and you will be happy you didn't waste much money.

 

With that said i like the kit from outley, i got the older model because it doesn't use AA's but its mostly the same. For a novelty item its great but the tubes are just to noisy, i have recreated the circuit and made some improvements but in the end the tubes just add noise. I might be able to digg up a good cmoy design, much better sound quality. 

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