[lampee]

Hi Everybody!

You may remember the M4 amplifier designed by j4cbo. You can find it http://www.head-fi.org/t/449462/the-m4-a-fully-differential-single-pcb-amplifier-with-relay-attenuator. This was a nice prototype, but since then, there were no developement in the project, no documentation, no nothing, but a circuit diagram.
I always thought, that it is a really great project, and since there were high interest I started to design basically the same. However I made some modifications:
- the design is through hole, no SMD is used
- no built in power supply
- no built in stepped attenuator
The aim of my project is different from j4cbo's, because I would like to design something which can be built by everybody.
In this design one could use the 2SJ76V and 2SK213V lateral MOSFET combo by eliminating the capacitors near the MOSFET and by putting 1K resistors to R3 and R13.
I have enclosed the schematic and the pcb design. Please don't let the name of the transistors confuse you, everything is exactly the same as in the M3 design. The PCB is only for one +/- channel of course.
 
Followup:
I have figured out the volume control and imput impedance problem. I attached the circuit and the PCB. The design need a 4 gang ALPS RK27 potentiometer, but one could use other too, like you can wire two delta1 like this one. If I am right, the imput impedance will be a constant 20K if I use 10K per gang RK27. Of course one could use a simple design like this http://www.raleighaudio.com/figure_17.htm. Eg. one could use two 5K resistors and one 10K potentiometer, but the input impedance would be changing between 10K and 20K.
If this attenuation seems OK, and nobody will correct me in the design, then I think I will build a prototype. Fortunatelly most of the wiring is on the bottom, so the through hole plating won't be a big problem during DIY PCB.
 
Please feel free to comment the design. Any recommendation is welcome!
 
The PCB of the amp:

Schematic of the amp:

PCB of the volume control:

Schematic of the volume control:

 

[FallenAngel]

Looks great, I'd certainly like to build one if you want another prototyper
 
Personally, I'd prefer not to have any integrated PSU or volume control.
 
The PSU looks very funky - looks good for onboard filtering, but why does it seem to take AC input?  Maybe I'm just not understanding the schematic.

 

[cfcubed]

Wondering if you're pursuing this here or there (amb forums): http://www.amb.org/forum/m3-headphone-amplifier-f15/fully-differential-m3-t1332.html?sid=1a360f1aa8040d0ec016df800659611c
Maybe you are moving the topic here.

 

[tranhieu]

Looks promising! Count me in if you need any tester

 

[zammykoo]

Very nice, I may be interested in building one of these. I am just curious how close is the pcb design to being finalized?

 

[lampee]

Thanks for the great interest. The PSU is not integrated, a sigma11 or any other single rail PSU could be used for it. Just like the original M3.
I will soon start to build a prototype, I will let you know how does it perform.

 

[kevin gilmore]

amb's site down at the moment, so i will post this here.
 
i think there are a number of major issues with the schematics above.
 
This with a real bipolar power supply would work much better.
 
http://gilmore.chem.northwestern.edu/balancedm3.pdf
 

 

[qusp]

indeed, why not use a bipolar psu, seems silly and tbh being smd was one of the best things about the design you drew inspiration from. the opamp will have to be smd, you cannot get dip8 in any fully diff opas. it really really confuses me this fear of smd, it really limits the design imo
 
nice choice of mosfets though