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croccodillo 25th January 2013 09:47 PM

New Portable Amplifier Project started
Hello all,

in the last two months I had collected everything I need to build a new portable headphone amplifier; in the meantime I learned a lot reading this forum, and now the project has started.

these are the main characterics:

- Lithium rechargeable batteries
- USB charger
- NO virtual ground (real ground - that means two batteries)

The amplifier must drive my Sony MDR-55.

Moreover, I don't have decided yet on the amplifier topology, I want to try both integrated and discrete topologies; so I ended up with a modulare solution.

What I mean is to build a motherboard which houses the two batteries with the USB charger, input/output jacks, the volume pot; the real amplifier will be mounted on a daughter board, connected to the motherboard with a series of pinheader.

I had already an Hammond 1553BB case, so decided to use it.

The schematic of the motherboard is the following:

From the left, there is the USB plug, PWS1 & PWS2 are two isolated 2WDC/DC converter, then there are two TP4056 battery charger modules.
The battery (G1 & G2) are connected through 4 diodes that permit to use the amplifier during the USB charge process.

T1 to T4 are used as automatic switches to connect the two batteries to the following circuit.
They are all SMD, so do not waste space on the upper side of the PCB.

The two batteries are Nokia BL-6P, cheap and good.
They are internally protected, and when one of them goes low, its disconnect itself; at that point the four transistors disconnect both the batteries, to protect the headphones.

Even more, with this circuit I can disconnect both the batteries with a single switch (ALPS RK097 potentiometer).

Next, come the PCB:\headphone\pcb_01.png

And today, I etched the real PCB (By the way, I have access to a CNC mill...):

And a mockup, just to understand how the amplifier will look:

This is a complete battery charger module; I will use two of them, installed under the two batteries, on the copper side of the PCB )I will of course remove the USB plug, cuase they will be directly soldered on the PCB itself):

I hope everything is clear: this is not a complete amplifier, this is only a motherboard.
The real amplifier circuit will be mounted on a daughter board, in the empty space between the batteries and the front potentiometer/plugs (please note the two rows of pins, close to the batteries and the front controls).
There will be two set of pin header, which will bring all the needed signal and power to and from the amplifier.
In this way I can experiment with every circuit I want.

The pinheader are spaced in a way I can even usa a perforated board, so I can try any idea without the need to etch a board.

Moreover, even the USB interface is brought to a pin header, so I could add a USB codec, in the future.

Hope you like it.


agdr 25th January 2013 10:20 PM

Hey they is an excellent design and project! I really like how you have separated the power supply section from the amp section and used a daughter card. That will allow all sorts of amp variations to use the same power supply. Plus provides some physical spacing/isolation between the supply and the sensitive amp parts.

The two lithiums with the charge controller chips and dc-dc converters are really the way to go. Much higher energy density than NiMH and the controller chips will allow for a fast charge.

paulb 27th January 2013 06:33 PM


Originally Posted by croccodillo (
PWS1 & PWS2 are two isolated 2WDC/DC converter

This looks excellent. Can you tell us the manufacturer and part number of these converters? Are they expensive?

T in AZ 28th January 2013 08:45 PM

what about the Vol control being close to the ear jack?

croccodillo 28th January 2013 10:09 PM


Originally Posted by paulb (
This looks excellent. Can you tell us the manufacturer and part number of these converters? Are they expensive?

First of all, I made a mistake: what I have are 1W/200mA output DC/DC converters (you can really find 2W version), I bought them on RS Components, but you can find them on Farnell or Mouser.
These things are quite standard, and cost about 4/5 Euro.
search for SIP4 package.

Just for reference:
RS Components

croccodillo 28th January 2013 10:10 PM


Originally Posted by T in AZ (
what about the Vol control being close to the ear jack?

I don't like either, but there's no space to move it far from the jack.
In the following days we'll see if this is a real problem...

croccodillo 28th January 2013 10:24 PM

And now, some new:

Today I've been able to mount all the components to the "motherboard", including modified batteries and battery chargers.
Please check the following pictures.

From Top:
As you can see I modified the batteries adding them two wires, and fixing everything with hot glue and teflon tranparent tape.
The underside.
Not too bad, for me. I'm quite proud of it, actually.
Here you can see the two battery charger modules; I draw the foot print for them into Eagle, but made a big mistake so had to connect them to the circuit using few resistor legs...
I'm not bullshitting you, everything works as expected, the two battery charger are working, the two blue leds will be lit during the whole charging process; when the batteries are full, two red led will replace the blue ones.
Details on the battery chargers
It fits perfectly!
I have to lower the mounting toers to center the PCB, but this is not too bad.
I was thinking to use a clear or smoke polycarbonate front panel (milled with my CNC), so I can check the led colors during the charging process.

Now, the difficult part: I have to decide which amplifier to built first!!! :D:D:D


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