Just finished building the smallest headphone amp.

Using a MAX9722B. Whole amp built with surface mounted components save the 7805 for power regulation. Power is courtesy of a 9V battery. Built the board using FR3 material and completely soldered by hand.

So far tested it on Koss portapro, Senn PX100 and SHP8900.

Very linear amplifier. Sometimes a bit too linear that it shows the flaws off the audio source and headphones. Had a problem with the phillips set as it had too harsh trebles and lack punch in the bass.

Sounded fine on the portapro and px100 though.

Here are some pics


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o--+ +--o \
3||E 115 |
3||E--o > 230
3||E 115 |
o--+ +--o /

For the center tap transformer, I have 2 x 115 V. Red black Red black.

If I connect the 2 red, do I need to short the 2 black or they are internally shorted?

sakaic,
A very commendable attempt there!
You can actually make the board even smaller, right? Would there be an improved/final version of the board?

You need to connect/short the blacks for 230VAC operations.

Ok.

My tranny have output 5V ac. Now If I need 10V AC, can I use resistive network, with the 5V across 1 meg and the output 10V across the 1 Meg at the top?

This post has been edited by ccschua: Jan 10 2009, 10:36 AM


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You're doubling the 5VAC to 10VAC using that resistive network?
I don't think it's as simple as that!

You're better off connecting the two tranny outputs in series for 10VAC.

Any specific reasons for a 10VAC requirements?

This post has been edited by jazzy939: Jan 10 2009, 10:49 AM

On a different subject matter, has anyone built a tube amp/pre-amp?
I am thinking about making one, no experience on tube.
Share your experience please. TQ.

BSL does.

To some extent. Tubes operate almost like FETs. If you know anything about FETs, Tubes isnt hard to figure out.

currently facing a problem with the charge pump during startup. working with their application engineer to solve this problem.

plan to either use 2 AA batteries or reduce the regulator to a SOT-223 sized regulator to reduce board size. The amp will probably be smaller than the batteries.

meanwhile working on a LME49600 based amplifier. That will be my mega amp.

This post has been edited by sakaic: Jan 10 2009, 02:36 PM

you mean you've been communicating directly with their engineers? that's cool!


Anyway, don't you want to add some caps to cope with the transients?

sure can, with 4/4mil track width/spacing FR4 dual layer with 0603 chip cap/chip resistor components.. sure can make it very tiny..

littleghost, cannot put too large value of caps due to esr buildup thus affecting the performance of the charge pump. charge pumps need caps at the supply, fly cap and output filter. the esr of all of them must be pretty low and also the values must be balanced to maintain performance.

this amp uses a charge pump to generate the negative voltage thus does not need a dual supply or a divider. the downside is that I have to use X7R ceramic caps.

Application engineers are the in people in between the customer and the design engineer. If there is anyone you would wanna talk to about any IC from any manufacturer, they are the people to talk to.

ar188, dual layer very troublesome for me at the moment. I'm just to lazy to line both the sides up precisely especially with track widths that small. pcbs are done by hand so dual layer needs some work.

But if considering the recommendations that you have made i think smaller than 10sen coin also possible. even now its smaller than 20sen coin if you don't count the 7805.

if you game to make this small board, let me know.. I sponsor you some of the SMD passive components. throw in some SMD 0805 blue LEDs for you also.

that'll have to wait. not until i solve the charge pump issue.

lucky i check. wrong connection. After I common black and red then OK.

Now another help required.

my toroid has 2 pairs of 18V, all in blue color. I need 18, 0, 18 connection. I measure the pair I got 18V each pair.

Now how to connect. If I mark the blue cable as 1,2,3,4. Should I common 2, and 3, and use 1 as +18 and 4 as +18.

This post has been edited by ccschua: Jan 10 2009, 10:07 PM

It should be the one adjacent to each other. black and red make sense then!

Follow similar arrangement like your primary coils wiring.

both 1 and 4 will be 18VAC when measured with reference to 2/3.

It will only be + and - after going through a bridge rectifier/full wave rectification. The DC voltage should be around 25VDC, give and take a few volts depending on diodes voltage drops and what nots..

I am having a problem. The 15V regulator LM 317T can not regulate incoming 25V to 15V. It can only manage 1.9V max. How to check if the regulator is working problem.

1.9V max? Then something is seriously wrong somewhere..
Get another LM317 then... No idea how to check the serviceability of the LM317...

Has anybody here DIYed SOHA II or beta22?

u sure the resistor value and trimmer pot value is correct?
if yes..check another lm317