They have arrived, PA150!

CJ900RR

Member
2006-01-29 10:09 pm
My goal to create my own version of the PA150 is on it's seccond stage. As allways it started with some (read many) hours in Eagle.

[IMGDEAD]http://www.leetmaa.se/misc/pa1501.jpg[/IMGDEAD]

I wanted to achive a good design that would be easy to assemble in a case when doing a bridge amplifier. Therefore the inputs and outputs are on both left and right side of the board. Some components should be left out depending on from which side you connect power, input etc.

They use servo-circut to null the DC-offset instead of trimpot's, so a +/-15V is needed.

[IMGDEAD]http://www.leetmaa.se/misc/pa1502.jpg[/IMGDEAD]

Today, 10 PCBs arrived from China (www.pcbcart.com, excellent supplier).

[IMGDEAD]http://www.leetmaa.se/misc/pa1503.jpg[/IMGDEAD]

They have NOT ben assembled yet, and therefore NOT TESTED. Sorry for poor quality on photo. Only mobile-cam avaible right now. If they work, I will order more boards and sell them if there is an intress. But as said, I need to test them first.

Size is 6.5 x 2.5 inch, or 16.5 x 6.35 cm.
 

CJ900RR

Member
2006-01-29 10:09 pm
AndrewT said:
Hi Cj,
does each chipamp need it's own output Zobel or does the parallel amp set up only need one Zobel between the summed output and the speaker terminals?


Hi Andrew.

I think that each PCB only need one Zobel between the summed output. The layout with servo-circuits, "snubber"-caps and decoupling-caps that will be soldered directly to the supply pins combined with well matched components should give a stabil construction.

I you have other opinions please feel free to say so :)
 

CJ900RR

Member
2006-01-29 10:09 pm
coldfuzion76 said:
Just curious how things are going on this one. I too want to build the pa-150 and was wondering if there might be a group buy after you get the kinks worked out. I missed out on the last one.


Things are going well i think. Test will follow later this week, if im lucky allready tomorrow. There will most likely be no groupbuy but i will offer them for sell at my webpage.

Better photo below.

[IMGDEAD]http://www.leetmaa.se/misc/pa1504.jpg[/IMGDEAD]
 

CJ900RR

Member
2006-01-29 10:09 pm
This will be the "real" design i guess. Zobel netwok on pcb as wanted by some. Personaly I dont like the Zobel, I think it has an interference on the sound, but this way people are able to use it if they want.

[IMGDEAD]http://www.leetmaa.se/misc/pa1505.jpg[/IMGDEAD]
 
zobels and servos have two entirely different functions.

The zobel gives the amplifier a proper load at higher frequencies to prevent oscillations which might damage it.

The servo replaces the capacitor in the feedback loop normally used to imit DC-gain to unity. Thus working at very low frequencies.

Otherwise amplifier gain * DC input offset might give several tens of volts DC on the output which can give severe problem with parallelled chips if not using large output resistors.

It is possible to trim output offset by injecting a small current into one of the inputs of the amplifier but offset drifts with time and temperature... the servo is always adjusting DC-offset to within the limits of the OPs own offset which is neglible in this application.
Also, the cost of low offset op good enough for this application is not very large (a few € at most for the amplifier) and neither does they demand a lot of PCB real estate.

Large output resistors reduces efficiency, creates a heat source on the PCB and is also negative as the Q-value of the loudspeaker is reduced (less "coupling" or "control") , it is the same effect as having a very long loudspeaker cable.

so both zobel and DC-servo is essential for the amplifier to operate safely.
 

CJ900RR

Member
2006-01-29 10:09 pm
Well one channel is up and running since 1 hour. Measured DC-offset with input to ground: 1.7mV, 0.4mV and 0.7mV. The servo-circuits does its job. No hum hizz or other bad sounds. Has been playing on a volume-level of 60% ca and the LM3886 does not even get hot. Supply voltage is about +/-32V. No Zobel is present what so ever (oh im a daredevil :cool: )

Two images, yet again sorry for bad mobile-shoots.

[IMGDEAD]http://www.leetmaa.se/misc/pa1506.jpg[/IMGDEAD]

[IMGDEAD]http://www.leetmaa.se/misc/pa1507.jpg[/IMGDEAD]