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PostPosted: 09 Oct 2010, 13:39 
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Hi Crazzy,

That is very interesting and extremely cheap to build. I already have a regulated 15V supply and an enclosure, so it would probably cost $10 to put together... :mrgreen:

Can you post a pic of your circuit board internals on that thread ? thanks...

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PostPosted: 14 Oct 2010, 04:42 
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I have decided to build a fully integrated amplifier based on P3A + P37A DoZ preamp.
The output caps of the preamp will be removed and replaced by wire links since the amp module already has an input cap. Tranny is a 500VA 18V dual secondary powering the dual supply PS based on the TDA 2050 amplifier. PS board has 5X 2200uF caps to form 11,000uF capacitance per channel. Each 2200uF cap is snubbed with a 100nF MKT cap to achive a "fast" and ultra low impedance PS.

Preamp module is my final version which will incorporate the best qualities of Mark's preamp module and my original module. Resistors are KOA carbon films, Fairchild transistors and an M-Cap 3.3uF input capacitor. It is powered from Rod's P05 regulated power supply providing +- 15V DC. A small 15VA EI transformer feeds 16V AC into the preamp PS module.

The P3A amp module will lie vertically, instead of horizontally as shown in the pic. A small fan operating very slowly will provide light ventilation.

Protection circuit is based on Jim's audio speaker protector which will disconnect the speaker if any DC voltage larger than 0.5V is detected. The 10 second turn on delay as well as the instant power off when amp is turned off will eliminate any turn on/off transients.

Wiring is not complete yet. Further layout analysis is required ! :mrgreen:
Can this beat a Krell KSA100 ? Only time will tell.. :D


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PostPosted: 14 Oct 2010, 10:47 
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P3A integrated amp layout V2. The amp module's heatsink acts as a separator for the PS section and the amp/preamp section. (and maybe offer a little bit of shielding ? ) Two smaller trans will be used; one to power the fan, and the other for the preamp. Totally overkill ! :eek: Main aim of this amp is to compete with other commercial offerings in the range of $5,000 - $10,000 and of course to provide reference grade sound.

Waiting for the preamp and preamp's PS components to arrive from farnell. I also ordered a Vishay dual gang 50K pot which will be mounted on the front right panel.

With so many things to install, this is going to be an amp from hell.
While waiting, I will probably pop in my Class D amp in another small chassis which will only take an hour to assemble up.

Please comment on the above layout. :thumbsup:


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PostPosted: 14 Oct 2010, 20:50 
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That is really looking great! I would consider your heatsink to be a bit small, and also the airflow won't be flowing through the fins the way you have it oriented, just my suggestion any way, maybe try and pull a better heatsink from a dead stereo or something.

Other than that it is looking good, and while you have been constructing your P3A, I have been planning on building my own, found a pair of 100MFD 50V and a pair of 330MFD 50V Black Gate capacitors and two 10000MFD 71V Nichicon Gold Tune power supply caps.

As for the chassis, I am going to try and make the sides from Maple, the front a piece of plexy glass painted black on one side, with a piece of ply wood or something to re-enforce it. Not sure about the back, but I am hoping to find a nice thick piece of aluminum that I can use for the bottom (I can use this as a heatsink too).

Your build is looking great dtsup1, keep it up, I want to hear the end result! :thumbsup:

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PostPosted: 15 Oct 2010, 06:39 
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Hi Crazzy ,

Before this heatsink, I tried a similar sized one but with thinner fins and it barely got warm. (no fan)
This one is slightly larger and rated at 1 deg/ watt which is what Rod recommended. Together with fan cooling, I am quite confident that it won't overheat. The main issue now is the transformer which gets quite hot during operation and will inevitable heat up the whole enclosure slowly. I will have to add another fan just to cool it down. :hot:

Enclosure is all marked out and drilled !!! A tiring 3 hours with simple drills. The IEC mains cut out was very tough. I had to drill 6 smaller holes, and use a file to shape it to a rectangle. How do you make a rectangular cutout in 3mm thick steel !!! ? :?

I also received the parts from farnell. An astonishing 1 day delivery. The preamp module was wired up and I added a 3.3uF Mundorf M-Cap input cap. Output cap was replaced with a wire link . Preamp PS uses Fairchild ultra low leakage, small signal diodes and 2,200uF vishay sprague electrolytics. Oops I almost forgot to solder in the preamp 22uF caps, it's all good now.


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PostPosted: 15 Oct 2010, 16:16 
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When I make a chassis I always try to use aluminum, for the IEC socket I drill four holes and usually use the rotary dremel tool to cut it out.

A 500VA transformer should not be getting hot? IE core type transformers are not nearly as efficient as toroid core types, you should get a 300VA 30X30 VAC Hammond toroid, likely to solve your problem and that is the recommended size for the project in the form of a toroid transformer. ;)

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PostPosted: 15 Oct 2010, 18:47 
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Hi Dinesh, Looking good! 8-)
What did you use to support the M-Caps?

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PostPosted: 16 Oct 2010, 00:59 
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Gio wrote:
What did you use to support the M-Caps?

Hi Gio,
There is a flat region created by the tops of the transistors and caps. I applied some hot glue to the capacitor and then pressed it against the flat area. It holds quite well, and the damping should be sufficient.

I installed the EI trans in the chassis without a layer of 3mm bitumen. There is a very slight vibration from the edges of the case. When I put the bitumen in, it was like a miracle. Absolutely no vibration/mechanical hum. Simply superb.

A mishap occurred when I readjusted the bias setting for the power trans. I accidentally shorted the Collector and Emitter pins on one trans ! :eek: There was a spark from the transistor and I immediately turned the power off. Damn my clumsy hands !! :bawling: There is no visual damage on any of the components ! I repowered the module for a brief second but it didn't work - both LED's are not lit.

The power transistors seem to be ok ( I hope!!). Most likely blown components are the input stage -
BD139/140 and BC546. I am going to get the parts now and will report in about the results.

Hope to get this quickly resolved as I have to do a demonstration of the integrated am at a local gathering next Wednesday!

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PostPosted: 16 Oct 2010, 07:02 
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Phew....*breathes a sigh of relief*.

After replacing 14 small signal transistors, the subtle green glow of the LED's were quite reassuring. Bias adjusted back to normal on both channels. I was almost going to send the module to a local amp repair expert !

NExt up: fitting all the boards and wiring of the chassis.

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PostPosted: 16 Oct 2010, 10:43 
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You should be using the 100 watt light bulb (dim light bulb tester) current limiting trick while you are testing and adjusting bias, if anything shorts out the light bulb will prevent destruction to the amplifier. If something like an output transistor gets shorted the bulb will light up at high intensity, if all is well the bulb will be lit for only a brief second from inrush current then should turn to a dim glow.

I should have suggested this before, but I hope your amplifier is ok now.

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