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My first class A attempt. Aleph 3
My first class A attempt. Aleph 3
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Old 3rd February 2019, 06:06 PM   #1
sifis_samos is offline sifis_samos  Greece
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Default My first class A attempt. Aleph 3

Hello from Greece... willing to try a pure class A amp, and after the encouragement by some local guys who have built many pass amps, i have decided to give a try to such an amp.. based on matterials that i already have, i decided to give a try to Aleph 3. I think that 30w@8 and 60w@4 is more than enough for me... also can be done in a logical size...

I dont have much knowledge into electronics, measurments etc, but i have a lot of experience into building-assembling and following plans ..

So i have started gathering the extra matterials i need.. I ordered a 500VA Torroid transformer , with dual windings of 20-0-20 .. so 2 separate windings of 250VA each, encased in a metal box for better appearance.. it will be in my hands in a few days. I will post photos..

On the search for PCB 's i couldnt locate something more apropriate , so i ordered from ebay these ones. (hopefully they are ok)

Click the image to open in full size.

Heatsinks i have 2 pieces of SK501 150 SA - these ones (around 0.34 K/W)


Click the image to open in full size.


I have a question regarding PSU, In general i love to use good quality matterials. and I already have plenty of good quality of materials to build a classical PSU.. I was thinking to use per chanel 4x RUR3060 diodes to form one bridge + 2x 47.000/80v Mundorf m-lytic caps + 0.47mF MKP ... i see in various places that people use various types of psu. for example here i see this

Click the image to open in full size.

Is this really critical? i mean to have dual bridges , chockes etc? i am asking because i wish to use matterials i already have..


Regarding the resistors, i see it requires 0.47ohm 3w.. What type? regular cement type? wirewound? metal film? or it doesnt make a difference?

Last but not least, transistors i guess that they should be matched, right?
Mounting requires anything special or the usual Mika insulators are ok?

I am sorry for the long post, but i will really apreciate any help given... Uppon receiving the various materials i have ordered I will update with photos of the construction.
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Old 3rd February 2019, 06:23 PM   #2
Zen Mod is offline Zen Mod  Serbia
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My first class A attempt. Aleph 3
PSU - if you have easy way for using chokes , go for CLC ...... but no more than , say , 0R25 of choke Rdc

if not , CRC wit 0R1 WW 5W resistor will do the job perfectly

mosfets :
group at positive rail - all matched
group at negative rail - all matched

input mosfets (LTP) matched

power resistors in amp itself - use 3W MOX
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Old 3rd February 2019, 08:06 PM   #3
gary s is online now gary s  Australia
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Zen Mod has it all covered, maybe small heat sinks on the IRF 9610 mosfets as well.
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Old 3rd February 2019, 08:27 PM   #4
sifis_samos is offline sifis_samos  Greece
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So, just to be sure, single bridge per Chanel using Rurg3060 - 47.000uf - 0.1ohm 5w in series - 470nf MKP... correct?
Mikas are ok?
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Old 3rd February 2019, 08:30 PM   #5
Zen Mod is offline Zen Mod  Serbia
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My first class A attempt. Aleph 3
dual bridge config is better , to be sure that you'll not have buzzing xformer if there is any dis-balance in rails current

CRC means 47mF-0R1-47mF
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Old 3rd February 2019, 08:52 PM   #6
gary s is online now gary s  Australia
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Mica washers plus heatsink compound is OK but messy for the output transistors - better still Keratherm pads are tidier and do the same job.

You can also use the 470nF MKP poly caps you mention to bypass the large PSU 47,000uF electrolytic caps per rail.
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Old 3rd February 2019, 09:03 PM   #7
sifis_samos is offline sifis_samos  Greece
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Quote:
Originally Posted by Zen Mod View Post
dual bridge config is better , to be sure that you'll not have buzzing xformer if there is any dis-balance in rails current

CRC means 47mF-0R1-47mF
Well this cant be done.. i have only 4 pcs of 47.000/80v Mundorf mlyticís..
If i dont do CRC whats the potential issue? Noise?
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Old 3rd February 2019, 09:08 PM   #8
Zen Mod is offline Zen Mod  Serbia
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My first class A attempt. Aleph 3
just put 47mF and that's it

you're good

but use dual bridges
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Old 3rd February 2019, 09:33 PM   #9
sifis_samos is offline sifis_samos  Greece
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Thanks a lot!
As soon as i start building i will be back.:-)
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Old 3rd February 2019, 10:29 PM   #10
gary s is online now gary s  Australia
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As ZM says, dual bridges feeding 2 off 47000uF per +/- rail with a smaller bypass cap on each rail to feed both channels will be fine. Oh, and don't forget the obligatory blue indication LED.
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