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Old 1st February 2017, 09:12 PM   #1
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Default The "150w" gainclone

First of all, I would like to say hello to the diyaudio community seeing this is my first post, and thread.

Hello!

For a while now i have been gathering parts to build myself a amp, because the old one that i am currently using doesnt sound as nice as it used to or compared to other ones.

I am building two PA150 board for a stereo system. I am planing on using the amp to drive two 8ohm speakers. I know that this amp is abit overkill for my speakers, but i want to have the option to use multiple speakers at the same time on the same channel (ie 3x8 ohm,2*6ohm etc.) and also when i use the amp just for two 8 ohm speakers it will not get as hot.

PA150 - 3x LM3886 PCB assembly guide

I have a few problems in building this amp, more like questions actualy:
I am using a 600va with 2x24v sec, a 50amp bridge, and a total of ~20k uF. Those are quite big numbers for me, and frankly i am quite scared of a falieure, so i am using a esp soft start board to limit the inrush current, but the board seems rather small for the job. What do you think guys, is it enough to keep everything in check?

Regarding the amplifier, i am using a 15 uF cap on the input just as in the article on Shine7. I'v read some different oppinions regarding the value of the input cap and I would like some light shed on this topic. Mainly whats the difference btwin a lower value vs a bigger one, and also a ballpark ideea of what a small value is xD

I also want to integrate some form of attenuation and or buffer on the input.
Attenuation, as in a volume pot or stepped attenuator(ladder type maybe? Its cheaper and found a promising one on ebay, eizz by the name). A buffer to drive some vumeters and the amp, some tips are highly welcome.

As much as i tried i couldnt figure what the input impedance of the amp is for sure. I keep getting 3.something using some formulae i found on mr Nuuk's website and others.

One more thing, can i use some thermal tape ( the one that you use on the video card vram for instance ) to isolate electrically and thermal-couple the chips to the heatsink? Im asking because i cant find mica washers in to-220-11 format anywhere.

I will come back with a edit or another post if I forgot something or have something new !

Thank you guys for your patience, I hope this is the right place to post (to me it seems like it) and please excuse any grammar misshaps, im writing this off my phone.
-Vld
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Old 1st February 2017, 10:37 PM   #2
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by druchivld View Post
....................I am using a 600va with 2x24v sec, a 50amp bridge, and a total of ~20k uF. Those are quite big numbers for me, and frankly i am quite scared of a falieure, so i am using a esp soft start board to limit the inrush current,.............
I use a soft start for nearly all my builds. That starts the transformer with a current limiter in the primary for a couple of hundred milliseconds.
I hear the transformer grumble a bit charging up the mains smoothing bank.
I often use 20mF for each amplifier, that's 80mF to be charged up to ~50V in a stereo amplifier and I don't bother with a slow charge circuit.
I have never blown a 25A/35A bridge rectifier.
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Old 2nd February 2017, 10:47 AM   #3
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@AndrewT I understand how a soft start works My concern is if the soft start board from Rod Elliott is too small or not. He says in his article that the board is perfectly capable of starting a 500VA transformer, but it looks abit small to me. Maybe I'm just a bit paranoid.

I dont quite understand what you are trying to tell me about +-20mf, are you trying to tell me you are using 20 000 uF per rail per amp? 80 000 in total?
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Old 2nd February 2017, 11:28 AM   #4
AndrewT is offline AndrewT  Scotland
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Yes, you are reading my comments correctly.
The soft start is just a resistance in the primary circuit that gets bypassed by a relay after a short delay. It does not need to be big, not even for a 1kVA transformer.

I have used much bigger capacitor banks than your 20mF and have never used a slow charge circuit.
The Krell Klone I built up had 75mF in the monoblock fed via a 25A bridge rectifier. But I only used this during extended testing. I don't have long term reliability data for this.
Most of my stereo amplifiers have 40mF or 45mF again charged via a 25A, or 35A bridge rectifier.
I have never blown a bridge rectifier.
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Old 2nd February 2017, 11:38 AM   #5
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Thank you for the heads up AndrewT! It is good to have you share your experience with us (implicitly me), because in some ways this is a learning process for me.
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Old 2nd February 2017, 12:48 PM   #6
AndrewT is offline AndrewT  Scotland
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I consider there is more learning in researching a project, than slapping it together to blow it up.

Some here claim people learn from their mistakes.
No, they learn from getting it right and that usually means read and ask.
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Old 2nd February 2017, 02:05 PM   #7
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I'd much rather research as much as I can beforehand, this is the reason I started this thread actually.
Yes, people learn from mistakes, but people also learn from research. In the diy audio section of learning, I think it's actualy safer and cheaper to learn from reasearch.

Any input regarding the other questions is welcome
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Old 3rd February 2017, 06:03 PM   #8
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well, one thing is sure, you can never go wrong by sticking to AndrewTs answers and suggestions. Danielwritesback (i hope i managed to spell it correctly..) is allso a guy to keep an eye on. and tomchr too.

i wrtie this meanwhile i don't allways 100% agree with them. even so if i would get stuck with something, i would blind trust them.
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Old 3rd February 2017, 06:07 PM   #9
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An inrush-prevention doesn't actually need a relay in many cases either. A resistor that decreases in resistance as it heats up is actually fully capable of driving a medium load device. Electronics doesn't have to be hard at all :P
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Old 3rd February 2017, 06:54 PM   #10
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Originally Posted by HumbleDeer View Post
An inrush-prevention doesn't actually need a relay in many cases either. A resistor that decreases in resistance as it heats up is actually fully capable of driving a medium load device. Electronics doesn't have to be hard at all :P
ntc inrush current limiters are certainly the simplest solution - but during operation they provide a hot spot of 80..120C - that does not suit everybodys taste.
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