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 Post subject: Making amplifier!
PostPosted: 10 Oct 2017, 14:34 
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Joined: 10 Oct 2017, 14:16
Posts: 3
Location: Zagreb, Croatia
Hi, guys! Im Vinko from Eastern Europe, im more IT guy, with basic knowledge in eletronics.

Recently I bought someone's unfinished project on the market with old things, it was cheap and tempting. In the box there was toroidal transformator 450VA 26-0-26/12-0-12, board with recitifier with two Philips 10.000 uF 63V, 2x VU meter LM3915 boards, one board with TL074CN(op amp i think), voltage regulator board with L7812CV, and something wich i doesnt know what is, it is board with relay and regulator. There is no amplifier boards inside.
I can put pictures later.

I would like to finish this project, but first problem is wich amplifier to chose depending on transformator, the 26V is problem maybe. Im open for yours sugestions.


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 Post subject: Re: Making amplifier!
PostPosted: 11 Oct 2017, 02:10 
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Joined: 28 Dec 2010, 22:07
Posts: 309
Vinko, hello.

neka_budala wrote:
Recently I bought someone's unfinished project on the market with old things, it was cheap and tempting. In the box there was toroidal transformator 450VA 26-0-26/12-0-12, board with recitifier with two Philips 10.000 uF 63V, 2x VU meter LM3915 boards, one board with TL074CN(op amp i think), voltage regulator board with L7812CV, and something wich i doesnt know what is, it is board with relay and regulator.


This is something I do from time to time...not to buy unfinished projects from others, but to buy damaged or forgotten home theater amplifiers from second hand stores. These items normally have decent components that are expensive to buy otherwise, and can get you started on an amplifier project on your own. Let's look at your transformer first...52v CT at 450 VA. Into a resistive load, you'd get 8.65A. Into a FWB rectifier you're looking more at 5.4A of continuous, all day long output. Let's say you ran this secondary into a FWB, center tapped with the two capacitors. This provides you with two voltage rails of roughly 37v (probably closer to 45v unloaded). This affords you a possible 85w into 8 ohms, or 171w into 4 ohms. You also have to remember that the power would be halved for two amplifiers.

Moving on, let's say you want to make twin 40 WPC amplifiers designed to play into an 8 ohm speaker system. Voltage requirements are 25.3v peak (easily do-able) with a current requirement of 3.162A Peak. The filter capacitors in the power supply will supply the peaks, as long as you can provide the value of the average current (roughly Peak * 0.636). In this case, it's a little over 2A. This transformer would easily provide 40 watts into two speakers. It's possible you'd want to double the capacitance in the power supply to stiffen up the response, esp. on the low end where the distance between peaks can get somewhat long. For running 4 ohm speakers, the amount of current will limit the amount of power you get out rather than the voltage. For the maximum amount of RMS output current, you could get about 36 watts out from the transformer (this is sine wave power into a 4-ohm load, coming from the transformer at maximum rated design).

Don't worry about regulating the voltage powering the amplifier. You don't need it for a power amp. Some people say you need it for a preamplifier, but I think that's more because of the ripple reduction of a regulator than the stability.

neka_budala wrote:
I would like to finish this project, but first problem is wich amplifier to chose depending on transformator, the 26V is problem maybe. Im open for yours sugestions.


If you're new to DIY audio, I would suggest building the power supply as a stand-alone unit (so you can try out different things) and use it to power a kit using chip amps. That way, you can get your feet wet with audio projects without the need for building an all-discrete supply. Here's some links that will let you play with these parts...

http://www.ebay.com/itm/Rectifier-POWER-SUPPLY-PSU-BOARD-FOR-AUDIO-POWER-AMPLIFIER-preamp-AMP-DIY-KIT-/140863277295?hash=item20cc1b08ef:g:u6EAAOSw0UdXvrbL

A rectifier / capacitor kit that will allow you to make two DC rails from that 52vCT secondary. I would recommend taping off the 12-0-12 secondary for now. You can use it later if you want to power a preamplifier, a bluetooth module, etc. If you don't draw any power from the smaller secondary, you can draw some more from the larger one.

http://www.ebay.com/itm/60W-LM3886TF-AC-20-28V-Sound-Audio-Amplifier-Mono-Digital-Power-AMP-DIY-Kit-CA-/142189384715?hash=item211b25d40b:g:Aj4AAOSwB09YNpX4

Here's a kit that should work well for you. You'd need two of them for a full stereo amplifier, as well as heat sinks for the chip. Remember that class AB amplifiers tend to be around 60-70% efficient, so the heat loss in the amplifier chip for a 40w amplifier will be considerable. You'll want to get larger aluminum finned radiators that will take the heat away from the small silicon die.

Ask if you have any more questions. I'll admit I covered just the power supply. The other items you scavenged from the project may or may not be useful. The relay could be made into a speaker protection circuit, and the VU meter boards might be useful in your finished amplifier.

Ed

_________________

You can't have too much heat sink.


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 Post subject: Re: Making amplifier!
PostPosted: 11 Oct 2017, 09:16 
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Joined: 10 Oct 2017, 14:16
Posts: 3
Location: Zagreb, Croatia
Ed, I really appreciate your sugestion and I will follow it. I can't tell you how greatfull I am to you. I will attach some images as soon I start to work on this project. Thx


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 Post subject: Re: Making amplifier!
PostPosted: 24 Oct 2017, 03:59 
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Joined: 23 Feb 2017, 02:02
Posts: 59
Any chip IC amp is good for ok, to good sound quality and cheap.

For top quality, discrete ocl amp circuits should be looked into and schematic must be good. If quality is poor then sound less than good ic amp.
I tried super class a before and its very good, I would recomend this as a high end build, however I don't think you are planning on such a expensive build with big heatsinks. Nor do you have the power supply for it.

You can always buy finished amplifier boards, quite a few on aliexpress and cheap. All you do is install amp onto heatsink and give it power supply.

If you have the money and cooling, throwing in a high power regulator can be a good idea.


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