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Author Topic: Hello... and some newbie questions  (Read 3714 times)

@rnie

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Hello... and some newbie questions
« on: January 10, 2009, 08:09:06 PM »
Hello, I recently discovered this forum on my search for a diy amplifier and it looks very interesting.
I have got a little bit of amplifier building experience from building the Noisy Cricket, but now I'm planning on making a more powerful amplifier. It will be driving an 8 Ohm speaker.

Now for the questions:

1) For the power amp I have ordered a mono LM3886-kit from www.chipamp.com. For the transformer I have found toroidals with 2*22V or 2*25V secondaries (2*22V:http://www.banzaieffects.com/Talema-TRT120222-pr-23395.html and 2*25V:http://be02.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2238049).
They are rated at 120 VA, I think this should be enough, but I just want to make sure. Could lower VA transformers be used? Also, which one is preferable, the 2*22V or 2*25V?

2) What thickness of wires is adviced for the power section ? How big should the fuse be? I was thinking 2A should be more than enough when the amp is working properly. And what is adviced for the wire thickness in the power amp itself?

3) Does anyone have some suggestions for a not-too-complicated preamp? I would have liked one which could do a cleanish amplification, but also has an overdrive option and has a tonestack I have found some posts referring to the Fender Blackface FET-version on http://www.redcircuits.com/Page120.htm, is this ok for clean and some overdrive? I am also open for other preamp suggestions.

Greets,

Arnaut
« Last Edit: January 11, 2009, 12:29:40 PM by @rnie »

@rnie

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Re: Hello... and some newbie questions
« Reply #1 on: January 31, 2009, 10:34:43 PM »
Hello again, I didn't do much on this project because of exams, but now I got some time. If somebody would be so kind as to take a look at my questions...

1) For the transformer VA rating, the assembly manual on chipamp says a 160 VA transformer is enough for the build, which I guess refers to the stereo version, so an 80 VA transformer should do the trick for my mono build, right?

2) For the wire thickness, can anyone tell me what thckness is usually used in this type of solid state amps?

3) Although I'm pretty sure I'm going with the preamp mentioned earlier, I'm still open for suggestions...

Greetings,

Arnaut

J M Fahey

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Re: Hello... and some newbie questions
« Reply #2 on: February 16, 2009, 10:20:23 PM »
Hi Arnie.
The ACvoltage in your power transformer should be around 0.707 times the DC supply voltage needed.
Example: if you need 28+28 volts DC your transformer should be around: 28x0.707= 19.796 VAC (x2) or around 20+20VAC.
VA rating: very roughly: minimum around 1.5 times your RMS output; say 80 V.A. for a 50 W amp. To be safe you should go for 2 times or more your output power: 100 or 120 VA . Use at least 4700 uF caps of at least 20% higher rating than DC voltage. For 28VDC: around 35 (or 40) volts. Higher ratings donĀ“t hurt at all.
Good luck.
J M Fahey