Quote from: J M Fahey on March 15, 2016, 07:50:18 PM
Tell us: what was the original power amp rating?
if a 30/60W MI amp or, say, 25+25W Hifi, fine.

I can't tell yet since I left it at home but the amp is a karaoke stereo amp using two TIP41's and two TIP42's
The traffo has 3 output winding, 18-0-18, 0-9, and 0-5-20. Looking at it, I think it can power at least two 100W channels (I doubt if it's RMS since the amp's cheap)

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You might build it on the table, un cased, just 1 channel first, and test it.

Then you decide whether to build it 25W or 25+25 .

can i build it on a solderless breadboard (except the PS and the chipamp itself)

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measure transformer EI iron stack, please use mm if at all possible.

I'll do it later at home

Should I go with panel mount fuse holder for the mains fuse and pcb mount for those between the ps and the powr amp?

 i do have transformers taken from an old amp and it has about 3 output windings. Is it okay to leave some windings unused? (since I'll only be using the 18+18 ct)
I already have these:
LM1875 chipamp
35A bridge rectifier
phenolic paper copper clad (is this ok or should i use fr4)
36vac ct traffo
large heatsink taken from a 300W amp connected to TIP41's and 42's
resistors (carbon film only)
caps (mylars and electrolytics)

please comment if these are enough

Edit: i don't know the fuses to use for the ps. And I already bought the Eminence Patriot Red White and Blues. I'm planning to use my amp with two speakers in parallel but impedance would not allow. Should I go for stereo?

Suppose I build an LM1875 power amp, how much current rating should my traffo have if I run it at +-25 volts? how bout at +-12v?
Let's say I'll be using the circuit in the ESP pages (project 72)
please don't give me VA, I don't fully understand it yet plus traffos sold here are rated in amperes not VA

I don't believe our PA system is up to the task, can't even hear much of the vocals, keyboard and guitar when our drummer's in the mood of rocking out a worship song.
I don't have a plan for the preamp yet except for the fetzer (so simple), but I won't stick to it long, i'll build a better one once I finished the power amp.
I have to thank all of you here for answering all my questions above. Right now, I'll try to start building the power amp and hope everything turns out good.
Ciao!!

thanks J.M., now i can move on to other concepts

anyway, would anyone discourage me in using that autotransformer? perhaps it gives too much voltage for the lm3886 and that my heatsinks (attached in above post) won't be able to handle dissipated heat?
I don't think I'll be using the 18-0-18 transformer for the lm3886 since it gives small wattage. Unless someone tell me +-25vdc is enough to obtain loud volumes from the lm3886 with a not so efficient speaker (good brands are hard to find in our country).

I really did a great mistake in making the title of this thread

oh, I just saw the light  :cheesy:
so all I really need is my transformer, bridge rectifier and filter caps to supply voltage to the +- rails of the power amp circuit. Got it now. Now I only need to do the math, I think (gotta use my major).
However, center tap means three wires (ac volts - 0 volts - ac volts), and all schematics I've seen show two wires from the transformer going to the two ac leads of the rectifier.

So, am I assuming it correctly that the two non-zero secondary volts goes to the AC leads of the rectifier which then creates the +-DC rail?
And that the zero (neutral) volts from the transformer is already the ground which will go to the ground plane of my power amp pcb?
Please confirm.

the only PS I've ever built is this one (attached image) from tonepad.com, the one using LM317
I use this and the large ordinary traffo (overkill) to power my pedals

I totally forgot I needed space for the heatsink of the LM317
Also, when I can't find a 240 ohms, I just connected two 120 ohms, explaining the standing resistors
Just so you know, I could make my designs and builds more decent in the future than that crappy looking PS, 

I've taken photos of available parts
Let me know what you think I could build with these. The autotransformer says 300W (Input 220V 50Hz Red-Red), Output (17-0-17, Yellow-Blk-Ylw), (42-0-42, Blu-Blk-Blu), (0-12, White-White). The ordinary traffo is the one I gave the specs above. I have bridge rectifier RS603M (datasheet attached) although I don't think needed rectifiers cost much and I prefer bridge type, two heatsinks, old amp case and some circuits I'm not familiar with (power amp section contains only one IC, HCF4052, don't know why it's here).

Hey Phil, in that chart, which one do you use for your PS circuit?
I have been using the full wave bridge cap input load there since what I understand is that those diodes receives AC and rectifies them to dc (forward diodes to positive and backwards to ground), capacitors filter pulsating dc to make it a flat line instead of wavy.
What I didn't expect is that this design may give voltage different from traffo's out (maybe this is the reason why my keyboard's PS which says 12vdc gives me a reading of 17.5 in my multimeter, or the transformer just don't give exact 12vac).

Another question, that full wave bridge cap input load circuit with the traffo, bridge rectifier and filter caps, if I make sure that they give the right dc voltage, are they all I need to power my power amp? Or do I need more components? Also, can I use a regulator here or is it unnecessary?

Sorry if I have so many questions  , hope you'll still answer.

thanks j.m.
Guess I need more reading. I'm not that well versed in electronics but I'm a good reader and theories don't bore me so I'll start from scratch.
Anyway, the power supply thing is really what confuses me, +-25v from an 18-0-18ac. I'll research that.
Also, can I use the remaining outs (0-5-20 and 0-9) of the transformer for my preamp? It will need a regulated ps inside the chassis, I think.
I can't find your suggested amps (lm383 and tda2003), how bout lm1875? is it good as guitar power amp?

Before I forget, I have built the ruby bassman amp already as practice amp.

Guys, thanks a lot for the info. I appreciate it. Will post my work when i start building. Right now I need to build ps just like R.G. said. Good luck to me.

thanks for the suggestions, although I really have made a decision to build it. I just like to see something I have built, rather than bought. How 'bout the LM3886 circuit in GGG? How much wattage does it give? Another thing I don't know is how to make a bipolar supply for it. I know how to build a bipolar supply but with limited current rating since I only know the 78xx or LM317 as regulators which handles 1.5 amperes only.
Anyway, I'm from the philippines so when I say low quality electronics, they're probably made in china or assembled here. I don't trust local stuff here.

right now, I have an old local made power amp which doesn't work so I tried to salvage the transformer. It's large and I measured its outputs, 220 v in, 18-0-18ac v out, 0-5-20ac v out, and another two black wire outputs measuring 9ac v when i connected each to the led of my multimeter
i also have two large heatsinks from damaged power amps, pc system unit case, bridge rectifiers which i think can handle large voltages, small components like 1/2 resistors, large caps, 2sa1943 and 2sc5200 transistors. I think remaining items won't cost me much more, i just need a complete schematic, better if layout of a power amp plus its power supply. I decided to build the fetzer valve for the preamp. Anyway, tnx phil for replying. I really need help for this.

we're using a PA system in our church right now. there's an 8 channel behringer mixing console, a low quality power amp and 2 passive speakers. The guitar, keyboard and vocals are all connected here. So, we don't have separate instrumental amps except that of the bass. I like to buy a fender champion 20 or 40 but I don't think it's volume can keep up with our drummer. So I decided to build something more powerful, 80 to 100 watts maybe?

Hi, can anyone share a schematic and layout of a power amp using tda7294 or bridged tda7294 coupled with a preamp that will give a Fender tone? I need to hear it over a loud drummer in a church setting with more or less 100 people so I think wattage depends on that. Also, I'm a newbie so please provide significant information including the power supply required. I won't build a combo, just a head. Please, I need your help.

Also, I only need one clean channel with no built in effects. I have built guitar pedals already so I think those are enough for getting overdrive sounds. My priority in this post is the power amp though coz I think it will be easier to build a preamp once I have a power amp.