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Solid State Amplifiers => Amplifier Discussion => Topic started by: exztinct01 on February 23, 2016, 10:26:05 PM

Title: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 23, 2016, 10:26:05 PM
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.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: phatt on February 24, 2016, 01:53:54 AM
What is wrong with the Amplifier/equipment are you using at the moment?
There might be easier/cheaper ways around your problems.
Phil.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 24, 2016, 03:25:22 AM
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?
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: phatt on February 25, 2016, 03:40:18 AM
This is no simple task;  :-\
You need a power supply to run it,
Transformers and rectification can be expensive,
A metal case,
A cabinet to suit,
Then PCB for power amp,
PCB for preamp,
Knobs, switches and many other ancillary bits.

Oh and a speaker box or boxes ,,
cost of speakers and wood is not cheap.

You then have to hope it all works and sounds right. :-X

My thoughts; FWIW,, if you already have pedals then build/buy a simple preamp and Stop;
Then go hunt down a big power amp and consider speaker boxes may well be cheaper to purchase second hand.

Others may have different views but if you do build a power amp,,,stay away from anything that says *Bridged* especially chip amps.

Phil.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 25, 2016, 08:57:55 PM
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.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: J M Fahey on February 26, 2016, 11:21:43 AM
right now, I have an old local made power amp
So where is local? ::)
Answer depends a lot on where you are.
As in: if you are in USA, forget building anything, just search Craigslist for a good but undesired guitar amplifier for cheap, cheaper than parts cost.
Undesired means anything not used by Jimi Hendrix and definitely solid state.

Quote
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

That means +/-25V rails.
An LM3886 can easily give you around 55W`RMS into 4 ohms, which is enough to play side by side with a drummer.
No, you won't drown him ... what's the purpose?
Just use 2 good 12"guitar speakers and your power and reach will be ample.
And the PA guy can still boost you during solos or something.

It's a well tries amplifier and you can find lots of PCBs and layouts on the Net.
Quote
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.
You might reuse the heatsin ks, you may even make a 2 x LM3886 , 2 x 55W amp , each driving its 4 ohms load, of course.

Quote
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.
Search for Rod Elliott's Guitar Preamplifier.

WAY better than the Fetzer and real Fendery sounding.
You'll have to order the inexpensive PCB from him or design and build your own.

EDIT: it will probably be WAY easier to repair your existing dead power amp, 90% of nthe job is already made for you, and add a preamp, either in the same chassis if it fits or outside.
Post some picture of what you have there.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: Vitrolin on February 26, 2016, 04:36:20 PM
i agree with fahey old power am with a guitar pre amp wiorks quite good.
for a time used a peavey cs-800... i was LOUD, and unlike many other guitarists i had no problem hearing myself if i turned up the volume, eventually the fan broke and the output stage overheated, not by my playing, when it was used for PA at a party.
its now waiting for some new output transistors.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: mexicanyella on February 26, 2016, 10:55:07 PM
It's possible the OP is specifically into building something himself regardless of cost, but as others have suggested, if the attraction here is mainly to get some cheap watts and get independent of the PA, used stuff is going to be a cheaper/quicker way to get there.

I gigged for awhile with a Tubeworks rackmount preamp ($100, classified want ads) and a 150-watt mono QSC rackmount power amp ($50, pawnshop) and a Fender  4 x 12 cabinet ($100? $125? somewhere in there...classified ads). Loud, cheap, and reliable. I think my rack case cost as much or more than any one component, though.

Right now I own a Peavey M-3000 mono PA power amp (130 watts/8 ohms, or 210 watts/4 ohms, or 300 watts/2 ohms) that I found on Craigslist...part of a church sound system that was getting updated to newer, smaller stuff. I got the M-3000 for $40, thrown in with the CS-400 a friend of mine wanted. Well worth the drive...and I've gotten a pretty cool Fendery clean out of that M-3000 feeding it with the line-out signal from a little Dean Markley practice amp, which was free.

If having separate components is not a big deal to you and you'd settle for a combo amp, just watch Craigslist for a used Peavey solid-state combo. Bandits, for example, are cheap and durable and have a clean channel with lots of headroom, which can get into Fender territory. I traded a pedal I didn't use for a Peavey Special 150 last week...like a Bandit with twice the output power. Initial tests suggest that it is more than capable at delivering loud clean tones at any kind of church levels I can imagine, and beyond. I think it could deliver loud clean tones into outer space.  Loud, cheap, simple and sounds good! 

So if having it be homebuilt is not the ultimate goal, keep your eyes open for used stuff.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 27, 2016, 11:38:15 AM
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.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: J M Fahey on February 27, 2016, 02:38:00 PM
All this was answered 4 posts above  ;)
Where I mention the +/- 25V rails .

If you don't know how to get them from a 18+18VAC transformer, I guess you have to make some reading.
That basic knowledge and lots more are what you need to build (ane even way more to design)  what you want.

The only way you can successfully build something without grasping basic theory is in a very well designed kit, where parts are properly laid out, you have good drawings showing PCBs, chassis, auxiliary parts (jacks/switches/transformer/fuse holder/heatsinks/mains cable/etc.) shown, also wiring, etc.
In that case you can "paint by the numbers".

Hey !! my Sister has no clue (she'll kill me if she reads this)  but she knits *beautiful* sweaters, strictly following instructions such as "3 knots top to bottom / switch needles / 27 "piqué" knots / back to original one ......... " , etc.

But if you have to make everything from scratch .... it's iffy.

Since you want to make your own, and I applaud you for it, go slower, step by step.

Build your Fetzer first (maybe you already did) , then Craig Anderton's 10W guitar amplifier, based on an LM383(obsolete) or TDA2003 (still available or recyclable from old car radios and such) , maybe the "GGG" one is the same, then some 3886 based one.

When you want us to check some schematic, *you*  post it here , or at the very least post a direct link, which when clicked straight opens it without fuss, we shouldn't search the Net to answer anybody.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 27, 2016, 05:44:14 PM
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.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: phatt on February 28, 2016, 05:11:24 AM
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.

Yes rectification has a few snags for the unwary. ;)
This chart will help but Read the Text First,, then the drawing will make more sense.

The 18-0-18VAC transformer is likely for a small power amp and would work fine for LM1875 but will not have enough power to keep up with larger venues.

If you want to go chip power stage then as JMFahey noted above,,,you need LM3886 or similar.
But be aware that the transformer will dictate the wattage ability of the amplifier.
I strongly recommend you spend time reading the ESP pages as it 's aimed at people just like yourself... hint ;)
Rod Elliot has the rare talent of being able to put complex stuff into layman's terms without going into complex maths unless absolutely needed.
He has helped me to build a stunning reverb circuit and yet I've never purchased any of his products.
Phil



Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 28, 2016, 11:11:36 AM
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.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 28, 2016, 11:23:22 AM
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,  :)
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: J M Fahey on February 29, 2016, 01:09:17 AM
99% of power amps use "raw unregulated" supplies, meaning transformer > diodes > filter caps > amplifier
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 29, 2016, 01:32:27 AM
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.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: J M Fahey on February 29, 2016, 07:19:08 AM
Yes.
In general (and this can be VERY confusing bto newbies), some things are "taken for granted" , "everybody knows it" , so are not drawn or if they are, in a very minimalistic way.

So it happens what you mention: 2 wires are shown going to the AC terminals of the rectifier bridge: check ; and center tap goes to grounf ... "why waste ink drawing that in full"? ... although in fact since ground is all over the place, drawing it as a solid line connecting all proper points messes the schematic, so just little ground symbols are vdrawn all over the place.

"It is known" that in practice all are joined by some **metal** , either the plain chassis itself, or some dedicated bus wire or plain plastic covered wire.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 29, 2016, 08:24:36 AM
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
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: phatt on February 29, 2016, 08:08:48 PM
Hey do not worry,, mistakes are one of the best learning tools. :tu:
As long as you avoid the Fatal mistakes you will be fine. 8|

Maybe start with someting in a kit form.
plenty of good ideas can be found here;
http://sound.westhost.com/projects-0.htm

Some reading here;
http://sound.westhost.com/amp-basics.htm

And the 100Watt Guitar Amp Here;
http://sound.westhost.com/project27.htm
You will need a transformer with 25-0-25VAC taps which means 25VAC x 1.414 = 35VDC rails.

That big donut transformer used in the right circuit will likely deliver way over 100Watts. <3)
But be careful you need the right circuit for those voltages. That transformer will deliver ~60-0-60VDC after rectification.
Phil.
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: PhredE on February 29, 2016, 09:01:43 PM
Hi all. Sorry, I haven't been around in a while.
I was lurking/spying on this thread, it's interesting (and I'll profess a bias toward Fender 'clean' tones..)

exztinct01, was just wondering..
It sounds like you're intent on building your own amp as a solution, but was wondering if you boosted your signal going direct into the mixer using an emulator type pedal/box (that does 'Fender tones') if that might help get you closer (?).  Don't know if your PA/mixer is up to the task, but if it is, then maybe that is a reasonable workaround (assuming you want one of course..) ?

I stick a Joyo American Sound in the front of my signal chain and eventually run it into my smaller Fender SS amp, and really love it.  It gets a range of good Fender-type tones and can really boost up the signal quite a bit (can be noisy... depending on settings). It supposedly has basic speaker emulation included as well.  I will attest, it can do a pretty good Bassman, good range of standard 'cleans' -- eg Twin,  etc. and even some classic tweed sounds as well.  I really love that thing.  It's the best $35 I've ever spent on anything gear-wise.  Reference:
http://www.amazon.com/Joyo-JF-14-American-Amplifier-Simulation/dp/B007L0CH8K

Well, it's just a thought (maybe not even a good  nor practical one).. good luck to you just the same.

Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: exztinct01 on February 29, 2016, 11:50:31 PM
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!!
Title: Re: TDA7294 Power Amp with Fender Clean Sound Preamp
Post by: PhredE on March 01, 2016, 12:55:46 AM
No worries. Good luck on the power amp.

And, doh! /smacks forehead ..you can always use the Joyo American in front of your amp to 'shape' the tone -- it really does Fender tones quite well <<forgot to mention the obvious in my earlier post. ..oh well.  You can even go straight out into headphones with it too.  It's a pretty versatile and interesting little beast.

Good luck. Let us know how it goes..