need tda7240 guitar amp schematic

[PRR]

> help us out with the voltage issue?

"If you know the supply is 12 volts then you could omit the LM338T" -Ian M.

The power-amp uses a "12V" chip. Taylor found an inexpensive yet ample 17V power supply. The LM338 is an ADD-ON to drop 17V down to 12V. If Mark has found a nice 12V supply, there is NO need for the LM338, just take positive 12V to the power-amp chip. There is NO need for any other changes, since 12V is the way it is supposed to work.

And BTW "any" car-sound chip could be used instead. They are all much the same. Some detail differences in external parts.... follow the data-sheet (yes you usually need a PDF reader).



Car-sound chips need a stronger signal than comes out of guitar. You need another stage. You also want a volume control. Taylor's TL072 stage and volume control is a fine example.

> i am only supplying it with 6 volts?

No. The supply is zero V and 12V. The signal must stay between these voltages; actually, between about 2V and 10V. Taylor set the DC voltage on the signal pins to half of the supply, 6V, which is correct or optimal.

[Jdansti]

Thanks for the information and good news Slacker and Paul!

So looking at the LM338T pin out and Taylor's PCB, Mark will need to place a jumper between where the regulator's Vin and Vout pins would go on the PCB and omit the 120R and 1k resistors that would normally connect to the Vout and Adj. He could leave the 100nf capacitor on the Vin.  Correct?  Should he substitute a larger electrolytic for the 100nf on the Vin to help filter the power supply?

[jogina111]

what would the effect if  no tweaks, replacements, etc.  are done, and just putting in that 12v 800mA  powes supply directly on the TG? Will the regulator chip malfunction?

[jogina111]

I mean even without skipping the regulator chip. What would happen if the regulator chip was supplied with a lower voltage than the voltage it supposed to regulate.? Would it still cut some volts or just let it pass?

[Jdansti]

I think the differential between Vin and Vout is around 1.25V, so 12V in would at most give you around 10.75V out. The value of the adjustment resistor was chosen to give 14V out when supplied with >15V in. If you decide to leave the regulator in, you might need to change the adjustment resistor to maximize the Vout given Vin~12V. Either way, I'll ship you all of the parts so you'll have an option.

BTW, the kit comes with a TDA7240, so I'll include it even though you already have one. I probably have all of the other parts in my stock to build another amp except for the LM338T, so I'll send you those too. You'll have enough stuff to build a second amp if you leave off the regulator and do your own PCB or perf. A suggestion: use the extras I send to build a second one and sell it. Use the proceeds to stock up on parts for more projects. Rinse and repeat.

Another suggestion: Once you get the ball rolling on selling a few pedals and reinvesting in parts, you might be able to supplement your household income.

[jogina111]

man, I cant wait for the package to come...

[Jdansti]

Patience young Grasshoppa!  I still need to receive it and re-ship it. 

[jogina111]

I found some wall warts but theyre all 12v 500mA..dang! :-) why do the TG need that much current?

[Gurner]

I found some wall warts but theyre all 12v 500mA..dang! :-) why do the TG need that much current?

I reckon that would work with an 8 ohm speaker

Back of a cigarette packet calculations...

12V rail ...at this voltage the most 'signal' you'll likely get across the load is 10pk to peak, which is 3.56V RMS (of course this assumes a sine wave).

For an 8 Ohm speaker 3.56/8 = 445mA max current required   .....not much spare capacity there for a 500mA PSU, but hey, if you're pushed it'll likely work.

Have a search for CCTV PSUs...they're often 12V & normally are spec'ed for high-ish current delivery.

[jogina111]

what does higher spare capacity means?

[Gurner]

what does higher spare capacity means?

The PSU you mentioned will supply up to 500mA, but my (rough) calculations shows that the amplifier circuit will draw about 450ma when operating at max  (in fact a touch higher due to the quiescent current of the other components on the circuit)  that means the circuit will be operating at very close to the maximum amount of current the PSU can supply  ...ideally you'd want the PSU to have more current delivery capacity spare/in reserve, but like I say if you're pushed - and don't clip the amplifier (ie don't use this amp  to distort the signal), it'll likely work.

[jogina111]

does that mean I cant use effects pedals? Would the amp distort by itself? And would higher current prevent the amp from distorting or it will give it more headroom? Sorry for the ignorance, still a noob noob in electronics.

[Gurner]

does that mean I cant use effects pedals? Would the amp distort by itself? And would higher current prevent the amp from distorting or it will give it more headroom? Sorry for the ignorance, still a noob noob in electronics.

Yes you can use effect pedals (distortion etc)...what I meant was, if using an 8 Ohm speaker, 12V PSU & a 12V 500mA PSU, you are best not using the TDA IC to do the distortion by pushing it to the max (else this will be too much for your 500mA PSU)

[Jdansti]

Gurner-I was typing this at the same time that you posted.

does that mean I cant use effects pedals? Would the amp distort by itself? And would higher current prevent the amp from distorting or it will give it more headroom? Sorry for the ignorance, still a noob noob in electronics.

Distortion in can be created in at least a couple of ways.

1) You can use a pedal that distorts the signal shape before it gets to the amp. In this case, you would normally still have a "normal" signal amplitude going into the amp and shouldn't cause any problems.

2). Amps can have "built in" distortion where the preamp overdrives the power amp. I haven't built a TG amp, but I suspect that like most amp designs, as you crank the volume closer to max, the preamp begins to push the power amp into distortion.  The TDA amps are solid state and would not sound nice like a tube amp if overdriven. Furthemore, as Gurner said, the more you push the TDA amp (by cranking up the volume), the more current it will draw. He calculated that you would max out in normal conditions at 450mA.  If you use a 500mA PS, you don't have much spare current capacity in your PS to handle the current if you cause the amp to draw more than 450mA. Keep in mind that the 450mA is an estimate. The 500mA capacity rating is also subject to error just like all components. If we assume that the PS current rating has an error of +/- 10%, then that tells us that we could max out somewhere between 450mA and 550mA. You could probably use the 500mA PS and avoid cranking up the volume near max. Ideally, you would use a PS that has more capacity than 500mA.

I looked at sending you a spare laptop PS that could do the job, but the shipping would be outrageous due to the weight.   Member 3hree6ixty in Singapore offered to help you with parts here: http://www.diystompboxes.com/smfforum/index.php?topic=98540.msg867594;topicseen#new. You might ask him what it would cost to send you a used or new laptop PS.

Edit: Fixed hyperlink.

[jogina111]

thanks for the explaination guys..john,yeah Iv'e pm'ed him for the psu and some lm386 and jfets. Maybe the shipping fee would be much cheaper since Philippines and Singapore is just a one way plane ride away.

[PRR]

> 12V rail ... 10V pk to peak, which is 3.56V RMS

The chip is two amplifiers in _bridge_ configuration. You get nearly 4V rms at each output, out-of-phase, so 8V rms across the speaker.

This is roughly 8 Watts in 8 ohms, 16 Watts in 4 ohms. (The chip is rated for 14.4V use, a hot car battery, and specs are taken with significant distortion, so it may say more on the data-sheet.)

> why do the TG need that much current?

It is a POWER amplifier, and significant power.

Your 12V 500mA (0.5A) supply only has 12V*0.5A= 6 Watts of DC power. With losses and sine-wave output, a linear audio amplifier can only deliver 3 or 4 Watts of undistorted output.

[Gurner]

> 12V rail ... 10V pk to peak, which is 3.56V RMS

The chip is two amplifiers in _bridge_ configuration. You get nearly 4V rms at each output, out-of-phase, so 8V rms across the speaker.

Good spot! I'd leaped into the maths overlooking that the TDA7240 is a BTL amp,

Therefore the aforementioned 10V is actually peak signal voltage across the load, or 7.07V RMS ........8(ohms)/7.07 = 1132mA ( therefore a 500mA PSU @12V *will* be totally insufficient - I'd be lookig to use a 12V 1.5A PSU at the least)

[jogina111]

would this thing help? :  www.extremecircuits.net/2009/08/ampere-or-current-booster-circuit.html?m=1

[Seljer]

would this thing help? :  www.extremecircuits.net/2009/08/ampere-or-current-booster-circuit.html?m=1

No, that would not help. You'd use such a circuit if you had a voltage regulator and your load pulled more current than the regulator could handle. Your actual power supply would still have to be able to supply the current.

[slacker]

Desktop PC power supplies will give you 12 volts at a few amps, if you can find an old one of those you could use that, It just takes a little work to make them work without the rest of the PC attached. They can be noisy, but for a chip designed to be used in cars this probably won't be a problem.