Want to build a TDA2040 amp.

[delus]

Hi everyone!

It's my first post here. I have been messing with electronics for almost a year now, made about half a dozen pedals, another half a dozen waiting on the shelf, and I also have an 18watter in progress.
Recenlty, the only amp I've been using is the Noisy Cricket (my first complete build), and I was amazed at how good a simple solid state amp can sound. So my interest turned to building a SS amp with enough power but still without the need for output transformer. The best option right now seems to be the TDA2040. Also I'm planning on putting a preamp such as the Umble plus an FX loop.

1) Can I use the schematic from general guitar gadgets for a bipolar power supply to power the amp?
If yes, I guess that the appropriate transformer would be one with 12V secondaries, and the regulators should be 7809 and 7909 so I can provide +9V and -9V?

2) If I want to make a complete amp with preamp and poweramp, which I can power straight from the wall, can I use an additional 7809 regulator with the appropriate additional capacitors and diode, to provide an extra power output of +9V to power the preamp from the same supply, or will I need an extra transformer to make a seperate supply?

3) Lastly, If I want to use two TDAs for a 30W amp how could I power it? The bipolar power supply schematic at GGG has an option of +-15V ouptut or 18V, whereas a 30W TDA amp needs +-16V according to the datasheet.

-Adam-

[teemuk]

I don't quite understand where else you would need a regulated power supply but the preamp section...?

Regulating the supply of the power amp will require quite a lot of power dissipation capacity from the regulators as they will conduct all the current drawn by the power amp. Most power amps will work just fine without a regulated power supply, in fact they likely work better without one.

The preamp won't require another power supply / another transformer, you can use the same supply that you use for the power amp. Just decouple it with an RC filter or regulate it like you obviously intent to.

Powering one, two, three or any number of power amps is pretty much similar. Hook their supply rails and common to power supply, be careful with "noding"; you want to supply them straight from the filter caps, kinda like wiring them in "parallel", and also have the commons of each board return individually to common ground point. The amp boards preferably should also include some additional filtering physically closer to the source of current draw.

Also, a scheme for bipolar +-15V supply isn't any different from scheme of +-16V bipolar supply or +-18V bipolar supply or even +-200V bipolar supply. The only differences are ovbiusly the component ratings.

Hope that helps.

[delus]

Thanks for the swift reply! After reading it, I took some time and looked at the power supply section of your book.
From what I understand in order to make a bipolar power supply, I have a number of options.
a)Use a center-tapped transformer and wire the center tap to ground.
b)Use a symmetrical dual transformer, which I don't think is quite neccessary in this case.
c)Use a transformer with single secondaries and make a virtual earth circuit.
The best option seems to be a)... right???

[delus]

Moving on with the theory before trying to build the amp, I believe I should use a 18-0-18 transformer, in order to provide the power amp with +-18VDC. If that is correct then my next concern is how much ampere-rating should the trasformer have (so I can order it from the local store). I don't know how much maximum current the TDA2040 draws, and I'm not sure where I can see it in the datasheet.
Does anyone know the maximum current draw of the TDA2040? Or to be more specific, two bridged Tda2040s powered by +-16 or 18 VDC, in order to provide 30W or so of output power?

I hope I'm not (that) tiresome, and please forgive my english if they are a bit lousy sometimes.

-Adam-

[joecool85]

Moving on with the theory before trying to build the amp, I believe I should use a 18-0-18 transformer, in order to provide the power amp with +-18VDC. If that is correct then my next concern is how much ampere-rating should the trasformer have (so I can order it from the local store). I don't know how much maximum current the TDA2040 draws, and I'm not sure where I can see it in the datasheet.
Does anyone know the maximum current draw of the TDA2040? Or to be more specific, two bridged Tda2040s powered by +-16 or 18 VDC, in order to provide 30W or so of output power?

I hope I'm not (that) tiresome, and please forgive my english if they are a bit lousy sometimes.

-Adam-

DO NOT use a 18-0-18 transformer.  Transformers output AC, when you rectify it to DC you need to multiply it by 1.4.  IE- a 10-0-10 transformer would put out +/-14v DC.

+/-20v is the max rating for the TDA2040.  To get 20w RMS on 4ohms you need +/-15v, I think that's a good target to aim for.  A 12-0-12 transformer would give you +/-16.8v, well below max but still above your 20w rating, plan on 22-23w RMS @ 4ohm, 14w @ 8ohm.

As for amps, you will want to be able to supply 2.5 x RMS wattage.  22w x 2.5 = 55w / 16.8v = 3.27amps.  You should be fine with only 3 amps though as this is rough calculations.  If you find one with VA ratings you will want at least 50VA.

Bridged TDA2040s will take double the power from the transformer (double the amps or VA, not voltage) and will net you 30w @ 8ohms.  I personally don't see this as worth the effort since you can get 22w on 4ohms with one chip.  If you HAVE to supply an 8 ohm load I suppose it could be worth it.

[delus]

Actually, i'm going to use it with a 16Ohm load. I have one cab with a 16 Ohm speaker, so I guess a TDA2040 will give around 8watt@16Ohm. Plz correct me if I'm wrong.

So if one chip draws 3.5A, for a bridged amp I'm gonna need a transformer of around 6 or 7 Amperes. I suspect it's gonna be quite bulky! It may not be THAT worth the effort, but I may go for it anyway.

Thanks a lot BTW! All that info have been really really helpful!  :tu:

[joecool85]

I personally wouldn't even build it with a 16 ohm load in mind, it just won't put out that much wattage.  Might be worth looking into a different power IC or a different speaker.

[delus]

I personally wouldn't even build it with a 16 ohm load in mind, it just won't put out that much wattage.  Might be worth looking into a different power IC or a different speaker.

Some time in the future I'm planning on getting myself another one of those cabs. This way I can form a mini stack of total 8Ohm load.

[joecool85]

I personally wouldn't even build it with a 16 ohm load in mind, it just won't put out that much wattage.  Might be worth looking into a different power IC or a different speaker.

Some time in the future I'm planning on getting myself another one of those cabs. This way I can form a mini stack of total 8Ohm load.

Well 8 ohm would certainly be much better, but a lot of these chips really do perform best at 4 ohm, albeit with a somewhat hotter chip temp.

[delus]

I found a nice 12-0-12 toroidal transformer rated 80VA. I haven't bought it yet, maybe tomorrow I'll go and buy it. I hope it can be capable of powering a 2xTDA2040 amp with 2 preamps and a couple of buffers (although I don't think the preamp section will draw a significant amount of current). Also the store owner suggested using filter capacitors >=3300uF on each rail. I'll post a schematic with the power supply section when I finish it.

Edit: Thank you very much for the support!

[J M Fahey]

Yes, that transformer will be fine and have some extra power to spare, good choice.
Use 2x4700x25 caps; and if you want, doubling them won't hurt at all.
Going beyond that will be overkill.
The raw 16+16V rails are perfect for experimenting.
You can bridge your 2040's into a single 8 ohm speaker, if that's all you have.

[delus]

So far I've made a schematic for the overall power supply of the amp.

On the AC side I placed a DPDT switch that connects the Live and Neutral wires from the wall and a fuse on the Live wire before the transformer.

On the DC side:
- I use a standard bridge rectifier to convert ACto DC.
- Optional 10nF caps across the pins of the rectifier. Are these necessary?
- 2x4700uF caps on each rail.
- A bleeder resistor placed between the rails. As for the value, I'm not sure yet, I just saw the particular value on tube amps. I'll propably change the value and place the pilot light there, too.
- C9-C12 were taken straight from the datasheet of the TDA2040.
- On the positive DC rail, past the large filter caps I placed 9V regulator taken straight from tonepad, to supply the preamp and any buffer I may need to place. Is C17 absolutely neccessary, or are C5 and C6 enough?

I hope I have the Caps' polarities correct.

[teemuk]

Optional 10nF caps across the pins of the rectifier. Are these necessary?

Some older rectifier diodes used to generate switching noise that bled from supply rails to the signal path or caused radio interference. Is the circuit noisy/noisier without those caps, does it cause interference when near other equipment? There's your answer.
They are sort of parts that may not matter at all if omitted but tend to be good design practice to fit in anyway. The cost of four 10nF caps is peanuts.

Is C17 absolutely neccessary, or are C5 and C6 enough?

C17 isn't neccessary. Somekind of filtering in close vicinity of the regulator's input pin might be though, depending how long wiring there is from the main filter caps to the regulator.

[delus]

Somekind of filtering in close vicinity of the regulator's input pin might be though, depending how long wiring there is from the main filter caps to the regulator.

Not long. They will be on the same board.

Ok, so I have a "good-to-go" on the power supply! I'll report back when it's done.
Thanks again!

[joecool85]

Somekind of filtering in close vicinity of the regulator's input pin might be though, depending how long wiring there is from the main filter caps to the regulator.

Not long. They will be on the same board.

Ok, so I have a "good-to-go" on the power supply! I'll report back when it's done.
Thanks again!

Looking forward to seeing your progress.  Are you making your own PCBs?