The "Forum Amp" design thread

[Taylor]

Here is the schem.

http://musicpcb.com/wp-content/gallery/pcbs/tiny-giant-schematic-copy.jpg

Parts list:

Quantity-part

Resistors
all 1/4 watt

1 - 100k
1 - 220k
2 - 1m
2 - 2.2R   [Marked "2R2" on the board - this is 2.2 OHMS]
1 - 1k
1 - 120R  [120 OHMS]

Capacitors
[cap types are just suggestions - if you prefer something else and you know what you're doing, feel free to substitute]

1 - 10uf electrolytic
3 - 100n film
2 - 1uf film
2 - 220n film
2 - 22uf electrolytic
1 - 470uf electrolytic

ICs

1 - TDA7240A
1 - LM338T
1 - TL072

Power supply:
Detailed info will be available in the PDF. You should be able to get a good supply for around $10 on ebay.

Q. Why did you use a dual opamp when only one opamp is used?

A. 2 reasons: dual opamps are much more common, so people are more likely to have them on their bench already, and because the TL072 is usually cheaper than the TL071 (by a couple of cents).

[phector2004]

Would heatsinks be a good optional component? i.e. for larger enclosures
Any suggestions?

[Taylor]

I'm using the enclosure as a heatsink. The reason is that unless you're using a much larger enclosure, you'd need to somehow cut a square in your box to have the heatsink sticking out. If you were going to build this in a wooden head case like a tube amp, then heatsinks would make sense. Probably best to have the sinks exposed to open air, rather then sealed up inside a box, so the ambient air can dissipate heat. Mounting to the enclosure is just a convenient solution to these issues.

This info will be in the PDF, but it's important to note that the regulator heat sink tab MUST be isolated electrically from the heat sink.

[tasos]

will you make a pcb for it?

[tuckster]

will you make a pcb for it?

http://musicpcb.com/future-projects/
I do think so

[tasos]

so i have to buy the pcb?isn't there a pcb ready to print or just the pcb in a picture?

[Gus]

Why are you using a regulator?  You might be able to use a power resistor and a cap (RC size the cap for feel)to drop the voltage and this might help the sound if it is used for guitar by adding a sag to the power supply.  If you do that add a resistor and cap at the PS input to the 72  decouple the 72 from the power amp rail a little.  Think of it like a Y to split the power two ways

I would change the input to a voltage divider 10K, 10K with a cap at the node to ground and a resistor from that node to the 72 + input.  This will allow you to easy change the input resistance you could even make the input resistor a 100K with a 1meg pot.  You might want to add a series resistor at the input(all part of making things work in the real world)

You could also use a lin pot for a volume control select the value to taper it  to a log/audio curve with the input resistance of the amp chip(70K from the data sheet).

[Taylor]

Although I appreciate that, the boards have already been ordered, so too late for any changes. The amp works exactly as I want it to, so I don't really see any reason to change it. It's not only for guitar, so I don't want sag. Again though, thank you for your suggestions.

[calpolyengineer]

Does this design really need a LM338T, or will a LM317T suffice?

[Taylor]

317 does up to 1.5A... TDA7240a says max repetitive current is 3.5A. I will try to measure actual current when in use in the real world, but I'm guessing the 317 will not be enough.

[Johan]

powersupplies to poweramps are usually unregulated as it means more components that needs cooling and sometimes/usually limit the possible output of the amplifier..wastefull use of energy..but I guess for such small and simple design it might be the better thing as you did.
personally, I'd just hook up the power chip to 19volts and regulated the TL072 voltage with a resistor a Zener and a cap...good job non the less..
J

[Ice-9]

Have the PCB's been finalized and ordered,  using the second part TL072 op amp could have powered another TDA7240 for stereo, that would be a nice option,  if only mono is wanted the extra components may just be left out.

[runmikeyrun]

wow, this is great, thank you so much for designing this!!  

Now, a couple of questions, please forgive me if they have been answered in one of the preceding 11 pages that i didn't read...

Do you recommend any certain type of tone stack?  And can I insert it right before the volume control?

How "flat" is the frequency response?

Would this play nice if i used a tube preamp (valvy) instead of the standard SS one in the schematic?

What are the approximate power ratings at 16, 8, and 4 ohms?  Do you think i'm ok driving a 2x12 4ohm cab with this?

Thanks again, great project!  

[Taylor]

powersupplies to poweramps are usually unregulated as it means more components that needs cooling and sometimes/usually limit the possible output of the amplifier..wastefull use of energy..but I guess for such small and simple design it might be the better thing as you did.
personally, I'd just hook up the power chip to 19volts and regulated the TL072 voltage with a resistor a Zener and a cap...good job non the less..
J

True, but most power amps are run on traditional linear power supplies. This is meant to run from a laptop SMPS, which will already be regulated, so I don't think it will limit the output. If we powered the chip off of 19.5v then we'd just have to get rid of al of that excess heat anyway, and risk damaging chips.

Have the PCB's been finalized and ordered,  using the second part TL072 op amp could have powered another TDA7240 for stereo, that would be a nice option,  if only mono is wanted the extra components may just be left out.

The idea here is that if you want stereo (as I do) you can use a second board. "Wasting" a single opamp makes more sense to me than wasting all the extra board space necessary to add an extra power amp to each chip, when most people won't use that.

wow, this is great, thank you so much for designing this!!  

Now, a couple of questions, please forgive me if they have been answered in one of the preceding 11 pages that i didn't read...

Do you recommend any certain type of tone stack?  And can I insert it right before the volume control?

Note that I will be doing a different PCB with integrated EQ and clipping preamp, so you may want to wait for that. If you want to add your own EQ, I would recommend that you use an active EQ and do it before the amp's preamp. Using a passive tone stack after the opamp but before the amp could cause weird loading issues I would think.

How "flat" is the frequency response?Would this play nice if i used a tube preamp (valvy) instead of the standard SS one in the schematic?

What are the approximate power ratings at 16, 8, and 4 ohms?  Do you think i'm ok driving a 2x12 4ohm cab with this?

The datasheet doesn't show the freq response, but it's down 3db at 30 hz and 25khz. I think we can say that it's certainly flat enough for guitar/bass/key use. It's meant for reproducing full range audio, so it will be flatter than just about any stompbox.

If I were you, I'd use the opamp stage still, and run your tube preamp into that. 20w at 4 ohm, 12 at 8 ohm. A 2x12 should be good.

[runmikeyrun]

awesome, thanks for the input!  i'll wait for the board with the eq.  As for the valvy, we'll see how it sounds first.

[therecordingart]

I'm dying over here, Taylor.....need PCBS!!!!! haha

[bassmannate]

Just got an idea...If I wanted to, could I use the other half of the dual op-amp to feed a second power amp chip? I realize that I'm not saving much in the way of cost ($0.80?) but I guess it has the potential to save some space if I wanted dual outputs.

[davidallancole]

Hi Taylor,

I was wondering if you considered this module at all:

http://www.electronics123.com/s.nl/it.A/id.325/.f

Different pre-amps would work well with this depending on what you need.  A high impedance buffer followed by volume control right into this module would work well for a pedal amplifier.  The module also works from a single sided power supply, so your use of a wall wart would still suffice.  You can get ~ 18 W with a 13.2VDC power supply and a 4 ohm load.  It would just need to be added to a pretty box then.

[armstrom]

You can spend a little less and get a 15W power amp kit like this: http://store.qkits.com/moreinfo.cfm/FK607 $10 is tough to beat.. and it will work with a run of the mill 12V wall wart.

I can vouch for that amp kit. I built one with a LM386 as the preamp and it sounded great. Nice and loud with a 6" weber signature speaker in a custom combo cabinet. Taylor's design is going to be better in terms of having a clean guitar-friendly preamp already integrated on board. If I wanted a clean guitar amp I would probably choose taylor's design.

[davidallancole]

That was what I was alluding to.  One of these kits may be an easier/cheaper method.  Pre-amp boards could be built up from a buffer/volume board to a full blown pre-amp and would then just need to be tacked onto the power module.