a couple of questions about a lm1875 guitar amp

[i2k92]

Originally Posted by neebster

14vac. Perfect.
I have a very large heatsink I intend to use, taken from an old broken amp.

I was concerned about the tone of the paia unit. I may go with five bands.
How about 5-Band Graphic Equaliser Kit (1044)

I don't think the freq selection on that particular EQ are well suited for guitar use. Check Boss GE-7 pedal, the freq are selected primarily for guitar tone shaping. You'll need to replace the opamp to remove the noise, after that it's a great & cheap tone shaping tool.

I still recommend Sansamp GT2 for preamp, drive/distortion and simulator, you can buy or build it (very popular in DIY musician scene). It's a proven simulator. The tone shaping if very good. Except if you're a tone god like Eric Johnson or David Gilmour, you won't need any EQ behind it.

One thing that will be difficult to achieve with SS amp is touch sensitivity (pick smoothly > clean, pick hard > overdrive), it requires tube amp or at least very good preamp.

[neebster]

OK, danielwritesbac. I'm sold on the idea of the TDA7294. Looking at the graphs on the datasheet, for 15w it looks like I will need an input voltage of 12v. Does that mean I need a transformer with a secondary voltage of 12/1.45=8.3v?

Thanks for all your help, guys!

[danielwritesbac]

Originally Posted by neebster

OK, danielwritesbac. I'm sold on the idea of the TDA7294. Looking at the graphs on the datasheet, for 15w it looks like I will need an input voltage of 12v. Does that mean I need a transformer with a secondary voltage of 12/1.45=8.3v?

Thanks for all your help, guys!

Less than a headphone amp?

Well, no. That's backwards. You're not building a tube amp either. See post #6 (headroom for live sound dynamics). And, as is typical for solid state, you run the voltage up high enough to avoid clipping but not high enough to roast the parts.

Transformer for TDA7294 with a 4 ohm speaker is a 22, 22 dual secondaries transformer or a 44vct (22,0,22 center tap). You could use a 20, 20 dual secondaries or a 40vct (20,0,20 center tap).

Your clean 15w for LIVE DYNAMICS from TDA7294 is something that it can barely do at full blast, so don't cut its voltage requirements--see the datasheet. You definitely don't want to use 100w all the time, but if you should happen to strike the guitar strings slightly harder, your 15w demand goes beyond 60w each time you do that. Headroom is the governing concept with solid state watts.

Once again, from post #6, don't forget to add 220uF onto both of the predrive Power pins for clean and cool operation. See the datasheet for pinout. http://www.st.com/internet/com/TECHN...CD00000017.pdf <--link
EDIT: With the Quasar Kit, you can simply locate the added 220uF caps underneath the 2200uF caps that come with the amp board.

[danielwritesbac]

Quasar Kits offers a matching power supply board in two sizes, but the 4 of 4700uF is the one you'd want for your project.

P.S.
Let's add a pair of 4.7k resistors and a pair of LED's to that power supply so you can tell if its on.

[neebster]

Well, if you're fairly certain that having up to 60 watts at 4 ohms won't blow my 15 watt guitar speaker, then I will go ahead with these components.

Thanks for all your input guys. I will post back when I have pics.

[danielwritesbac]

Speaker protection is part of design.

You can certainly under-size the input filter cap to restrain the amplifier to the operational bandwidth of the guitar. This bass blocker will protect the speaker. Little polyester caps, which are great fun anyway, are certainly good for guitar amp input caps. They are very cheap. Try several sizes, solo and in combination. This part is fun!

You can also add an output capacitor, series to the speaker, as a sort of bass blocker (exactly a speaker crossover component). The Nichicon PW family make excellent clean results. Its also guaranteed DC protection so that an upset amplifier doesn't kill the speaker from offset. Its the least expensive DC protector.

There are many examples of 15w rated speakers in high power applications when using a smaller bandwidth, such as the Tang Band 6-1/2" hi-fi woofer employed as the midbass in 200 watt party speakers.

TDA7294 is no more, no less able to blow up a speaker than LM1875. Voltage, use, current, and design govern that factor.
However, in high demand live sound applications, TDA7294 is less likely to blow up itself --its more sturdy and its a 4 ohm rated part.
I'm saying that while both chips could drive that speaker, TDA7294 can withstand more exuberance.

[neebster]

I apologize for my limited knowledge regarding this. My electronics experience is limited to replacing parts on some Fender amps. So I can solder, follow a schematic, and not get zapped by big caps, but as for engineering knowledge .

So, I am wondering, if I use the output capacitor, should I still use the reduced-value input filter capacitor?

And also, really for both of these, but especially the output cap, what is a good set of values I can start with?

Guitar is what would be used, so we don't need frequencies below 82.4 hz (the lowest fundamental).

Again, thanks so much for your help with this. I'm already working on the enclosure!

[danielwritesbac]

Okay, over at the radio shack is a nice little collection of green polyester caps. You can get the 0.01uF (10nF), the 0.022uF (22nF), the 0.047uF (47nF) and the 0.1uF (100nF) to experiment with solo and in parallel combination for input cap. And, you might as well get their 0.22uF (220nF) because those are generally useful for output zobel.

You're going to want their biggest bag of 1/4w carbon resistor variety pack too.

Their Gel Flux works to help fix errors and to help solder stubborn parts--excess washes off with a drop of dishwashing liquid in a small container of 90% alcohol. This makes for beautiful easy soldering, after the mess is removed. Its worth the slight bit of trouble for excellent solder connections that don't accidentally run together in tight spaces. Its also a heck of a lot faster. . . except for the cleanup.

The white thermal paste there, will also be useful--scrape/squeegie it on thin.

As far as an output cap for DC protector and bass blocker, a place to start is 470uF (Use Nichicon PW or any "low impedance" model if you can). You can try adding (parallel) another electrolytic cap to that for slightly different effect. Some of the little polyester added parallel to output cap can make a difference to treble. Really its going to take some on-hand parts for experimentation as to the best output cap. Do this as the last step, when the amp is up and running and you can very easily make these decisions.

You can make the "fingers off" lamps with a pair of 4.7k resistors on CT (0v line) hooked series to LEDs, one LED for V+, one for V-. Just observe that the polarity of the LEDs is going to be the same as the nearby caps. If you decide to cable these LEDs to the front panel, use a twisted wire, such as a couple strands of cat5. These will continue to light as they drain the power supply down to about 5v. When the light is out, the power supply is still able to shock, but not with the force of an arc welder.

That kit has a slight problem with the circuit board--just a simple matter of too much board. The simplest fix is to mount the board to the side of the heatsink so that the excess board doesn't block the airflow. They've apparently made the kit so that a stereo pair mounts one on each side of the heatsink. Well, you'll see it. The good news is that it attaches very securely so the chip can't spin off the insulator. I'd put several washers between the board mounting holes and the heatsink to avoid direct contact with that hot heatsink (no need to get the circuit board so hot).

Other option is to take the 1/2" drill bit and well ventilate that huge excess of board behind the chip so that air isn't blocked from the heatsink. You'll see it. That board is extremely strong too.

Be sure to allow yourself some under-board space, since, if you want clear audio from that kit, you're going to have a pair of 220uF stuck underneath, right under the kit's 2200uF. The kit is really very bad without that upgrade, so be sure to buy the pair of 220uF. Again, Nichicon PW, or the famous Panasonic or any "low impedance" cap will be better than a random choice, and that's true at any small signal areas, including this "signal support" area of the power circuit.

Enclosure needs cool air intake underneath heatsink and hot air output above heatsink. So be sure to plan for that.

[danielwritesbac]

Got your kit up and running yet?
I've got one of them and could go through it with you, if you want.