Slightly OT: Practice Amp Question

[tcobretti]

I apologize if this is off-topic, but I posted this in the lounge and didn't get a response.

I have an old stereo with some LM1875 chips in it that I want to use to build an amp.  I have some questions about the data sheet and would really appreciate the help.  I've built pedals, but this is my first project that plugs into the wall.  Here's a link to the data sheet.  http://the23rdcentury.com/lm1875.pdf

1.  Power supply - I have a 30v/0.4a power supply from a printer that I was thinking of using.  Will it work, and how do I figure that out for myself?  I also have the transformer that came in the stereo, which is 17.5v.  It has two 17.5v leads and one gnd as the output, so it is an AC transformer, right?  Which would be more appropriate?  I would like as many watts of power from the amp as possible.

2.  On the data sheet there are two schematics titled Typical Applications.  Am I right that the top one is AC and the bottom one is DC?  Which would you guys recommend?

3.  Since I have only worked with pedals, I am unsure when to use higher wattage resistors/ higher voltage caps.  Do all the caps have to be 30v if I'm using a 30V power supply?  Do the resistors have to be 30w if they are after the output of the chip?

4. How likely am I to fry myself with this project?  I've read thru the previous safety posts, but those all seem to pertain to HV tube amps.  Is a 2200uf cap in a 30v circuit enough to shock the hell out of me?

Thanks very much.  I know it gets old answering simple questions that I should probly know the answer to before I start messing around with this stuff in the first place (run-on sentence?).

travis

[Joecool85]

I apologize if this is off-topic, but I posted this in the lounge and didn't get a response.

I have an old stereo with some LM1875 chips in it that I want to use to build an amp.  I have some questions about the data sheet and would really appreciate the help.  I've built pedals, but this is my first project that plugs into the wall.  Here's a link to the data sheet.  http://the23rdcentury.com/lm1875.pdf

1.  Power supply - I have a 30v/0.4a power supply from a printer that I was thinking of using.  Will it work, and how do I figure that out for myself?  I also have the transformer that came in the stereo, which is 17.5v.  It has two 17.5v leads and one gnd as the output, so it is an AC transformer, right?  Which would be more appropriate?  I would like as many watts of power from the amp as possible.

You need +/- 30v DC.  When you rectify AC (all transformers are AC btw), you multiply voltage by 1.4, +/-30v means you have +30v and -30v, so 60volts.  Your 17.5v tranny is just right, and assuming it came out of the stereo you got the chips from, thats why lol.  17.5v x 1.4 = 24.5v x 2 =49v.  That should give you just about 20w RMS on that chip.

2.  On the data sheet there are two schematics titled Typical Applications.  Am I right that the top one is AC and the bottom one is DC?  Which would you guys recommend?

Do the DC one.

3.  Since I have only worked with pedals, I am unsure when to use higher wattage resistors/ higher voltage caps.  Do all the caps have to be 30v if I'm using a 30V power supply?  Do the resistors have to be 30w if they are after the output of the chip?

The caps should be 60v+ caps, you are essentially working with 49volts here.  No on the resistors.

4. How likely am I to fry myself with this project?  Not likely if you pay attention to what you are doing and are safe.  I've read thru the previous safety posts, but those all seem to pertain to HV tube amps. 

I know, thats why I created ssguitar.com

Is a 2200uf cap in a 30v circuit enough to shock the hell out of me?

Yes, it could (in theory) kill you.  But it's unlikely, just don't touch it when it's hot, and drain it before you work on the board.

Thanks very much.  I know it gets old answering simple questions that I should probly know the answer to before I start messing around with this stuff in the first place (run-on sentence?).

travis

My number one advice is to get onto ssguitar.com, this is what we do.  Researching is your friend.

http://www.ssguitar.com

[tcobretti]

Thanks very much for the help.  I didn't know ssguitar existed, else I'd have checked it out first, so thanks for pointing out another repository of knowledge.

Thanks again for the help, I know answering stupid questions gets old.

[Joecool85]

Answering stupid questions is nice actually, keeps me on my toes.  And besides, it wasn't that long ago I was the one with the dumb questions.

**edit**
I should note that you will still need to think about what chassis you want to use, heatsink and preamp.

**edit 2**
Oh yeah, and ssguitar has only been around for a tick over a month, so its not surprising you didn't know about it.  On that same note, we already have 32 members and over 650 posts.  It is already a booming wealth of SS amp knowledge, and is growing extremely fast.