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Basic Electronics » line-level output, regulator ICs and op-amps??

March 31, 2012
by JamesFysh
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Since picking electronics up about a month ago, I think I've started to get a fairly good grasp on some of the basic concepts. I'm not ready / interested in MCUs just yet, at the moment I'm really really keen to learn the analog stuff, because I'm getting really interested in analog synthesizers (stuff like the Korg monotron fascinates me. Plus they give you the schematics for free!).

I'm still trying to wrap my head around how I'm going to build my own synthesizer (making an oscillator from a 555-chip is one thing, making "effects" via numerous oscillators and other circuits to actually make something that sounds good... hmm). Anyway, there are a few questions that have been rolling around in my head for long enough now that I think I need some hints from someone with a little more experience. So...

  1. Is there an IC available to take +/- 15V and convert it down to (consumer) line-level, which is just shy of .9V peak-to-peak (that is, +-0.447v)?

  2. Is there any way to build a protective circuit that I could put between some experiment I build and an amplifier, to protect the amp in the inevitable event that I make a mistake? Happy to blow up some ICs on my breadboard, not so happy to ruin a perfectly good home stereo amplifier. Or maybe I should just build a basic amplifier for testing with? I certainly do want to understand how to build amplifiers.

  3. TL072 op-amps.. I could not find anything in the datasheet (or I don't know enough to know what to look for/how to read the datasheet) to figure out how much current these chips can handle. Are they suitable for building an actual amplifier to drive speakers? Or are they just suitable as a pre-amp (i.e. to bring the output of a microphone/instrument up to a suitable level for an amplifier to use)?

  4. 7805T and 7905T: The 7905T is referred to as a Negative regulator (7805T is a Positive regulator). Does this mean I can use on of each of these, driven by an e.g. 9V battery or other DC source, to obtain a ground, +5V and -5V? Or is there more to it (i.e. would I need two isolated DC sources)?

One other thing; I salvaged a little transformer from an old clock-radio and I'm wondering a little bit about it. It's quite handy because it has a power cord running right into it that is fully insulated and probably impossible to electrocute myself on. There are then two leads (orange and yellow, if that means anything?!?) coming out of one side and giving about 7Vpp AC, and 4 (black, red, white, blue) on the other side, giving (black, red) ~30Vpp, (black, white) ~35Vpp and (black, blue) ~50Vpp.

I built a little bridge rectifier, and smoothed the output with a 1000uF electrolytic and had a lot of fun looking at the output on the scope at each stage!

Anyway, it's about 1" by 1" by 1", and I'm just wondering, what sort of current is it likely to be able to handle? I think it would make a nice little power supply for a project (I'm thinking of putting the output into a 7812T and 7912T to hopefully obtain +/-12V DC). I'm not sure how much current I'm likely to be drawing, probably less than 1A?

Anyway, this post is getting long and rambly.. I'll keep some questions for next time :)

March 31, 2012
by Akeshish
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Hello James,

Im currently finishing my junior year in electrical engineering and understand almost all your questions but i will let some of the more qualified remembers answer some of the more specific questions. ( i want to get involved in the community :) so i will give you my 2 cents)

For questions #2, why not try using a fuse? You know what current you should be pulling and if you by chance create a short the only down side will be the fuse will blow.

April 01, 2012
by JamesFysh
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Hey Akeshish,

I did think about using a fuse, but was unsure if you can get fuses in such small values (I think it would have to be just a few milliamp, if that??). Maybe that's just 'cause I've only ever seen car / house fuses and they're usually rated at 3A - 25A :)

Also, to answer question #3 myself - I noticed on one of the TL07x datasheets that I found, that there is an output rating given, in milliwatts. So obviously the TL072 chips are not going to be suitable for constructing an amplifier, only for performing (low-distortion) pre-amplification.

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