New member saying hi and a simple question about amplifying a musical instr.

[steve f]

Hi All!
I new here. This is my first post. This looks like a great site and I'm looking forward to talking tech with you all and learning much here.

I have a BS in EE but I've been working doing power system controls for more years than I care to admit to. I've been getting the bug lately to hobby with audio, TV and perhaps ham radio again ( I had a tech class license years ago). I still understand all the basics but scratch my head when looking at more advanced audio/ radio stuff. I did a few cool projects with pic's about 8 years ago too.

OK, here's my first question...
I have an old Fender Rhodes electric piano. I want to build an amplifier to use with headphones. The keys hit tuning forks that vibrate near a coil - just like a pickup for a guitar. I'm sure there are numerous circuits readily available online but I want to design one myself. That's half the fun right?

What's the typical voltage a thing like a guitar pickup puts out? I just tried putting a meter on the cord from the piano while banging the keys but couldn't read anything. Don't know why. I also tried looking at it with an O-scope. Still didn't see any voltage but I haven't used the scope in so long I might be doing something wrong. I'll re-read the owners manual for it tonight. Also how much output power into the headphones should I design for?

Can anyone recommend some way of learning audio/ TV electronics geared for someone like me who doesn't need to start from the basics of Ohm's Law and how transistors work, etc. ?

Thanks !
Steve

 

[flat5]

I'm guessing 5 to 15 milivolts at an impedance of a few hundred ohms. Your 'scope should see it. Start with an opamp so you can tailor the gain and impedance as needed. Follow it with an IC power amp.

0.05 watts is safe for most real headphones but might be too much for "buds". It is probably too loud at that power anyway.

To learn more, Google on specifics and check the FAQ in this forum for books.

 

[steve f]

Thanks ! I will search the FAQ section. I also saw another thread sort of similar to what I want to build.

Now here it is, my first post and I already have to admit I did something stupid. I couldn't read the voltage on a meter or scope because I had the volume turned all the way down on the piano. Atfer I turned it up to full volume and banged on about 8 keys I saw a spike up to about 75 mv on the multi-meter.

 

[Mikebits]

What's the typical voltage a thing like a guitar pickup puts out?

100 millivolts or more.

 

[audioguru]

Some guitar preamp circuits say that one volt RMS is possible for a guitar played loudly if the preamp has the recommended input impedance of 1M or more.

 

[Mikebits]

Some guitar preamp circuits say that one volt RMS is possible for a guitar played loudly if the preamp has the recommended input impedance of 1M or more.

It depends on the pickup. High powered do reach a volt at the expense of sound quality. Pickup flavors are offered in low, med and high output. Head bangers most likely go for the latter. My first home made guitar used med power humbuckers as I like the noise cancelling that humbuckers provide.

It is a matter of opinion I suppose.

 

[flat5]

Design the preamp for very soft playing allowing headroom for loud peaks.

What max dynamic range is easily achieved from a preamp?

 

[Blueteeth]

When doing my wireless guiltar project in uni, I measured the output of my SG up to 2V RMS...but then again, it could go very low. That was with a simple impedence buffer after it straight into an oscilloscope.

For your app, I would experiment with opamps, and power amplifiers.

I would say an opamp with variable gain and high input imepedence, followed by an audio power amplifier, such as the 'ol LM386.

The order would be 'opamp buffer -> opamp preamp (inverting if you wish to boost, or attenuate the signal, non-inverting if you only wish to amplify it, positive gain) -> headphone amp.

There are many example circuits of all the above three, so it would just be a case of experimenting, and plugging them altogether. I wouldn't worry about any form of EQ, its gain you need to work out.

If you really want to get fancy then you could add a simple 'clipping detector' to see if your gain is too high.

Blueteeth.

Ps. Gotta love the fender rhodes...I could spend all day playing one of those..

 

[steve f]

Thanks again. I'll dig out the data sheet for a LM386. I think I have a few laying around in the parts box from a project I did a few years ago. I'll play with some numbers and do a schematic and post it for comments this week sometime.