First project 1W hybrid practice amp questions...

[GibsonGM]

That's looking pretty sharp, Funk!  Nice work so far.

*Best practice* is to use the plastic "marshall type" enclosed jacks for an amp build.  You would have ONE ground in the amp, and all others travel there, which is called "star grounding" if it's done right.  The panel (chassis in a larger amp) would be grounded by one thing only, your choice which it is.   For your build here, being so simple and not high-powered, I don't think you'll find much  improvement by doing that.....I believe you'll be ok if the jacks  ground to the front panel.   But for assurance, I'd also be sure the input jack and any pots going to ground actually have a WIRE going there (allow the panel to 'get grounded' by touching....don't use IT for ground).    If a jack rotates loose without a real ground wire on it, it sounds like CRAP - "bzzzzzzzzzzzzzzzz!".

Alternatively, you could just cover the back of the panel with something insulative, like plastic film, so it doesn't get grounded by jacks, but you would allow ONE thing to touch it to ground it (so it will act like a shield)...does that make sense?   Again, not a big build, it may not matter much at all but it's good to know about for later builds.   

Carry on, we look forward to hearing it!!  If you still get 16V when the regulator is connected, and it heats up too much (DEF. want a heat sink...), you can add a dropping resistor between wall wart and regulator....

[funkjunk2]

Update,

The Dumpster dive Ruby Tubey was a fail.  Not sure where it went wrong, suspect bad caps in the power supply section.  I don't have a meter to test capacitance. I couldn't get the preamp/tube stage to light. I have to admit, trying to use scrap PCB boards and re-purpose the circuits was probably taking it a bit too far and I should have bought some vero board or something as it was very time consuming to then end an epic fail.

I didn't let that discourage completely though and I have persevered just to get something working so the boy could play.  I figured I would simplify and get as basic as possible until I had something that worked and made some noise.

I backed up to a simplified version of an LM386 amp, not sure what it is called but found it on instructables. http://www.instructables.com/id/Portable-Guitar-or-iPod-Amplifier-Amp-9v-LM3/


Since it was only three Caps, an LM386 and a switch, I figured it would be tough to screw up.  Used a 9V battery as power source.  I used a single 4.7uf cap for the distortion circuit between pins 1-8.

At first I was disappointed as the output from the speaker was very distorted and minimal even for a low power amp.  It seems the speakers we salvaged were not very good for this particular application.  They apparently saw some abuse in previous life.  I hooked the little amp up to an old bookshelf speaker and it came to life.

Knowing the speakers were the issue I swapped them for another set which is working much better.  After that was done, we added a multi-position selector switch and added 2.2uf and 10uf caps in the distortion circuit. (now has a clean channel and 3 levels of distortion)

Not the best sounding amp I ever heard but it is just as good as a lot of the $39-$59 store bought amps and nearly as loud while surprisingly more quiet in the hiss/hum department.

My son is very pleased with the outcome of our little project.

We plan to build on this and make changes and modifications as we go.  I still want to revisit the tube idea as well as time allows.  I will have some pictures of the completed box soon, it still needs some finishing up and we definitely need to work on an alternative to the 9v battery although it is nice to have it completely portable.

Thanks to all who helped along the way and provided insight. 

[Johan]

http://www.diystompboxes.com/smfforum/index.php?topic=107868.20
late in the game, but if you're going to keep experimenting with this, you might take a look at this.( the hand written schematics.  )
With some tinkering, output chip could be any chip amp from 1 to 100 watt
not much more complex than a tube screamer, push pull tube stage.
j

[Renegadrian]

2822 are fun, I have built a couple in the past...I also like to get more power, with the super easy 2030a.

[funkjunk2]

Thanks for the replies and ideas.  We are continuing to tinker with the amp.  As of right now, we have the LM385 in use as a pre-amp stage and to produce the distortion.  It is then fed into a little amplifier I salvaged out of a set of computer speakers.  It is based on the TEA2025B chip.  I have split the input to feed both channels of the amp.  Each channel has a dedicated output speaker.  I'm not sure if this is better than running it bridged mono or not.  Something more to fiddle with.  Our focus right now is relocating the on board power switch to the front panel, and upgrading to the higher input voltage LM386n4 from the one we currently have.  This will allow using a single power supply at about 15V and getting everything properly mounted.  Right now I am powering each stage separately, one off a 6VDC wall wart and one with a 9.6vdc Remote control vehicle battery pack that is rechargable.  End goal is to have a 15V power wall wart for home use and the 9.6V battery for mobile use that he can take to school, etc.

We have some cleanup to do on the switches and pots to get them all matched up and mounted at the proper depth.

I am working on a full schematic and am open to suggestions on how to make it sound better. the 2025A amp schematic is not finished yet.

I tried using a 50k pot between pins 1/8 for distortion adjustment and removing the capacitors but that didn't work as well as the original setup.  Perhaps I needed to leave the cap circuit in place with the pot?  Our "clean" setting is very clean but not very loud, I think I need to add a .1uf cap in that circuit.

Since this is not a "hybrid" project anymore, and tubes would probably be added as a outboard unit or pedal, I think I will retitle a new post to continue.

Here's some pics.

[GibsonGM]

Nice! Pretty snazzy.  You can get some standoffs, and mount those PCB's right to the wood....

[funkjunk2]

Thanks, and yep, I have some standoffs.