Nabo's Mini-head/Talkbox Amp - Builder's log / Q & A

[NaBo]

Inspired by Paul's cool photojournal type pages, I thought I might make this thread for whatever questions I have that deal with this project (I've started too many for individual ones already  :?), and just to let people know how I've gone about this.

It's based on a Crate GFX-15 practice amp.  Replaced the transformer, rehoused, and added stuff to make a head-like amp that can be used to drive a cab, or set on the floor to switch to a talkbox.

Here's my diagram of what the finished product should look like:



Build photos tomorrow!

[aron]

Nice!

I made a talkbox long ago. I used some sort of poisonous PVC tubing. I nearly passed out from the acrid taste onstage!

[NaBo]

Thanks Aron!  ... and...  :shock: .... I better watch what I use for tubing!

Funny story-  I made all the sizing measurements with a ruler we had in the house, and gave the measurements to my dad.  He measured the wood with a tape measure out in the garage, and when I came out and measured, I was like "what?!?!?!  you're off by almost a 1/4", everywhere!!!" ... after a bunch of head scratching, we found the problem.  The ruler was long  :roll:...  Never buy anything from Buffalo.

Anyway, after straightening out the problem and recalculating the dimensions I need, we cut all the wood today.  It'll be 18" long, 5 1/2" wide, and 4" tall... so still a decent stomping height.

I've decided how to group the pieces for aluminum foil shielding.  The front and bottom will be covered in one piece, grounded by the input jack.  The back and sides will be together, grounded by the output jack.  And the top will be by itself, grounded by the speaker jack.

I'd like the top to screw off (where the most stuff is anchored, like the chassis and transformer) so I can still tinker with it and repair it after it's finished, so, I ended up having a problem with my plan to have the AC cord out the back panel.  The cord is gonna be permanently fixed to stuff anchored to the top, and the back is gonna be permanently fixed to the bottom.  My solution to this is probably gonna be having the cord enter right at the seam of top and bottom, or even through the top... unless someone has a better suggestion?  Are there 3-prong panel mount AC jacks I can remove from the inside but screw into place from the outside?

[spudulike]

Nice!

I made a talkbox long ago. I used some sort of poisonous PVC tubing. I nearly passed out from the acrid taste onstage!

Try the tubing they sell in home-brew beer shops.  :wink:

[NaBo]

This evening I was pondering exactly how the switching should be done... I thought "Hmmm... i have two live amps here.  If there's no guitar connected and the input isn't grounded, that will equal serious hum.  If I only ground the talkbox amp while it's bypassed, that's not good enough.  Normal stompbox wiring need not apply.  I'll have to ground both inputs when they aren't being used.  And how shall I power that bypass indicator LED?"

Here's what I came up with:



Not sure of current limiting resistor... that's just what the power indicator LED's is.  I'll have to see what works best with the ones I have.

If all that's right I will be pleased.  Confirmation please?  

[NaBo]

WELL, sanded everything, drilled holes, contact cemented aluminum foil on the inside, and glued/screwed most of it together.

Since the wood is too thick for jacks and switches, I drilled some big holes through it and im gonna screw some metal on.  This will also be resonsible for grounding the aluminum shielding.

It's turning out pretty nice.  I'll have to remember to take my camera out to the garage tomorrow.  I'm hoping to get the painting done tomorrow, and start wiring it up.  It looks kinda cool all sanded... unfortunately its just veneered particle board so the cut edges dont look so cool.  I might decide to paint all the half inch edges and do the rest with a candy colour or lacquer.  I'll have to see what we have around here.

Finally some pictures tomorrow.  If I'm real ambitious, maybe of the finished product, too.  Then maybe someone will wanna read this  

[spudulike]

People are reading this  :wink:

The wiring diagram looks ok at first scoot - grounds and signal routing look ok to me. Looking forward to pics of an interesting project (fuzz boxes get tiring after a bit  :lol: )

[NaBo]

LOL!  Good to know... I sometimes wonder if anyone is actually finding a good use for the stuff I put out there.  I'm not the sort who needs regular praise to continue making my work available, but it's still nice to hear someone's at least reading every so often  :wink:!  Thanks!

Without further ado, here's those real live pictures I promised!  All shielded with the bits of metal (from a tea can and altoids tin ) affixed behind the holes.





I'm about to glue & screw the front/bottom piece to the back/sides piece, and then I'll do a little test-fitting.  I'll snap a couple pics then before slapping paint on it.

I'm not gonna get to show it all painted tonight however, since I'm painting two colour... but tomorrow it looks like the structural elements will all be in place and good to go.  Then it's just a matter of etching a little fuse board and wiring it all up!

[NaBo]

WELL!!!  Everything but the top is now one piece!  Only got as far as the mockup tonight since daylight and ambition ran out on me at 10.  Pictures speak a thousand words, and I've said too many myself in this thread, so here's 10 more... i know its a lot but i hand picked them from about 50  ... I'll put links instead of the pics:

1 - Together at last
2 - Lookin pretty squared away there, soldier
3 - How almost everything mounts to the top
4 - Oh, now thats just beautiful
5 - Nice face, buddy!
6 - I'd love to turn you on.  The hole beside it shows the original power indicator led
7 - I see you in there!  I can still manipulate your knobs though!
8 - Stompbox or head?  ... BOTH!
9 - That's right.  Nicely protected stompy.
10 - Baby's got back.  That hole near the top is where the power cord will be, and it too will be anchored to the top piece with a homemade bracket.

Tomorrow, we paint.  I have white, green, and yellow at my disposal.  I think i'll go with white on the faces, green on all half-inch sides.  It'll look like this............. unless someone can suggest a better a better colour scheme before tomorrow (its a .bmp so you can fill in the colours and post if you want  ).

[NaBo]

Well, with each coat of paint taking a minimum of 2 days to dry, I've had plenty of time to just sit and ponder this project.

I've read all about the dangers of turning on a tube amp without a load connected...  and I've read that solid state amps are usually okay without a load...  But I've been thinking, since this thing had its speaker connected inside the chassis, it's conceivable that the people at crate never intended for it to be run without a load.

If you look at the schem here, there's three output locations- the headphones jack, the external speaker jack, and then where the original speaker clipped onto the board.  The first two are switching jacks so when nothing is connected to either of these two, it would go to the original speaker (of course).  I was thinking maybe I should use a low value, high wattage resistor as a dummy load on the original speaker out, just to be safe.  Perhaps someone more knowledgeable about this could let me know if this is necessary with my particular amp?

[NaBo]

Hmm...  Well I got impatient and bought a 10 ohm, 10 watt resistor from radi-  i mean circuit city today while killing time at the mall.  Should i use it in this case or not?  I'm sure it cant hurt, but is it necessary?  Someone?  Please?  Answer a poor soul's question?

P.S.  Started on the green today. Paint is taking forever to dry, about 3 or 4 days.  It's messy, so im counting on it sanding nicely.

[illphil]

Im also looking to build a talkbox and am wondering the best way to do this, i would preferably like to make a powered 1 so that there is not hassle with either another amp or safety.

Was wondering if anyone had any good advice for this, so far from the forums it seems that most people are using seperate amps for their talkboxes if this is the best way then i may consider it but want to know the best kind to use.

[NaBo]

hrm... not quite sure what you mean.  a powered one?  do you just mean the amp and horn driver in the same enclosure?  or a mic and preamp in the same enclosure too?

There's tons of info and links at //www.generalguitargadgets.com, just click talk box under filters.

It's best to have a dedicated amp for several reasons - switching loads on an active amp is dangerous or tricky business, plus your driver probably wont be rated for as high a wattage as your speaker cabinet so you'll likely need to use an attenuator (or low settings)...  A dedicated amp, say, 10 - 30 watts should be perfect - you only need to get it as loud as a speaking voice since theres a mic right by your mouth anyway.  With this way you're only A/B switching the inputs to two amps, instead of the load on your main amp.  Easier and safer.

As for my project - the paint job turned out like crap :evil: ... I stripped it all and am just gonna paint it all white.  Looks more "elegant" that way I think.  I have about a week off, so hopefully it'll be completed before I start working again.  Hopefully... cuz then I gotta finish my cabinet before school kicks up again.

[illphil]

Thanks Nabo

I did mean the amp and horn driver in the same unit, i have a seperate pa system, i have a choice of 2 amps i could modify to use, 1 is an old tiny guitar amp and the other is a 30w bass amp. Im just wondering really if its possible to make it with anything smaller than a combo style head really.

Would it not need to be a bit higher than a speaking volume to be heard over everything else, my mic is set up for singing.

[NaBo]

Uh... I guess it depends on how loud you talk or sing?  I've hooked up my ruby to the compression horn and it worked... and that thing's only a half-watt.

Anyways, painting is done, and it looks pretty good.  All the wiring is complete... but... this thing is popping fuses!!!

I first tried it with a 1.5 amp fuse... the on/off LED lit up, then it went off and i smelled the unmistakable odor of a burnt fuse.  So i took it out of its box, shoved in a .4A fuse (just cuz i have a bunch) make sure all the volume/gain pots were turned to 0... plugged in the original speaker, plugged in a guitar...   turned it on... no noise from the speaker, no led, burnt fuse.  Tried a 1 amp fuse outside of the chassis too, with the dummy load resistor removed... same thing

I'm 100% sure the wiring going to the board is correct...  I think something prolly happened to the on-board components the first time this thing had its fuse blow (the time months ago with the internally fused original xformer...).  Any clues as to what it could be?

I think I'm gonna try calling Crate people tomorrow and asking what the original transformer was fused at... just to be sure... blah.  I hate when things dont work the first time!

[aron]

Good luck. I hate when things break.

[NaBo]

I'm about ready to give up...  i took a close look at everything and the only thing that looks bad on the board is the very heart of the amp, the TDA2030.  The plastic casing looks a bit bulgey and three of the legs have that brownish singed look to them.  I don't think thats normal.      

And I have a confession to make.  I just tried it unfused... albeit "carefully"...  (i know, still not smart, considering how much i payed for the new transformer, not to mention all the other parts that could splode) and D13 and D14 started smoking so i immediately turned it off.  I have no problem replacing recitifier diodes, and the TDA2030 with a brand new one... but could that amplifier really be the ONLY problem?  The TDA got toasted the first time (aka the thermal fuse in their original transformer didnt protect it...), and it is now in such a state that its maybe shorting the power supply and drawing too much current (like, infinite amps...) for a reasonable fuse (or the diodes) to handle?  I guess i just need a nudge in the right direction here... how should i proceed?  The only thing that I could do myself with any confidence that it'll be fixed is pull every single component and replace it with new ones... i just dont know any better.

It's times like this i wish i actually knew what i was doing... instead of just being an ambitious chump who can read some component values and diagrams, with a club-tipped cheap soldering iron in hand, and loads of spare time.   :roll:  

[R.G.]

A quick reading of the datasheet and your schematic points to a couple of things.

(1) is the heatsink tab of your TDA2030 grounded? Does it connect to the chassis and signal ground, perhaps unintentionally? Check with an ohmmeter. If it does, the chip is guaranteed to die, 100% certainty, if you power it because you're shorting the negative power supply through it. Use your ohmmeter and check it out.
(2) your layout might be killing you, or some flaw in connections, by causing high frequency oscillation.

Check out (1) first, then check back.

[NaBo]

:shock:

That's exactly it... damn this guy is good!!!  There's continuity between heatsink (connected to V- through pin 3) and ground... VERY BAD.  Well it appears in any case that the TDA2030 is indeed quite dead...  now to figure out HOW V- is connected to ground?

Apart from the nifty signal routing and LED wiring... nothing was changed.  The only (intentional) difference in off-board wiring is that instead of having two wires connected to the chassis - 1 from board, 1 from third prong & center tap - I twisted, soldered and heatshrinked all those wires together and ran one single wire to the chassis.  That shouldn't be an issue though............... right?

I'll have to triple check my jack wiring... because im also measuring continuity between pin 4 (output) and pin 3 too...  I also just noticed that pin 4's solder joint looks burnt.  If the jack wiring is alright, then i guess I'll pull the TDA2030 and see if they've shorted inside the case somehow... starting with an already-singed-looking amp chip probably wasnt the best way to guarantee success... :?

Thanks so much RG!  I'm sure I'll get it going soon enough with some guidance from you!

[R.G.]

The reason I went to that one first was that I toasted a few 2030's myself once upon a time.

It can be hard to figure out where the continuity path is. You're right, you can't tell anything about it with a bad chip in there. It's probably shorted inside. Get a new one installed **but don't fire it up** until you can find out where the short is.

The heatsink tab of the 2030 is mechanically tied to something. This is usually one of
(a) an on-board formed sheet metal wingding
(b) an aluminum finned heatsink on the chassis
(c) the chassis metal itself.

The real advantage of (a) is that you can insulate the sheet metal from the grounded chassis, so you don't have to use insulation at the chip. (b) usually requires insulation of the chip, as insulating a finned aluminum thingie is harder and error prone. (c) absolutely needs an insulating wafer on the chip.

You might put the chip in, but do not connect it to the whatever that the heatsink tab bolts to, nor even let them touch. Now use your DMM and see if it shows continuity to ground. If not, GOOD. The problem was contact to the thing it bolts to, and you can use insulating wafers and shoulder washers, etc. to bolt it on but leave it electrically isolated, and test by DMM whether you got it right before firing it up.