Help on creating a practice amplifier - Missing thread

[morgoth2006]

My old posts seem to be misssing, I'll post here my failures and successes when I get home later in the week.
Cheers

[J M Fahey]

Hi morgoth.
Don't worry, it's normal  :duh :loco
The Forum just recently switched Servers, it was a "moving" experience, a lot of things got lost or at least misplaced (as in real life moving)
In Argentina we have a Popular Saying: "Cada tres mudanzas = 1 incendio"
Meaning every time you move, you lose 1/3 of your belongings, as in a house fire.

[joecool85]

My old posts seem to be misssing, I'll post here my failures and successes when I get home later in the week.
Cheers

I apologize.  As Juan said, it was due to the move.  That thread appeared to have been corrupted due to the move so I had to delete it.

Please, if anything else is missing or not working properly, let me know.

[morgoth2006]

Well, I finnally got some spare time to get back to this. Work has a funny way to ruin all the fun   xP
I got to build my amp, but not without some missaps, as you can see here
I was careless and shorted the output capacitor, instant fireworks... I replaced the TDA2040 with a TDA2050 that I had also bought and the end result is this:

Notice that I still have a 3,5 stereo input jack attached, I used that as input from my Ipod. I connected a 3,3k resistor to the left and right channel and sent it to the input (is that correct or is there a better way?).
The rest of the amp:






I am sorry about the low quality but my camera is at my inlaws and my PDA has a crappy camera.
The cabinet is covered in a tolex like material that I got from a car upholsterer. It's lighter than the one in my fender but it gives a nice look. The faceplate is transparent acrylic but it's not finished, It only has the power switch and the input jack. You can see it in pic nº 4. I will put a sound lever meter (LM3915) with 19 LEDs in the middle and the pots for the preamp.
My largest problem is still the power tranformer. The best thing I have now is a PSU from a HP printer (I believe it was JMF that sugested that one), and it gives 32v and 3,6 A.
The speaker is 10'' 8ohm with unknown watts. The guy I got it from said it should be 120W RMS, but I have no idea. The size of the magnet looks nice so  I don't think the TDA2050 will blow it.

Opinions are welcome

[bry melvin]

You didn't have specifics on the speaker, but usually a 120 Watt 10" speaker is designed for bass/pa not guitar.

With TDA20X0 amps I have best results (with guitar) with Alnico 10" (eminence or jensen) in the 30 Watt range. I usually use 2 of these. They are expensive though. I Think weber signatures are more economical

Actually I AM using that kind of setup currently with a pair of slightly modified Fender Frontman 15g amps (variable voicing DI added) and a 2X TDA2050 power amp (stereo) as a studio amp

With a good preamp you should be able to get some really nice sounds out of your project,

FWIW that Ppwer amp runs the 2050s at 40V and 4 ohms output but with a MUCH larger heatsink which is mounted to the back of the chassis.

The heatsink I used with TDA2050s is Actually a large aluminum one used originally as a passive heatsink (no fan) on HP Pentium 3 business model computers

[morgoth2006]

Actually I think the speaker was from a PA system. I got it for free so it was a good option for me. I can also get some Jensen Alnico 30W for around 50€ (JP10Q). I can't find Weber signatures in Portugal, maybe they are more common in America.
You run them at 40V and many amps? I tested the amp with it's current heatsink for 10 minutes at full power and the heatsink was only barely warm to the touch.
I also have some larger heatsinks salvaged from old PIII, either passive or with fan. If the amp gets too warm I'll put one of those in.

[joecool85]

Looks good, nice job.  I don't see a problem using the ipod as an input device with those resistors.  Although I am curious as to how you figured out those specific values as I don't know how to calculate what would be "right" in that scenario.

[morgoth2006]

Thanks Joe If I had to change anything it would be the depth of the cabinet. I made it 30 cm but maybe 20-23cm would make it more standard.
The resistor value, I have no idea how I got  it  I think JMF in the old thread mentioned this configuration but I can't remember what were his values. I ended up on using 3,3k because I had them on my table and it felt like a good idea.
If someone could enlighten us with the math involved?

[J M Fahey]

Hi.
Not that much math here, simply that it's not good to put two *outputs* in parallel, which is what you would do by joining left and right Ipod outputs, so each of them must have a reasonable resistor in series, both the same value of course.
The value is not critical, but it must not be so low as to negate its effect nor so high as to attenuate the signal needlessly.
Values between 1K and 10K work well.
If you use 1/8 watt resistors they usually fit into any normal "guitar" plug (1/4"=6.35mm).

[morgoth2006]

A couple of weeks gone by and some more work done. It could have been more advanced but my Ibanex RX played a trick on me... it started humming like crazy every time I touched the strings...The amp was quite silent when unplugged but every time I plugged and touched the guitar... and near my quad core... well it looked my old spectrum loading tapes (load "" ftw  )
I ended up building a little lm386 amp with an opamp pre that is battery powered just to rule out mains interference and that one hummed too so I decided to quit the laziness, and completely stripped my guitar and redid all the wiring. For good measure I shielded all of the inside with conductive paint and aluminum foil.
I am now quite happy, finally its quite silent, except the volume pot that has reached the end of it's life. I've allready ordered some from ebay that should arrive in 2 weeks.
A bit offtopic: I noticed some loaded pickguards from china for like 20€, and I'm tempted to get one. Has anyone ever tried one of those? What looks too good to be true usually is but 3 pickups, 3 pots, 3 pot covers and the pickguard for 20€ is damn tempting... I might even buy it just to build a cigar box guitar!

[bry melvin]

those pickguards are probably very close or the same as an SX pickguard I tried from Rondo guitars (19 USD).

To be truthful the ONLY loaded pickguards I have ever been completely happy with were those from Carvin, and "Golden Age" from Stewart MacDonald Luthier supply