Colorsound Mighty Atom 7W battery powered practice amp - help with caps & diodes

[Roly]

Yep, copy the preamp; modernise the power amp; use a couple of dry 6 volt "lantern cells" or a 12 volt gel cell for portable, and 12-18V plugpack for home/recharge (with suitable recharge cct).

[J M Fahey]

Dear Roly, thanks for posting and simulating, but please correct your schematic (and re-simulate) by making the bright input cap 2200pF (written as 2.2N in the original schematic). (yes, it looks like a capital N ; they should have used small caps)
Yea, I know, Vox had a peculiar way of writing parts values inside the (too small)  box and in confusing column way.
Oh well.

[Roly]

Vox Escort preamp related stuff...
Rev 1.1 (thanks JM)

[fursteak]

I too have one of these little gems . I believe that  mine says Rochester N.Y. on the front sticker/label . I sounded great   and yes it has/had 2 of the large 9 volt batteries . one volume and one tone . to power it up you just  put the guitar cord into the jack . there also was a speaker out on the back . I was hoping  to get mine fixed as it had incredible tone. I have been reading thought the slew of messeges but havent finished them all . I was hoping somebody  came up with a schematic that I can bring  to a repair guy as I really am not one .  Is anybody  confident in what  has been submitted here? The only  reason I found you guys was that  everyonce in a while I try a search  for Mighty Atom on yahoo. Alas I have found hope!

[br00klynbear]

This is all amazing, you guys!  Thanks for the schematics and tests; it all looks pretty promising.  I can't believe posting this thing on here got such a thorough and careful analysis. You guys are truly champions, and I really appreciate the response!

My main question at this point is how to integrate the preamp circuit with a TDA2030 or 2050.  Would it really be as simple as replacing the original opamp and use the same pinouts?  Is it still acceptable to use 9/18V power?  And will using a newer amp chip alter the tone in any way?  I'm assuming not, as the "preamp" stage has already been resolved, but I'm just curious.  Again, not as thorough an electronics engineering type as y'all, but I'm trying to learn!   :duh

[Roly]

Thanks for the thanks.

Is anybody  confident in what  has been submitted here?

Well looking at the Atom pix and the Vox circuit I agree with JM that they are very similar if not actually identical.  I didn't examine the chipamp portion since it appears to be one that is no longer current, and the OP related to the preamp section.

Am I confident about the preamp?  Well since JM spotted my component value error (now corrected) and a one transistor preamp is hardly rocket surgery, I think I can answer "yes".

While any repairer likes to have a circuit any tech worth their salt should have no problems at all with this amp without one, and while I can't speak for JM or Enzo I'll bet that like me the vast majority of gear they have fixed over the years has been without the luxury of a circuit.  Given parts, any tech who can't knock this over in an hour or two isn't worthy of the title.

Would it really be as simple as replacing the original opamp and use the same pinouts?

If a dead TBA810 is your problem, and you can obtain a replacement, yes, otherwise no.  JM suggested the TDA2030 which is functionally similar with a higher power capacity, but the pinout is entirely different.

Note that while this chipamp has a higher power capacity that doesn't mean that you will get more power, only that it is much more robust in this application, the actual power output being set by the supply voltage and speaker impedance, both of which will be the same.

Is it still acceptable to use 9/18V power?

Yes; the TDA2030 will work on supplies from 6 to 18 volts.

And will using a newer amp chip alter the tone in any way?

Very unlikely.  The dominant factor in a small amp like this is the speaker and the cabinet it is in. In fact one way to give any small amp like this "legs" is to hook it up to a much bigger speaker cabinet; it may seem a little louder, but it will certainly sound "fuller".

The other main factor is the rather low input impedance of the preamp which will tend to make any passive guitar sound a bit dull.  While the tone control, very similar to that used in the "Big Muff", has a good range and can compensate to some extent the damping effect of the input on the guitar means some tonality is lost and cannot be recovered.  One way around this is to use a high impedance buffer such as the AMZ FET buffer, however if you have an active guitar (one with a battery in it) then it already has an inbuilt buffer and it shouldn't be an issue.

HTH