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Modding a cheap solid state amp for more clean headroom.

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  • #46
    Koreth, thanks for your reply! Sorry, if I revived this old thread, looks like what you started back in 2009 is exactly what I want to do now myself. Only you know about this stuff while I, well, I know almost ...nothing.

    Anyway, I'd like to start with a schematic of this amp. Should I refer to the one you already posted or wait for you to find the final(?) one you mentioned. Then I will trace my amp and see if it is identical or not in which case I will post the differences. I think it is important for me to understand how this thing works before I start mucking with it, it will also be easier to follow expert advice from you guys. BTW, I am also subscribed in this thread Modifying a First Act MA104 which you also started as well back then.

    Thank you very much!

    PS: Sorry, what exactly does " IIRC " mean?
    Last edited by Dimi Pana; 10-22-2011, 02:11 AM.

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    • #47
      It wouldn't hurt to start tracing the circuit out yourself while I'm looking for those schematics. I have no idea where those images are at present and it may take a bit before I find them. Though from the gut shots, the two amps *look* the same, the circuit could have changed. Such a change might be a niggling minor detail that isn't worth worrying about, or it could be a huge gotcha. I'd hate to be recommending a change that made things worse or blew up the amp. Or, for all either of us knows, First Act themselves may have made changes to the circuit similar to what I did already. Though it might look scary and nonsensical at first, the more of the circuit you get traced out, the more it will start to make sense, and the easier it will be to understand what what each part of the circuit does. From there, the how and why of the mods recommended will be much clearer.

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      • #48
        Koreth -

        I agree with you 100%.

        I will start tracing the circuit.

        I am also not in a hurry, this is probably going to be this winter's project (lol).

        And trust me if you (or anyone else for that matter) suggest something that blows up the amp, the only person I will blame for wasting the mythical amount of $4.98 USD will be me, myself and -for sure- the ugly face I see in the mirror every morning when I brush my teeth (LOL).

        Sorry if my jokes are bad...

        BTW, is there any easy to use PC software I can use for documenting the tracing process?

        Thank you soooo much !!!

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        • #49
          Hello all!

          Since this exact topic also appears in THIS thread I will post my replies in both, so here it goes:

          --

          Thank you all for your feedback, it is very much appreciated.

          Koreth, I have followed your advice and started tracing the circuit on paper. It is not easy. The way I understand "nodes" vs. "imagery" is try not to visually transfer a picture of the board but starting from one side, move from junction point to the next one making sure I "transfer" all lines, components, etc., that start from it.

          Another -more fun- thing I did is hook this amp up to different speakers. I found two old 6" (rated 6Ω at 20W) speakers from an old home stereo and tried the different combinations:

          1. One speaker (6Ω) --> It sounded almost the same or maybe a little "better". I noticed less distortion from the speaker but it still rattled when played hard. Overall, I'd say it was marginally better.

          2. Two speakers in series (6+6=12Ω) --> Sounded clearly "bigger", but noticed an output difference in the volume pot. There was also less speaker rattling, unless I played unrealistically hard, i.e. intentionally hit the strings hard in an ..."unmusical" way.

          3. Three speakers in series (6+6+4=16Ω) --> This is exactly the previous setup (#2) with the addition of the original 4" speaker (4Ω , 8W) from the amp. Again the apparent sound was bigger, fuller and not as loud. I could not make the speaker cones rattle no matter how hard I played, but like I said this setup was noticeably NOT as loud as the previous ones.

          However, one thing I noticed in all setups is that the amp generated distortion did not really change in timbre, and yes it still sounded cheap, not natural. Also bear in mind that I did not solder the wires to the speaker terminals but rather used wire with crocodile clips. Do you think this is a factor?

          --

          Finally, I was complaining that with no instrument plugged-in and all pots turned down, this amp produces a strange noise when turned ON, something between a hum and a buzz. I tested the wall wart output and found that even though the DC output is rated at 14V , 500mA , in reality it outputs more that 20 Volts. I then found a wall wart rated at 12V , 1.5A tested it and found in reality it outputs around 18V. I then reversed the polarity (had to, the amp requires the pin to be negative and the sleeve to be positive) and used that instead of the FirstAct supplied one.

          Guess what? With this PSU the noise is not gone but is significantly reduced. Any comments on that?

          --

          So it appears that a better(?) , cleaner (?) PSU will definitely help, as well as a bigger and better speaker/cabinet configuration.

          I will continue tracing the PCB and get back to you with more. 'Till then thanks again to ALL of you!!!

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          • #50
            I registered just for this post because i picked up one of these with a FirstAct guitar off my local craigslist for $25. i couldnt pass it up. The guitar is surprisingly good afte ra few tweaks on the truss rod and adjusting the bridge a little the thing plays better then some guitars ive paid 3X as much for.

            To the amp, i just decided to take the 2 diodes out. I played around with a couple of little plugs and poked in a few different diodes and LEDs but i decided that it was better with them out. It is a very nice clean sounding little amp. Ofcorse the first mod was to ditch the little speaker and hook a jack to the back for a larger speaker (4X12) is what i have it hooked to.

            For what it is and what i have into it im more then happy with its tone. Even after removing both the clipper diodes (1448s i believe) i can turn the gain all the way up and still manage to get a little distortion out of it. A more 70s rock type of distortion.

            Im thinking about cutting the box in half and just use it as a little pocket head. Run a pedal infront of it and its a nice little portable amp, fits in the case or in the gig bag. The cleanest sound i can get is crankign the volume pretty high then slowly cracking the gain open just a hair and its as clean as any clean amp ive ever used.

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