I've done some more tweaking of the circuit. I first tried replacing C8 with a .01, thinking it would get me more highs. It did, slightly, but it didn't cure the muffled/muddy sound I'm still getting from the amp, and worse reduced the audible effect of sweeping the tone knob to almost nothing.

So I did some more reading and finally figured out how to use LTSpice to do more than just draw circuit diagrams. I made a schematic of just the tone stack starting at C7 and ending at the wiper of VR3, then did an AC analysis with sweeps for a few different positions of the wiper of VR3 to see just what the heck the tone stack is doing. I'm actually amazed by its simplicity. At one end of VR3's sweep the tone stack is a simple high cut filter. At the other end, it is a notch filter with the notch gradually shifting forward flattening out to smoothly transition into the high-cut filter. Thinking about how I run my EQ's on the other amp, it actually makes sense. Clean sounds tend to have a mid-range scoop to counter the mid-range emphasis that magnetic pickups supposedly have for a more natural, balanced sound. But when running heavy distortion, I tend to pull my highs back a bit to keep the distortion from sounding too harsh. So yeah, once again, the First Act designers knew exactly what they were doing.

By changing C8 to the same value as C10, I pretty much flattened out the mid-range notch. All lows and mids with rolled off highs sounds pretty muffled. Playing with a few values in the simulation I found out I actually want C8 to be a larger value than C10. Yes, a larger C8 rolls off more highs, but the greater the difference, the notch gets narrower and shifts it down in the frequency range. There are other places in the circuit I can get my highs back or reduce lows to keep the amp from sounding muffled. Also, changing C7 to a smaller value has a negligable (<-3dB) on the bass until the cap gets down into the .1µF range and lower. If I change C5 and C7 to 1µ, I get the bass down about -1.5 dB. I think I'd have to change C15 and C24 to 1µF to as well to see a -3dB reduction in bass. It'd be easier to change the input caps at each gain stage from .022µF to .01µF, methinks. So I'm not going to bother with C7 just yet. I'll play around with the values of C8, C10 and R7 in the simulation and see if I can't tweak the tone stack to my liking in the simulation, then I'll try it and report my findings.

Yes, I know I'm wasting my time. This is a fun learning experience.

So then an opamp, output chip or whatever else with a supply voltage rating of 18V used as a voltage amplifier could only swing a max of 18Vpp assuming it received an 18V supply, but one with a +-18V supply rating could swing 36Vpp, again assuming it received its maximum rated supply voltage.

In the various schematics I've looked at, I don't know that I've ever seen any op amp or chip amp run at its maximum rated voltage supply. This makes sense to me as running things at their max all the time leaves no room for error. If ever there was a momentary surge from the power supply for whatever reason, you'd risk blowing the device. I'd like to think that the manufacturer's ratings are somewhat conservative, but better safe than sorry. So, that said, is there a general rule that's considered good design or best practices when deciding what voltages to run devices at, (e.g. so many volts under the max, a fixed percentage of the max, etc.)?

While I would generally agree with that, until their forward voltage the diodes should do nothing. So the 1N4148s should have no effect on the signal until the signal coming out of the op amp reaches 1.3-to 1.4Vpp. The bridge humbucker on my guitar isn't that hot as is evidenced by there being no clipping with the gain knob is turned down until the volume knob (located after the clipping diodes) is turned up. As such I want to look at the gain of the output stage or the opamp before it. But I'll try lifting D2 and D3 to see if that makes a difference.

I've been looking at datasheets for things like op amps and chip amps the past few days, and some of the specs in them confuse me a bit. Some say something like 18V, others say +-18V. Are these meaning the same thing or something different. I assume that 18V means it can have a total of 18V across it's power supply terminals before taking damage, and that +-18V would mean that the V+ terminal can be +18V, and the V- terminal can -18V, for  total of 36V across its power supply terminals, but we all know what happens when you assume. Am I on the right track here or am I off in la la land?

I was wondering about that TDA2003. Unless I'm misunderstanding things, I figure with 14V supply and a gain of 32 as it is currently configured, the hottest signal you could feed to it without breaking up would be in the neighborhood of 1.5Vrms. Basically, I could feed it directly with my guitar, and it would probably still breakup if I used the bridge pickup.

I took a look at the Jensen MOD series, nice. Unfortunately, there isn't even enough room to mount even the 5" speaker without building a new cabinet. A damn shame. I really like the sound of the 15W 6" version from the sound clips on the website.

Stock, the MA104 comes with a tiny, 4" speaker of unknown manufacture.

I agree that replacing the power supply, preamp, power amp, and cabinet, I'm building a new amp. But that's not what I'm trying to achieve here. Thus far I've changed the speaker and replaced two components to give the gain knob a useful range. Hardly building a new amp, IMO. Now if I get to the point where I'm adding or removing op-amps in the preamp, changing the 2nd stage into a tonestack or something, replacing the power supply and changing the TDA2003 for something bigger, than yes I am building a new amp and would do better to start from scratch. But that's not what I want to do. What I want to do is reduce the output of the preamp slightly so it isn't overdriving the power amp, get some more highs and tweak the distortion character slightly. The latter I suspect I can do with a few dollars and a few minutes with my soldering iron. The former would require more parts, more time, more planning and knowing more than I do now to pull off successfully.

Greetings all.

I have a First Act MA104 that I purchased from Wal-Mart years go when I first started playing guitar. I hated it's tone pretty quickly, as even with the gain knob all the way down, and the volume set very low, even vintage single coil pickups would send the amp into a nasty sounding breakup very quickly. When I found another amp (a Ross Systems 22) at a garage sale with a slightly better sounding distortion and the ability to clean up when the gain knob was turned down, the MA104 was shoved into a corner and forgotten about.

Fast forward to the present: The Ross 22 has since died and been cannibalized for parts and my main amp is a 100W Mesa/Boogie. The Mesa will remain my main performance and rehearsal amp for the foreseeable future, but a 100W all tube half-stack isn't very practical for bedroom practice, never mind the impracticality of lugging a big heavy head and cabinet across town twice a week for rehearsals. So the MA104 has been brought out of its corner and put to use again. Stock, it still sounds gross, but unlike years ago, I now own and know how to operate a soldering iron.

First Act wasn't forthcoming with a schematic, so I traced the circuit out and made my own in LTspice. It is attached. I'm pretty sure the diagram is complete, but I could have made some mistakes in tracing the circuit. So if anyone sees anything that doesn't make sense, please lemme know. Though the output chip is labeled "UTC2003", I'm pretty sure it is a TDA2003, which UTC makes. The power ratings and test circuit seem to be close to what's in the amp and First Act's claimed power output for this amp.

Thus far, I've made a few mods. I spliced a 1/4" plug onto the speaker out and used one of the 10" 40W 8Ohm speakers left over from the Ross 22. Overkill for a 4W amp, I know, but the 10" speaker sounds a lot better than the little 4" speaker it came stock with. With the help of the fellows at Music-Electronics-Forums, I lowered the gain in the 1st op amp stage. With the stock values, the gain of the 1st stage was over 300, enough for even a vintage single coil to overdrive the op amp hard against it's power rails. I changed R2 from 270Ohm to 1KOhm, and C4 from 10µ to 1µ. The gain knob has a much more usable range from clean to barely breaking up to full distortion now.

At this point I'm looking to achieve the following:

• More clean headroom. Even when the gain knob is all the way down, when the volume knob is swept past 4, there starts to be a bit of breakup on the attack that I don't like. Past 4 on the volume knob, the amp stops getting louder as quickly and starts getting dirtier. Since the volume control is after the gain and tone controls, I'm guessing that the 2nd op amp stage is making the signal from the 1st stage too hot and it is overdriving the output chip. The voltage divider on the output of the 2nd stage barely attenuates the signal before it hits the output stage. I've got few ideas here:
  
  
  • Reduce the gain of the 2nd stage
  • change the voltage divider to attenuate the output of the 2nd stage more.
  • Reduce the gain of the output stage
  • A combination of all or some of the above
  
  
• More highs. When I run the amp clean, I dime the tone control. When I turn up the distortion, I turn it down to tame some of the harshness that comes when the turning the gain up.That's all well and good, but when the tone control is dimed, it still isn't enough highs. The amount of highs with the tone control turned up is short of "pleasantly warm." I'd like it to be able to go all the way up to "sparkly bright", if possible. I wouldn't mind if it went a little bit into "piercingly bright", as I can always turn the tone knob down. I've got a few ideas here too:
  
  
  
  • Lower the value of or remove C3 in the 1st stage
  • Lower the value of C8 in the tone stack
  • Lower the value of or remove C13 in the 2nd stage
    
  • Some combination of the above
  
  
• Tweak the distortion character of the amp. I want to do this after I have the above two addressed. I'd like to play with LEDs both as the hard clippers and in the negative feedback loop of the 1st stage as soft clippers, as I've discovered through modifying one of my pedals that I prefer the clipping sound made by LEDs to that of 1N4158s.

Hopefully all that isn't tl;dr. Your input on the above would be greatly appreciated.

Greetings. I found this place via music-electronics-forum.com (Ampage?). I have some threads over there with some of my questions, but I figured this place would be a good place to cross-post with some of them since this place has a Solid state focus, and some of my music gear is purely solid state.

Should I throw my immediate questions into this thread or start another one in another section of the forums appropriate for it?