Quote from: teemuk on February 18, 2008, 03:56:20 PM
Third question: Again the gain of the first amplifier stage varies throughout the bandwidth. Mostly it's R57 / R43 +1 but at higher frequencies the parallel branch of R44 again begins to have effect and increases the gain. At very high frequencies C81 in turn decreases the gain.

The gain of the following stage varies according to switch position. In clean mode (and pot at max) it's about R65 (270K) / R80 (33K). I think the switch labels should be reversed (it's clean in the portrayed position). In distortion mode the gain is approximately R65 (270K) / R88 (470R) but the maximum amplitude is naturally limited by the clipping LEDs that are no longer in series with the high-value resistance. All these are naturally just approximate values that are somewhat modified by the lesser parallel branches.

If you really want to completely figure out the behaviour then I feel it's best that you get acquainted with some kind of circuit simulator software. I, for one, hate calculating all the stuff involved with pen-and-paper method.

i thought when in clean mode, gain should be:
parallel VR1, R85 and R86 / parallel (R87 + R88) R89

and when in distortion mode, just omit R86 and R87.

but i have no idea on how to calculate gain with that network (i can't even calculate ProCo Rat's Gain bcos of the resistors in parallel)

lets just omit the leds, and it will be easier to me.

BTW, thanx for your answers and your patience.

Hi, i have this very odd schematic and i wanna know just 3 things. first, what the hell is this symbol?

L7


second, how much is the total resistance of the input, so i can calculate the total gain (just for the second op-amp)



i'm guessing, first, with the switch open, is:
Parallel R52 & R53,
Parallel R50 & R51, in series with R92
and both values in parallel, right?

third, wanna know the gain of the first and second amp (asuming pot is at max)




and final, any of you knows 'bout any good poweramp (just the IC), i think is called chip-amp, which can delivers 100 watts?

ever tried one of the MG series? they r solid state (100%) but sounds so full of life you'll think you're playin' thru a valve-amp or hybrid. sounds good to me. try before you buy, but first, press the FDD switch.

now its clear. i wont risk my amp he he he. thanx a lot for your help and for the schem.

Quote from: teemuk on February 10, 2008, 07:54:57 PM

6) Could you rephrase this question? What is it that you exactly want to know?
7) Sure you can. But if you really need the schematic (of MG15CD) I can just email it to you.

6. i don't know 'bout impedance or resistance when it comes to speakers. my MG uses a 8ohm  speaker, and sometimes i read the manual of other amps (MG100, JCM800, ...) and they keep tellin' me "don't use less than 4ohm spk, use one cabinet @ 16ohm or 2 cabinets @ 8ohm or something. let's assume i build my own 100w amp and i wanna build my own 4x12 cabinets (say 1 or 2 cabinets). what do i need to know about the impedance of each speaker or the total cabinet, or the impedance of both, etc.

7. i'd love to see that schematic and then tryin to mod mine (i need more crunch) and/or make my preamp.

thanx a lot.

edited:

d'ya have any schematic of any superior model, just wanna take a look at the clean/crunch and od1/od2 channels.

Hi. I'm new at this forum and just wanted to know your opinions bout the MG series. i have 2 MG15 (CDR and DFX) and i'm really glad with their sound. i dont like the digital FX on the MG15DFX, i just bought it for suck a low price, so i can open this little box and trace the schematic and do my own 4-channel preamp (and possibly a 100w SS-amp based on this ). so i have a few questions:

1. d'ya have any opinion 'bout this amp?

2. the preamp is powered by bipolar 16v, if i make a 100w poweramp, do i need to change the voltage of the preamp?

3. if i make just a rackmount preamp version of this (i really like its sound, but i need a more channels for a thrash sound) can i connect it to the FX Loop Return?

4. can i use any (like, ANY) 100w power amp? or do i need to change some stuff?

5. if any of you guys, have a 100w version of the MG series, can u check whats the voltage of yer preamp? (wanna know if the keep using 16v in the other models)

6. do ya have any info bout the speakers? i dont know 'bout using a 8-ohm cab or just one 8 ohm speaker. i mean, i need info of that (i dont know anything 'bout speakers)

7. if I trace the schematic of the preamp, poweramp and FDD (i wont trace the DFX or REverb thing) can i upload it here?

its nice to have a place like this (for SS amps) 'cos i personally think valves are overated and, since im kinda cheap and lazy, i dont wanna change valves every year, and i really love the MG sound (i've played a few SS amps and i hate 'em).

Thanx a lot, and excuse my bad english.