Marshall MG15 Series

[THChrist]

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.

[teemuk]

1) No comment.
2) No. The dual 16 V is pretty much generic operating voltage for OpAmp circuits. Whether it's couple volts higher or lower doesn't really matter much. You can change the voltage but you can't go much higher than this (±18V is the maximum for most opamps) and lower voltage means decreased headroom and earlier clipping. The circuit is designed to operate on a specific voltage so no need to alter that.
3) Very likely yes. The answer to this pretty much depends on the FX loop's properties, which vary from amp to amp. Do you like the sound when you listen it through the CD Input/Line out because that's what you get if you only replicate the preamp section? Do note that the power amplifier and the speaker setup have a great part in shaping the sound of that amp.
4) Most power amplifiers should work but to get most out of the combination (and to retain the proper sensitivity of preamp's volume controls) you need to match the power amp's input sensitivity to the output amplitude of the preamp (or vice versa).
5) Don't have one but I'm pretty sure it's about ± 15 – 16 V. See answer number 2.
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.

[THChrist]

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.

[teemuk]

The total resistance for two or more resistors in series is

R1 + R2 + R3 +... Rn

The total resistance for two or more resistors in parallel is

1 / ((1/R1) + (1/R2) + (1/R3) + ... (1/Rn))

Impedance Z is equal to resistance but changes with frequency. Therefore in the case of loudspeakers the parameter stated is the "nominal" impedance that is sort of the "average" resistance.


Solid-state and tube amplifiers behave differently when driving a load:

A stereotypical solid-state amplifier provides maximum output power to its minimum rated load. In theory, if the load impedance is doubled the amount of output power drops to half. Similarly in theory, if the load impedance is halved the output power doubles up. Never use a load that is lower than the minimum rating: That will very likely damage your amplifier and in reality, due to many reasons, doesn't likely give more output power either. In reality the output power when doubling up the load impedance doesn't necessarily halve up either. Never, ever have a short circuit loading the amplifier! Increasing the load impedance is safe in most solid-state amplifiers although not in all of them.


A stereotypical tube amplifier works in a manner that makes it provide a constant power to its rated load. Very often the load rating is also switchable. Load impedance that is lower than the stated load rating does not damage the amplifier as likely as is the case with solid-state amplifiers but it provides no significant benefits either. Higher load impedances may damage the amplifier. So, a stereotypical tube amplifier behaves exactly in an opposite way to a stereotypical solid-state amplifier. Never, ever omit the load! In reality, the speaker's impedance may rise as much as ten times higher than the nominal impedance and most tube amplifiers tolerate this condition rather well. Anyway, it is best to follow the practice that you match the speaker system's nominal impedance to the load rating of the amp, just in case.

I hope that clarified things a bit. I'll email you the schematic.

[THChrist]

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

[dynac0mp]

I've got one of these, and for me the clean channel is great.  I'm looking for more of the same in a larger form factor with less fluff... anybody have suggestions on a serious amp that is simply clear and low noise?

[THChrist]

u mean something like, a bigger MG just for clean? maybe u can use any MG or rackmount your mg15.

[dynac0mp]

See my other post for more info...
http://www.ssguitar.com/index.php?topic=717.msg4663#msg4663

[THChrist]

i said that bcos i dont quite understand what you mean (language issues, english is not my first language). i love the clean tones u can have with a MG, and i love the dirty tones, sounds pretty good. but the clean channel, with my Epi LP, using Dimarzios Super Distortion and Super 2 with 2 push-pull pots for Series/parallel sounds like glory. especially when using the bridge (SuperDist) in parallel, it has the crunch of a strat, not as twangy, but pretty cool.

[R Santoso]

I bought Marshall MG15CDR  more than a year ago, I like the clean sound quality and the Overdrive. Specially for the FDD system is very interesting, but I really want to know the idea of the FDD and how it works. If anybody can share me the schematic I would really appreciate. May be Mr Teemuk can help me and email me the schematic. Thank you very much.

[josip]

MG15CD: If anybody can share me the schematic I would really appreciate. Thank you very much.

[d4v1d5hu13r]

About #6

Make it a small combo. 50 Watts is actually all you'll need. Matter of fact, see if you can get your hands on the Vox Valvetronix hybrid schem. They sound like a fully tube amp and only use one 12ax7. I'm big on hybrids because of versatility and sound quality.

I plan to buy a Vox AD50VT 1x12 combo mostly because of the power/size/weight comparison to my fully SS 120 watt stack.

[J M Fahey]

Dear Teemuk: it would be great if you could post some MG schematic here, especially in the 60 to 100W range. I understand they´re basically "tubeless valvestates", I wonder what they replaced the 12AX7 with. Probably nothing too different from a 9004 preamp, but I haven´t repaired one yet, so I don´t know for sure.  Thanks.

Hola TChrist, ¿de dónde sos?

[teemuk]

I'm a bit cautious about sharing schematics at public forums (they're copyrighted material after all) but I suppose that providing links to files that are shared by someone else isn't technically breaking any laws... So, here's one example from the more complex and higher power end of the series:

http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf

You can see that circuits of these amps are pretty "basic" stuff and consist of usual OpAmp stages, various types of typical analog filters and stuff like diode clipping etc. There's really nothing fancy inside. Lower power models? Well... just think that you're building a simpler and cheaper amp and to do that you remove some of the features and substitute the power amp chip with a lower power model. That's pretty much it.

The only somewhat peculiar thing in these amps is the rather unorthodox (inverting) power amplifier circuit that has a quite cunning hi-pass filter – and the "FDD", of course. You can find the same circuit topology from some Valvestates as well (in those cases without the FDD feature). Marshall had some specific name for this type of power amp circuit and they even claimed that it reproduces some tube amp characteristics (don't they always). They don't exactly specify what characteristic so I can't really comment on that - except by saying that I didn't find that circuit to have any noticeable difference in comparison to multitude of other chip-based (guitar) power amps.

[THChrist]

Dear Teemuk: it would be great if you could post some MG schematic here, especially in the 60 to 100W range. I understand they´re basically "tubeless valvestates", I wonder what they replaced the 12AX7 with. Probably nothing too different from a 9004 preamp, but I haven´t repaired one yet, so I don´t know for sure.  Thanks.

Hola TChrist, ¿de dónde sos?

hola M Fahey, soy de Peru, tu eres argentino por lo que leo (creo...)

sorry for the off-topic.