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Need a nice sounding pre-amp for metal

Started by weatherlght, January 15, 2010, 07:00:41 AM

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J M Fahey

Hi rowdy
QuoteSome may disagree, but I think FET's in a source follower configuration should have as high a Vth as practical and as high a Idss as practical.
Well, I *do* agree. ;)
In fact, I always buy Fets in lots of 100 or more and nmeasure them all.
The ones with Vp (pinch-off) from 1.3 to 1.9V are labelled "preamp"; the ones around 2.5V "general purpose" and those above 3.5V "switching/compression/source follower"


Here it is. MG15 CDR.

IMO, Distortion Channel is good for metal (and nothing else, actually). Clean channel is awfully dull.


That MG15 pre-amp looks similar to the Guv'nor pedal also produced by Marshall. Guess the sound could be similar. But a believe there are two important differences that could make the distortion sound different;

- Guv'nor uses two red LEDs to produce (symmetrical) clipping, with one green an one red LED, which the MG15 pre-amp uses, the clipping should be asymmetrical if I get this right?

- Instead of connecting the diodes to the ground like the Guv'nor, the diodes is at the feedback (not sure about the thermology here so feel free to correct me if I'm wrong) of the op-amp. This usually produce a softer clipping than putting them between the signal and ground. But the MJ4558 use in the MG15 is described as an "high gain op-amp" in the data-sheet (Guv'nor uses the old familiar TL072). So I don't know, I guess it can clip quite hard anyway.

I'm also looking for a good metal pre-amp and this one really looks interesting. One that sounds like the ProCo Rat (best metal distortion ever IMO) would probably be better, but maybe this one can work.

J M Fahey

In practical use RC4558 and TL072 sound the same, with the 072 being *slightly* brighter.
The Rat is excellent if plugged into a good guitar amp, preferably a good "metal" tube one.
On its own , driving straight a chip amp or into an effects loop the sound is, well, "raw", maybe suitable for Punk.


I'm seriously considering to build the MG15 pre-amp, using the schematic attached schematic. There is some things I need to know before a start however. Two questions that might be obvious, but I want to for sure.

- The reverb circuit and jackets after the pre-amp made me a bit confused. However I'm only going to build the pre-amp, and then use it to drive a lm3886 chip. So I'm not really sure exactly what part of the schematic that includes this one. I've marked part what I think could work as a pre-amp if built at the attached schematic, but maybe I've missed something (except the PSU).

- Some pots wasn't marked at the schematic. This wasn't very hard to figure out, but if someone else could look at too it would be nice.


I could not find the 2n5484 mosfets shown at the schematic :( I found a few bf256 which are a replacement for 2n5485 and 2n5486 i believe. Will they work fine for a nice metal distortion sound? Or they will ruin it? What transistor would be best for this DR.Boogy preamp?


I wouldn't worry too much about the substitution. Just make sure that the cut off voltage and Idss for the bf256 are about the same as the 2n5486. Keep in mind that even JFET's with the same part number will vary quite a bit. For the Dr. Boogey, get some j201's if you're using a 9v supply voltage. For Q5 (gauss markov schematic) you might use a MPF102. If doing the mensur version which uses a much higher supply voltage, use 2n5457's or something like that. You might use transistor sockets to try different FETs. I wish I did. Also, checkout J M Fahey's post above. He's got a pretty good idea. The measured characteristics of an individual JFET are more important than the part number printed on it.


hi everybody!

im new here, just wondering cause i cant access Davelectro schem for the marshall
i need this really bad  and it would e a real help if anybody could provide one



A Distortius Maximus clone might be pretty good for metal, too.

Alexius II

My vote also goes for Dr.Boogey. :D

I haven't tried Mensur's version, but even the standard Gaussmarkov version sounds great to me :tu:
Although it is more common to build it as a "distortion stompbox", I actually did it as a preamp for my practice solid-state amp (TDA2003). If you want to hear it: I made a short sound sample with all knobs at 12 o'clock = 50%. I used a 2n5457 for input transistor (Q1) and for buffer (Q5), the other three are J201.

You can clearly (pun intended) hear that even with gain at 50%, it is distorted as hell (a bit too much for my taste)  ;D

I usually use it with lower gain and somewhat more scooped eq, and It sounds realy great (I use Ibanez with dimarzios, and Marshall 1960AV cab)

J M Fahey