Randall RG100ES Emu

[CLAAS]

If I remember correctly,
there`s the
"Preamp from Hell"
from a Russian Site.
Sorry,no Link

[Marcos - Munky]

The "Preamp From Hell" was in my site. Actually, it's just a redraw of the Red channel of an RG100, the same one that was posted by Ian in this topic. That layout I had in my site had an error, but I don't remember what was the error.

[caspercody]

The ony thing I noticed was it is missing the 68k resistor at the input. I re did your schematic in Eagle, and now have a pcb layout if anyone wants to try it?

[deaconque]

Sorry guys,  I've been busy with a couple of guitar builds lately so I haven't been able to get this one built yet.  It's next on my list though so it should get built soon.

[Ed G.]

I'd try to integrate a charge pump IC to get it at least up to 18v. That way it can also be used on any pedalboard with a 9v power supply.

[space_ryerson]

Also, I built the X2FET, which is a Randall X2 emu. And this is a very easy, and sounds good pedal. Does anyone know the big differences between the X2 and RG100?

Thanks
Rob

The biggest differences between the X2FET and the RG-100 is that the X2FET uses J201's, where the RG uses 2N5484's for fets. The X2FET also uses 1N4148 for clipping diodes, and the RG-100 uses 1N914's. There are other subtle differences between the two. As an experiment, you could try omitting the tonestack on the X2FET to see how much more volume you get. If you don't want to omit the tonestack, you could add a makeup gain stage. On the original RG-100, check out Q4 and the presence control surrounding it. In my opinion, that presence control is a big part of the 'RG sound' for me.

edit: I forgot to mention another difference; an extra jfet! I remember talking to some Randall folks around the time when they were designing the warhead, and they mentioned that they were trying to cram even more gain out of it when compared to a RG-100. That would make sense, given that there's an extra jfet, and if the X2FET is an accurate reproduction, the use of J201's, as opposed to 2N5484's.

[caspercody]

I finished making the Preamp from Hell, and it is very nice. It is probably the quietest Jfet emu I have made. It is not as high gain as the X2FET, and I would say it is medium to (low) high gain. I am using (2) 9v batteries and works fine. I tried it with the 33k resistors but sounded a little un biased. So replaced with 100k trim pots and dialed them in. Sounds way better that way. Might try replacing one or two stages with J201s to see if I get any more gain.

[space_ryerson]

Also, keep in mind that jfets vary wildly from one to the next, so try re-arranging what you have or try other 2n5484's (or J201's). I have four old Randall RG's, three of which were built in the same six month period, using the same PCB and same lot of parts. I replaced the electrolytics in those three at the same time, so they would still have identical components, but yet they sound very different from one another, and the amount of gain they get really varies. One really goes into high gain territory, and one only achieves what I would describe as medium/low gain!

[teemuk]

If you want to "emulate" the design then don't change it.

If you convert from 24V to 9V it will decrease headroom and this is a BIG issue because after that the tone won't be the same no longer. You can rebias the FETs for 9V but while you're at it you should also re-scale their gain for similar breakup with 9V supply. Then rescale all the capacitors for similar frequency response. Omitting those changes - it no longer sounds like the original circuit.

The Zener diodes are for clipping signal peaks above approximately 5V. They can be omitted if the solid-state shunting diodes are wired on permanently but if you keep the switch for the diodes you also need the Zeners to retain the original clipping characteristics of the circuit. Without them the signal at the point can swing from +12 to -6 volts. Zeners would limit that to +5 - -5V Damn sure omitting that clipping mechanism will effect how the following change overdrives and to what kind of offset levels the signal biases.

[caspercody]

I am running (2) 9v batteries. Which is 18v. I know it is not 24v, but sounds good.

But what that extra 6volts make that big of a difference?

[meffcio]

Any chance for a PCB?

[caspercody]

Send me a PM with your email address.

[12afael]

years that I don´t post here, I did a layout some years ago but I never built it, so if someone try it please post if it work.

http://sites.google.com/site/plexilandia/

cheerz

Rafael

[meffcio]

I'm confused now. What was used in the original? 1n914 or 1n4148? 2n5458, 2n5484 or any other?
And btw - hitting the sustain switch will actually cause a big volume drop, yes? And a volume pot even before the tonestack is kinda strange for me..

[caspercody]

The original uses 1N914. And yes big volume drop. I will send you copy of the original schematic.

Did you get the other files I sent?

[space_ryerson]

I'm confused now. What was used in the original? 1n914 or 1n4148? 2n5458, 2n5484 or any other?
And btw - hitting the sustain switch will actually cause a big volume drop, yes? And a volume pot even before the tonestack is kinda strange for me..

The original uses 1n914's for clipping diodes, and 2n5484's for jfets. There is indeed a big volume drop when the clipping diodes are switched in (the sustain switch), which was addressed in later Randall designs with iirc a makeup gain stage. I like the sound of it without the makeup gain stage, and besides, I've never had an issue with the RG's not being loud enough! As for stompboxes, that's another story. If it were me, if I were making a pedal out of this, I'd move the volume control to right before the output. I also agree with teemuk about the scaling for 9v operation. That's where I had left off with my version. Well, that and what to do with the tonestack. I got a fairly close response curve to what I like about original tonestack with a big muff style tonestack in Duncan's tonestack calculator.

Out of curiosity, has anyone had a look at what's in MXR's Dime Distortion circuit-wise?

[meffcio]

Did you get the other files I sent?

Yup, thanks.

Anyway, I've got a question to all who built this thing. Does it sound... um... hi-fi? I mean is the sound nice to ears, or more fizzy and grainy etc? I need a distortion preamp that sounds better than my friend's Peavey Bandit 112 ;p

[meffcio]

http://www.speedyshare.com/files/21227833/randall.zip
Another design, someone should check it.

[meffcio]

The pots are log, lin, or what? 

[meffcio]

Bump.

[EDIT]
Aww, I see...

Gain - 50k log
Master - 10k lin
Treble - 50k lin
Bass - 50k log
Middle - 2k lin
Presence - 2k lin