Thursday, 8 May 2014

Madbean Rump Roast

Another request for a discontinued Madbean board, this one based on the Dr. Z Carmen Ghia preamp.

Info about the original project and board:
The Rump Roast finds its origins in the preamp section of the Dr. Z Carmen Ghia amplifier. The Carmen Ghia is a very understated and elegant design consisting of a single tone and gain control. This simplicity makes it an ideal candidate for pedal conversion. Standard practice in porting an amp design into a pedal is to use JFET transistors, such as the J201 or 2N5457, in place of pre-amp tubes. While it is impossible to correctly match the dynamics or compression of a tube cooking at a couple hundred volts, discreet emulation in a pedal does offer some nice tones and at least retains some of the character of the amp it emulates.

In the case of the Rump Roast, the design is broken down into two JFET gain stages with a simple cut control for tone and attenuator for gain in between. Some modifications have been made to “pedalize” this design. C1 has been added to trim off a little top end of the guitar signal (above 15kHz) due to the amount of boost on tap. C2 and C3 is a single 680n cap in the Carmen Ghia. These were broken into two parallel 220n and 470n caps which are more widely available for pedal building.

The tonal range has also been altered and enhanced. In the amp, C5 is a 250pF cap. While this does offer a very wide range of cut control (the tone pot works to cut high end as it is turned up), I found that at its lowest setting it was far too bright for a pedal. Therefore, this was changed to a 680pF cap to tame it a little. Additionally, a switch was added to put a 68n cap in parallel to provide another range of cut control. Together, the two ranges offer a lot of flexibility in shaping the tone of this design.

Lastly, an option was added to use a master volume at the end of the circuit. This offers even more flexibility. Without the volume control, the Rump Roast acts as a boost, providing a huge amount of gain to cook the front end of an amp. With the volume control added, it becomes more like an overdrive. This lets you tame the volume, at the cost of some of the overall gain. It is possible to make these options switchable, depending on what your needs are at any given time. See the wiring diagrams below where this is illustrated.

The controls are as follows:
TONE: This cuts the amount of high end as you turn the control up.
GAIN: This controls a wide range of boost. At minimum, it will be close to (or slightly above) bypass signal. As you turn it up you will get a large amount of boost.
VOLUME: If used, this will allow you to make the Rump Roast behave more like an overdrive than a boost.

A 500kA pot may be substituted for the Gain control.
Other JFETS may also be used, but be sure to check pin-out before soldering.





Now verified by Miro.
" Sounds very good with quite unconventional tone controls (including the bright switch). However. I didn't like the volume control's feel and range, so i swapped the 220K for 120K and volume pot for 200K linear. This way the lower gain settings become usable. Other good change could be to up the 10K at gain 1 to 22K-47K to make more of its sweep usable. Sounds very good indeed.




Clean first, gain at 12 o'clock and ending with gain maxed. Signal path as follows:

Tokai Telecaster w/ Seymour Duncan STL-1 on bridge -> this circuit -> Laney VC15-110 -> AudioTechnica MB2K -> Behringer shitmix -> M-Audio 2496 -> Ardour2 -> mp3

All non-hifi noise is due to my shitty bench gear rather than from the circuit itself. "

9 comments:

  1. Man, that was quick. Thanks!

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  2. Now it's time for the Dr. Z MAZ 18....

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  3. Jesus christ mate, have you been at the redbull again?!... rapid fire layouts!

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    1. Ha ha. Yes but I mixed it with brandy so don't know whether to fall asleep or go for a run around Manchester.

      Totally unrelated but have you seen Last Vegas? Morgan Freeman's character is drinking vodka Red Bull and says it's like getting drunk and being electrocuted at the same time. Made me lol! :o)

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    2. Haha, No I haven't seen it but I will definitely have to now...

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  4. I have one of these on my pedalboard right now, made with a MadBean etched board.

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  5. Thanks for this one, I really, really love the sound from it so far! It does the low to mid gain overdrive sound very nicely on it's own, and unlike a lot of amp-in-a-box pedals I've built so far (admittedly not a huge amount), it reacts really nicely being fed with another overdrive/distortion. Shows that a low parts count can still sound great.

    (Though I nearly tore the decal off in an incident with some blu-tak - seems there must be at least one screwup each build, no matter how careful I think am!)

    And some pics - please don't sue me Dr Z! I've not managed to make a 1590a build look pretty inside yet, but at least they actually work consistently now...
    Inside
    https://dl.dropboxusercontent.com/u/3035771/Pedals/rump%20roast/P1000488.JPG
    https://dl.dropboxusercontent.com/u/3035771/Pedals/rump%20roast/P1000490.JPG

    Outside
    https://dl.dropboxusercontent.com/u/3035771/Pedals/rump%20roast/P1000492.JPG
    https://dl.dropboxusercontent.com/u/3035771/Pedals/rump%20roast/P1000493.JPG

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    1. That's great, thanks for the pics

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  6. I just built this one tonight on a whim. Wow! What a great little circuit this is! No bias problems with the j201s (trimmers were easily set by ear). The amp-like response and overdrive grind sounds really good. And the bright switch works amazingly well - I loved some of the sounds I was getting from my neck pickup of my Squier Fender 51 Pawnshop guitar (a hot single coil strat pickup) in the bright mode.

    For those who want a great sounding "amp-in-a-box" that's easy to build, try the Rump Roast - you won't be sorry. :)

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