BAJA MDR overdrive pedal 9v 060218

bajaman · Post by **bajaman** » 06 Feb 2018, 06:23

Here is the latest in the series - the Mesa Dual Rectifier high gain channel.
Not built yet but should work fine

have fun

cheers
bajaman

Manfred · Post by **Manfred** » 06 Feb 2018, 13:57

By comparison, the 1N4148 and BC547B junction curve traces.

Frank_NH · Post by **Frank_NH** » 06 Feb 2018, 14:56

Interesting, Manfred. Recently, I measured bunch of random silicon NPN transistors (wired as diodes) in my stash with my DVM set to the diode mode. Most seem to have about a 0.6V - 0.7V forward voltage. I wonder how germanium would compare to silicon.

Ice-9 · Post by **Ice-9** » 06 Feb 2018, 20:08

Hi Bajaman, great work yet again, I've just had a quick look over the schematic but wonder if IC1B will bias correctly without a resistor from a point on the non inverting input to Vb ?

bajaman · Post by **bajaman** » 06 Feb 2018, 20:31

I've just had a quick look over the schematic but wonder if IC1B will bias correctly without a resistor from a point on the non inverting input to Vb ?

my mistake in the drawing - here is the correct schematic
cheers
bajaman

Ice-9 · Post by **Ice-9** » 06 Feb 2018, 20:35

bajaman wrote:
I've just had a quick look over the schematic but wonder if IC1B will bias correctly without a resistor from a point on the non inverting input to Vb ?

my mistake in the drawing - here is the correct schematic
cheers
bajaman
Baja MDR overdrive pedal 9v 060218.GIF

, yeah that'll sort it,

bajaman · Post by **bajaman** » 07 Feb 2018, 04:23

I just built this today in an old jfet DR Boogie enclosure - used the existing tone stack and added a small gain stage after it to recover the signal level - works surprisingly well and the output level when the tone stack is engaged is only slightly louder than when it is bypassed

enjoy

bajaman

FausterCraster · Post by **FausterCraster** » 07 Feb 2018, 07:48

A tone control is something that complete these pedals series .... Baja Characters Series awesome !!!!

Why not use the Tech21 tone stack too ?
Maybe just over-complicating but can be more suitable for all seasons... opinions?

bajaman · Post by **bajaman** » 07 Feb 2018, 10:21

even a very simple tone control such as the DS-1 or Big Muff Pi may be worth considering

temol · Post by **temol** » 07 Feb 2018, 12:34

bajaman wrote:Here is the latest in the series

What? Where's the Peavey, Engl.. ?

T.

marcao_cfh · Post by **marcao_cfh** » 07 Feb 2018, 19:06

Maybe this is the latest like the most recent one, not the last one of the whole series. At least that's what I'm hoping for

Optical · Post by **Optical** » 07 Feb 2018, 23:47

Yes a 5150 preamp would be a great one.

I'm actually surprised there is any difference between the dual rec and the SLO pedals, given the preamps are all but identical.
The mesa has a different power amp and NFB circuit, does your pedal design take the power amp characteristics into account too?

bajaman · Post by **bajaman** » 08 Feb 2018, 02:02

I'm actually surprised there is any difference between the dual rec and the SLO pedals, given the preamps are all but identical.
The mesa has a different power amp and NFB circuit, does your pedal design take the power amp characteristics into account too?

No - these pedals only imitate the frequency and phase responses of the preamp section up to the passive tone stack.
yes - i was surprised that the response of the mesa DR is quite different to the Soldano SLO - incidentally the original Peavey 5150 was very similar to the SLO as well with an additional two gain stages which introduce the hiss that these amps are noted for

bajaman · Post by **bajaman** » 08 Feb 2018, 02:04

What? Where's the Peavey, Engl.. ?

Which Engl would you like to see ?
cheers
bajaman

temol · Post by **temol** » 08 Feb 2018, 10:54

bajaman wrote:Which Engl would you like to see ?

Engl Savage (Blackmore)

T.

bajaman · Post by **bajaman** » 09 Feb 2018, 10:09

Here are the plots for the frequency and phase measured at the output just before the tone stack - the red line is the Mesa Dual rectifier schematic simulation and the green line is the baja pedal simulation.
As you can see they are very closely matched between 100Hz and 100kHz.
cheers
bajaman

Frank_NH · Post by **Frank_NH** » 09 Feb 2018, 13:44

Very cool, Bajaman. Are you using Spice to simulate the real amplifiers? I've been doing that with some of the old Vox solid state preamps - the main issue is how to translate the original circuit to a 9v solid state pedal. With discrete transistors, it's relatively straightforward, but with tubes...hmmm.

Another quick question. How important is it to match the phase? I've modeled preamps in LTSpice and have mainly looked at waveforms (output versus input sine waves at specific frequencies) and the magnitude of AC frequency sweep.

bajaman · Post by **bajaman** » 09 Feb 2018, 22:02

Very cool, Bajaman. Are you using Spice to simulate the real amplifiers? I've been doing that with some of the old Vox solid state preamps - the main issue is how to translate the original circuit to a 9v solid state pedal. With discrete transistors, it's relatively straightforward, but with tubes...hmmm.

Another quick question. How important is it to match the phase? I've modeled preamps in LTSpice and have mainly looked at waveforms (output versus input sine waves at specific frequencies) and the magnitude of AC frequency sweep.

I use a program called Multisim by National Instruments - I use the tube amplifier schematics to derive a working model and then try to get the same frequency, phase and clipping response characteristics as this with a much lower 9v supply rail. Often this requires lowering individual gain stages considerably to avoid early clipping of the 9v supply rail

I feel the phase response is just as important as the frequency response in defining the "sound' of these pedals. The frequency response is only showing you a two dimensional realisation of the amplifier's response - the phase angle shows you what is happening in the 3rd dimension - a sine wave is not a "roller coaster" in reality it is a "helix".

Try to visualise this waveform in 3 dimensions and you will see that the zero crossing points are actually the same height and depth as the "roller coaster" two dimensional sine wave BUT below and above the flat sheet of paper

I hope that makes sense for you all

cheers
bajaman

Frank_NH · Post by **Frank_NH** » 09 Feb 2018, 23:20

Thanks. In looking MDR schematic, I can see the correspondence between the op amp gain stages and the tube stages in the real amp. I suppose there is an "art" to modeling the tubes with op amps but I'm assuming this process could be applied to most any amp. (Like a Vox AC-30???

) Well, I suppose you also need to factor in the power amp to get fully authentic tones, but the two projects that I've built so far on the breadboard sound great. Hope to get the Highway 1 built on vero this weekend!

J0K3RX · Post by **J0K3RX** » 11 Feb 2018, 16:26

Steve, All other projects have just been shit canned, for the moment. I am gonna start on a couple of these and see how they sound!

I like the optional tone stack idea! Maybe you could also try a "Raw control" so you can mix the raw signal and tone stack..

BAJA MDR overdrive pedal 9v 060218 [documentation]