GDIY passive 'Mojo' box

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ruffrecords

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I see lots of talk about transformer 'Mojo' and how nice it would be to have a passive box that allowed you to dial some in. I seem to recall some guy on gearslutz selling such a box for quite a high price with the guts all potted so nobody can copy it. I should not be too hard to come up with a design for a passive line level box with an undersized transformer, maybe a couple of diodes and some pots to tweak things that will allow all kinds of Mojo to be added to the signal.

Thoughts?

Cheers

Ian
 

TwentyTrees

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Hi Ian,

That's an excellent idea. I had a Handsome Audio 'Zulu' for a while (that might be the passive, potted box you mention?) - it did some very interesting things to the audio and I had a lot of fun with it, but it ultimately didn't gel with my workflow at the time due to the lack of bypass (probably because changing most settings altered the output level, and there was no trim / makeup gain). So yes, something a bit more than just adding transformers into the chain would be very interesting!

Cheers,
Andy
 

emrr

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There’s a bunch of different things on the market, and a small army of people building and selling one-offs as they can.  I’d do a survey of as much as possible.  I sell pairs of vintage repeat coils on a regular basis to people looking for for....something.  What I may think is cool about any type is almost never what people want to chase, so I let them decide rather than COLORING the conversation. 
 

ruffrecords

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Thanks guys, keep the ideas and data coming. Yes, Zulu rings a bell. I checked out their web site. $750 for a couple of channels. I am sure we can beat that.

Cheers

Ian
 

ToBSn

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nice idea. such a thing is also on my diy todo list.

found this a while back:

https://audioxpress.com/article/you-can-diy-build-the-mojo-maestro

it‘s more an saturation box using diodes.
 

ruffrecords

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ToBSn said:
nice idea. such a thing is also on my diy todo list.

found this a while back:

https://audioxpress.com/article/you-can-diy-build-the-mojo-maestro

it‘s more an saturation box using diodes.

Excellent link. Ethan knows his stuff. I think a circuit similar to his with an added transformer would be just the ticket.

Cheers

Ian
 

ToBSn

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I have bunch very small Haufe and Pikatrons lying around.
They mostly made for standard line level 1:1.

What exactly happens when a audio transformer goes into saturation?
 

ToBSn

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Here are my possible candidates.
Haven’t measured this guy’s yet.
I don’t know if they really underrated or not.
 

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ruffrecords

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ToBSn said:
Here are my possible candidates.
Haven’t measured this guy’s yet.
I don’t know if they really underrated or not.

TRIAD also make some similar sized transformers, all with steel cores. I used a 10K:10K one to provide a balanced floating input to a VU meter driver. It worked OK. They also do 600:600 version with excellent low frequency response.

Cheers

ian
 

JMan

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Forgive me if this is exactly what you are describing, or if I’m being completely naive, but when I read this my mind immediately went to the ITB plugin True Iron from Kazrog.  When I’m working in the box, I often find myself reaching for that plugin when I need a bit of, as the French say...”I don’t know what.” 🤣 (Thanks for the Austin Powers clip above!).  The main features are selectable transformer types, a drive/saturation section, and a unity gain/boost switch.

I have to say, the idea of having that kind of thing in honest, physical form (I hate when dsp plugins call themselves “analog” processors, it just feels so wrong) sounds super appealing.  Not sure if this project is shaping up to be something like that, but...maybe it would be possible to consider some of those features?  Or maybe that is more complicated than it seems in the analog realm, I don’t know, you guys are the experts.
 

ruffrecords

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Yes, the True Iron plug - in  is very much like what I have in mind. I don't know about being able to provide the unique tones of specific transformers but certainly there would be controls to alter the degree of drive (crush/strength) and the wet/dry ratio.

Cheers

ian
 

gyraf

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Good idea. Did that once, was an eye-opening process.

But we should first agree what we're chasing.

If we're looking for simple saturation or "linearly coherent" harmonic generation, there's not really much use for the transformer - this can be done at resistor-diode-capacitor level, simpler and cheaper. And yes, even including some of the first-order "tape saturations", that mainly saturates low frequencies (emphasis / deemphasis wrapped around a clipper) and filters distortion products in the highs. Easy and cheap enough, even in digital.

But the harmonic generation in the above "predictable" type of circuits is probably not what we really want, is it? It's usable alright, and even convincing at first - but, at least to my ears, it gets boring quickly. In this regard, the linear harmonic generation is no different from what is done in (recent, good) digital simulations.

What I like in analogue is the introduction of the slightly chaotic component. Not chaotic like noise, but chaotic in the sense of being non-predictable in the particular despite being generally predictable on macroscopic level of working. In other words, it (nearly) always does what you want and expect - it's just that you can't/shouldn't-be-able-to predict in each instance exactly what will happen. Outcome will be distributed in ways or patterns that we can think of as attractors instead of determined. The chaotic component itself stems from miniature (too small to control) variations of initial conditions resulting in rather large variations in outcome, messing up perceived determinism (note that it's still 100% deterministic, just outside of our ability to control and predict).

From outside, feels like magic. Perhaps "organic" more precise

So, where to get that?

The simplest chaotic system consists of an inductor and a diode

"Chaos in a diode" (Andrew Missert and Peter Thompson):
https://www.pas.rochester.edu/~advlab/reports/thompson_missert_choas.pdf

"Chasing Chaos with an RL-Diode Circuit" (Junaid Alam and Sabieh Anwar, 2010):
https://www.google.dk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwj14beZ6NvsAhVSzIUKHTOOCv0QFjACegQIBBAC&url=https%3A%2F%2Fadvlabs.aapt.org%2Fwiki%2FFile%253A2597&usg=AOvVaw00nJUX48SORHur3wjwzJlm

I haven't pursued this all the way down the rabbit-hole, but to me this direction of explanations makes much more sense for describing the subjectively experienced behavior of certain transformer-based circuits that we like a lot. Like "no, it's not happening in the tube, nor in the transformer, so what is it"?

For the hardware, may I suggest the humble Monacor LTR-110 as a candidate? Good core size for our opamp-driven endeavors, many many different winding ratios possible, Core material of some crappy type with just the right amount of Barkhausen grittiness at low levels like reverb tails

/Jakob E.
 

ruffrecords

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The Monacor seems specified for signal levels up to 5V (+16dBu so it might be too hard to drive it into saturation. I suspect a transform specified for around 0dBu would be a better bet.

Edit: I think a first step would be to test a couple of transformers and look at the spectra they produce at various levels, frequencies and source impedances.

Cheers

Ian
 
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