GDIY passive 'Mojo' box

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skipwave

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Very very nice and all passive, can you share in a easy way to understand?

He said based on Mojo Maestro


Definitely try the author’s suggestion of trying a small transformer before the clipper circuit, it’s interesting what a little transformer saturation can do with certain sources.
 

SWAN808

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I tried the mojo maestro colour module and whilst I think Ethan Weiner has some great videos, this box/circuit sounds pretty gritty and harsh...I don't think it proved his point that you don't need expensive circuits for quality saturation...not to my ears at least...probably there is a better one which still could be cheap...
 

andow

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Very very nice and all passive, can you share in a easy way to understand?
As mentioned, it's basically the circuit in the above link. I just added the suggested input transformer, a second diode in series and different caps for the filter.
 

skipwave

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The audio samples of the Mojo Maestro definitely sound like simple guitar pedal clipping when the circuit is pushed.

Not cheap, or DIY, but the Handsome Audio ZULU is the real mojo box. I still can’t believe it’s passive.

I understand the appeal of passive for DIY ease, but with a couple of opamps you can add pre-emphasis and de-emphasis that results in more distortion in the critical speech intelligibility frequency range in a tape-ish way. You could achieve similar passively with the HPF before clipping and blend that with the clean signal.

The popular “tape sat” circuit is cheap to build, fun to mod, and I think it sounds gf almost too subtle. I added small transformers for core saturation distortion and it’s a pleasant vintage-izer.

 

SWAN808

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thanks for the tip on the tap sat circuit...it looks similar to the clip sat I saw Roger Foote using...I might give that a try...
 

SWAN808

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Does anyone know with this type of diode ladder circuit if there is a way to soften the transfer curve so the clipping is softer - a lot of these circuits seem like clippers which clip too obviously at certain levels...what I'd like to do is have the saturation over a wider range of amplitude...I was hoping there might be a way of tweaking this behaviour with the components...
 

ruffrecords

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Does anyone know with this type of diode ladder circuit if there is a way to soften the transfer curve so the clipping is softer - a lot of these circuits seem like clippers which clip too obviously at certain levels...what I'd like to do is have the saturation over a wider range of amplitude...I was hoping there might be a way of tweaking this behaviour with the components...
Saturation implies further increase in level is not possible which is the same as clipping.

What I think you are asking for is something that changes gain with amplitude but does not clip. This is quite easy to do with diodes simply by adding a resistor in series, The transfer function wil then at some point make a smooth transition from one gain to a lower one. The point at which this occurs is defined by the diode forward characteristic. There is no reason why you could not add a second transition to an even lower gain at some higher level. This could be achieved by using two additional diodes in series with their own resistor so that the transition takes place at a higher input level. This is basically a form of what is known as piece wise linear approximation.

Cheers

Ian
 

bowanderror

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Saturation implies further increase in level is not possible which is the same as clipping.

What I think you are asking for is something that changes gain with amplitude but does not clip. This is quite easy to do with diodes simply by adding a resistor in series, The transfer function wil then at some point make a smooth transition from one gain to a lower one. The point at which this occurs is defined by the diode forward characteristic. There is no reason why you could not add a second transition to an even lower gain at some higher level. This could be achieved by using two additional diodes in series with their own resistor so that the transition takes place at a higher input level. This is basically a form of what is known as piece wise linear approximation.

Cheers

Ian
I was reading about this recently and found a good set of blog posts about something called the "Nachbaur Diode Limiter" (or here for newer post). It uses a diode ladder to sequentially switch in a group of resistors in parallel on the bottom of an attenuator. I haven't given it a shot yet, but there are links to spreadsheet calculators for approximating a given transfer curve.
 

SWAN808

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SWAN808

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Saturation implies further increase in level is not possible which is the same as clipping.

What I think you are asking for is something that changes gain with amplitude but does not clip. This is quite easy to do with diodes simply by adding a resistor in series, The transfer function wil then at some point make a smooth transition from one gain to a lower one. The point at which this occurs is defined by the diode forward characteristic. There is no reason why you could not add a second transition to an even lower gain at some higher level. This could be achieved by using two additional diodes in series with their own resistor so that the transition takes place at a higher input level. This is basically a form of what is known as piece wise linear approximation.

Cheers

Ian
I think I must have a mistaken idea of saturation...that could be because I am a 'DAW Generation' producer...where we encountered many processors first via plugins and saturation plugins became 'de-rigueur' due to the clean ITB sound...but saturation I thought was low level harmonics which were created without coming close to clipping...but the description of literal saturation being an overloaded level makes sense.

However, is there a term which describes the addition of harmonic content without clipping/overdrive? Lets say the sound of running a signal though tubes like the Culture Vulture without audible distortion.
 

scott2000

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However, is there a term which describes the addition of harmonic content without clipping/overdrive? Lets say the sound of running a signal though tubes like the Culture Vulture without audible distortion.
Maybe a term here you're looking for?

 
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ruffrecords

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I think I must have a mistaken idea of saturation...that could be because I am a 'DAW Generation' producer...where we encountered many processors first via plugins and saturation plugins became 'de-rigueur' due to the clean ITB sound...but saturation I thought was low level harmonics which were created without coming close to clipping...but the description of literal saturation being an overloaded level makes sense.

However, is there a term which describes the addition of harmonic content without clipping/overdrive? Lets say the sound of running a signal though tubes like the Culture Vulture without audible distortion.
I think you are right. I am an electronic engineer by training so words like saturation have a very specific meaning for me. The explosion of project studios in the last century has meant there are lots of people using electronic gear who pick up their technical knowledge informally. There is nothing wrong with that but it does tend to lead to a distortion (sic) of well defined technical terms. And I think the term you are looking for is harmonic distortion.

Edit: Things like the culture vulture do add harmonic distortion which is measurable even if it is not audible. Studies undertaken many years ago found that most people could not hear harmonic distortion until it reaches about 5% despite what the golden ear brigade would tell you.

Cheers

Ian
 

okgb

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This is interesting, I've long wanted something to get rid of the non audible [ or musical ] transients that cause the peak leds to light, sometimes it's low frequency stuff as well like handling noise on a bass gtr. Can't make a steep enough passive shelf for the low end but clipping the high end at different levels sounds promising, thanks for bringing this up!
 

jensenmann

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This is interesting, I've long wanted something to get rid of the non audible [ or musical ] transients that cause the peak leds to light, sometimes it's low frequency stuff as well like handling noise on a bass gtr. Can't make a steep enough passive shelf for the low end but clipping the high end at different levels sounds promising, thanks for bringing this up!
This asks for a tape deck ;-)
 

Tubetec

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I'm not sure 5% THD is ever inauduble , I always thought 0.1% was the generally accepted limit of audibillity . Of course the ears sensitivity to distortion varies with frequency also , low bass frequencies in particular can have % points thd and go completely unoticed by ear .
Depending on topology , tube gear were often spec'd upto 5% , triodes especially as even 5% on the peaks might not be judged objectionable . Take your modern day ic op amp based audio circuit , you can bet 5% THD from such a circuit is going to be very objectionable due to a completely different balance of THD products .

Could be the people they did the testing on had no concept or understanding of THD , the results of that test will come out very different from tests with 'HiFi' tuned ears Im sure. Just goes to show ,even under the most rigourous conditions a group of HIFI afacionados may have an underlying bias that the ordinary listening public dont .

Be kinda nice to develop a series of simple test tones , each with added harmonics of varying order and level relative to fundamental , and have people vote on a scale of how objectionable the sound of each was .
 
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ruffrecords

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I'm not sure 5% THD is ever inauduble , I always thought 0.1% was the generally accepted limit of audibillity .
Depending on topology , tube gear were often spec'd upto 5% , triodes especially as even 5% on the peaks might not be judged objectionable . Take your modern day ic op amp based audio circuit , you can bet 5% THD from such a circuit is going to be very objectionable due to a completely different balance of THD products .
I did say the studies were made many years ago and the first one I know of was exclusively tubes and the bandwidth most probably did not extend to 20KHz either. But 5% is definitely the figure that sticks in my mind. Op amps with their huge quantities of NFB are different because the onset of distortion is very sharp so it will be hard to set a system up for just 5% distortion and no more. I know of a later study that tested different transfer functions where the exact distortion could be measured and then conducted listening tests and I am pretty sure the minimum distortion in programme material that could be heard by regular folk was well over 1%. I will see if I can find these studies.

Cheers

Ian
 

Tubetec

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Id say the modern day kids with loud earphones and MP3's are probably deaf or immune to THD by now , so again an underlying bias that probably makes them a poor reference as test subjects , Still would be interesting to compare trained ears to ears used to squarewaves to see whats judged tollerable at opposite ends of the scale .
 

sahib

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I had searched for info about this very topic for my dissertation.

" It has been found that harmonic distortion below 400 Hz is harder to detect than harmonic distortion above 400 Hz [12, 27]. This can be partially explained by the fact that the threshold of hearing increases at low frequencies. Additionally, temporal effects have an impact on the perception distortion due to the finite time resolution of the ear. In studies conducted by Moir [12], it was found that the "just detectable" distortion decreased with increased presentation time. Specifically, it was found that for a 4ms tone burst distorted by clipping, the just detectable distortion reached approximately 10%, while increasing the presentation time to 20ms reduced the just detectable distortion level to 0.3% [12] " .

Source :
Page 16 / http://hifisonix.com/wordpress/wp-content/uploads/2017/11/Perceptual-Levels-of-distortion.pdf

I might be remembering wrong but I think in one of his audio amplifier design books Douglas Self mentions the threshold of audibility of THD to be 1%.
 
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