GDIY passive 'Mojo' box

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Check out the Iron Age Audio Portia Street Stomp, it's a passive transformer in a box, with primary and secondary loading pots to get different colors from the driving and receiving amplifiers. I wonder what potentiometer values they used. There's a demo video on YouTube by Seawell Studios if anyone wants to hear it.

There's also the Avedis TransDriver which seems somewhat similar. Maybe not as much color as some of the other things mentioned in this discussion.
 
Check out the Iron Age Audio Portia Street Stomp, it's a passive transformer in a box, with primary and secondary loading pots to get different colors from the driving and receiving amplifiers. I wonder what potentiometer values they used. There's a demo video on YouTube by Seawell Studios if anyone wants to hear it.
This seems a reasonable box. Primary series resistance will definitely alter distortion and secondary load will alter frequency response and possibly the distortion of the driving amp. They say they use a low inductance transformer. I wonder which? I have a couple of small low inductance 600:600 transformers taken from an old broadcast mixer. Might be worth a try with them
There's also the Avedis TransDriver which seems somewhat similar. Maybe not as much color as some of the other things mentioned in this discussion.

Definitely a similar device and a similar price too.

Cheers

Ian
 
This seems a reasonable box. Primary series resistance will definitely alter distortion and secondary load will alter frequency response and possibly the distortion of the driving amp. They say they use a low inductance transformer. I wonder which? I have a couple of small low inductance 600:600 transformers taken from an old broadcast mixer. Might be worth a try with them
We use one output transformer in nearly all of our designs. It's wound for us by Cinemag, not too dissimilar from a Jensen JT-123 or an API 2503. Less turns than a Cinemag CMOQ-2S, though all of these are in the ballpark.

The unit itself is pretty subtle, and the effect of secondary knob changes a lot depending on the driving amp. The ratio switch makes it a lot more fun, but also makes A/B a little bit difficult because of the level change. It's definitely a spin the knobs with your eyes close kind of thing.

Here's a bunch of rough frequency responses when I was playing with the prototype in 1:1 and 2:2.
 

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We use one output transformer in nearly all of our designs. It's wound for us by Cinemag, not too dissimilar from a Jensen JT-123 or an API 2503. Less turns than a Cinemag CMOQ-2S, though all of these are in the ballpark.
Cool. I was not aware you are with Iron Age Audio
The unit itself is pretty subtle, and the effect of secondary knob changes a lot depending on the driving amp. The ratio switch makes it a lot more fun, but also makes A/B a little bit difficult because of the level change. It's definitely a spin the knobs with your eyes close kind of thing.

Here's a bunch of rough frequency responses when I was playing with the prototype in 1:1 and 2:2.
The subtlety is clear from the frequency response graphs. Just a dB or so variation.

Thanks for the info about the transformers used.

Cheers

Ian
 
Spent some time messing with magnets & a homemade DI. Magnets are smallish but reasonably powerful. 2 magnets seemed to give the most dramatic results. I left one at a fixed position on the side of the transformer and moved the other around to different positions atop the xfo. At one particular position, it acted like a high-pass filter--very noticeable. Moving away from that spot eased the low-frequency attenuation, and at certain spots there was also a noticeable increase in the relative level of even-order harmonics.

Test signal was a 40 Hz square wave input from a synth. I used Izotope Insight's Spectrum Meter in my DAW to view the signal.
 
This was description of Neve Silk was recently posted:
That's really interesting - thanks for providing the video link. I would love to see the approach quantified and schematised. How can filtered feedback be applied to only affect the distortion characteristics and not the overall signal?
 
That's really interesting - thanks for providing the video link. I would love to see the approach quantified and schematised. How can filtered feedback be applied to only affect the distortion characteristics and not the overall signal?
There is a schematic floating around here somewhere, and a few explanations on how it works, if memory serves correct
 
I was wondering if it would be possible for someone to draw just the Silk section?

Would it be possible to make something similar as a standalone device?
that is essentially already there in the document you posted. could it be made to be standalone? probably, but there would definitely be some legwork involved, for a product that is essentially still in production. so you might have a hard time getting someone to do that. all that being said, theres nothing to stop you from breadboarding it and seeing what you come up with. Relevant section is here:
1636576349851.png
 
While the dc-offset into the transformer is straight forward (sw pin 5/6 components, as well as the 1uF and R108), the frequency dependant cancellation of transfromer feedback is a looking a little strange (at least to me). How is that s-k lopass filter part balancing against C66 (when silk is off)?
How is that filtering compensated to result in a bandwith up to 100kHz?
Of course the cap readings of 10n could be incorrect...but still-
In the NEVE promo video the "Silk Red" is described to work on the upper frequencies and "Blue" on the lower.
 
I've been thinking about having a go at making something along these lines but my knowledge of transformers is a bit limited.

There's some info floating around about the SSL Fusion too, which also has transformer saturation. Apparently they're using something like a Bourns LMNP 1001 (this could be wrong though). See here for a brief discussion with some shots of the Fusion's internals.
 
I’m going to explore adding pots for adding resistance to the primary and secondary of a 600:600 ohm transformers. Any suggestions for pot values?
As discussed earlier in this thread, series resistance in the primary will increase distortion. Not much you can do on the secondary side. Anything you do there is reflected to the primary and only alters the load seen by whatever is driving the transformer.

Cheers

Ian
 

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