GDIY passive 'Mojo' box

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monkeyxx

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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.
 

rock soderstrom

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ruffrecords

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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.
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
 

dogears

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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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ruffrecords

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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
 

hodad

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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.
 

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